Propiolic Acid in Solid Nitrogen: NIR- and UV-Induced cis → trans Isomerization and Matrix-Site-Dependent trans → cis Tunneling

Abstract

Propiolic acid (HCCCOOH, PA) was studied experimentally by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. The vibrational spectra of the cis and trans monomers (OCOH dihedral equal to 0 and 180°, respectively) were measured and assigned. The trans-PA monomer was produced by selective vibrational excitation of the cis-PA monomer molecules trapped in different matrix sites. Broadband in situ UV irradiation (λ > 235 nm) of matrix-isolated PA yielded as product the higher-energy trans conformer, with no other photoproducts being detected. Two cis–cis dimers were also identified in the matrixes and characterized structurally and vibrationally. trans-PA was found to decay back to cis-PA in the dark, by tunneling, and the different lifetimes of the higher-energy PA conformer resulting from pumping different matrix sites and different experimental conditions (using a filter blocking the higher-energy IR radiation of the spectrometer source and without using such a filter) were discussed.

Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide

“…Substituent effects have been shown to affect tunneling rates before, most prominently in systematic studies on carboxylic acids [54–61] and highly reactive hydroxycarbenes [62–69] . In hydroxycarbenes +I‐donating substituents like methyl result in reduced tunneling half‐lives for [1,2]H‐shifts towards the corresponding aldehyde [69] .…”

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

“…Table 1) with n 14 shifted to higher wavenumbers. 34 This sensitivity of the energetic order of n 9 and n 14 also extends to quantum chemical calculations, rendering the n 9 -n 14 energy sequence excellent for benchmarking, which will be explored in the following section.…”

“…32 Propiolic acid (HCRC-COOH), as the smallest unsaturated, aliphatic carboxylic acid, constitutes with only seven atoms and fifteen relatively stiff internal degrees of freedom an excellent intermediate between the well studied formic and the larger saturated carboxylic acids. The vibrational spectrum of its dimer has been studied in solid rare gas matrices, 33,34 and in the gas phase using low-resolution infrared 19 and jet-cooled Raman spectroscopy. 29,35 Building on a previous study, 29 we will present and scrutinise the vibrational spectrum of the propiolic acid homodimer in the CQO stretching region by means of FTIR and Raman jet spectroscopy and characterise the vibrational spectrum of the formic-propiolic acid heterodimer for the first time, which has been thoroughly investigated with microwave spectroscopy.…”

“…The first vibrational detection of a cis-carboxylic acid was achieved for formic acid in a cryogenic argon matrix by Pettersson et al in 1997, 40 nearly sixty years after the first vibrational characterisation of its trans-form. 41 Ever since, these matrix isolation studies have been extended to a multitude of other acids 34,[42][43][44][45] and matrix hosts. 44,[46][47][48] One advantage of this technique are the long storing times achievable, which enable the formation of higher-energy conformers via light-induced isomerisation.…”

“…30,[51][52][53] 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.…”