Matrix Isolation‐Vibrational Circular Dichroism Spectroscopic Study of Conformations and Non‐Covalent Interactions of Tetrahydro‐2‐Furoic Acid

Abstract

The conformational landscape and aggregation behaviour of tetrahydro‐2‐furoic acid (THFA) were investigated by using matrix isolation‐vibrational circular dichroism (MI‐VCD). The well‐resolved experimental MI‐IR and MI‐VCD features in an argon matrix at 10 K allow one to identify two dominant monomeric conformations as trans‐THFA where the hydroxyl and carbonyl groups of COOH are at opposite sides, as well as one cis‐conformer. At 24 K and 30 K deposition temperatures, the experimental IR and VCD spectral features reveal further growth of the binary THFA aggregates. Systematic conformational searches identified three vastly different binary binding topologies, resulting in a few hundred stable (THFA)2 conformers. Interestingly, the main binary structures observed correspond to an unusual type of structure which is made of two trans‐THFA subunits, in contrast to the usual double H‐bonded ring binary structures, identified in a previous solution study. The present work showcases the power of MI‐VCD spectroscopy in revealing unusual structures formed in a cold rare gas matrix.

Raman Optical Activity of N‐Acetyl‐L‐Cysteine in Water and in Methanol: The “Clusters‐in‐a‐Liquid” Model and ab Initio Molecular Dynamics Simulations

“…The trans ‐COOH configuration allows the OH of the carboxylic group to form a strong intramolecular hydrogen bond with the O atom of the carboxamide group. A similar reason leads to the strong preference of the trans ‐COOH configuration in tetrahydro‐2‐furoic acid [5] and the detection of dimers which are made of trans ‐COOH in a supersonic jet expansion [56] and in a cold rare gas matrix [46] . A quantum theory of atoms in molecules (QTAIM) analysis shows that such an intramolecular hydrogen bond energy is about 40 kJ mol −1 , indicating a strong attractive force between the ether oxygen and OH on the carboxylic acid group.…”

“…A similar reason leads to the strong preference of the trans-COOH configuration in tetrahydro-2furoic acid [5] and the detection of dimers which are made of trans-COOH in a supersonic jet expansion [56] and in a cold rare gas matrix. [46] A quantum theory of atoms in molecules (QTAIM) analysis shows that such an intramolecular hydrogen bond energy is about 40 kJ mol À 1 , indicating a strong attractive force between the ether oxygen and OH on the carboxylic acid group. The same QTAIM analysis shows no specific bond critical point between the carboxylic acid oxygen and OH group for the cis-COOH configuration, further supporting the observation that the trans-COOH configuration is strongly preferred.…”

Rotational Spectroscopy of 2‐Furoic Acid and Its Dimer: Conformational Distribution and Double Proton Tunneling

“…The thermodynamically favored binary conformers with two cis ‐COOH THFA subunits were detected, so were the kinetically favored and thermodynamically moderate binary conformers with one trans ‐ and one cis ‐COOH THFA subunits. Interestingly, the most strongly kinetically favored binary conformers with two trans ‐COOH subunits were not observed in a jet expansion, but were detected in the matrix isolation IR and VCD spectra [8] . Assuming a kinetically controlled process, one would expect the abundances of I – I , I – II , I – III , II – II , II – III , and III – III to be 26 %, 48 %, 2 %, 22 %, 2 % and 0 %, respectively, based on the experimental conformational abundances of I (51 %), II (47 %) and III (2 %).…”

“…Interestingly, the most strongly kinetically favored binary conformers with two trans-COOH subunits were not observed in a jet expansion, but were detected in the matrix isolation IR and VCD spectra. [8] Assuming a kinetically controlled process, one would expect the abundances of I-I, I-II, I-III, II-II, II-III, and III-III to be 26 %, 48 %, 2 %, 22 %, 2 % and 0 %, respectively, based on the experimental conformational abundances of I (51 %), II (47 %) and III (2 %). Therefore, the situation with the FA dimer is greatly simplified because there is no competition between the kinetic and thermodynamic processes, i. e. both processes favor binary geometries containing only cis-COOH conformers, I-I, I-II, and II-II.…”

“…In addition, a closely related system, the hydrogenated FA, i. e. tetrahydro-2-furoic acid (THFA), was recently studied using rotational spectroscopy and matrix isolation IR and vibrational circular dichroism spectroscopy. [7,8] The THFA monomer was shown to strongly favor the trans-COOH conformation, in contrast to FA. When THFA interacts with itself, water, or another chiral molecule such as propylene oxide, the conformational distributions were shifted by different degrees towards the cis-COOH conformation, [7,9,10] highlighting the importance of environmental perturbation.…”