Ab initio interpolated potential energy surface and classical reaction dynamics for HCO++H, HOC++H, and deuterated analogues

Abstract

Classical simulations of the reactions between HCO+/COH+ and hydrogen atoms, as well as their deuterated variants, have been carried out on an ab initio interpolated potential energy surface. The surface is constructed at the quadratic configuration interaction with single and double excitation level of ab initio calculation. At low energies we observe reaction channels associated with the isomerization of the cation, hydrogen/deuterium exchange, and the combination of isomerization with exchange. The HCO+/DCO+ ions only undergo exchange, and deuteration is more facile than the release of deuterium. The COH+/COD+ ions undergo isomerization or isomerization combined with exchange, the latter being the dominant reaction channel. Deuteration is again more facile than the release of deuterium, in combination with isomerization. These results are consistent with experimental measurements and with hypotheses on the deuteration of molecules in the interstellar medium.

The dynamics of the H2+CO+ reaction on an interpolated potential energy surface

“…The cis form of HCOH + is also accessible, lying around 15 kJ mol −1 higher than the trans-HCOH + energy with a cis-trans isomerization barrier around 75 kJ mol −1 above the trans-HCOH + energy. 37 Hydrogen atoms can dissociate from cis-HCOH + to form HCO + and HOC + , with the energies of the saddle points being similar to those for reactions from trans-HCOH + .…”

“…42 As in the work of Moyano et al, 37 calculations were initially performed at the QCISD/ 6-311G͑d , p͒ level of theory. Indeed, after adding 495 data points to the interpolation selected from classical trajectories simulating the H 2 +CO + reaction, the 1404 data points calculated by Moyano et al describing the H 2 CO + , HCOH + , H + HCO + , and H + HOC + regions of the PES were added to the data set, whereupon the grow procedure from the H 2 +CO + channel was continued.…”

“…The other paths to HOC + ͑via COH 2 + or vw 3 ͒ cannot occur without tunneling at low temperatures. Note that the isomerization barrier from HCO + to HOC + lies substantially above the entrance channel energy, 37 although it is thought that interactions with other neutral species can reduce the barrier height of this isomerization. [45][46][47] …”

“…Note that on this scale the classical isomerization barrier lies above 320 kJ mol −1 . 37,44 The HCO + to HOC + tunneling rate can be estimated by considering tunneling through an Eckart barrier. Using high accuracy information about the transition state 44 gives a transmission probability through an appropriate asymmetric Eckart barrier 50 of the order of 10 −16 .…”

“…Moyano et al 37 studied the interaction of H with HCO + and HOC + , which constitute the product channels of H 2 +CO + . The goal of that study was to estimate the isomerization rate constant from HOC + to the more stable HCO + in collisions with H atoms.…”