The role of the cubic and quartic Jahn–Teller coupling in the X˜2E ground electronic state of the methoxy radical CH3O

Permutation invariant polynomial neural network based diabatic ansatz for the (E + A) × (e + a) Jahn-Teller and Pseudo-Jahn-Teller systems

In this work, the permutation invariant polynomial neural network (PIP-NN) approach is employed to construct a quasi-diabatic Hamiltonian for system with non-Abelian symmetries. It provides a flexible and compact NN-based diabatic ansatz from the related approach of Williams, Eisfeld, and co-workers. The example of H₃ ⁺ is studied, which is an (E + A) × (e + a) Jahn-Teller and Pseudo-Jahn-Teller system. The PIP-NN diabatic ansatz is based on the symmetric polynomial expansion of Viel and Eisfeld, the coefficients of which are expressed with neural network functions that take permutation-invariant polynomials as input. This PIP-NN-based diabatic ansatz not only preserves the correct symmetry but also provides functional flexibility to accurately reproduce ab initio electronic structure data, thus resulting in excellent fits. The adiabatic energies, energy gradients, and derivative couplings are well reproduced. A good description of the local topology of the conical intersection seam is also achieved. Therefore, this diabatic ansatz completes the PIP-NN based representation of DPEM with correct symmetries and will enable us to diabatize even more complicated systems with complex symmetries.

High-fidelity first principles nonadiabaticity: diabatization, analytic representation of global diabatic potential energy matrices, and quantum dynamics

Nonadiabatic dynamics, which goes beyond the Born-Oppenheimer approximation, has increasingly been shown to play an important role in chemical processes, particularly those involving electronically excited states. Understanding multistate dynamics requires rigorous quantum characterization of both electronic and nuclear motion. However, such first principles treatments of multi-dimensional systems have so far been rather limited due to the lack of accurate coupled potential energy surfaces and difficulties associated with quantum dynamics. In this Perspective, we review recent advances in developing high-fidelity analytical diabatic potential energy matrices for quantum dynamical investigations of polyatomic uni- and bi-molecular nonadiabatic processes, by machine learning of high-level ab initio data. Special attention is paid to methods of diabatization, high fidelity construction of multi-state coupled potential energy surfaces and property surfaces, as well as quantum mechanical characterization of nonadiabatic nuclear dynamics. To illustrate the tremendous progress made by these new developments, several examples are discussed, in which direct comparison with quantum state resolved measurements led to either confirmation of the observation or sometimes reinterpretation of the experimental data. The insights gained in these prototypical systems greatly advance our understanding of nonadiabatic dynamics in chemical systems.

Thermal Decomposition of CH3O: A Curious Case of Pressure-Dependent Tunneling Effects

The thermal unimolecular decomposition of a methoxy radical (CH₃O), a key intermediate in the combustion of methane, methanol, and other hydrocarbons, was studied using high-level coupled-cluster calculations, followed by E,J-resolved master equation analyses. The experimental results available for a wide range of temperature and pressure are in striking agreement with the calculations. In line with a previous theoretical study that used a one-dimensional master equation, the tunneling correction is found to exhibit a marked pressure dependence, being the largest at low pressure. This curious effect on the tunneling enhancement also affects the calculated kinetic isotope effect, which falls initially with pressure but is predicted to rise again at high pressures. These findings serve to reconcile a set of conflicting results regarding the importance of tunneling in this prototype unimolecular reaction and also motivate further experimental investigation. This study also exemplifies how changes in the energy redistribution due to collisions manifest in the tunneling rates.

Simulation of Ã(2)A(1)←X̃(2)E laser excitation spectrum of CH3O and CD3O

A theoretical calculation of the laser-induced fluorescence excitation spectrum from X̃(2)E→Ã(2)A1 is carried out for CH3O and CD3O using a transition dipole moment surface expanded up to second order. The vibronic form of these operators is obtained using symmetry arguments. The Ã(2)A1 vibrational levels are calculated using Van Vleck perturbation theory, and the latter is used to adjust harmonic constants of the potential to match experimental fundamentals. The CH3O fit force field is tested on CD3O. For both molecules the transition energies are well reproduced, but there are systematic differences between experimental and theoretical intensities. The origins of these differences are discussed.

Decomposition of Tertiary Alkoxy Radicals

Rate coefficients of β-scission reactions in tertiary alkoxy radicals, R(CH3)2CO (R = methyl, ethyl, tert-butyl and neo-pentyl) have been estimated via density functional theory (DFT) calculations in conjunction with statistical unimolecular rate theory. For tert-butoxy, results obtained by employing different basis sets are compared with experimental values, indicating that UB3LYP/6-31G(d,p) excellently predicts kinetic data. Rate coefficients for inter- and intramolecular hydrogen abstraction are also reported. Depending on R, the β-scission rate may vary by orders of magnitude. The predicted temperature dependence of the alcohol-to-ketone product ratios for alkoxy radical decomposition in a hydrocarbon environment is in remarkably close agreement with the corresponding ratios measured on the product mixtures from decomposition of tert-butyl and tert-amyl peroxyacetates in solution of n-heptane.

Slow electron velocity-map imaging of negative ions: applications to spectroscopy and dynamics

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

Jet-cooled infrared spectrum of methoxy in the CH stretching region

Observations of the jet-cooled infrared spectrum of CH(3)O in the CH stretching region have been extended, down to 2756 cm(-1) and up to 3003 cm(-1). In the lower frequency extension, a single vibronic band has been assigned. In the higher frequency region, the spectrum becomes complex above 2900 cm(-1) and remains so until near 2970 cm(-1) where it rapidly becomes sparse. Including the single vibronic band previously reported, a total of four bands have been assigned. Two bands including the original one follow a perpendicular DeltaP = +1 rotational selection rule and the other two bands follow a parallel DeltaP = 0 selection rule. In addition to these in the congested region between 2900 and 2970 cm(-1), ten isolated sub-bands (two P'' = -1/2, two P'' = +1/2, and six P'' = +1.5) have been assigned, but it has so far not been possible to connect these together to form bands. Taken together these observations suggest that there are strong vibronic couplings between the two CH stretching vibrations and the overtone and combination levels in the region.

Slow electron velocity-map imaging photoelectron spectra of the methoxide anion

High resolution anion photodetachment spectra are presented for the methoxide anion and its fully deuterated counterpart. The spectra were obtained with slow electron velocity-map imaging. Improved electron affinities are determined for CH3O as 1.5690+/-0.0019 eV and for CD3O as 1.5546+/-0.0019 eV. The spectra resolve many features associated with spin-orbit and vibronic coupling that were not seen in previous photodetachment studies. Photoelectron angular distributions taken as a function of detachment wavelength for the ground vibronic state transitions are recorded and are consistent with the removal of a nonbonding, p-type electron localized on the oxygen atom. Several hot bands and sequence bands are observed for the first time, providing insight into the vibrational structure of the methoxide anion. The results are compared to recent calculations of the anion photoelectron spectra that incorporate bilinear coupling terms among the methoxy vibrational modes and are found to be in reasonable agreement.