We calculated the photoinduced decomposition of various nitrotoluene molecules, resulting in the formation of atomic carbon, at the B3LYP/6-311++G(d,p) level of theory using Gaussian 09. In addition, we used TD-DFT (B3LYP/6-311++G(d,p)) to calculate the excitation energies. The results confirm our previously reported experimental results. Specifically, we show that the absorption of 226 nm (5.49 eV) light can lead to the decomposition of nitrotoluene molecules and the formation of atomic carbon. One 226 nm photon is sufficient for the dissociation of carbon from 2-NT and 4-NT molecules. During the dissociation process, the CH3 group provides the dissociated carbon atom and the NO2 group accepts the H atoms from either the CH3 group or the benzene ring before carbon exits the molecular system. For the second and third carbon dissociation of 2-NT, the energy barriers are 6.70 eV and 7.43 eV, respectively, and two 226 nm photons would need to be absorbed by the molecule. If extra NO is present during the first carbon dissociation of 2-NT, it gets involved in the last two decomposition steps and forms a C=NH-N=O structure which stabilizes the decomposition products and lowers the energy barrier from 5.22 eV to 4.70 eV. However, for the second and third carbon dissociation of 2-NT, the NO molecules have no apparent effect. For nitrotoluene molecules with two or three NO2 groups (i.e., 2,4-DNT, 2,6-DNT, 3,4-DNT, and 2,4,6-TNT), the first carbon dissociation energies are between 5.26 eV and 5.57 eV. The carbon dissociation pathways for these molecules are similar to those of 2-NT. In 2,4-DNT, the lowest energy barriers for the second and third carbon dissociation are 6.54 eV and 6.60 eV, respectively, which are about 1 eV higher than the energy barrier for the first carbon dissociation. In case of 2,4-DNT/NO and 2,4,6-TNT/NO, NO acts as a catalyst in the first carbon dissociation processes and forms a C=NH-N=O structure which lowers the energy barriers by 0.48 eV and 0.89 eV, respectively.

The investigation of nitrotoluene molecules is of fundamental interest to the field of energetic materials and has been widely reported.1–14 Besides developing a better understanding of the combustion and decomposition properties, developing a better understanding of the photodissociation processes of these molecules may also lead to the development of laser-based detection methods of explosives. Typical photodissociation products that have been reported previously include: CnHm, NO2, NO, O, OH, CO, etc.7 

As part of an investigation to develop a better understanding of potential approaches for the detection of explosive molecules, we recently reported on the experimental observation of neutral carbon during the photodissociation of various nitrotoluene molecules.15 Using a time-resolved fluorescence technique, we observed emission from nitric oxide (NO) and neutral carbon (C) when we excited trace amounts of 2-NT, 4-NT, 2,4-DNT, 3,4-DNT, and 2,6-DNT in nitrogen and air. Furthermore, when the gas mixture contained extra NO, the fluorescence signal from C increased significantly. Also, we observed that the carbon fluorescence after photoexcitation of di-nitrotoluene molecules lasts longer than the carbon fluorescence after photoexcitation of mono-nitrotoluene molecules. The fact that carbon has not previously been optically detected as a dissociation product is most likely due to its emission wavelength overlapping with that of NO and that time-resolved techniques are required to differentiate between the fluorescence from these two fragments.

While the dissociation of NO from nitrotoluene molecules (energy barriers of ∼2.89 eV) has been widely studied,2,16,17 we are not aware of any previous investigations on the carbon dissociation from nitrotoluenes. To better understand our experimental data for the carbon formation, we now perform theoretical calculations of molecular geometries and energies for the Franck-Condon structures, transition states, intermediate states, and final products for carbon dissociation along the ground electronic state potential energy surfaces, and explore possible decomposition mechanisms. When subjected to UV laser light, nitrotoluene molecules can absorb one or two photons and get excited into higher electronic states. Subsequently, through a series of conical intersections among different electronic states, the molecules can rapidly relax to the ground electronic state S0, while the excitation energy is nonadiabatically transferred to the vibrational and rotational excitation of S0.18 This vibrational and rotational energy, stored in the molecules, is then used for the following dissociation reaction. The specific reaction pathways mainly depend on the energy-barrier height for each transition state and the relative energy differences between the Franck-Condon structures and the final carbon-dissociated products. In addition, we calculate the second and third carbon dissociation mechanisms for 2-NT and 2,4-DNT. Furthermore, we study the potential surfaces for the molecular decomposition of 2-NT, 2,4-DNT and 2,4,6-TNT in the presence of NO molecules to determine how the formation of carbon is affected.

Our results show that in many cases, the absorption of a single UV photon is sufficient to overcome the energy barrier for the dissociation of a carbon atom. We also show that in several cases the presence of extra NO can reduce the energy barrier, thereby enhancing the probability for carbon dissociation. We further investigated four reference molecules and compared the computational results with experimental results.

The calculations for the molecular decomposition, resulting in the formation of carbon, are executed at the B3LYP/6-311++G(d,p) level of theory using Gaussian 09, and TD-DFT (B3LYP/6-311++G(d,p)) is used to calculate the excitation energies. In addition, some points in the decomposition pathway are calculated in MP2/6-311++G(d,p) to verify the accuracy of our calculations. No symmetry restrictions are used for the calculations, and equilibrium geometry calculations are conducted taking the total charge as neutral and the spin multiplicity as 1. Following the dissociation, the most stable state of carbon is the triplet state (spin multiplicity equals 3). In the presence of NO, the spin multiplicity for the whole system for the most stable configuration equals 2.

Analytical frequency calculations are used to characterize critical points (minima and transition state structures), and an intrinsic reaction coordinate (IRC) algorithm, implemented in Gaussian 09, is used to calculate minimum energy paths. A relaxed scan optimization algorithm, in which all geometrical parameters, except for the specified bond distance, are optimized and electronic energies are monitored as the specified bond is elongated, is used to find the transition states and intermediate states along the reaction pathways. In the scans, structures with peak potential energies are hypothesized to be a transition state, while structures with potential energies in a valley are hypothesized to be an intermediate state. To verify these hypotheses and to obtain a more accurate potential energy surface for the transition/intermediate states, the molecular structure provided in the scan is used as the initial structure in the subsequent optimization calculation.

Schematic one-dimensional projections of the ground state potential energy surfaces of the various molecules, with locations and potential energies (the presented energies are not corrected for zero-point energy) for different critical points along the minimum energy reaction path, are shown in Figures 1–21 (yellow is carbon, blue is hydrogen, purple is nitrogen, and red is oxygen), where arrows indicate possible carbon dissociation channels. The structures of all the critical points are shown and small arrows near the transition states (TSn) show the imaginary frequencies. The Frank-Condon geometries are the optimized structures of the various molecules with minimum potential energies. TSn are all the transition states in the reaction pathway and IMn are all the intermediate states.

