The hydroxyl radical is an important atmospheric oxidant, and a significant source of its production occurs through alkene ozonolysis. This takes place via a cycloaddition reaction and subsequent fragmentation to form an energized carbonyl oxide (for example, CH3CHOO), known as a Criegee intermediate, which can then either react with another atmospheric species or decay and, in doing so, produce the hydroxyl radical. Here, we examine the dissociation dynamics of a prototypical Criegee intermediate by characterizing the translational and internal energy distributions of the OH radical products, which reflect critical configurations along the reaction pathway. Experimentally, the kinetic energy release to OH products is ascertained through velocity map imaging. Theoretically, quasi-classical trajectories are performed on a new full-dimensional, ab initio potential energy surface. Both experiment and theory show that most of the available energy flows into internal excitation of the vinoxy products. The isotropic angular distribution of OH fragments indicates that dissociation occurs in ≥2 ps, in agreement with theory.

UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociation to two spin-allowed product channels: CH3CHO X(1)A' + O (1)D and CH3CHO a(3)A″ + O (3)P. The O (1)D and O (3)P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O (1)D action spectrum closely follows the previously reported UV absorption spectrum for jet-cooled CH3CHOO [Beames et al. J. Chem. Phys. 2013 , 138 , 244307]. Velocity map images of the O (1)D products following excitation of CH3CHOO at 305, 320, and 350 nm exhibit anisotropic angular distributions indicative of rapid (ps) dissociation, along with broad and unstructured total kinetic energy (TKER) distributions that reflect the internal energy distribution of the CH3CHO X(1)A' coproducts. The O (3)P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasing O (3)P yield. Excitation of CH3CHOO at 305 nm, attributed to absorption of the more stable syn-conformer, also results in an anisotropic angular distribution of O (3)P products arising from rapid (ps) dissociation, but a narrower TKER distribution since less energy is available to the CH3CHO a(3)A″ + O (3)P products. The threshold for the higher energy CH3CHO a(3)A″ + O (3)P product channel is determined to be ca. 88.4 kcal mol(-1) from the termination of the TKER distribution and the onset of the O (3)P action spectrum. This threshold is combined with the singlet-triplet spacings of O atoms and acetaldehyde to establish the dissociation energy for syn-CH3HOO X(1)A' to the lowest spin-allowed product channel, CH3CHO X(1)A' + O (1)D, of ≤55.9 ± 0.4 kcal mol(-1). A harmonic normal-mode analysis is utilized to identify the vibrational modes of CH3CHO likely to be excited upon dissociation into the two product channels.

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A 2 Σ + ← X 2 Π transition band. However, the electronic quenching of NO (A 2 Σ + ) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO (A 2 Σ + ) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO (A 2 Σ + ) with H 2 , N 2 , and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H 2 is incapable of electronically quenching NO (A 2 Σ + ) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO (A 2 Σ + ) with N 2 and CO are highly anisotropic and demonstrate evidence of electron transfer from NO (A 2 Σ + ) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO (A 2 Σ + ) + CO and NO (X 2 Π) + CO. In contrast, for ON + N 2 , the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor-scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch-bend Fermi coupling for a variety of systems, it is observed that the anharmonic couplings are robust; their values are similar for the four molecules studied as well as for ethane and methanol. Similar results are obtained with both density functional theory and coupled-cluster calculations. This robustness suggests a new parametrization of the model Hamiltonian that reduces the number of fitting parameters. In contrast, the harmonic contributions to the Hamiltonian vary substantially between the molecules leading to important changes in the spectra. The resulting Hamiltonian predicts most of the major spectral features considered in this study and provides insights into mode mixing and the consequences of the mixing on dynamical processes that follow ultrafast CH stretch excitation.