Very different CH3 internal rotation barriers in the syn- and anti- forms of thioacetic acid: Microwave measurements and energy decomposition analysis
Smith, CJ
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https://hdl.handle.net/2142/100577
Description
Title
Very different CH3 internal rotation barriers in the syn- and anti- forms of thioacetic acid: Microwave measurements and energy decomposition analysis
Author(s)
Smith, CJ
Contributor(s)
Mo, Yirong
Zhang, Huaiyu
Leopold, Ken
Huff, Anna
Issue Date
2018-06-20
Keyword(s)
Small molecules
Abstract
Rotational spectra of two conformers of thioacetic acid (CH3COSH) have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for both the syn- and anti- forms of the parent species, as well as for five isotopologues which include 34S and 13C substitution on the methyl and carboxyl atoms. Spectra were fit using two different internal rotation fitting programs, XIAM and BELGI-Cs, and comparisons between their performances will be discussed. The experimental internal rotation barriers for the parent syn- and anti-thioacetic acid obtained from BELGI-Cs are 69.3(10) and 435.2(22) cm−1 , respectively, and compare favorably with the computed values of 83 and 344 cm−1 at the M06-2X/6-311+G(d,p) level of theory. An energy decomposition analysis using the block localized wavefunction method indicates that the steric (including both Pauli and electrostatic) repulsion between the -SH and CH3 groups, which is further enhanced by the pi-conjugation from the -SH to the carbonyl group, is largely responsible for the large difference in the internal rotation barrier between the syn- and anti- forms.
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