FIG. 1.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2-NT after excitation with a 226 nm laser photon.

FIG. 1.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2-NT after excitation with a 226 nm laser photon.

Close modal
FIG. 2.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 4-NT after excitation with a 226 nm laser photon.

FIG. 2.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 4-NT after excitation with a 226 nm laser photon.

Close modal
FIG. 3.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2-NT after excitation with a 226 nm laser photon. Four possible decomposition pathways are shown.

FIG. 3.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2-NT after excitation with a 226 nm laser photon. Four possible decomposition pathways are shown.

Close modal
FIG. 4.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2-NT after excitation with a 226 nm laser photon. Four possible decomposition pathways are shown.

FIG. 4.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2-NT after excitation with a 226 nm laser photon. Four possible decomposition pathways are shown.

Close modal
FIG. 5.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

FIG. 5.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 6.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

FIG. 6.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 7.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

FIG. 7.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2-NT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 8.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

FIG. 8.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

Close modal
FIG. 9.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 3,4-DNT after excitation with a 226 nm laser photon.

FIG. 9.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 3,4-DNT after excitation with a 226 nm laser photon.

Close modal
FIG. 10.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,6-DNT after excitation with a 226 nm laser photon.

FIG. 10.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,6-DNT after excitation with a 226 nm laser photon.

Close modal
FIG. 11.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

FIG. 11.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

Close modal
FIG. 12.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

FIG. 12.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2,4-DNT after excitation with a 226 nm laser photon.

Close modal
FIG. 13.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

FIG. 13.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 14.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

FIG. 14.

Schematic one-dimensional projection of energy surfaces for the second carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 15.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

FIG. 15.

Schematic one-dimensional projection of energy surfaces for the third carbon dissociation of 2,4-DNT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 16.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4,6-TNT after excitation with a 226 nm laser photon.

FIG. 16.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4,6-TNT after excitation with a 226 nm laser photon.

Close modal
FIG. 17.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4,6-TNT/NO after excitation with a 226 nm laser photon.

FIG. 17.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of 2,4,6-TNT/NO after excitation with a 226 nm laser photon.

Close modal
FIG. 18.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of toluene after excitation with a 226 nm laser photon.

FIG. 18.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of toluene after excitation with a 226 nm laser photon.

Close modal
FIG. 19.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of nitrobenzene after excitation with a 226 nm laser photon.

FIG. 19.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of nitrobenzene after excitation with a 226 nm laser photon.

Close modal
FIG. 20.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of benzene after excitation with a 226 nm laser photon.

FIG. 20.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of benzene after excitation with a 226 nm laser photon.

Close modal
FIG. 21.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of CO2 after excitation with a 226 nm laser photon.

FIG. 21.

Schematic one-dimensional projection of energy surfaces for carbon dissociation of CO2 after excitation with a 226 nm laser photon.

Close modal

The reaction coordinates depicted in Figure 1 include C-N, N-H, C-C, N-O, C-O and O-H bond lengths of the active site of 2-NT. S0 is the optimized structure of 2-NT with minimum potential energy on the S0 state, and Sc is the carbon-dissociated product on the ground state. Starting from S0, and following absorption of a 226 nm (5.49 eV) photon, 2-NT transfers an H atom from the CH3 group to the nearby O atom of the NO2 group through a transition state TS1 to the intermediate state IM1 with an energy barrier of 1.93 eV. Subsequently, the C atom of the CH2 group and the O atom of the ONOH group, which formed in the first step, move closer together and create a C-O bond through TS2 by conquering 1.34 eV of energy. During this process, a five-membered ring is formed which shares a C-C bond with the benzene ring. Next, one of the H atoms of the CH2 group combines with the OH group and an H2O molecule is formed through TS3 with an energy barrier of 1.65 eV. The intermediate state IM3 with a dissociated H2O molecule is 1.02 eV more stable than the S0 structure. Then, on the five-membered ring, the H atom of the CH structure transfers from the C atom to the O atom through TS4 by surmounting an energy of 3.96 eV and forms IM4. Next, the five-membered ring opens through the N-O bond and the H atom transfers from the O atom to the N atom through TS5 with an energy barrier of 0.07 eV. Finally, on the benzene ring, the C=C=O structure re-arranges to C=O=C and the outside carbon atom dissociates from the molecular system.

If the dissociated C atom is in the triplet/singlet state, the carbon-dissociated final product Sc is 5.22 eV/5.62 eV higher in energy than the S0 structure. To experimentally observe the C dissociation, one 226 nm photon needs to be absorbed by the 2-NT molecule.

The structure of 4-NT is similar to that of 2-NT, except that the NO2 group is located opposite to the CH3 group. To transfer an H atom from the CH3 group to the NO2 group, the H atom needs to pass two carbon atoms on the benzene ring as shown in TSn (n=1-4) and IMn (n=1-4) in Figure 2. An H2O molecule is dissociated after two H atoms are transferred from the CH3 group to one of the O atoms of the NO2 group (IM4). Next, the second O atom from the NO2 group connects to a C atom on the benzene ring, forming a four-membered ring via TS5 with an energy barrier of 0.24 eV. Subsequently, the four-membered ring opens via the N-O bond and the H atom transfers from the CH2 group to the N atom on the benzene ring by passing two carbon atoms. Finally, the C atom of the initial CH3 group dissociates from the molecular system and the structure of the carbon-dissociated final product Sc of 4-NT is the same as that of 2-NT. If the dissociated C atom is in the triplet/singlet state, the carbon-dissociated final product Sc is 5.36 eV/5.75 eV higher in energy than the S0 structure. This energy is 0.13 eV lower than the energy provided by a 226 nm photon.

In summary, the CH3 group provides the dissociated carbon atom, and the NO2 group accepts the H atoms from either the CH3 group or the benzene ring before the carbon atom leaves the system. H2O dissociation is an important step during the overall carbon dissociation process. The energy barriers for the carbon dissociation of 2-NT and 4-NT are 5.22 eV and 5.36 eV, respectively, when the carbon atom is in the stable triplet state, and absorption of a single 226 nm photon provides enough energy for the carbon dissociation of these molecules.

The reaction coordinates depicted in Figure 3 include C-H, C-C and O-H bond lengths of the active site of the molecule. Arrows in Figure 3 indicate four possible second carbon dissociation channels for 2-NT, shown in red (1), orange (2), blue (3), and black (4), respectively. Sc,0 is the Frank-Condon geometry of the first carbon-dissociation product, Sc, of 2-NT, see Figure 1, and S2c is the second carbon dissociation product on the ground state.

