LABORATORY DETECTION OF THE {\em TRANS-GAUCHE} CONFORMER OF ETHYL FORMATE
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Date
2010
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Publisher
Ohio State University
Abstract
Ethyl formate has two coordinates of conformational flexibility, in the ester (O=C-O-C) and ethyl (C-O-C-C) dihedral angles. Two conformers, one with a {\em cis} ester and {\em trans} ethyl orientation, the other with a {\em cis} ester and {\em gauche} ethyl orientation, have been previously detected by rotational spectroscopy. \textbf{46}, 4605 (1967).} In addition, the {\em cis-trans} isomer, the lowest-energy conformer, has recently been detected in the SgrB2(N) hot core.., \textit{A\&A} \textbf{499}, 215 (2009).} The third conformer of ethyl formate, with a {\em trans} ester orientation and {\em gauche} ethyl orientation, is significantly higher in energy than the {\em cis-trans} conformer according to electronic structure calculations (1900 cm$^{-1}$, or 1330 K), but there is a barrier of 2870 cm$^{-1}$ (2000 K) for this conformer to relax into the more stable {\em cis} ester potential well, and so local thermodynamic equilibrium between these conformers is not expected in the interstellar medium. Similar behavior is found for the {\em trans} ester conformer of methyl formate, for which a tentative detection in SgrB2(N) was presented at this meeting last year, with a column density roughly 1\% of that of the more stable {\em cis} isomer.., RH15, 64th International Symposium on Molecular Spectroscopy (2009).} Here we report the laboratory detection of {\em trans-gauche} ethyl formate using Fourier transform microwave spectroscopy; its low population at room temperature equilibrium has been enhanced by the use of a pulsed discharge nozzle. The spectrum is complicated by a low barrier (140 cm$^{-1}$) to tunneling between equivalent structures.
Description
Author Institution: Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904; Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, and School of Engineering \& Applied Sciences, Harvard University, 29 Oxford St., Cambridge MA 02138