Raman spectrum and torsional potential function for isoprene conformational enthalpy difference for 2-cyclopropylpropene
✍ Scribed by T. G. Towns; L. A. Carreira; R. M. Irwin
- Publisher
- John Wiley and Sons
- Year
- 1981
- Tongue
- English
- Weight
- 599 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0377-0486
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✦ Synopsis
Abstract
A polarized band at 223.7 cm^−1^ in the vapor phase Raman spectrum of isoprene has been observed and assigned to the 2‐0 transition of the vinyl torsion originating in the s‐cis conformation. This observation has been complemented with previously reported data to afford the determination of the vinyl torsional potential function in the form
magnified image
The potential constants are (cm^−1^±2σ): V~1~=112±18; V~2~=2742±34; V~3~=294±18; and V~4~=−150±16. Modifications to a standard Spex model 1401 Raman spectrophotometer to allow digitization of spectra and automated operation are described in detail. The modifications have facilitated a precise deduction of the conformational enthalpy difference in both the liquid and the vapor states of 2‐cyclopropylpropene; they are, respectively, (cm^−1^±2σ)246±6 and 280±40. The results are discussed in terms of different mechanisms for resonance stabilization, particularly in light of a cyclopropyl group as one of the ‘conjugated’ moieties. The effect of a methine methyl‐substitution on the torsional potential function is discussed in terms of the hyperconjugation concept.