Starting from Sc,0, on the six-membered ring, an H atom transfers from one CH structure to the nearby C atom of another CH structure (channels (1), (2), and (4)) or to the nearby O atom of the C=O structure (channel (3)) by surmounting transition state energies between 3.31 eV and 4.50 eV. Subsequently, the molecular structure rearranges for channels (1) – (3), and the initial six-membered ring breaks into a CH-C-CH triangle ring combined with a five-membered ring by sharing a CH-CH bond (3+5 ring structure). There are two transition states in channels (1) and (2), TS2 and TS3, that are involved in the 3+5 ring formation. The respective energy barriers are 0.08 eV (channel (1), TS2), 1.94 eV (channel (1), TS3), 0.25 eV (channel (2), TS2), and 2.08 eV (channel (2), TS3). In channel (3), the formation of the 3+5 ring has one transition state, TS2, with an energy barrier of 2.44 eV. In channel (4), the 3+5 ring forms in IM1 and then another H atom transfers from the CH2 structure to the CH structure by surmounting 1.98 eV in TS2. Finally, the carbon atom on the triangle ring dissociates from the molecular system, resulting in the second carbon dissociation of 2-NT. If the dissociated carbon is in the triplet state, the energy of the final carbon-dissociated product S2c is 6.70 eV – 7.52 eV higher compared to the energy of the Sc,0. If the carbon atom is in the singlet state, the energy of the final carbon-dissociated product S2c is 7.09 eV – 7.92 eV higher than the Sc,0. For the second carbon dissociation, the stable aromatic ring of 2-NT has to break into a five-membered ring which requires about 1.5 eV more energy than the first carbon dissociation.

The reaction coordinates depicted in Figure 4 include C-H, C-C and O-H bond lengths of the active site of the molecule. Arrows in Figure 4 indicate four possible third carbon dissociation channels for 2-NT, shown in red (1), orange (2), blue (3), and black (4), respectively. S2c,0 is the Frank-Condon geometry of 2-NT after the second carbon dissociation (S2c in Figure 3), and S3c is the third carbon-dissociated product on the ground state.

Starting from S2c,0, on the five-membered ring, an H atom transfers from the CH structure to: (i) the nearby C atom of another CH structure (channels (1) and (2)), (ii) the nearby O atom of the C=O structure (channel (3)), or (iii) to the nearby C atom of the C=O structure with the opening of the five-membered ring (channel (4)). The energy barriers for these four channels are 3.59 eV – 4.00 eV. Subsequently, the carbon atom dissociates from the molecular system which is the third carbon dissociation of 2-NT. In channels (1) – (3), the final decomposition product forms a four-membered ring and the energy of the final product S3c in these channels is 7.43 eV (triplet state) for channels (1) and (2), and 8.00 eV (triplet state) for channel (3). In channel (4), the final decomposition product forms a chain structure with a CHO group on one side and the energy of this final product S3c is 10.76 eV (triplet state) higher than the energy of the initial second carbon dissociation structure S2c,0. Dissociation of a third carbon atom requires the five-membered ring to break which requires an even higher energy than that needed for the dissociation of the second carbon of 2-NT.

Because the energy barriers for the second and third carbon dissociation of 2-NT are greater than 6 eV, these processes require the absorption of two 226 nm laser photons by the molecule. Breaking the stable aromatic structure is responsible for the larger amount of energy required for the second and third carbon dissociation compared to the first carbon dissociation energy of 2-NT.

The reaction coordinates depicted in Figure 5 include C-N, N-H, C-C, N-O, C-O, O-H, C-H and N-N bond lengths of the active sites of 2-NT and NO. The Frank-Condon geometry S0,NO is the optimized structure of 2-NT with NO nearby (2-NT/NO), and Sc,NO is the carbon-dissociated product on the ground state. The carbon dissociation of 2-NT/NO has a decomposition mechanism that is similar to that of pure 2-NT. However, NO molecules interact in the process during the last two steps, lowering the decomposition energy. Starting from S0,NO and following laser absorption, an H atom is transferred from the CH3 group to the nearby O atom of the NO2 group through transition state TS1 to the intermediate state IM1 with an energy barrier of 1.92 eV. Subsequently, the transferred H atom moves from one oxygen atom to the other oxygen atom in the NO2 group with an energy barrier of 0.79 eV. Next, the C atom of the CH2 group and the O atom of the ONOH group connect through TS3 with an energy barrier of 0.81 eV and form a five-membered ring (same as for the pure 2-NT carbon decomposition). Next, one of the H atoms of the CH2 group combines with the OH group on the five-membered ring and forms an H2O molecule through TS4 by surmounting 1.71 eV. The intermediate state IM4, formed in this process with a dissociated H2O molecule, is 1.06 eV more stable than the S0,NO structure. Next, the five-membered ring opens via the O-N bond with an energy barrier of 2.06 eV, followed by a rearrangement of the C-CHO structure and formation of a C-O-CH structure through TS6, requiring 3.53 eV. Subsequently, the N atom of the C-N structure and the CH of C-O-CH structure move closer together and form an N-CH bond. With the formation of this N-CH bond, a second five-membered ring is created in the system through TS7 with an energy barrier of 0.30 eV. The intermediate state IM6 with the newly formed five-membered ring is 2.66 eV more stable than the S0 structure. Next, on the five-membered ring, the H atom transfers from the C atom to the nearby N atom through TS8 with an energy barrier of 3.28 eV. The NO molecule takes part in the next step when the second five-membered ring opens through the C-NH bond. The N atom of the NO molecule combines with the N atom of the C-NH structure through TS9 by conquering 3.12 eV. Finally, the C atom of the C-O-C structure, which initially came from the CH3 group of 2-NT, dissociates from the molecular system. If the carbon atom is in the triplet/singlet state, the energy of the final carbon-dissociated product Sc,NO is 4.70 eV/5.10 eV higher than the energy of the S0,NO structure. Compared to pure 2-NT, the energy barrier for carbon dissociation is 0.5 eV lower.

There are several similarities between the carbon decomposition pathways of pure 2-NT and 2-NT/NO. First, a five-membered ring is created and then an H2O molecule is dissociated from the molecular system. The energy of the intermediate state with H2O dissociation is about 1 eV more stable than the initial S0 or S0,NO structure. The main difference for the carbon dissociation of pure 2-NT and that of 2-NT/NO is the involvement of NO in the last two decomposition steps, forming stable C=NH-N=O structures. In this structure, the C-N bond is 1.38 Å, the N-N bond is 1.36 Å, and the N=O bond is 1.21 Å (i.e., the N=O bond is a double bond, and the C-N and N-N bonds have characters of both single and double bonds). A large π-bond forms in the molecular system which includes: the remaining aromatic ring after carbon dissociation, the C=O bond on the aromatic ring, and the C=NH-N=O structure, making the dissociation product more stable than that of pure 2-NT. As a result, the carbon dissociation becomes easier when NO is present.

