Vibrational states of deuterated trans- and cis-formic acid: DCOOH, HCOOD, and DCOOD

Abstract

The vibrational eigenenergies of the deuterated forms of formic acid (DCOOD, HCOOD, and DCOOH) have been computed using the block-improved relaxation method as implemented in the Heidelberg multi-configuration time-dependent Hartree package on a previously published potential energy surface [J. Chem. Phys. 148, 064303 (2018)] generated at the CCSD(T)-F12a/aug-cc-pVTZ-F12 level of theory. Fundamental, combination band, and overtone transition frequencies of the trans isomer were computed up to ∼ 3000 cm-1 with respect to the zero point energy and assignments were determined by visualisation of the reduced densities. Root mean square deviation of computed fundamental transition frequencies with experimentally available gas-phase measurements are 8, 7, and 3 cm-1 for trans-DCOOD, HCOOD, and DCOOH, respectively. The fundamental transition frequencies are provided for the cis isomer of all deuterated forms; experimental measurements of these frequencies for the deuterated cis isotopologues are not yet available and the present work may guide their identification.

Increasing the weights in the molecular work-out of cis- and trans-formic acid: extension of the vibrational database via deuteration

“…S3 in the ESI †). However, the good agreement of all cis-n 2 bands with the highlevel prediction of Aerts et al 13 (deviations below 3 cm À1 ) does not support this conjecture. Besides, close to the cis-band position of cis-DCOOD (2145 cm À1 ), there are no other hot bands in the DCOOH spectrum and vice versa (Fig.…”

“…A recent example of a high-level variational anharmonic ab initio study on the formic acid monomer is an MCTDH study by Aerts et al from 2020 13 who have characterised the cisand trans-conformers of all three deuterated isotopologues. Due to the lack of environment-free experimental data on the higher-energy structure, the accuracy of their description could solely be evaluated for the global minimum trans-form.…”

“…27 This Fermi resonance is also present in trans-DCOOH as well as in trans-HCOOD but seemingly absent in trans-DCOOD. For the cis-conformers, this Fermi resonance is not predicted, [11][12][13] which makes a combination of both species valuable for vibrational benchmarking.…”

“…3). Subtle mass changes such as 13 C isotopic substitution 28 might allow for more insight, though this is experimentally too elaborate without further theoretical support.…”

“…As an important contributor to atmospheric chemistry, 1,2 the formic acid monomer, particularly its global minimum trans-conformation, has received much attention over the years both from the experimental [3][4][5][6][7][8][9][10] as well as the theoretical [11][12][13] side. Despite the low abundance of merely 0.1% at room temperature, the first gas phase detection of the higherenergy cis-conformer was achieved with microwave spectroscopy in 1976 by Hocking, 14 enabled by the threefold larger dipole moment of cis-(3.79 D) compared to trans-HCOOH (1.42 D).…”