Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: A Trajectory Surface-Hopping Study

Abstract

The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS­(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully’s fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O–O bond leads to major products CH3O­(2E) + OH­(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O–H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0 leading to ground-state products CH3OO­(X 2A″) + H­(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO­(1 2A′) + H­(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm.

Deciphering the Influence of Ground-State Distributions on the Calculation of Photolysis Observables

“…An exciting and counter-intuitive feature of both theoretical and experimental distributions is the long tail at low kinetic energy that appears only for the 193 nm excitation energy. The experimental study of Thelen et al 71 lacks any explanation of this feature, while the theoretical work of Mahata and Maiti 72 did not predict the tail. We analyzed the TSH trajectories leading to very low translational kinetic energies for the released OH and noticed that these OH fragments exhibit large vibrational amplitudes.…”

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Computational simulations of the dynamics must first describe the energetically accessible electronic states by mapping potential energy surfaces, conical intersections, and regions of intersystem crossing 23,24 and then must propagate the nuclear dynamics of the molecules as their structures evolve over these excited-state surfaces, using either classical or quantum mechanical descriptions of the nuclear motions. 25,26 Simulations of this type can be used to unravel the electronic states responsible for bond breaking, to characterize nonadiabatic dynamics as molecules switch between different electronic states, and to reveal unusual mechanisms such as the "roaming" dynamics of molecules that explore long-range, weakly bound regions of their potential energy surfaces. More computationally efficient methods based on master equation treatment of reaction kinetics are also finding applications in modeling photodissociation, 27 while statistical theories offer computationally low-cost methods to predict the outcomes of photochemical reactions that occur from long-lived intermediates, which include energized ground-electronic-state molecules.…”

Spectroscopic Characterization and Photochemistry of the Atmospherically Relevant Methanesulfenic Acid

“…Therefore, it is very likely that the photolytic generation of H 2 CS•••H 2 O from CH 3 SOH proceeds via 1,3-hydrogen migration from the carbon atom to the oxygen atom. The 49,50 To understand the photochemistry of the water complex H 2 CS•••H 2 O, the matrix containing the 254 nm photolysis products of CH 3 SOH was subjected to an ArF excimer laser irradiation at 193 nm. The resulting IR difference spectrum (Figure 3A) shows further decomposition of CH 3 SOH.…”

“…Consistent with the selective deuteration at the endocyclic hydrogen atom, the observed H/D-isotopic shift of −205.8 cm –1 for the δ­(H 2 O) mode shows better agreement with the calculated shift for the corresponding isotopomer (−211.7 cm –1 ) than that for the other isotopomer (−174.6 cm –1 ) with deuteration at the exocyclic hydrogen atom. Therefore, the concerted decomposition mechanism of CH 3 SOH in the matrix is different from the photochemistry of CH 3 OOH, for which the primary photodissociation channel is the cleavage of the O–O bond in yielding CH 3 O· and ·OH radicals. , …”