Zero- and High-Pressure Mechanisms in the Complex Forming Reactions of OH with Methanol and Formaldehyde at Low Temperatures

Abstract

A recent Ring Polymer Molecular Dynamics study of the reactions of OH with methanol and formaldehyde, at zero pressure and below 100 K, has shown the formation of long lived complexes, with long lifetimes, longer than 100 ns for the lower temperatures studied, 20-100 K (del Mazo-Sevillano et al., 2019). These long lifetimes support the existence of multi collision events with the He buffer-gas atoms under experimental conditions, as suggested by several transition state theory studies of these reactions. In this work we study these secondary collisions, as a dynamical approach to study pressure effects on these reactions. For this purpose, the potential energy surfaces of He with H 2 CO, OH, H 2 O and HCO are calculated at highly accurate ab initio level. The stability of some of the complexes is studied using Path Integral Molecular dynamics techniques, determining that OH-H 2 CO complexes can be formed up to 100 K or higher temperatures, while the weaker He-H 2 CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra shows new features which could help the identification of the OH-H 2 CO complex. Finally, the He-H 2 CO + OH and OH-H 2 CO + He collisions are studied using quassi-classical trajectories, finding that the cross section to produce HCO + H 2 O products increases with decreasing collision energy, and that it is ten times higher in the He-H 2 CO + OH case.

Quantum Effects on the D + H3+ → H2D+ + H Deuteration Reaction and Isotopic Variants

“…58 Work to analyze the pressure effect with the buffer gas, He, is being done. 78 The challenge is to calculate the tunneling rate constant, k tunnel (T ), and the redissociation rate, k rediss (T ), under similar conditions, since they involve different processes and degrees of freedom. TST methods do not properly take into account the nature of these long lived resonances and more adapted multidimensional quantum method must be designed for this purpose.…”

Neural Network Potential Energy Surface for the low temperature Ring Polymer Molecular Dynamics of the H2CO + OH reaction

“…What is different in RPMD calculations, is that there is an increasing probability of trapped trajectories as temperature decreases below 300 K. Those trapped trajectories are formed following an orbit similar to the QCT orbit of Fig. 9 as those shown in previous RPMD calculations in this system 29,30 : the long range interaction deviates the initial trajectories for rather long impact parameters, increasing the end-over-end angular momentum and the rotation of each of the two reactants. These trajectories get then trapped orbiting continuously keeping the relative orientation of the two reac-tants fixed, close to the geometry of reactant well, RC1.…”

“…The free polymer propagation was done using a Fourier transform (FT) 81 , and, being nearly exact, allows a large time step. In the previous RPMD calculations 29,30 it was also found that, for temperatures below 100 K, many trajectories were trapped in the complex well in the reactants channel. Those trapped trajectories lived for more than 500 ns, and were too long to be finished.…”

“…In this line, we have studied the dynamics of the reactions of OH with H 2 CO and CH 3 OH [27][28][29][30] . For this purpose, full dimensional potential energy surfaces (PESs) were fitted to accurate ab initio calculations 27,28 .…”

Path-integral approximations to quantum dynamics

“…This is effectively what is done in RPMD rate theory [29,30,41], where the t > 0 RPMD dynamics serves as an artificial, quantum-Boltzmann-conserving, classical dynamics, which gives a lower bound estimate of the t → 0 + flux through the optimal dividing surface [41,[78][79][80][81]. 9 For this reason, RPMD rate calculations give good approximations to the quantum rate coefficients for direct reactions [10,29,30,38,39,[82][83][84][85][86][87][88] and also for 'capture' reactions [89][90][91][92] involving statistically decoupled direct (capture and release) steps.…”