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Microwave Spectroscopy of Chiral Astrochemical Candidate Vinyloxirane: The Missing Gauche Conformer
Journal article   Peer reviewed

Microwave Spectroscopy of Chiral Astrochemical Candidate Vinyloxirane: The Missing Gauche Conformer

Afnan Alasmari, Chris Medcraft, Kaidan Rolfe, Courtney Ennis and Evan G. Robertson
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, Vol.128(29), pp.5969-5979
25/07/2024
DOI: https://doi.org/10.1021/acs.jpca.4c03560
PMID: 39004902
Appears in  Recent Faculty of Science and Engineering Publications

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Abstract

molecular structure chemical calculations alkyls energy levels electromagnetic radiation
The recent detection of a chiral molecule, propylene oxide, in the interstellar medium provides impetus for investigation of related analogues as candidates for discovery of a second chiral species. Vinyloxirane (VO) shares many of the characteristics of propylene oxide that favored its remote detection such as modest size, appreciable dipole moment and modest adsorption to water ice. The microwave spectrum of vinyloxirane at room temperature has been studied in the 18 - 26 GHz region. Rotational transitions of the previously undetected gauche-1 conformer have been assigned and fitted. The quantum number range of anti conformer transitions has been greatly expanded, providing improved molecular constants. Vibrational satellite transitions were assigned and fitted for the lowest frequency nu(27) torsion mode, for 2 nu(27), and for the nu(26) C=CC bend of anti VO, along with nu 27 satellites of gauche-1. The rovibrational analyses were assisted by anharmonic vibrational calculations at B3LYP-D3/aug-cc-PVTZ, B2PLYPD3/cc-pVTZ, and DSDPBEP86/cc-pVTZ levels. Experimental peak intensities provide a population ratio N (gauche-1)/N( anti )of 0.36 +/- 0.06, corresponding to a Gibbs free energy difference of 2.5 +/- 0.4 kJ mol(-1) in favor of the anti conformer, in agreement with the density functional theory results. In the gauche-1 conformer, the torsional angle between vinyl group and oxirane ring, tau C=CCM, is optimized at -35 degrees to favor an intramolecular CvinylH(11)...O interaction. The gauche-2 conformer with tau(C=CCM) of + 74 degrees suffers from CvinylH11...HCcyclopropyl repulsion so that it is calculated to be 8-9 kJ mol(-1) higher than gauche-1. Reinterpretation of earlier Raman spectra suggests that the gauche-2 conformer had not been observed as reported.

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