Argon-matrix-isolation Raman spectra and density functional study of 1,3-butadiene conformers

s-trans, s-cis and gauche conformers of 1,3-butadiene have been studied usi ng density functional theory and the coupled-cluster method using double su bstitutions (CCD). Matrix isolation Raman and IR data for the minor conform er were obtained and are used in combination with the theoretical results t o resolve earlier ambiguities in vibrational assignments. Based on high-qua lity Hessians, new harmonic stretching force constants are reported for the carbon backbone of s-trans-1,3-butadiene. For the minor conformer the best unsealed root mean square error of the calculated frequencies for the s-ci s and gauche geometries are 17.5 cm(-1) and 7.4 cm(-1), respectively, prima rily due to a better agreement of the gauche results for the vibrations at 983 cm(-1), 596 cm(-1) and 470 cm(-1) which depend strongly on the torsiona l angle. Although this points towards the gauche form rather than the s-cis form, the calculated transition dipole moment directions at the CCD/6-311G (d,p) level confirm the earlier conclusion that the minor conformer has C-2 v symmetry in the matrix. It is concluded that either the better agreement between the frequencies calculated for the gauche form and the observed val ues is coincidental, or that the molecule is indeed nonplanar in the matrix and tunnels very rapidly between the two mirror-image forms (or its lowest vibrational level lies above the barrier).