Vibrational spectra of cis and trans but-2-enes: assignments, isolated CH stretching frequencies and CH bond lengths

Infrared and Raman spectra have been obtained from 11 isotopomers of cis and trans but-2-enes in various phases ( c - and t -CH 3 CHCHCH 3 , CH 3 CHCDCH 3 , CD 3 CHCHCD 3 , CD 3 CHCDCD 3 , CD 3 CDCDCD 3 ; c -CH 3 CDCDCH 3 ). Nearly all the fundamentals are securely assigned. Isolated CH stretching frequencies ν is CH are observed or deduced from the spectra and used to predict CH bond lengths and dissociation energies. The olefinic CH and methyl CH s bonds are stronger in the cis compound than those in the trans , in keeping with substituent effects previously observed in propene, and with molecular mechanics predictions. Comparison of ν is CH values with local mode fifth overtone spectra for cis -CH 3 CHCHCH 3 indicates that the part of the latter associated with the CH a bond cannot be readily interpreted.