The recent discovery of benzonitrile (C6H5CN), one of the simplest nitrogen-bearing polar aromatic molecules, in the interstellar medium motivates structural characterization of the benzonitrile-containing molecular ions as potential precursors for nitrogen-containing complex organics in space. Herein, we present mass-selected ion mobility measurements combined with density functional theory (DFT) calculations to reveal, for the first time, the structures of the benzonitrile dimer radical cation, the protonated dimer, and the protonated hydrated small clusters in the gas phase. The measured collision cross sections of the investigated ions in helium are in excellent agreement with the calculated values of the lowest energy DFT structures. Unlike the dimer radical cations of nonpolar aromatic molecules which adopt parallel sandwich configurations, the (C6H5CN)2·+ displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen and hydrogen atoms. The protonated dimer has the structure of a proton-bound dimer (C6H5CNH+NCC6H5) where the bridging proton connects the nitrogen atoms in the two benzonitrile molecules resulting in a calculated collision cross section of 101.1 Å2 in excellent agreement with the measured value of 103.3 Å2. The structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules. By locating the proton on the lower proton affinity water molecule, the resulting hydronium ion can be fully solvated by forming three ionic hydrogen bonds with the benzonitrile molecules. These unique structural motifs could be useful for the molecular design and recognition involving charged aromatic systems and also for the search of nitrogen-containing complex organics in space.

Dimer radical cations and protonated dimers of aromatic molecules are of fundamental importance in chemistry, biochemistry, and astrochemistry. For example, dimer radical cations constitute the smallest intermolecular units that carry a delocalized positive charge and thus provide the basis for photoconductivity and ferromagnetism in organic materials.1–3 They can also play critical roles in other diverse areas such as protein structures, base pair stacking in DNA, drug design, and crystal packing of aromatic molecules.4–7 Dimer cations of aromatics may also play important roles in astrochemistry, where they may be responsible for much of the interstellar extended infrared (IR) bands.8–13 Protonated dimers and clusters of aromatic molecules play important roles in ion solvation and may result in forming stable hydrated structures that could be related to proton solvation in the condensed phase.13–15 

In homodimer radical cations of nonpolar aromatic molecules such as benzene and naphthalene, the unpaired electron is considered to be equally distributed between the two moieties providing extra charge resonance stabilization in addition to the ion-induced dipole and dispersion interactions.3 The charge resonance stabilization is considered to be at a maximum in sandwich-like structures.16–20 Therefore, the dimer cations of nonpolar aromatic molecules are believed to adopt parallel sandwich configurations on the basis of maximizing the charge transfer resonance interactions.16–20 Density functional theory (DFT) calculations, at an all-electron level and without any symmetry constraint, predicts that the benzene dimer radical cation has two nearly degenerate ground state structures with the sandwich configuration more stable than the T-configuration by only 1.6 kcal/mol.19 Ion mobility experiments indicate that only one structure is observed for the mass-selected dimer cation with a measured collision cross section in helium of 71 Å2 at room temperature in good agreement (within 2.4%) with the calculated cross section for the sandwich dimer.19 Similarly both DFT calculations and ion mobility experiments indicate that the (naphthalene)2·+ and naphthalene·+(benzene) dimer radical cations have stacked sandwich structures.20,21

In the present work, we investigate whether highly polar aromatic molecules will form stacked or T-shaped dimer radical cations or adopt other ion-dipole and hydrogen bonding structures. To answer this question, we study the structure of the benzonitrile (C6H5CN or BN) dimer radical cation where the monomer (BN=C6H5CN) has a large dipole moment (4.18 D)22 and therefore, is capable of forming ion-dipole or hydrogen bonding structures in the dimer radical cation. We also examine the structure of the even-electron protonated dimer H+(BN)2 to compare with the structure of the odd-electron dimer radical cation (BN)2·+. Finally, we investigate the effect of hydration on the structures of the protonated benzonitrile dimer and trimer to see if stable solvated hydronium ion structures can be formed. Herein, we provide the first experimental evidence of the structures of the benzonitrile dimer radical cation and protonated dimer and report the observation of a stable hydronium ion solvated by three benzonitrile molecules H3O+(C6H5CN)3 in the gas phase. The interest in the structures of the benzonitrile dimer and hydrated cluster ions is also motivated by the recent discovery of benzonitrile, one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium.12 The structures of the benzonitrile dimer radical cation, protonated dimer, and the hydronium ion solvated by benzonitrile molecules, determined for the first time in the present work, could act as precursors for nitrogen-containing complex organics thus providing a chemical link to the carriers of the unidentified infrared bands.11,12,23 It is hoped that the present results motivate the search for these ions in the cold-core Taurus Molecular Cloud 1 (TMC-1), which has long been known to possess a rich chemistry dominated by unsaturated carbon-chain molecules containing nitrile (R—C≡N) groups.12,24,25

