Far-Infrared Synchrotron Spectroscopy and Torsional Analysis of the Important Interstellar Molecule, Vinyl Alcohol

Abstract

We report far-infrared spectra (100−600 cm −1 ) covering the torsional bands of the important interstellar molecule, vinyl alcohol. We observed the fundamental and first two hot bands of syn-vinyl alcohol and the fundamental and first hot band of anti-vinyl alcohol, whose infrared spectrum has not been previously reported. The hot bands for syn-vinyl alcohol were incrementally shifted by ∼40 cm −1 from the fundamental, implying a high degree of anharmonicity in the torsional potential in the vicinity of the global minimum. The opposite holds true for anti-vinyl alcohol, as evidenced by the relatively small red shift (∼2 cm −1 ) of the hot band with respect to the fundamental. These characteristics were captured by ab initio calculations of the torsional potential, which we fit a seven-term Fourier series expansion to, and used along with the calculated torsional inertias to solve for the energy levels and wave functions. The potential was empirically refined by fitting the three leading terms in the expansion to the observed torsional frequencies, which resulted in a 3-fold decrease in the root-mean-square deviation between theory and experiment. The resulting energy difference between the ground states of the two rotamers is 4.0 kJ/mol, which is approximately consistent with the measured intensity ratio of the two ground-state torsional bands and with microwave spectroscopic measurements (4.5 ± 0.6 kJ/mol) [

Submillimeter spectroscopy and astronomical searches of vinyl mercaptan, C2H3SH

“…In particular, the energetics computed at the W2-F12 level by Karton & Talbi (2014) for the syn isomer of vinyl alcohol are within ∼1 kJ/mol of the values presented here. The energy difference between the syn and anti isomers has been examined theoretically (Nobes et al 1981) and experimentally; the value presented here (4.6 kJ/mol) is in excellent agreement with both the microwave (4.5 ± 0.6, Rodler (1985)) and the far-infrared semi-experimental determination (4.0 kJ/mol, Bunn et al (2017)). The sulfur analogues [C 2 ,H 4 ,S] on the other hand are near-isoenergetic at the current level of treatment: with the exception of CH 3 CHS, the three remaining isomers are within ∼0.5 kJ/mol of one another, with anti-vinyl mercaptan being the highest in energy (5.3 kJ/mol).…”

Computational Evidence Suggests That 1-Chloroethanol May Be an Intermediate in the Thermal Decomposition of 2-Chloroethanol into Acetaldehyde and HCl

“…Experimental microwave spectra of vinyl alcohol were reported by Saito [36] for the syn-conformer, and by Rodler [37] for the anticonformer. Bunn's et al data [35] agree well with the experimental values, while our own calculations afford C-O distances slightly too short (i.e. the C-O bond is slightly too strong) but the overall discrepancy is not significant.…”

“…1. For the stable structures, when they are available, the first entry correspond to the experimental values, followed by other accurate calculations and, at last, our own calculations at DFT and, eventually, MP2 levels, according to the following specifications: (a) 2ClEtOH, microwave spectroscopy [28], electron diffraction [29], B97X-D/cc-pVTZ calculations in this paper, B97X-D/aug-cc-pVQZ and MP2/aug-cc-pVTZ calculations [31]; (b)1ClEtOH, B97X-D/cc-pVTZ calculations in this paper, B97X-D/aug-cc-pVQZ and MP2/aug-cc-pVTZ calculations [31]; (c)vinyl alcohol syn-conformer microwave spectrum [36], CCSD(T)/cc-pVQZ [35] and our own B97X-D/cc-pVTZ calculations; (d)vinyl alcohol anti-conformer microwave spectrum [37], CCSD(T)/cc-pVQZ [35] and our own B97X-D/cc-pVTZ calculations;…”

“…Vinyl alcohol (VA) is a molecule that has also been studied several times and which, besides its interest in multiple industrial reaction paths, is also of considerable interest as an interstellar molecule. The most recent publication, to our knowledge, is that of Bunn et al [35], who studied the far IR spectra of both anti-and syn-VA, using a CCSD(T)/cc-pVQZ chemical model to obtain the structure of both species. Experimental microwave spectra of vinyl alcohol were reported by Saito [36] for the syn-conformer, and by Rodler [37] for the anticonformer.…”

Astronomical Search of Vinyl Alcohol Assisted by Submillimeter Spectroscopy

“…VA can exist in two rotameric forms, depending upon the value of the CC–O–H dihedral angle ϕ: for the syn conformer, ϕ = 0° and the hydrogen of the hydroxy group is in the plane of the molecule on the side of the double bond, whereas in the anti case, there is a torsion of 180° of the hydroxy group. The interconversion between the two forms is hindered by an energy barrier of 21 kJ/mol, with the syn rotamer lower in energy than the anti rotamer by 4.6 kJ/mol . The conversion of syn -VA to acetaldehyde (CH 3 CHO) is an example of keto–enol tautomerism; the reaction is exothermic by 40.5 kJ/mol, making acetaldehyde much more thermodynamically stable.…”