Conformational preferences and internal rotation of methyl butyrate by microwave spectroscopy

• Published in: Journal of molecular spectroscopy Vol. 337; no. C; pp. 51 - 58
• Main Authors: Hernandez-Castillo, Alicia O., Abeysekera, Chamara, Hays, Brian M., Kleiner, Isabelle, Nguyen, Ha Vinh Lam, Zwier, Timothy S.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.07.2017; Elsevier
• Subjects: Chemical Sciences; Conformational analysis; Large amplitude motions; Methyl butyrate; or physical chemistry; Physics; Rotational spectroscopy; Spectroscopy; Theoretical and; Methyl butyrate; Large amplitude motions; Rotational spectroscopy; Conformational analysis; conformational analysis; methyl butyrate; rotational spectroscopy; large amplitude motions
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/j.jms.2017.03.016

[Display omitted] •Supersonic jet FT microwave spectra in the ground vibrational state.•Methyl butyrate exists under jet conditions in both Cs and C1 symmetry forms.•Molecular and internal dynamics parameter determined with very high accuracy.•2D potential energy surface for conformational preferences.•Collisional removal of population from the high energy conformers is observed. The broadband rotational spectrum of methyl butyrate from 8 to 18GHz, recorded using a chirp-pulsed Fourier transform microwave (FTMW) spectrometer, was combined with high resolution FTMW measurements over the 2–26.5GHz region to provide a comprehensive account of its microwave spectrum under jet-cooled conditions. Two low-energy conformers, one with a fully extended, heavy-atom planar anti/anti structure (a,a), and the other with a gauche propyl chain (g±,a), were assigned in the spectrum. Torsional A/E splittings due to the internal rotation of the methoxy methyl group were resolved for both lower energy conformers, and were fitted using the program XIAM and BELGI, providing an estimate of the barrier to methyl internal rotation of V3≈420cm−1. The conformational landscape of methyl butyrate occurs on a two-dimensional potential energy surface, which was mapped out by quantum chemical calculations at the B2PLYP-D3BJ/aug-cc-pVTZ level of theory. The low torsional barrier about the CC(O)O bond leads to collisional removal of population originally in the (a,g±) and (g±,g±) minima into the (a,a) and (g±,a) minima, respectively, during the cooling in the expansion. Analysis of experimental intensities in the spectrum provide percent populations downstream in the expansion of 41±4% (a,a), and 59±6% (g±,a).