INFRARED AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, AB-INITIO CALCULATIONS AND VIBRATIONAL ASSIGNMENT FOR 1,2-PENTADIENE (ETHYL ALLENE)

The infrared (3500-50 cm(-1)) and Raman (3500-20 cm(-1)) spectra of 1, 2-pentadiene, H2C=C=C(H)CH2CH3 (ethyl allene), have been recorded for both the gaseous and solid states. Additionally, the Raman spectrum of the liquid has been obtained with qualitative depolarization values. In the fluid phases both the cis and gauche conformers have been ident ified, with the gauche rotamer being the predominant form although it may not be the conformer of lowest energy. In the solid state only the cis conformer remains after repeated annealing of the crystal. The as ymmetric torsion of the cis conformer is observed as a series of Q-bra nch transitions beginning at 103.4 cm(-1) and falling to lower frequen cy. An estimate of the potential function governing conformer intercon version is provided. A complete assignment of the normal modes for the cis conformer is given and several of the fundamentals are assigned f or the gauche rotamer. Ab initio electronic structure calculations of energies, conformational geometries, vibrational frequencies, and pote ntial energy functions have been made to complement and assist the int erpretation of the infrared and Raman spectra. In particular, the tran sitions among torsional energy levels for both the symmetric (methyl) and asymmetric (ethyl) motions have been calculated. The results are c ompared to the corresponding quantities for some similar molecules.