As shown in Figure 6, the second carbon dissociation of 2-NT/NO is similar to that of pure 2-NT. The first step is the transfer of an H atom and the second step is the formation of a triangle ring combined with a five-membered ring (IM2). Similar to the first carbon dissociation of 2-NT/NO, NO is involved in the last two steps, forming a stable C=NH-N=O structure. The final carbon-dissociated product S2c,NO is 7.45 eV/7.85 eV higher for the triplet/singlet state than the Sc,0,NO structure. Compared to the second carbon dissociation of pure 2-NT (6.70 eV – 7.52 eV), the presence of NO does not lower the energy barrier for the second carbon dissociation of 2-NT/NO.

As shown in Figure 7, the first step for the third carbon dissociation of 2-NT/NO is hydrogen transfer followed by the opening of the five-membered ring. For the four reaction channels described in Figure 7, NO is only involved in channel (4) which forms a C=NH-N=O structure in IM2. The energy of the final carbon-dissociated product S3c,NO is 7.17 eV – 10.64 eV for the triplet state (7.56 eV – 11.04 eV for the singlet state) as compared to the S2c,0,NO structure. Compared to the third carbon dissociation energy of pure 2-NT (8.00 eV – 10.76 eV), NO does not have an apparent effect on the third carbon dissociation in 2-NT/NO.

In summary, in the presence of extra NO, it is involved in the last two steps of the first carbon dissociation of 2-NT, forming a C=NH-N=O structure which stabilizes the decomposition products and lowers the energy barrier by about 0.5 eV. This calculation result is consistent with the experimental observations. However, for the second and third carbon dissociation of 2-NT, the theoretical results indicate that the presence of NO does not have any apparent effect on the energy barriers.

2,4-DNT has two NO2 groups and one CH3 groups connected to the benzene ring, one NO2 group is located adjacent to the CH3 group and the other NO2 group is located opposite the CH3 group. The reaction coordinates depicted in Figure 8 include C-N, N-H, C-C, N-O, C-O, C-H and O-H bond lengths of the active site of 2,4-DNT. The Frank-Condon geometry S0 is the optimized structure of 2,4-DNT with minimum potential energy on the S0 state, and Sc is the carbon-dissociated product on the ground state. Starting from S0, and following photon absorption, 2,4-DNT transfers an H atom from the CH3 group to the nearby O atom of the NO2 group through a transition state TS1 to the intermediate state IM1 with an energy barrier of 1.88 eV. Subsequently, the C atom of the CH2 group and the O atom of the ONOH group create a C-O bond through TS2 with an energy barrier of 1.32 eV, and a five-membered ring is formed, as shown in IM2 in Figure 8. Next, one of the H atoms in the CH2 group combines with the OH group and forms an H2O molecule through TS3 with an energy barrier of 1.74 eV. The intermediate state IM3 formed in this process, with a dissociated H2O molecule, is 1.02 eV more stable than the S0 structure. Next, the five-membered ring opens via the O-N bond with an energy barrier of 2.18 eV, and then the C-CHO structure on the benzene ring rearranges and forms a C-O-CH structure through TS5 by surmounting 3.42 eV. Next, the N atom of the C-N structure and the CH group of the C-OCH structure move closer together and form a second five-membered ring through the formation of the N-CH bond via TS6 with an energy barrier of 0.88 eV. The intermediate state IM6 with the benzene ring and newly produced five-membered ring is 2.63 eV more stable than the S0 structure. Finally, on the five-membered ring, an H atom transfers from the C atom of the CH structure to the nearby N atom through TS7 with an energy barrier of 3.72 eV, and then the C atom dissociates from the molecular system. In the triplet/singlet state, the final carbon-dissociated product Sc is 5.57 eV/5.97 eV higher in energy than the S0 structure. This energy is about 0.1 eV higher than the energy of a 226 nm laser photon. While this difference would indicate that two 226 nm laser photons are required to bridge this gap, considering the accuracy of the calculation, one laser photon might be enough to observe the carbon dissociation in the laser absorption experiment of 2,4-DNT.

The carbon decomposition of 2,4-DNT is similar to that of 2-NT. Initially, a five-membered ring is created followed by the dissociation of an H2O molecule, forming an intermediate state which is about 1 eV energy more stable than the initial S0 structure. The energy barrier of the carbon dissociation of 2,4-DNT is 0.35 eV higher than that of 2-NT. The extra NO2 group on the benzene ring which is located opposite the CH3 group does not get involved in the dissociation mechanism and its existence does not affect the energy barrier for the carbon dissociation.

3,4-DNT has two NO2 groups and one CH3 group connected to the benzene ring, neither one of them adjacent to the CH3 group. The carbon dissociation of 3,4-DNT is similar to that of 2,4-DNT, including the formation of the five-membered ring (IM6) and the H2O dissociation (IM7). However, the CH3 and NO2 groups of 3,4-DNT are separated by a CH structure. To form a five-membered ring, the CH3 group needs to pass the CH group and then move to the NO2 group, see Figure 9, from TS1 to IM6. In the triplet/singlet state, the final carbon-dissociated product Sc is 5.34 eV/5.74 eV higher than the S0 structure.

2,6-DNT’s two NO2 groups are located on either side of the CH3 group. The carbon dissociation of 2,6-DNT is similar to that of 2-NT, and only one NO2 group is involved in the process. In the triplet/singlet state, the final carbon-dissociated product Sc of 2,6-DNT is 5.26 eV/5.66 eV higher than the S0 structure, and one 226 nm laser photon is sufficient for the carbon dissociation.

If two NO2 groups are present in the nitrotoluene molecules, the position of the NO2 groups with respect to the CH3 group may affect the detailed decomposition process, however, it won’t change the overall carbon dissociation mechanisms. 2,4-DNT, 3,4-DNT and 2,6-DNT all have carbon dissociation pathways similar to that of 2-NT, and the second NO2 group in the system does not lower the energy barrier for the carbon decomposition. As is well-known, compared to nitrotoluene molecules with a single NO2 group, nitrotoluene molecules with two NO2 groups are more reactive in the NO and NO2 decomposition reaction. However, for carbon dissociation, the increased number of NO2 groups does not make a difference.

The reaction coordinates depicted in Figure 11 include C-H, C-C and O-H bond lengths of the active site of the molecule. Five different decomposition reaction pathways are shown in red (1), orange (2), green (3), blue (4), and black (5) in Figure 11. Sc,0 is the Frank-Condon geometry of the first carbon dissociation product Sc of 2,4-DNT, and S2c is the second carbon-dissociated product on the ground state.

