Abstract
The cis-trans-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.
Highlights
The spectroscopic characterisation of carboxylic acids and their hydrogen bonded clusters has attracted considerable attention over the years
In line with the results presented here for the propiolic acid dimer, it was assigned to a combination vibration of the symmetric and antisymmetric OH torsion by Kollipost,[24] which is predicted to exhibit significant oscillator strength.[26]
The torsional isomerism of carboxylic acid monomers is an attractive target for performance tests of quantum chemical calculations, as the global minimum trans-form can be scrutinised, and higher-energy regimes of the potential energy hypersurface, for example the cis-rotamer
Summary
The spectroscopic characterisation of carboxylic acids and their hydrogen bonded clusters has attracted considerable attention over the years. The propiolic acid monomer has a significantly lower cis–transenergy difference compared to formic acid, resulting in a room temperature abundance that is an order of magnitude higher (about 1%), yet vibrational data on the cis-rotamer are so far restricted to matrix isolation studies.[34,44] The global minimum trans-rotamer has been examined with low-resolution gas phase infrared spectroscopy,[54] in the liquid phase with infrared and Raman spectroscopy,[54] with matrix isolation spectroscopy,[33,34,44] and Raman jet spectroscopy[29,35] focussing mostly on fundamentals. Following the approach that allowed for the detection of cis-formic acid using a heatable Raman jet set-up,[30,52,53] we will present the first perturbation-free vibrational data for cis-propiolic acid in addition to a manifold of vibrational information beyond fundamentals on trans-propiolic acid, highlighting that the torsional isomerism of the propiolic acid monomer as well as its homo- and heterodimer with formic acid can prove to be meaningful reference systems for theory
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