The calculation of the anharmonic modes of small- to medium-sized molecules for assigning experimentally measured frequencies to the corresponding type of molecular motions is computationally challenging at sufficiently high levels of quantum chemical theory. Here, a practical and affordable way to calculate coupled-cluster quality anharmonic frequencies using second-order vibrational perturbation theory (VPT2) from machine-learned models is presented. The approach, referenced as “NN + VPT2”, uses a high-dimensional neural network (PhysNet) to learn potential energy surfaces (PESs) at different levels of theory from which harmonic and VPT2 frequencies can be efficiently determined. The NN + VPT2 approach is applied to eight small- to medium-sized molecules (H2CO, trans-HONO, HCOOH, CH3OH, CH3CHO, CH3NO2, CH3COOH, and CH3CONH2) and frequencies are reported from NN-learned models at the MP2/aug-cc-pVTZ, CCSD­(T)/aug-cc-pVTZ, and CCSD­(T)-F12/aug-cc-pVTZ-F12 levels of theory. For the largest molecules and at the highest levels of theory, transfer learning (TL) is used to determine the necessary full-dimensional, near-equilibrium PESs. Overall, NN + VPT2 yields anharmonic frequencies to within 20 cm–1 of experimentally determined frequencies for close to 90% of the modes for the highest quality PES available and to within 10 cm–1 for more than 60% of the modes. For the MP2 PESs only ∼60% of the NN + VPT2 frequencies were within 20 cm–1 of the experiment, with outliers up to ∼150 cm–1, compared to the experiment. It is also demonstrated that the approach allows to provide correct assignments for strongly interacting modes such as the OH bending and the OH torsional modes in formic acid monomer and the CO-stretch and OH-bend mode in acetic acid.

The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simulations. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5.9 ± 1.5 × 10 −10 (T /300 K) −0.5±0.2 cm 3 s −1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0.08 ± 0.03(T /300 K) −0.8±0.7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower.This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the ratelimiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel.An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels. a) rvborgmailbox@us.af.mil b) m.meuwly@unibas.ch

Understanding the formation of molecules under conditions relevant to interstellar chemistry is fundamental to characterize the chemical evolution of the universe. Using reactive molecular dynamics simulations with model-based or high-quality potential energy surfaces provides a means to specifically and quantitatively probe individual reaction channels at a molecular level. The formation of CO2 from collision of CO­(1Σ) and O­(1D) is characterized on amorphous solid water (ASW) under conditions typical in cold molecular clouds. Recombination takes place on the subnanosecond time scale and internal energy redistribution leads to stabilization of the product with CO2 remaining adsorbed on the ASW on extended time scales. Using a high-level, reproducing kernel-based potential energy surface for CO2, formation into and stabilization of CO2 and COO are observed.

The full reaction pathway between the syn-CH3CHOO Criegee Intermediate via vinyl hydroxyperoxide (VHP) to CH2COH+OH is followed for vibrationally excited and thermally prepared reactants. Reactivity along the entire pathway was characterized from an aggregate of more than 10 μs of reactive MD simulations using energy functions with accuracies at the Møller–Plesset second order level of theory. Reaction times for OH elimination are on the nanosecond time scale, and the energy dependence of the rates is consistent with experiments in the jet. The actual rates depend on the O–O dissociation energy (D e OO = 31.5 kcal/mol at the MP2/aug-cc-pVTZ level or D e OO = 23.5 kcal/mol closer to earlier CASPT2 calculations). Also, the initial preparation of the reactants influences both the VHP formation/OH elimination rates and the OH yields. For most conditions with initial vibrational excitation 80% or more of syn-CH3CHOO progress to OH elimination on the 5 ns time scale. However, for internally cold conformational ensembles generated at low temperature (50 K) only 20% to 30% of VHP eliminate OH on the 5 ns time scale which increases to 55% to 67% depending on excitation energy from simulations on the 15 ns time scale. For thermal preparation of syn-CH3CHOO, which is relevant in the atmosphere, 35% of the trajectories lead to OH-elimination within 1 ns. This compares with experimentally reported yields of 24% to 64% in a collisional environment. The estimated thermal rate at 300 K is 103 s–1, extrapolated from results at 500 to 900 K, is consistent with the experimentally measured rate of 182 ± 66 s–1.

Feshbach resonances are fundamental to interparticle interactions and become particularly important in cold collisions with atoms, ions, and molecules. In this work, we present the detection of Feshbach resonances in a benchmark system for strongly interacting and highly anisotropic collisions: molecular hydrogen ions colliding with noble gas atoms. The collisions are launched by cold Penning ionization, which exclusively populates Feshbach resonances that span both short- and long-range parts of the interaction potential. We resolved all final molecular channels in a tomographic manner using ion-electron coincidence detection. We demonstrate the nonstatistical nature of the final-state distribution. By performing quantum scattering calculations on ab initio potential energy surfaces, we show that the isolation of the Feshbach resonance pathways reveals their distinctive fingerprints in the collision outcome.