The experiments were performed using the VCU mass-selected ion mobility spectrometer (schematic is given in Fig. S1, supplementary material). Details of the instrument can be found in several publications19–21 and only a brief description of the experimental procedure is given here. The essential elements of the apparatus are jet and beam chambers coupled to an electron ionization (EI) source, a quadrupole mass filter, a drift cell, and a second quadrupole mass spectrometer. The neutral BN clusters are generated in the source chamber by supersonic pulsed adiabatic expansion.20–22 During operation, a vapor mixture consisting of 86 Torr benzonitrile (Aldrich, 99.9%) in 3500 Torr helium (ultrahigh purity, airgas 99.99%) is expanded through a conical nozzle (500 µm in diameter) in pulses of 300-400 µs duration at a repetition rate of 100 Hz. The jet is skimmed and passed into the second chamber, which is maintained at 2 × 10−6 Torr, where the clusters are ionized by 45 eV EI ionization. The ions are injected into the drift cell containing 1.0–3.0 Torr helium at different temperatures (175-350 K) using an injection energy of 13.0 eV (lab frame).

The lowest energy structures of the cluster ions were calculated using DFT at the B3LYP/6-311++G** level within the Gaussian 09 suite of programs.26 These structures were used to obtain average collision cross sections in helium using the ion size scaled Projection Approximation (PA) method within the Sigma program.27–29 

Figure 1 displays the mass spectra of the ions injected into the drift cell containing pure helium at different temperatures which show that the dimer radical cation (BN)2·+ is the major ion injected along with a small amount of the monomer radical cation (BN)·+ most likely generated by the dissociation of the dimer by the injection energy. At low temperatures, the additions of up to three water molecules on the dimer radical cation are observed resulting from the presence of a trace water vapor impurity in the drift cell. At 173 K, the water-containing peaks disappear due to the freezing of the water vapor in the cell and only the benzonitrile dimer along with small amounts of the monomer and trimer radical cations are observed.

FIG. 1.

Mass spectra obtained following the injection of the benzonitrile radical cation (C6H5CN, (BN), m/z 103) and benzonitrile dimer radical cation (C14H10N2, (BN)2, m/z 206) generated by 45 eV EI ionization of the neutral clusters produced by a supersonic beam expansion of benzonitrile vapor in helium as a carrier gas. The P/V value in the drift cell is 0.20 Torr/Volt, and the temperatures are as indicated in (a)–(d).

FIG. 1.

Mass spectra obtained following the injection of the benzonitrile radical cation (C6H5CN, (BN), m/z 103) and benzonitrile dimer radical cation (C14H10N2, (BN)2, m/z 206) generated by 45 eV EI ionization of the neutral clusters produced by a supersonic beam expansion of benzonitrile vapor in helium as a carrier gas. The P/V value in the drift cell is 0.20 Torr/Volt, and the temperatures are as indicated in (a)–(d).

