Experimental evidence for the role of the $πCO∗$ orbital in electron transfer to gas phase acetic acid $CH3CO2H$⁠: Effects of molecular orientation

High energy ionizing radiation produces a plethora of near-thermal energy electrons which are amazingly destructive to biological molecules.¹ For amino acids and their simpler constituents, formic and acetic acid, this damage results from resonant electron attachment near 1.3 eV giving a transient negative ion that can dissociate to a reactive H atom and a carboxylate anion.^2–7 Although the initial attachment is considered a $π∗$ shape resonance centered on the CO bond, the OH bond must be broken, and recent calculations by Rescigno et al.⁸ for formic acid suggest that this requires a symmetry-breaking molecular deformation. We present experiments on electron transfer from K atoms to oriented acetic acid showing that electron transfer favors the $CO2H$ end of the molecule and that the electron is most likely transferred to the $π∗$ orbital. The donor atom facilitates the symmetry-breaking deformation production of acetate ion, largely corroborating the theory.

Formic acid and acetic acid are the simplest carboxylic acids found in the atmosphere,⁹ in interstellar clouds,¹⁰ and in the pioneering studies of Miller¹¹ replicating the conditions of a primitive Earth atmosphere. They serve as prototype systems for the amino acids because of the carboxylic acid moiety –COOH. In the gas phase they attach free electrons^3–7,12,13 at energies ≈1.5 eV forming transient negative ions which can fragment to give H atoms and the formate (acetate) anion $RCOO−$⁠, where $R=H$ or $CH3$⁠. The electron is attached in a shape resonance, apparently entering the $π∗$ lowest unoccupied molecular orbital (LUMO) (although some calculations suggest this is the $LUMO+1$⁠).¹⁴ But to break the OH bond it must then migrate to the $σOH∗$ orbital. The calculations of Rescigno et al.⁸ for formic acid show the electron entering the $π∗$ orbital of $A″$ symmetry requiring a polyatomic out-of-plane distortion to dissociate along a second electronic surface of $A′$ symmetry.

We report electron transfer from K atoms to oriented $CH3CO2H$ that support the conclusions of Rescigno et al.⁸ We study the reaction

in crossed beams where the $CH3CO2H$ molecule [HAc] can be oriented in space to present either the $CH3$ “end” or $CO2H$ end to the incoming K atom. Attack at the $CO2H$ end of the molecule forms more acetate anions, and the steric asymmetry suggests that the electron is most likely transferred to the $πCO∗$ orbital. The atomic donor can distort the molecule, enhancing the probability of an $A″-A′$ surface crossing to give acetate as the major product anion.

Beams of fast K atoms and acetic acid (HAc) molecules cross inside a coincidence time-of-flight mass spectrometer (TOFMS), as shown in Fig. 1.¹⁵ K atoms are accelerated to 5–25 eV (laboratory) by charge exchange inside an oven, providing enough energy for individual ion detection. Residual ions are removed from the primary beam by charged plates outside the oven. Acetic acid vapor at $23°C$ is mixed with He at 180 torr and expanded from a nozzle at $150°C$ to break up dimers, calculated from the equilibrium constant¹⁶ to be $<1%$ at $150°C$⁠. (No dimer signals are observed.) The beam travels along the axis of an inhomogeneous electric hexapole field 1.4 m long,¹⁷ and molecules are deflected in this field depending on their rotational state. For symmetric top molecules the interaction energy $W$ is¹⁸ $W=−μEMK/[J(J+1)]=−μE⟨cosθ⟩$⁠, where $⟨cosθ⟩$ is the average cosine of the angle between the top axis and the electric field. Molecules in low-field seeking states $MK<0$ move toward the axis and are focused with a concomitant increase in intensity. Molecules with $MK=0$ are unaffected; those with $MK>0$ are defocused.

Even though HAc is an asymmetric top, the barrier to internal rotation of the $–CO2H$ group is low¹⁹ $(170cm−1)$⁠, and the $–CO2H$ group spins about the $CH3$ group effectively averaging the acid group about the symmetry axis. The molecule appears “symmetric” with $μA=0.86D$ and some rotational states have first order Stark effects.¹⁹ Similar considerations hold for nitromethane $CH3NO2$⁠, where the barrier to internal rotation is $2cm−1$ and the molecule behaves as a symmetric top.^20–22 These molecules are focused in the hexapole field as strongly as $CH3CN$ (a symmetric top with $μ=3.93D$⁠) and the relative signal increase $(SHVon−SHVoff)/SHVoff=ΔS/SHVoff$ for these three molecules is ≈1, where $SHVon$ is the signal with hexapole voltage (HV) on, etc. For asymmetric tops without internal rotation such as bromo- and chlorobenzene²³ $ΔS/SHVoff≈0.1$⁠, showing that acetic acid and $CH3NO2$ behave as “symmetric tops.” The steric asymmetry observed supports this conclusion. We roughly estimate $⟨−cosθ⟩≈0.52$ (compared to 0.39 and 0.43 for $CH3Br$ and $t-C4H9Br$⁠)²⁴ by assuming that the $–CO2H$ group freely rotates, averaging the rotational constants $B$ and $C$⁠,¹⁹ with dipole moment equal to $μA=0.86$⁠. The average $⟨−cosθ⟩$ is then calculated by averaging over hexapole transmission and population and orientation of each state.²⁴ 

