^{1}, Deborah L. Crittenden

^{1}, Scott H. Kable

^{1}and Meredith J. T. Jordan

^{1,a)}

### Abstract

Previous experimental and theoretical studies of the radical dissociation channel of acetaldehyde show conflicting behavior in the HCO and product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of into HCO and fragments over the barrier on the surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) *ab initio* calculations. 20 000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and products are vibrationally cold; there is a small amount of HCO rotational excitation and little rotational excitation, and the reactionenergy is partitioned dominantly ( at threshold) into relative translational motion. Close to threshold the HCO and rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reactiondynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.

One of the authors (D.L.C.) acknowledges the financial support of an Australian Postgraduate Research Award. This work has also been supported in large by Grant No. A00104447 from the Australian Research Council and by grants of computer time from the Australian Partnership in Advanced Computing (APAC) National Merit Allocation Scheme. The authors would like to thank Professor Tim Softley for comments on his recent experimental work on acetaldehyde photodissociation.

I. INTRODUCTION

II. METHODS

A. Potential-energy surface construction

B. Trajectory simulations

III. RESULTS

A. Energetics and PES construction/PES topology

B. Dynamics

1. Vibrational distributions

2. Rotational distributions

3. Translational energy distributions

IV. DISCUSSION

A. Comparison with experiments

1. Vibrational distributions

2. Rotational distributions

3. Translational distributions

B. Comparison with previous theoretical studies

1. Vibrational distributions

2. Rotational distributions

3. Translational distributions

C. The transition from impulsive to statistical dynamics

V. SUMMARY

A. The shape of the HCO angular momentum distribution

B. The average HCO rotational excitation

C. vibrational excitation

### Key Topics

- Excitation energies
- 42.0
- Ab initio calculations
- 22.0
- Dissociation energies
- 22.0
- Boltzmann equations
- 19.0
- Dissociation
- 19.0

## Figures

Schematic diagram of the photodissociation of triplet acetaldehyde to and HCO. represents the excitation energy, is the electronic energy barrier for the forward dissociation reaction, is the electronic energy barrier for the reverse, association reaction, is the electronic dissociation energy, is the excess energy at the transition state, and is the total available energy to the and HCO products. The inset is the transition state structure for the photodissociation of triplet acetaldehyde, as calculated at .

Schematic diagram of the photodissociation of triplet acetaldehyde to and HCO. represents the excitation energy, is the electronic energy barrier for the forward dissociation reaction, is the electronic energy barrier for the reverse, association reaction, is the electronic dissociation energy, is the excess energy at the transition state, and is the total available energy to the and HCO products. The inset is the transition state structure for the photodissociation of triplet acetaldehyde, as calculated at .

Approximations to the minimum-energy path profiles for the dissociation of acetaldehyde, using UB3LYP/cc-pVTZ optimized geometries.

Approximations to the minimum-energy path profiles for the dissociation of acetaldehyde, using UB3LYP/cc-pVTZ optimized geometries.

Vibrational energy distributions for HCO and calculated for five different excess energies (labeled in ). The data and labels for the highest and lowest excess energies are shown in bold. The energy of the lowest vibrational levels in HCO and , the HCO bend at (Ref. 94), and umbrella mode ( or ) (Ref. 95), respectively, are indicated by arrows.

Vibrational energy distributions for HCO and calculated for five different excess energies (labeled in ). The data and labels for the highest and lowest excess energies are shown in bold. The energy of the lowest vibrational levels in HCO and , the HCO bend at (Ref. 94), and umbrella mode ( or ) (Ref. 95), respectively, are indicated by arrows.

Rotational energy distributions for HCO and calculated for the labeled five different excess energies. The distributions for the highest and lowest are shown in bold. The distributions change from symmetric “Gaussian shaped” at low energy to more asymmetric “Boltzmann shaped” at higher energy.

Rotational energy distributions for HCO and calculated for the labeled five different excess energies. The distributions for the highest and lowest are shown in bold. The distributions change from symmetric “Gaussian shaped” at low energy to more asymmetric “Boltzmann shaped” at higher energy.

Total translational energy distributions calculated for five different excess energies (labeled in ). The data and labels for the highest and lowest excess energies are shown in bold. The energy of the exit channel is shown as an arrow and indicates that most of the exit channel energy is partitioned into translational energy.

Total translational energy distributions calculated for five different excess energies (labeled in ). The data and labels for the highest and lowest excess energies are shown in bold. The energy of the exit channel is shown as an arrow and indicates that most of the exit channel energy is partitioned into translational energy.

Natural log of the HCO population as a function of HCO rotational energy for three different excess energies (labeled in ). The top panel shows the classical trajectory results and the lower panel shows experimental results, taken from Ref. 46.

Natural log of the HCO population as a function of HCO rotational energy for three different excess energies (labeled in ). The top panel shows the classical trajectory results and the lower panel shows experimental results, taken from Ref. 46.

Amount of average energy partitioned into internal and external degrees of freedom of HCO and as a function of excess energy . The average energy has been separated in an impulsive reservoir, which is determined by the fixed exit channel [panel (a)], and statistical reservoirs [panel (b)] that increase with increasing excess energy (see text).

Amount of average energy partitioned into internal and external degrees of freedom of HCO and as a function of excess energy . The average energy has been separated in an impulsive reservoir, which is determined by the fixed exit channel [panel (a)], and statistical reservoirs [panel (b)] that increase with increasing excess energy (see text).

## Tables

Vibrationless relative energies with respect to the equilibrium structure, for the forward and reverse energy barriers, and , respectively, and the dissociation energy for the photodissociation of triplet acetaldehyde. See text and Fig. 1 for definition of energetics.

Vibrationless relative energies with respect to the equilibrium structure, for the forward and reverse energy barriers, and , respectively, and the dissociation energy for the photodissociation of triplet acetaldehyde. See text and Fig. 1 for definition of energetics.

Energy partitioning amongst rotational, , vibrational, , and relative translational, , degrees of freedom for photoproducts initiated with energy at the transition state and with energy at the product configuration.

Energy partitioning amongst rotational, , vibrational, , and relative translational, , degrees of freedom for photoproducts initiated with energy at the transition state and with energy at the product configuration.

Experimental energy partitioning amongst rotational, vibrational, and relative translational degrees of freedom for photoproducts. The values of are as reported in the original work, while assumes a exit channel barrier.

Experimental energy partitioning amongst rotational, vibrational, and relative translational degrees of freedom for photoproducts. The values of are as reported in the original work, while assumes a exit channel barrier.

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