Discerning Near-Isoergic Isomers. A Matrix Isolation Infrared and ab Initio Study of the Propargyl Alcohol Dimers

Abstract

This study reports the first infrared spectroscopic investigation of the dimers of propargyl alcohol (PA) using matrix isolation infrared spectroscopy and ab initio computations. Computations indicated a number of isomers for the dimer, with the two most stable structures being nearly isoergic. Interestingly, our matrix isolation experiments were able to discern both of these isomers, which were corroborated by computations performed at the M06-2X/6-311++G(d,p) level. Identifying these two isomers was significant, since the two most stable and near-isoergic isomers can be expected to play an important role in the structure of the propargyl alcohol ices, an understanding of which is of significant importance in the study of astrochemical environments. The global minimum structure differed from its near-isoergic local minimum, in the dimer architecture; the former making a three-point hydrogen-bonded contact, O-H···O, O-H···π, and C-H···π, and the latter having two. The global minimum had been observed in molecular beam experiments, though not the near-isoergic local minimum. In both isomers, the propargyl alcohol monomer units were in their gauche conformation. AIM and NBO analyses were also performed to understand the nature of interaction in the dimers. The results of this study may well have significant implications in interpreting the PA spectra in astrochemical environments.

Predicting OH stretching fundamental wavenumbers of alcohols for conformational assignment: different correction patterns for density functional and wave-function-based methods

“…In addition four weak downshifted bands at 3581, 3562, 3532 and 3507 cm À1 are observed in our work, which likely originate from two dimer structures previously assigned in a matrix study. 89 In these dimers both hydroxy groups donate a hydrogen bond each and their OH stretching transitions are thus shifted out of the monomer spectral region. The spectrum in the full monochromator range (3720-3270 cm À1 ) is available in the ESI.…”

Understanding benzyl alcohol aggregation by chiral modification: the pairing step

“…The most stable structural motif O g π features cooperatively interacting OH• • • OH• • • π hydrogen bonds and is reminiscent of the global minimum of the propargyl alcohol dimer 63,64 . The het variant is slightly less stable despite somewhat stronger hydrogen bonds (based on predicted downshifts and intensity enhancements), indicating a compensating role of dispersion interactions.…”

“…An interesting test case could be the dimers of propargyl alcohol, for which two conformers were recently assigned in an FTIR matrix study. 64 In the nomenclature used here they correspond to homO t π and homO g π structures, with the former having weakly and the latter strongly coupled (assigned only 20 cm −1 apart) OH stretching modes.…”

Intermolecular Complexes and Molecular Conformations Directed by Hydrogen Bonds: Matrix Isolation and Ab Initio Studies

“…33 Hydrogen-bonded complexes with multiple interactions were also observed in the PA dimer system. 34,35 Studies on the PhAc-C 2 H 2 system yielded four minima on the potential surface which are shown in Fig. 6.…”