The energized hexapole thus acts as a filter, passing those molecules in states with $⟨cosθ⟩<0$⁠. These molecules travel adiabatically from the hexapole field into a uniform field ≈300 V/cm defined by two identical and oppositely charged Wiley–McLaren TOFMSs. The TOFMSs lies in the plane of the crossed beams and $E$ is roughly along the relative velocity. Reversing the polarity of the TOFMS reverses $E$ and the direction of orientation. In this uniform field the negative end of the molecule points toward the negatively charged TOFMS. The beams are continuous and all voltages are constant (no time zero) but each electron transfer event produces an ion pair simultaneously. The positive ion signal starts a time to digital converter (TDC), and the negative ion signal (delayed $4μs$ to allow detection of electrons) stops the TDC, giving the difference in flight times between the positive and negative ions.

Signals (coincidence TOF spectra) for each laboratory energy are acquired for positive or negative end attack with the hexapole field on and off for each orientation. If the hexapole field is off a randomly oriented beam is transmitted and its signal is used to eliminate any differences in collection or detection efficiency arising from different TOFMS polarities.²⁴ The experimental conditions are computer controlled in random sequence.

Figure 2 shows coincidence TOF mass spectra: electron transfer to $CH3CO2H$ produces $K+$ ions and negative ions $CH3−$⁠, $OH−$⁠, and $CH3CO2−$⁠. Except for $m/e=0$ and 32, these signals depend on the hexapole focusing voltage and are thus from HAc monomer. Hydroxyl ions and acetate ions are well known in solution, but $CH3−$ ions $(m/e=15)$ are not and could be confused with $O−$ $(m/e=16)$⁠. The TOFMS resolution is limited by spatial and geometric constraints, making the distinction between $CH3−$ at $m/e=15$ and $O−$ at $m/e=16$ problematic. Deuterium substitution of the acid hydrogen shifts the $OD−$ to $m/e=18$⁠, and perdeuteration shifts both $CD3−$ and $OD−$ to $m/e=18$⁠, verifying that the light ions are $CH3−$ and $OH−$⁠. Signals at $m/e=59$ from $CH3CO2–H$ and $CH3CO2–D$ are identical and due to acetate ion, showing that the acid H(D) atom is lost as also observed in formic acid.⁵ The signal-to-noise ratio is too low to detect complementary fragments such as $CH3CO2H−$⁠, $COOH–$⁠, $CH3CO−$⁠, or $H−$⁠. Further measurements on thresholds and steric effects used $CH3CO2D$ to maximize the peak separation between $CH3−$ at $m/e=15$ and $OD−$ at $m/e=18$⁠.

Each mass peak has satellites at lower mass (especially $m/e=59$⁠) apparently caused by grids in the TOFMS. The energy (and orientation) dependence of the satellite peaks at $m/e=59$ is the same as the dependence of the main peak, and perdeuteration shifts the pattern as a whole. Extensive experiments were also conducted with a different TOFMS with the drawout field normal to the plane of the beams.²⁵ This TOFMS has mass peaks with different satellites (different grids) but the satellite pattern is the same for all masses and we conclude these are satellites.

We calibrate the energy by determining nominal laboratory thresholds (Fig. 3) for the acetic acid ions and for $CN−$ from separate $CH3CN$ calibration runs (for positive and negative orientation each with HV on and HV off), and for $O2−$ from the background. These are plotted versus laboratory thermodynamic thresholds to give corrected laboratory energies that are then used to calculate center-of-mass (CM) energies for the $K–CH3CO2D$ system. The signals above threshold do not behave as resonances.

