An Investigation of the Rotamers of Butadiene by High-Resolution Infrared Spectroscopy

A wide-ranging investigation of high-resolution infrared spectra of 1,3-butadiene was undertaken with the prime objective of finding subbands in the rotational structure attributable to the gauche rotamer, from which information about the molecular structure could be derived. A weak band near 750 cm−1, which had previously been assigned to the gauche rotamer, has been shown to be a difference band for the trans rotamer. In support of the analysis of this band, the rotational structure, including two hot bands, was analyzed in the C-type band for the ν12(au) mode near 525 cm−1 and in the C-type band near 162 cm−1 for the ν13(au) mode. An unsuccessful attempt was made to extend the analysis of the B-type component of the A/B-type band for ν17(bu) near 3000 cm−1. With the exception of the two weak Q branches at 463.82 and 462.16 cm−1, no new evidence for the gauche rotamer in the gas phase was found.