Issue 32, 2021

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation


The cistrans-isomerism of the propiolic acid monomer (HC[triple bond, length as m-dash]C–COOH) is examined with linear Raman jet spectroscopy, yielding the first environment-free vibrational band centres of a higher-energy cis-rotamer beyond formic acid (HCOOH) in addition to all fundamentals and a large number of hot and combination/overtone bands of the trans-conformer. Two near-isoenergetic trans-fundamentals of different symmetry (CC[double bond, length as m-dash]O bend and OH torsion) prove to be a sensitive benchmarking target, as their energetic order is susceptible to the choice of electronic structure method, basis set size, and inclusion of vibrational anharmonicity. For the infrared- and Raman-active C[double bond, length as m-dash]O stretching fundamentals of the cyclic (C2h) trans-propiolic acid dimer, resonance couplings are found that in part extend to the Cs-symmetric heterodimer of trans-propiolic and trans-formic acid. Exploratory vibrational perturbation theory (VPT2) calculations show that all perturbing states involve displacements of the OH moieties located on the doubly hydrogen bonded ring. The comparison of the infrared spectra of the propiolic acid dimer and its heterodimer with formic acid to that of several other carboxylic acid dimers from the literature reveals a notable similarity regarding a non-fundamental dimer band around 1800 cm−1, which in most cases is so far unassigned. VPT2 calculations and a simple harmonic model suggest an assignment to a combination vibration of the symmetric and antisymmetric OH torsion.

Graphical abstract: CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation

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Article information

Article type
15 Jun 2021
28 Jul 2021
First published
05 Aug 2021
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2021,23, 17208-17223

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation

K. A. E. Meyer and A. Nejad, Phys. Chem. Chem. Phys., 2021, 23, 17208 DOI: 10.1039/D1CP02700J

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