Starting from Sc,0, on the six-membered ring, an H atom transfers from the CH structure to: (i) the nearby C atom of the C-NO2 structure (reaction channel (1)), (ii) the nearby C atom of the CH structure (reaction channel (2)), (iii) the nearby N atom of the C-N structure (reaction channel (3)), (iv) the nearby O atom of the C-O structure (reaction channel (4)), or (v) the nearby O atom of the C-NO2 structure (reaction channel (5)) by conquering transition states TS1 with energy barriers of 2.92 eV – 3.56 eV to the intermediate state IM1. Subsequently, the molecular structure rearranges and the initial six-membered ring breaks into a CH-C-CH triangle ring which combines with a five-membered ring (from IM1 to IM3 in Figure 11). Finally, the carbon atom on the triangle ring is dissociated from the system which is the second carbon dissociation of 2,4-DNT, and the final dissociation products S2c in the five channels all have a five-membered ring in the molecular system. If the dissociated carbon is in the triplet state, the energy of the final carbon-dissociated product S2c is 6.54 eV – 8.09 eV higher compared to the energy of Sc,0. If the carbon is in the singlet state, the energy of the final carbon-dissociated product S2c is 6.93 eV – 8.49 eV higher than Sc,0. Based on the theoretical calculation, the energy barrier of the second carbon dissociation is at least 1 eV higher than that of the first carbon dissociation of 2,4-DNT.

The reaction coordinates in Figure 12 include C-H, C-C and O-H bond lengths of the active site of the molecule. Five different decomposition reaction pathways are shown in red (1), orange (2), green (3), blue (4), and black (5). S2c,0 is the Frank-Condon geometry of the first carbon-dissociated product S2c of 2,4-DNT, and S3c is the third carbon-dissociated product on the ground state.

Starting from S2c,0, an H atom transfers from the CH structure on the five-membered ring to (i) the nearby C atom of the C=O structure with the opening of the five-membered ring (reaction channel (1)), (ii) the nearby C atom of the CH structure (reaction channels (2) and (3)), (iii) the nearby O atom of the C-NO2 structure (reaction channel (4)), and (iv) the nearby O atom of the C=O structure (reaction channel (5)). The energy barriers for these five channels range from 3.45 eV to 3.93 eV. Subsequently, one of the C atoms is dissociated from the molecular system which is the third carbon dissociation of 2,4-DNT. In channels (1) and (2), the final decomposition products are in a chain structure with an O=N-O-C=O group on one side. In the triplet state, the energies of the final products S3c are 6.60 eV and 6.69 eV higher than the energy of S2c,0, respectively. In channel (3), the final decomposition product forms a six-membered ring which owns four carbon atoms, one N atom and one O atom from the NO2 group. In the triplet state, the energy of this final product S3c in channel (3) is 7.08 eV. In channels (4) and (5), the final dissociation products form a four-membered carbon ring. In the triplet state, their energies are 7.19 eV and 7.32 eV. For the dissociation of the third carbon of 2,4-DNT, the five-membered ring must be broken, requiring an even higher energy than the dissociation of the second carbon.

In 2,4-DNT, the lowest energy barriers for the second and third carbon dissociation are 6.54 eV and 6.60 eV, respectively, which are about 1 eV higher than the energy barrier for the first carbon dissociation. Compared to 2-NT which needs 6.70 eV for the second carbon dissociation and 7.43 eV for the third carbon dissociation, the second and third carbon dissociation of 2,4-DNT requires less energy. The extra NO2 group in 2,4-DNT, which forms a stable O=N-O-C=O structure in the system after dissociation, may be responsible for these lower energy barriers.

The reaction coordinates depicted in Figure 13 include C-N, N-H, C-C, N-O, C-O, O-H, C-H and N-N bond lengths of the active site of 2,4-DNT and NO molecule. The Frank-Condon geometry S0,NO is the optimized structure of 2,4-DNT/NO with minimum potential energy on the S0 state, and Sc,NO is the carbon-dissociated product on the ground state.

The carbon dissociation process of 2,4-DNT/NO is similar to that of pure 2,4-DNT. However, the extra NO molecules participate in the process and lower the decomposition energy barrier. Starting from S0,NO, and following photon absorption, 2,4-DNT transfers an H atom from the CH3 group to the nearby O atom of the NO2 group through transition state TS1 to the intermediate state IM1 with an energy barrier of 1.86 eV. Subsequently, the transferred H atom moves from one oxygen atom to the other oxygen atom of the NO2 group by conquering 0.84 eV energy. Next, the C atom of the CH2 group and the O atom of the ONOH group move closer together and create a five-membered ring through TS3 with an energy barrier of 0.84 eV. Then, one of the H atoms of the CH2 group combines with the OH group on the five-membered ring and produces an H2O molecule through TS4 with an energy barrier of 2.09 eV. The intermediate state IM4 formed in this process with a dissociated H2O molecule is 0.94 eV more stable than the S0 structure. Next, the five-membered ring opens via the O-N bond while the NO molecule moves closer and connects to the N atom which originated from the newly broken N-O bond by surmounting 1.04 eV energy. Following this process, the C-CHO structure on the benzene ring rearranges and forms a C-O-CH structure through TS6 by conquering 4.21 eV energy. Next, the H atom of the C-O-CH structure transfers to the nearby N atom of the C=NH-N=O structure as shown in Figure 13 through TS7 with an energy barrier of 0.32 eV. Finally, the C atom of the C-O-C structure which initially came from the CH3 group in S0,NO dissociates from the molecular system. In the triplet/singlet state, the final carbon-dissociated product Sc,NO is 4.68 eV/5.07 eV higher than the S0,NO structure, and one 226 nm photon is required in the 2,4-DNT/NO system.

Similar to the carbon dissociation process of pure 2,4-DNT, a five-membered ring is created in the 2,4-DNT/NO molecular system and the energy of the five-membered ring structure with H2O dissociation is about 1 eV more stable than the initial S0,NO structure. The main difference between the carbon dissociation of pure 2,4-DNT and that of 2,4-DNT/NO is the participation of NO during the decomposition steps and the formation of a stable C=NH-N=O structure which helps to form a large π bond in the molecular system, stabilizing the decomposition product. The energy barrier for carbon dissociation of 2-NT/NO is 0.52 eV lower than that of pure 2-NT, while in 2,4-DNT/NO, the energy barrier of carbon dissociation is 0.89 eV lower than that of pure 2,4-DNT. Thus, NO molecules have an similar effect in the carbon dissociation processes of 2-NT and 2,4-DNT.