To emphasize the effects of the target’s orientation we define the steric asymmetry factor^24,26 $G=(σ−−σ+)/(σ−+σ+)$⁠, where $σ−$ and $σ+$ are the cross sections for negative end and positive end attacks. If reaction only occurs at the negative end of the molecule, $G=+1$⁠, and if there is no difference in reactivity, $G=0$⁠. Figure 4 shows the steric asymmetry factors for $Ac−$ and $OD−$⁠. Attack at the negative end (⁠$CO2H$ end) of the molecule clearly favors the formation of $Ac−$⁠. All of the data points have $G>0$⁠, and $G$ is essentially constant.

Figure 4 shows that the molecule is oriented before the collision and that $Ac−$ is more likely to be formed upon attack at the $CO2H$ end of the molecule. Although $G$ for $Ac−$ is small, it is never negative and is constant over the energy range studied. Small constant values of $G$ are the signature of electron transfer to $π∗$ orbitals^21,22,27 in clear contrast to the behavior observed for transfer to the $σ∗$ orbital in tert-butyl bromide $(CH3)3CBr$⁠, where $G$ is large and energy dependent.^24,28 Many calculations suggest that $πCO∗$ is the LUMO in acetic acid^4,14,29 or formic acid^8,30 consistent with electron transmission experiments^2,7 and with our observations that electron transfer favors the $πCO∗$ orbital. But the electron must somehow migrate to the $σOH∗$ orbital to complete the reaction and break the O–H bond. This is similar to the behavior observed in $CH3CN$ (Refs. 27 and 31) where the electron enters the $πCN∗$ orbital and migrates to the $σCC∗$ orbital, breaking the C–C bond and producing $CN−$⁠.

Rescigno et al.⁸ examined the electron attachment in formic acid concluding that a transient $π∗$ anion is formed with $A″$ symmetry. A symmetry-breaking deformation is necessary for the electron to migrate to the $σOH∗$ orbital to form the products $HCOO−+H$ of $A′$ symmetry. A similar deformation is likely to be necessary for HAc. In the present experiments this deformation is easily provided by the K donor atom, and we have shown elsewhere that the electron donor can distort the molecule from its neutral geometry.^22,32

The results presented here are similar but different from those using free electrons to attach to various carboxylic acids.^3–5,12,33 In both experiments the cross sections are low(!) and $CH3CO2−$ is formed, but other decomposition channels for the two processes are completely different. The lowest energy process in electron attachment to acetic acid⁴ is a resonance at 0.75 eV producing $CH2O2−$ (⁠$m/e=46$⁠, isobaric with $HCOOH−$⁠). The cross section is similar to that for the resonance at 1.5 eV which produces Ac. A higher energy resonance¹³ (6.7 eV) produces $H−$⁠. We find no signals from HAc at $m/e=46$ for $CH2O2−$ or $D−$ at $m/e=2$ with a sensitivity of ≈3% of the $Ac−$ signal. We conclude that the donor atom distorts the molecule and allows the very stable $Ac−$ to be formed at the expense of these other products. This finding is similar to that in $CH3CN$ where the electron initially enters the $πCN∗$ orbital but must migrate to the $σCC∗$ bond to form $CN−$⁠. The ratio of $CN−$ to $CH2CN−$ is vastly different in the two experiments, ≈100:1 for electron transfer,^27,31 but ≈1:100 for electron attachment.³⁴ The donor atom facilitates the $Π-Σ$ surface crossing in $CH3CN$⁠, channeling the reactive flux into the production of $CN−$⁠. In the attachment experiments, that interaction is absent, the $Π-Σ$ surface crossing is not facile, the channel leading to $CN−$ is blocked, and the reactive flux proceeds to another channel, formation of $CH2CN−$⁠. Thus for the present case in acetic acid, we expect the atomic donor to facilitate the surface crossing to an $A′$ surface yielding $CH3CO2−$ at the expense of other products, and indeed it does. In effect, the donor atom opens the door to the low energy but symmetry forbidden channel.

Finally, it is clear that the steric asymmetry for formation of $OD−$ is different from that for $Ac−$ and that $OD−$ is more likely to be formed by attacking the $CH3$ end of the molecule where the only $σ$ orbitals are available. The energy dependence might imply that transfer is to a $σ$ orbital, but the scatter in the data is too large for us to conclude that.

In summary, we observe $Ac−$ as the main product in electron transfer collisions between K atoms and oriented HAc molecules. Production of $Ac−$ is favored by attacking the $CO2H$ end of the molecule, and the steric asymmetry indicates that the electron enters a $π∗$ orbital. The $π∗$ electron must migrate to the $σ∗$ orbital on OH in order to cleave the O–H bond, and the K core facilitates that migration.

We thank Cesar Skalany for laboratory assistance and Peter Harland for helpful discussions. We also thank the National Science Foundation and the ACS-PRF for financial support.