Close modal

The mobility K of an ion is defined as30 

K=vdE
(1)

where vd is the drift velocity and E is the field across the drift region. The reduced mobility K0 (scaled to the number density at standard temperature and pressure, STP) is given by

K0=P273.15760TK,
(2)

where P is the pressure in Torr and T is the temperature in Kelvin. Equations (1) and (2) can be combined and rearranged to give

td=l2273.15T7601K0PV+t0,
(3)

where l is the drift length (5 cm in our system), td is the measured mean arrival time of the drifting ion packet corrected for the non-Gaussian shape of the arrival time distribution (ATD) peak,19,t0 is the time the ion spends outside the drift cell before reaching the detector, and V is the voltage across the drift cell. All the mobility measurements were carried out in the low-field limit (E/N < 5.0, where E is the electric field intensity and N is the buffer gas number density, and E/N is expressed in units of Townsend (Td) where 1 Td = 10−17 V cm2).30 

Mobility measurements are made by injecting a narrow pulse of ions into the drift cell. The ion gate located just prior to the cell entrance chops the pulse into a narrow, 30–50 µs wide packet, which enters the drift cell. The injection energies used in all the experiments are slightly above the minimum energies required to introduce the ions into the cell against the helium flow. Upon exiting the cell, the ions are collected and refocused into the second quadrupole for analysis and detection. The signal is collected on a multichannel scalar with the zero time for data acquisition set to the midpoint of the ion gate trigger. Mobility is determined according to Eq. (3), by plotting td versus P/V. The ATDs and the td versus P/V plot for the benzonitrile dimer radical cation are displayed in Fig. 2(a). The repeated mobility measurements yield a reduced mobility of K0 = 5.67 ± 0.15 (0.28) cm2 V−1 s−1 at 304 K. The uncertainty given here is ±0.15 in the repeated measurements of the reduced mobility, and the value in parenthesis represents a 5% experimental error in the mobility measurements. Figure 2(b) displays the ATDs of the benzonitrile dimer radical cation obtained at a fixed P/V ratio (0.20 Torr/V) and measured at different temperatures. The relatively broad ATDs suggest the presence of several structural isomers with close values of collision cross sections at all temperatures. At the lowest measured temperature of 173 K, the ATD reveals a collision cross section in the range of 104–113 Å2 with an average value of 108.4 Å2 corresponding to the maximum ATD peak. However, at the highest measured temperature of 344 K, the ATD suggests a range of Ω344K (collision cross section at 344 K) between 88 and 97 Å2 with the maximum ATD peak resulting in Ω of 92 Å2.

FIG. 2.

(a) Arrival time distributions (ATDs) and td versus P/V plot of the benzonitrile dimer radical cation at 304 K. The inset shows the td versus P/V plot. (b) ATDs for the benzonitrile dimer radical cation at drift cell conditions of 0.20 Torr/V and different temperatures as indicated.

FIG. 2.

(a) Arrival time distributions (ATDs) and td versus P/V plot of the benzonitrile dimer radical cation at 304 K. The inset shows the td versus P/V plot. (b) ATDs for the benzonitrile dimer radical cation at drift cell conditions of 0.20 Torr/V and different temperatures as indicated.

Close modal

According to the kinetic theory,30 Eq. (4) relates the mobility of an ion to the average collision cross section of the ion with the buffer gas according to

K=1N18π12161m+1mb12zekBT12Ω1,1avg,
(4)

where N is the buffer gas number density, m is the mass of the ion, mb is the mass of a buffer gas atom, z is the number of charges, e is the electron charge, kB is Boltzmann’s constant, and Ω1,1avg is the average collision integral. The measured mobilities of the benzonitrile dimer radical cation at different temperatures are used to calculate average Ωs for the ion in helium using Eq. (4), and the resulting values are listed in Table I as Ωexp2). The lowest energy structures of the benzonitrile dimer radical cation calculated at the B3LYP/6-311++G** level of theory26 with the corresponding binding enthalpy (ΔH°) and free energy (ΔG°) with respect to dissociation to BN radical cation (C6H5CN+.) and neutral BN molecule (C6H5CN) are shown in Fig. 3. These structures are used to obtain average collision cross sections using the ion size scaled Projection Approximation (PA) method within the Sigma program.29 The method, also known as PSA (projected superposition approximation), computes molecular collision cross sections as a projection approximation28 modified to account for collective size and shape effects by a superposition of atomic potentials and by the inclusion of a shape factor.27,29 The calculated average collision cross sections in helium at the temperatures of the measured mobilities are given in Table I.

TABLE I.