The schematic one-dimensional projections of the ground state potential energy surfaces for the second carbon dissociation of 2,4-DNT/NO (Sc,0,NO) and the third carbon dissociation of 2,4-DNT/NO (S2c,0,NO) are plotted in Figures 14 and 15. As shown in Figure 14, for the second carbon dissociation, an H atom transfers from the CH structure to the nearby carbon of the C-NO2 structure through TS1 with an energy barrier of 3.57 eV. Subsequently, the carbon atoms rearrange and form a C-C-NH triangle ring by surmounting 1.80 eV energy through TS2. Finally, one of the carbon atoms on the C-C-NH triangle ring dissociates from the system. In the triplet/singlet state, the carbon-dissociated product S2c,NO is 7.59 eV/7.98 eV higher than the Sc,0,NO structure. During the carbon dissociation process, NO does not get involved in the decomposition steps. Compared to the second carbon dissociation energy of pure 2,4-DNT (6.54 eV – 8.09 eV), the presence of NO does not affect the second carbon dissociation of 2,4-DNT.

As shown in Figure 15, for the third carbon dissociation of 2,4-DNT/NO, the first step is hydrogen transfer and the second step the opening of the five-membered ring which is similar to the third carbon dissociation of pure 2,4-DNT. For the listed five reaction channels in Figure 15, NO gets involved in reaction channels (3) and (4) and forms a C=NH-N=O structure. In the triplet/singlet state, the final carbon-dissociated product S3c,NO is (7.09 – 8.39) eV/(7.49 – 8.79) eV higher than the S2c,0,NO structure. Compared to the third carbon dissociation energy of pure 2,4-DNT (6.60 eV – 7.32 eV), the presence of NO does not help the third carbon dissociation of 2,4-DNT.

In the presence of NO, these molecules act as catalysts in the first carbon dissociation. Similar to the carbon dissociation mechanism of 2-NT/NO, NO helps to form a C=NH-N=O structure, and lowers the energy barrier by 0.89 eV. However, the presence of NO molecules does not affect the second and third carbon dissociation.

2,4,6-TNT has three NO2 groups and one CH3 groups connected to the benzene ring, with two of the NO2 groups located adjacent to the CH3 group and the third NO2 group located opposite the CH3 group. The reaction coordinates depicted in Figure 16 include C-N, N-H, C-C, N-O, C-O, C-H and O-H bond lengths of the active site of 2,4,6-TNT. The Frank-Condon geometry S0 is the optimized structure of 2,4,6-TNT with minimum potential energy, and Sc is the carbon-dissociated product on the ground state.

Starting from S0, and following photon absorption, 2,4,6-TNT transfers an H atom from the CH3 group to the nearby O atom of an NO2 group through transition state TS1 to the intermediate state IM1 with an energy barrier of 1.83 eV. Subsequently, the C atom of the CH2 group and the O atom of the ONOH group create a C-O bond through TS2 with an energy barrier of 1.22 eV and a five-membered ring is formed as shown in IM2 in Figure 16. Next, one of the H atoms of the CH2 group combines with the OH group and forms an H2O molecule through TS3 with an energy barrier of 1.79 eV. The intermediate state IM3 formed in this process, with a dissociated H2O molecule, is 1.22 eV more stable than the S0 structure. Next, on the five-membered ring, the H atom moves from the C atom to the O atom through TS4 with an energy barrier of 4.19 eV. Subsequently, the five-membered ring opens via the N-O bond and the H atom moves again from the O atom to the N atom via TS5 by surmounting 0.15 eV. Finally, the C atom, initially from the CH3 group and now from the C=O=C structure, dissociates from the molecular system. In the triplet/singlet state, the final carbon-dissociated product Sc is 5.37 eV/5.77 eV higher than the S0 structure.

The carbon decomposition pathway of 2,4,6-TNT is similar to that of 2-NT and 2,4-DNT. Initially, a five-membered ring is created followed by the dissociation of H2O leading to an intermediate state that is 1.22 eV more stable than the initial S0 structure. Only one NO2 group on the benzene ring is involved in the carbon decomposition pathway. Compared to 2-NT and 2,4-DNT, the energy barrier for the dissociation of carbon from 2,4,6-TNT is 0.15 eV higher than that of 2-NT and 0.2 eV lower than that of 2,4-DNT. Thus, the extra number of NO2 groups on the benzene ring does not affect the energy barriers for carbon dissociation.

A schematic one-dimensional projection of the ground state potential energy surfaces (S0,NO) of 2,4,6-TNT/NO, is plotted in Figure 17. The reaction coordinates depicted in Figure 17 include C-N, N-H, C-C, N-O, C-O, O-H, C-H and N-N bond lengths of the active site of 2,4,6-TNT/NO.

The Frank-Condon geometry S0,NO is the optimized structure of 2,4,6-TNT/NO on the S0 state, and Sc,NO is the carbon-dissociated product on the ground state. The carbon dissociation mechanism of 2,4,6-TNT/NO is similar to that of pure 2,4,6-TNT. Starting from S0,NO, and following photon absorption, 2,4,6-TNT transfers an H atom from the CH3 group to the nearby O atom of the NO2 group through transition state TS1 to the intermediate state IM1 with an energy barrier of 1.82 eV. Subsequently, the C atom of the CH2 group and the O atom of the ONOH group move closer and form a five-membered ring through TS2 with an energy barrier of 0.36 eV. Next, one of the H atoms in the CH2 group combines with the OH group on the five-membered ring and an H2O molecule is dissociated through TS3 with an energy barrier of 1.58 eV. The intermediate state IM3 formed in this process with a dissociated H2O molecule is 1.31 eV more stable than the S0 structure. Next, the five-membered ring opens via the O-N bond and the NO molecule moves closer and connects to the N atom of the newly broken N-O bond by surmounting 1.88 eV, forming a C=NH-N=O structure (IM4). Following this process, the H atom of the C-CHO structure moves to the N atom of C=NH-N=O structure through TS5 by conquering 2.22 eV. Finally, the C atom initially on the CH3 group dissociates from the molecular system and the final carbon-dissociated triplet/singlet product Sc,NO is 4.89 eV/5.29 eV higher than the S0,NO structure, and the dissociation energy is 0.48 eV lower compared to that of pure 2,4,6-TNT.

In summary, similar to 2-NT/NO and 2,4-DNT/NO, NO molecules lower the energy barrier for the carbon dissociation of 2,4,6-TNT/NO by forming a C-NH-N=O structure. When NO is present, it acts as a catalyst and lowers the energy barriers for carbon dissociation by 0.48-0.89 eV compared to that of pure 2-NT, pure 2,4-DNT, and pure 2,4,6-TNT. The number of NO2 groups in the molecular system has no relationship with how much the energy barrier is decreased by NO in the carbon dissociation reaction.

For reference purposes, toluene, nitrobenzene, benzene and CO2 molecules are also studied under the same experimental condition, however, no carbon dissociation is observed in these molecules. The paragraphs below explain the experimental observation through theoretical calculations.

