Spectroscopic characterization of the elusive gauche-isoprene by high resolution microwave spectroscopy

Westerfield, J. H.

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https://hdl.handle.net/2142/100525 Copy

Description

Title

Spectroscopic characterization of the elusive gauche-isoprene by high resolution microwave spectroscopy

Author(s)

Westerfield, J. H.

Contributor(s)

• McCarthy, Michael C.
• Shipman, Steven
• Baraban, Joshua H.
• Nguyen, Thanh Lam
• Changala, Bryan
• Porterfield, Jessie P.

Issue Date

2018-06-19

Keyword(s)

Large amplitude motions, internal rotation

Abstract

The microwave spectrum of isoprene has been investigated in a cryogenic (4-7 K) He buffer gas cell (12- 26.5 GHz) and in a room temperature waveguide (8.7-26.5 GHz). Rotation about the single C-C bond converts the lower-energy trans rotamer to the higher-energy gauche-conformer via an unstable, planar, cis transition state. As in butadiene, it is counterintuitive that gauche is more stable than cis because it has long been believed that planarity is required for π electron delocalization, a factor that often imparts greater stability. In standard jet experiments, observation of the higher-energy gauche conformer has proven challenging. However in the buffer gas cell, collisions with cold He result in rapid but gentle conformational cooling, allowing for straightforward observation of the gauche-isoprene rotamer. The rotational spectrum of gauche is complex owing to the combined effects of rotational line splitting (0+ / 0− from gauche-gauche inversion) and A/E splitting of the methyl rotor. On the basis of new theoretical calculations, steric hindrance of methyl rotation is predicted to be lower in the gauche than in the trans conformer (V3 barrier approximately 653 cm−1 or 7.8 kJ/mol). In addition to the on-going spectroscopic analysis, efforts are now underway to better characterize the energetics of C-C bond rotation in isoprene, with the goal of ultimately understanding the factors that result in the greater stability of the gauche relative to the cis isoprene rotamer.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/100525

DOI

10.15278/isms.2018.TK05

Copyright and License Information

Copyright 2018 J.H. Westerfield

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