Experimental-based mobility (K0,exp) and collision cross sections (Ωexp) (shown as bold values) of the benzonitrile dimer radical cation in helium at different temperatures (K) and calculated cross sections (Ωcalc) using the PSA method29 for the lowest energy structures with the associated changes in Gibbs free energy (ΔG, kcal mol−1) obtained at the B3LYP/6-311++G** level of theory.26 Units for mobility (K0) and collision cross-section (Ω) are cm2 V−1 s−1 and Å2, respectively.

IIIIIIIVVVI
TK0,expΩexpΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔG
344 5.64 92.2 95.3 10.3 92.6 10.8 86.1 10.1 98.8 11.3 99.0 10.5 80.4 4.8 
304 5.67 94.6 97.2 11.5 94.5 11.8 88.8 11.2 102.1 12.1 101.8 11.3 82.4 6.2 
250 5.87 100.6 101.3 13.1 97.9 13.2 92.6 12.7 106.9 13.2 106.4 12.5 85.5 8.1 
205 6.44 101.4 104.9 14.4 102.8 14.3 96.1 13.9 112.2 14.1 110.9 13.5 90.3 9.7 
173 6.55 108.4 106.6 15.4 106.8 15.2 100.1 14.8 116.7 14.8 115.7 14.3 94.5 10.8 
IIIIIIIVVVI
TK0,expΩexpΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔGΩcalc.−ΔG
344 5.64 92.2 95.3 10.3 92.6 10.8 86.1 10.1 98.8 11.3 99.0 10.5 80.4 4.8 
304 5.67 94.6 97.2 11.5 94.5 11.8 88.8 11.2 102.1 12.1 101.8 11.3 82.4 6.2 
250 5.87 100.6 101.3 13.1 97.9 13.2 92.6 12.7 106.9 13.2 106.4 12.5 85.5 8.1 
205 6.44 101.4 104.9 14.4 102.8 14.3 96.1 13.9 112.2 14.1 110.9 13.5 90.3 9.7 
173 6.55 108.4 106.6 15.4 106.8 15.2 100.1 14.8 116.7 14.8 115.7 14.3 94.5 10.8 
FIG. 3.

Lowest energy structures, binding enthalpies (ΔH°), and free energies (ΔG°) of the benzonitrile dimer radical cation isomers calculated at the B3LYP/6-311++G** level of theory. The corresponding collision cross sections in helium (Ω173K) calculated at 173 K using the PSA method.29 

FIG. 3.

Lowest energy structures, binding enthalpies (ΔH°), and free energies (ΔG°) of the benzonitrile dimer radical cation isomers calculated at the B3LYP/6-311++G** level of theory. The corresponding collision cross sections in helium (Ω173K) calculated at 173 K using the PSA method.29 

Close modal

The structures shown in Fig. 3 represent several energy minima on the potential energy surface of the BN dimer radical cation within a relative energy range of 4 kcal mol−1, and therefore, they may coexist under the temperature range of the current mobility experiments (173 K–344 K). The lowest energy structure (I) displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen atom of one molecule and the ortho hydrogen atom of the second molecule. The structure is similar to that of the neutral dimer determined from the rotationally resolved laser induced fluorescence and also predicted by ab initio MP2/6-31G(d,p) calculations.31,32 The stability of this double hydrogen bond structure arises from a dipole–dipole interaction which leads to anti-parallel orientations of the dipole moments of the two molecules. The second lowest energy structure (II) displays a non-planar displaced anti-parallel orientation with the nitrogen atoms separated by 2.78 Å between the planes of the two molecules. The calculated collision cross sections corresponding to structures (I) and (II) are in excellent agreement with the experimental values at the five measured temperatures. At the lowest measured temperature of 173 K where the mobility resolution is the highest, the cross sections of structures (I) and (II) are within 1.7% of the measured cross section. On the other hand, the calculated cross sections for the single hydrogen bond structures (IV) and (V) are about 7% higher than the measured value at 173 K. Also, the head-to-tail parallel structure (VI) has a calculated cross section smaller by more than 12% than the measured value at 173 K. Similarly, the slipped parallel structure (III) is 7.7% smaller than the measured cross section at 173 K. These results clearly indicate that the BN dimer radical cation does not adopt the parallel sandwich configuration of the aromatic dimer cations of non-polar molecules such as benzene and naphthalene.16–20 This is also consistent with the higher free energies of the parallel structure (VI) than hydrogen-bonding structures (I), (II), (IV), and (V) at all the measured temperatures. Since the measured cross section is within 1.7% of the calculated values corresponding to structures (I) and (II) which have the lowest free energies at 170-250 K, it can be concluded that the BN dimer radical cation adopts either the planar double hydrogen bonded structure (I) or the non-planar displaced anti-parallel structure (II). Although the hydrogen bonding structure IV has slightly lower free energies than structures I and II at higher temperatures 300-350 K, the calculated collision cross section is larger than the experimental value at these temperatures. Therefore, we may conclude that structures I and II are the major contributors to the measured collision cross sections at low temperatures in addition to some contribution from the hydrogen bonding structure IV at higher temperatures.