A schematic one-dimensional projection of the ground state potential energy surfaces (S0) of toluene is plotted in Figure 18. The Frank-Condon geometry S0 is the optimized structure of toluene with minimum potential energy on the S0 state, and Sc,NO is the carbon-dissociated product on the ground state. As shown in Figure 18, the H atoms transfer from the CH3 group to nearby carbon atoms on the benzene ring and then the carbon atom, initially from the CH3 group, dissociates from the molecular system. In the triplet/singlet state, the energy barrier for this carbon dissociation is 8.03 eV/8.43 eV, which is 2.81 eV higher than the carbon dissociation energy of 2-NT. If NO2 groups are present in the molecular system, they can accept H atoms from the CH3 group. Without the presence of NO2 groups, the H atoms on CH3 group have to transfer to the benzene ring before carbon dissociation can occur. In this case, the stable aromatic structure of toluene with a higher energy barrier has to be break.

A schematic one-dimensional projection of the ground state potential energy surfaces (S0) of nitrobenzene is plotted in Figure 19. The Frank-Condon geometry S0 is the optimized structure of nitrobenzene with minimum potential energy on the S0 state, and Sc,NO is the carbon-dissociated product on the ground state. For the carbon dissociation of nitrobenzene, the benzene ring breaks into a five-membered ring combined with a triangle ring. Subsequently, the carbon atom on the triangle ring dissociates from the molecular system. The energy barrier for the carbon dissociation of nitrobenzene is 7.96 eV/8.35 eV in the triplet/singlet state, which is 2.74 eV higher than the carbon dissociation energy of 2-NT. Without a CH3 group present, the dissociated carbon has to originate from the benzene ring, requiring the stable aromatic structure to break.

In summary, to lower the carbon dissociation energy, both a CH3 group and an NO2 group should exist in the molecular system. CH3 groups provide the carbon for the dissociation (i.e., the benzene ring does not need to open), while NO2 groups help with the molecular stabilization, resulting in an easier carbon dissociation.

Meanwhile, as shown in Figures 20 and 21, the energy barrier for the carbon dissociation of benzene and CO2 is 8.43 eV and 11.32 eV, respectively. These energies are significantly higher than those required for the carbon dissociation of 2-NT. As a result, no carbon dissociation is found in these molecules, consistent with the experimental observations.

The excitation energies of the investigated molecules are calculated via TD-DFT, and the results are shown in Figures 22 and 23, and summarized in Table I. Although TD-DFT is not a very accurate method for the calculation of excitation energies, CCSD yields values of 5.18 eV and 6.48 eV for the first and second excited states of benzene, which are close to the actual values of 5.39 eV and 6.05 eV, respectively.19 Nitrobenzene and nitrotoluene molecules with one NO2 group (2-NT and 4-NT) have similar excitation energies. The theoretical TD-DFT spectra show lower excited states at 4.5 eV to 4.8 eV, and higher excited states at 5.7 eV to 5.9 eV, with no other excited states in-between. A 226 nm photon has an energy of 5.49 eV which is enough for the excitation of the lower excited states. Combined with some vibrational excitations, some higher excited states may also be excited.

FIG. 22.

Calculated excitation energies of 2-NT (top left), 2-NT/NO (top right), 4-NT (middle left), 2,4-DNT (middle right), 2,6-DNT (bottom left), and 3,4-DNT (bottom right).

FIG. 22.

Calculated excitation energies of 2-NT (top left), 2-NT/NO (top right), 4-NT (middle left), 2,4-DNT (middle right), 2,6-DNT (bottom left), and 3,4-DNT (bottom right).

Close modal
FIG. 23.

Calculated excitation energies of 2,4,6-TNT (top left), nitrobenzene (top right), toluene (middle left), CO2 (middle right), and benzene (bottom left).

FIG. 23.

Calculated excitation energies of 2,4,6-TNT (top left), nitrobenzene (top right), toluene (middle left), CO2 (middle right), and benzene (bottom left).

Close modal
TABLE I.

Comparison of excitation energies near 226 nm laser for different molecules.

Excited state energies (eV)
Pure MoleculeMolecule in presence of NO
2-NT 3.76, 4.14, 4.33, 4.75, 5.70 2.24, …, 4.94, 5.05, 5.09, 5.13, 5.16, 5.32, 5.41 
4,NT 3.79, 4.32, 4.33, 4.49, 5.73  
2,4-DNT 3.78, …, 4.97, 5.15, 5.27, 5.43, 5.97 1.63, …, 5.15, 5.17, 5.19, 5.20, 5.22 (x2), 5.24, 
  5.27, 5.28 
2,6-DNT 3.77, …, 4.84, 5.38, 5.44, 5.55  
3,4-DNT 3.71, …, 4.95, 5.21, 5.38, 5.46, 5.56  
2,4,6-TNT 3.78, …, 5.02, 5.18, 5.19, 5.20, 5.25, 5.28, 5.30  
Nitrobenzene 3.78, 4.30, 4.32, 4.71, 5.90, 6.03  
Toluene 5.26, 5.79, 5.88  
Benzene 5.39, 6.05, 6.60  
CO2 8.67  
Excited state energies (eV)
Pure MoleculeMolecule in presence of NO
2-NT 3.76, 4.14, 4.33, 4.75, 5.70 2.24, …, 4.94, 5.05, 5.09, 5.13, 5.16, 5.32, 5.41 
4,NT 3.79, 4.32, 4.33, 4.49, 5.73  
2,4-DNT 3.78, …, 4.97, 5.15, 5.27, 5.43, 5.97 1.63, …, 5.15, 5.17, 5.19, 5.20, 5.22 (x2), 5.24, 
  5.27, 5.28 
2,6-DNT 3.77, …, 4.84, 5.38, 5.44, 5.55  
3,4-DNT 3.71, …, 4.95, 5.21, 5.38, 5.46, 5.56  
2,4,6-TNT 3.78, …, 5.02, 5.18, 5.19, 5.20, 5.25, 5.28, 5.30  
Nitrobenzene 3.78, 4.30, 4.32, 4.71, 5.90, 6.03  
Toluene 5.26, 5.79, 5.88  
Benzene 5.39, 6.05, 6.60  
CO2 8.67  

The excitation energies of nitrotoluene molecules with two or three nitro groups (i.e., 2,4-DNT, 3,4-DNT, 2,6-DNT and 2,4,6-TNT) that are closest to the energy of a 226 nm photon are 5.15 eV, 5.21 eV, 4.84 eV, and 5.25 eV, respectively. These energies are higher than those of molecules with a single NO2 group. The additional NO2 groups can create more electronic states in the molecule, resulting in a more readily photon absorption process. When NO is present, its effect on the excitation energies is similar to that of extra NO2 groups, see Table II. Comparing pure 2-NT with 2-NT/NO, and pure 2,4-DNT with 2,4-DNT/NO, pure 2-NT absorbs at 4.75 eV while 2-NT/NO absorbs at 5.09 eV, 0.34 eV higher, and pure 2,4-DNT absorbs at 5.15 eV, while 2,4-DNT/NO absorbs at 5.24 eV. To check the accuracy of the DFT calculations, MP2/6-311++G(d,p) is applied to calculate the carbon dissociation energies of 2-NT, 2-NT/NO, and toluene molecules. The resulting energies are listed in parentheses next to the DFT-based energy values in Table II. The difference in calculated energies between these two methods is less than 0.25 eV, indicating that DFT yields reasonable dissociation energies.