Figure 4 displays the mass spectrum of the ionized binary BN-water clusters H+(BN)n(H2O)m generated by the co-expansion of benzonitrile-water vapor mixture in helium. The major ions injected into the drift cell containing pure helium are the protonated dimer (BN)2H+ and the hydrated protonated dimer H+(BN)2(H2O) and trimer H+(BN)3(H2O). The hydrated protonated trimer (BN)3(H2O)H+ exhibits a significantly enhanced ion intensity which could be explained by the stability of the structure consisting of a hydronium ion core solvated by three benzonitrile molecules H3O+(BN)3. The stability of a hydronium ion core solvated by three acetonitrile molecules has been reported based on several mass spectrometry studies, but no direct experimental evidence has been provided for this structure.33,34

FIG. 4.

Mass spectrum obtained following EI ionization of the clusters formed by the co-expansion of a benzonitrile-water vapor mixture in helium and the injection of the cluster ions into a drift cell containing 2.6 Torr helium at 301 K. Note: a = BN, b = H3O+(BN), c = H3O+(H2O)(BN), d = (BN)2+OH(H2O), e = (BN)3, f = (BN)3+OH(H2O), and g = (BN)3+OH(H2O)2.

FIG. 4.

Mass spectrum obtained following EI ionization of the clusters formed by the co-expansion of a benzonitrile-water vapor mixture in helium and the injection of the cluster ions into a drift cell containing 2.6 Torr helium at 301 K. Note: a = BN, b = H3O+(BN), c = H3O+(H2O)(BN), d = (BN)2+OH(H2O), e = (BN)3, f = (BN)3+OH(H2O), and g = (BN)3+OH(H2O)2.

Close modal

The ATDs and td versus P/V plots for the protonated benzonitrile dimer (BN)2H+ and the hydrated protonated dimer (BN)2H+(H2O) are displayed in Fig. 5. It is clear that a significant fraction of the hydrated protonated dimer (m/z 225) dissociates by the loss of a water molecule inside the drift cell resulting in the shorter arrival time peak of the protonated dimer (m/z 207). The mobility of the hydrated protonated dimer is calculated using the longer arrival time peak corresponding to m/z 207. The dissociation of the hydrated protonated dimer suggests the presence of a weakly bond isomer which results in the loss of a water molecule upon the injection into the drift cell.

FIG. 5.

ATDs for the (a) protonated benzonitrile dimer and (b) hydrated protonated benzonitrile dimer at the drift cell conditions provided and 304 K.

FIG. 5.

ATDs for the (a) protonated benzonitrile dimer and (b) hydrated protonated benzonitrile dimer at the drift cell conditions provided and 304 K.

Close modal

Unlike the hydrated protonated benzonitrile dimer, the hydrated protonated trimer (BN)3H+(H2O) does not show any evidence of dissociation as shown in the ATDs and td versus P/V plots displayed in Fig. 6. The ATDs exhibit single narrow peaks which suggest the presence of one isomer with a significant binding energy which could have the structure of the hydronium ion core solvated by three benzonitrile molecules.

FIG. 6.

ATDs for the hydrated protonated benzonitrile trimer ion under the conditions provided at 304 K. The inset of the ATD displays the mean arrival time of each ion as a function of P/V.

FIG. 6.

ATDs for the hydrated protonated benzonitrile trimer ion under the conditions provided at 304 K. The inset of the ATD displays the mean arrival time of each ion as a function of P/V.

Close modal

The lowest energy structures of the protonated benzonitrile dimer and the hydrated protonated dimer (BN)2H+(H2O) and trimer (BN)3H+(H2O) calculated at the B3LYP/6-311++G** level of theory26 are shown in Fig. 7. The corresponding ΔH° and ΔG° for the formation of the protonated dimer (BN)2H+ from the association of protonated benzonitrile (C6H5CNH+) and a neutral BN molecule, the formation of the hydrated protonated dimer (BN)2H+(H2O) from the association of the protonated dimer (BN)2H+ and a neutral water molecule, and the formation of the hydrated protonated trimer (BN)3H+(H2O) from the association of the hydrated protonated dimer (BN)2H+(H2O) and a neutral BN molecule are also given in Fig. 7. The measured mobilities of the protonated benzonitrile dimer and the hydrated protonated dimer (BN)2H+(H2O) and trimer (BN)3H+(H2O) are used to calculate average Ωs for the ion in helium using Eq. (4), and the resulting values are listed in Table II as Ωexp2). The calculated average collision cross sections in helium for the structures shown in Fig. 7 are also given in Table II.

FIG. 7.

Two lowest energy structures (i) and (ii) of (a) protonated benzonitrile dimer (C6H5CN)2H+, (b) hydrated protonated dimer (H2O)H+(C6H5CN)2, and (c) hydrated protonated trimer (H2O)H+(C6H5CN)3 calculated at the B3LYP/6-311++G** level of theory and the corresponding ΔH° and ΔG° for (a) the formation of the protonated dimer (C6H5CN)2H+ from the association of protonated benzonitrile (C6H5CNH+) and a neutral C6H5CN molecule, (b) the formation of the hydrated protonated dimer (H2O)H+(C6H5CN)2 from the association of the protonated dimer (C6H5CN)2H+ and a neutral water molecule, and (c) the formation of the hydrated protonated trimer (H2O)H+(C6H5CN)3 from the association of the hydrated protonated dimer (H2O)H+(C6H5CN)2 and a neutral C6H5CN molecule. The collision cross sections (Ω298K) calculated at 298 K using the PSA method.29 

FIG. 7.

Two lowest energy structures (i) and (ii) of (a) protonated benzonitrile dimer (C6H5CN)2H+, (b) hydrated protonated dimer (H2O)H+(C6H5CN)2, and (c) hydrated protonated trimer (H2O)H+(C6H5CN)3 calculated at the B3LYP/6-311++G** level of theory and the corresponding ΔH° and ΔG° for (a) the formation of the protonated dimer (C6H5CN)2H+ from the association of protonated benzonitrile (C6H5CNH+) and a neutral C6H5CN molecule, (b) the formation of the hydrated protonated dimer (H2O)H+(C6H5CN)2 from the association of the protonated dimer (C6H5CN)2H+ and a neutral water molecule, and (c) the formation of the hydrated protonated trimer (H2O)H+(C6H5CN)3 from the association of the hydrated protonated dimer (H2O)H+(C6H5CN)2 and a neutral C6H5CN molecule. The collision cross sections (Ω298K) calculated at 298 K using the PSA method.29 

Close modal
TABLE II.

Experimental mobility (K0) and collisional cross sections (Ωexp) for the protonated benzonitrile dimer (C6H5CN)2H+, hydrated protonated dimer (H2O)H+(C6H5CN)2, and hydrated protonated trimer (H2O)H+(C6H5CN)3 and the calculated cross sections (Ωcalc) using the PSA method29 for the structures shown in Fig. 7.

K0,expΩexpΩcalc.2)
Cluster [m/z](cm2 V−1 s−1)2)iii
(C6H5CN)2H+ [207] 5.19 103.3 101.1 100.9 
(H2O)H+(C6H5CN)2 [225] 4.77 112.3 111.4 110.3 
(H2O)H+(C6H5CN)3 [328] 3.58 149.5 153.8 141.5 
K0,expΩexpΩcalc.2)
Cluster [m/z](cm2 V−1 s−1)2)iii
(C6H5CN)2H+ [207] 5.19 103.3 101.1 100.9 
(H2O)H+(C6H5CN)2 [225] 4.77 112.3 111.4 110.3 
(H2O)H+(C6H5CN)3 [328] 3.58 149.5 153.8 141.5 