TABLE II.

Carbon dissociation energies for different molecules calculated in B3LYP/6-311++G(d,p). For comparison, the values listed in parentheses are from MP2/6-311++G(d,p)-based calculations.

C dissociation energy (eV)
Pure moleculeCombined with NO
2-NT 5.22 (5.44) 4.70 (MP2) 
2-NT 2nd C dissociation 6.70 7.45 
2-NT 3rd C dissociation 7.43 7.17 
4-NT 5.36  
2,4-NT 5.57 5.06 
2,4-NT 2nd C dissociation 6.54 7.59 
2,4-NT 3rd C dissociation 6.60 7.09 
3,4-NT 5.34  
2,6-NT 5.26  
2,4,6-TNT 5.37 4.89 
Toluene 8.03 (8.08)  
Nitrobenzene 7.96  
CO2 11.32  
Benzene 8.04  
C dissociation energy (eV)
Pure moleculeCombined with NO
2-NT 5.22 (5.44) 4.70 (MP2) 
2-NT 2nd C dissociation 6.70 7.45 
2-NT 3rd C dissociation 7.43 7.17 
4-NT 5.36  
2,4-NT 5.57 5.06 
2,4-NT 2nd C dissociation 6.54 7.59 
2,4-NT 3rd C dissociation 6.60 7.09 
3,4-NT 5.34  
2,6-NT 5.26  
2,4,6-TNT 5.37 4.89 
Toluene 8.03 (8.08)  
Nitrobenzene 7.96  
CO2 11.32  
Benzene 8.04  

The carbon dissociation energies for the different molecules are summarized in Table II. For 2-NT, 4-NT, 2,4-DNT, 3,4-DNT, 2,6-DNT and 2,4,6-TNT, the energy barriers range from 5.22 eV to 5.57 eV. Considering potential errors between theoretical calculations and actual energies, one 226 nm laser photon absorption may be sufficient for the carbon dissociation of all these molecules. An increased number of NO2 groups does not make the carbon dissociation any easier. For the second and third carbon dissociation of 2-NT and 2,4-DNT, the energy barriers range from 6.54 eV to 7.43 eV which means these reactions are more difficult to happen in our experimental system, as at least two 226 nm laser photons need to be absorbed by the molecule. In the presence of NO, the NO molecule acts as a catalyst in the first carbon dissociation reaction. For 2-NT, 2,4-DNT and 2,4,6-TNT, NO molecules lower the energy barrier for the first carbon dissociation by 0.48 eV to 0.52 eV, making the reaction easier to happen which is consistent with the experimental observations. However, NO molecules do not affect the energy barriers for the second and third carbon dissociation. The carbon dissociation energy of toluene and nitrobenzene is around 8 eV which means both, CH3 groups and NO2 groups contribute to the carbon dissociation of nitro-toluenes.

The potential surfaces for the molecular decomposition with formation of carbon have been calculated for 2-NT, 4-NT, 2,4-DNT, 3,4-DNT, 2,6-DNT, 2,4,6-TNT, toluene, nitrobenzene, benzene, and CO2, at the B3LYP/6-311++G(d,p) level of theory using Gaussian 09, and their excitation energies have been calculated using TD-DFT. For nitrotoluene molecules with a single NO2 group (2-NT and 4-NT), the energy barriers of their first carbon dissociation are 5.22 eV and 5.36 eV, respectively, when the carbon atom is in the stable triplet state. One 226 nm laser photon absorption is sufficient for the carbon dissociation of 2-NT and 4-NT. During the carbon dissociation process, the CH3 group provides the dissociated carbon atom and the NO2 group accepts the H atoms from either the CH3 group or the benzene ring before the carbon atom leaves the system. For the second and third carbon dissociation of 2-NT, the energy barriers are 6.70 eV and 7.43 eV, respectively, and two 226 nm laser photons would need to be absorbed by the molecule.

If NO is present during the first carbon dissociation of 2-NT, it gets involved in the last two decomposition steps and forms a C=NH-N=O structure which stabilizes the decomposition products and lowers the energy barrier from 5.22 eV to 4.70 eV. However, for the second and third carbon dissociation of 2-NT, the NO molecules have no apparent effect.

For nitrotoluene molecules with two or three NO2 groups (i.e., 2,4-DNT, 2,6-DNT, 3,4-DNT, and 2,4,6-TNT), the first carbon dissociation energies are between 5.26 eV and 5.57 eV. The carbon dissociation pathways for these molecules are similar to that of 2-NT, but the extra NO2 groups do not lower the energy barriers for the carbon decomposition. In 2,4-DNT, the lowest energy barriers for the second and third carbon dissociation are 6.54 eV and 6.60 eV, respectively, which are about 1 eV higher than the energy barrier for the first carbon dissociation. In case of 2,4-DNT/NO and 2,4,6-TNT/NO, NO acts as a catalyst in the first carbon dissociation processes and forms a C=NH-N=O structure which lowers the energy barriers by 0.48 eV and 0.89 eV, respectively.

The carbon dissociation energy of toluene and nitrobenzene is around 8 eV, indicating that both, the CH3 group and the NO2 groups contribute to the carbon dissociation of nitrotoluene molecules. Compared to 2-NT and 4-NT, the extra NO2 groups of 2,4-DNT, 2,6-DNT, 3,4-DNT and 2,4,6-TNT lead to excitation energies closer to the 226 nm laser photon (5.49 eV) energy. When NO is present, the NO molecule can affect the excitation energy, making the laser absorption easier as well.

The theoretical results confirm our experimental observation for various nitrotoluene molecules. Using 226 nm laser photons, we observed emission from neutral carbon. When extra NO is present, the carbon dissociation energy barrier is lowered, increasing the probability for the observation of carbon. The computational verification of the observed time differences between mono-nitrotoluenes and di-nitrotoluenes will require further work.

This work was supported was supported by ONR (N000014-16-1-2088).

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