The lowest energy structure of the protonated BN dimer shown in Fig. 7 exhibits a proton-bound dimer configuration (BNH+NB) where the bridging proton connects the nitrogen atoms in the two BN molecules.35 The calculated collision cross section of this structure (101.1 Å2) is in excellent agreement with the value obtained from the measured mobility (103.3 Å2). Interestingly, the other hydrogen bonding structure, shown in Fig. 7(a–ii), has a similar cross section of 100.9 Å2. However, this structure lies at a significantly higher energy than the proton-bound dimer and therefore, it is unlikely to be formed under the thermalized conditions of the mobility experiment. Therefore, based on the energetic and mobility considerations, it can concluded that the protonated BN dimer observed in our experiment has the proton-bound structure displayed in Fig. 7(a–i).

In the hydrated protonated benzonitrile, BNH+(H2O), because of the higher proton affinity of benzonitrile (194 kcal/mol)36 compared to water (165 kcal/mol),36 the proton is expected to be more associated with BN than with water. This suggests that the hydrated protonated dimer (H2O)H+(C6H5CN)2 could consist of a proton-bound dimer (BNH+NB) associated with a water molecule through a C—H⋯OH2 hydrogen bond similar to structure (b-ii) shown in Fig. 7. However, by locating the proton on the water molecule the resulting hydronium ion is now capable of forming two stronger ionic hydrogen bonds with the benzonitrile molecules, as shown in structure (b-i) displayed in Fig. 7. The significant dissociation of the hydrated protonated dimer (H2O)H+(C6H5CN)2 as shown in Fig. 5(b) suggests the presence of a significant population the weakly bound isomer (b-ii) shown in Fig. 7. Therefore, the hydrated protonated benzonitrile dimer formed in our experiments appears to have two structures representing the weakly bound kinetically favorable structure (b-ii) and the most stable thermodynamically favorable structure (b-i) shown in Fig. 7. The evidence for the formation of the two structures comes from the ATDs which exhibit double peaks corresponding to the undissociated and dissociated structures at longer and shorter times, respectively, as shown in Fig. 5(b).

The lowest energy structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules, as shown in structure (c-i) displayed in Fig. 7. This stable structure has a collision cross section (153.8 Å2) in excellent agreement with the experimental-based value of 149.5 Å2. Structure (c-ii), in addition to its significantly smaller negative ΔH° (−4.8 kcal/mol as compared to −18.0 kcal/mol for structure (c-i), has a collision cross section (141.5 Å2) smaller than the measured value of 149.5 Å2. The presence of the lowest energy structure of the hydrated protonated trimer is also supported by the lack of dissociation of the injected ion in the drift cell and the narrow ATD peaks shown in Fig. 6 which most likely correspond to a single stable structure. Therefore, we conclude that the hydrated protonated trimer consists of hydronium ion core solvated by three benzonitrile molecules through the formation of three ionic hydrogen bonds.

In conclusion, the structures of the benzonitrile dimer radical cation, the proton-bound dimer, and the hydronium ion core solvated by benzonitrile molecules have been determined by the mass-selected ion mobility measurements and DFT calculations. Unlike the dimer radical cations of nonpolar aromatic molecules which adopt parallel sandwich configurations, the benzonitrile dimer radical cation (C6H5CN)2·+ displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen and hydrogen atoms. The protonated dimer has the structure of a proton-bound dimer (C6H5CNH+NCC6H5) where the bridging proton connects the nitrogen atoms in the two benzonitrile molecules resulting in a calculated collision cross section of 101.1 Å2 in excellent agreement with the measured value of 103.3 Å2. The structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules. By locating the proton on the lower proton affinity water molecule, the resulting hydronium ion can be fully solvated by forming three ionic hydrogen bonds with the benzonitrile molecules. These unique structural motifs could be useful for the molecular design and recognition involving charged aromatic systems and also for the search of nitrogen-containing complex organics in space.

See supplementary material for the description of the mass-selected ion mobility system (Fig. S1).

This work was supported by the National Science Foundation through Grant No. CHE-1463989.

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