FAR-INFRARED SPECTRUM, AB-INITIO CALCULATIONS, AND CONFORMATIONAL-ANALYSIS OF 1-PENTYNE

The far infrared spectrum of 1-pentyne, CH3CH2CH2C=CH, has been record ed in the gas phase. The fundamental asymmetric torsional transitions have been observed at 114 and 109 cm(-1) for the gauche (synclinal) an d trans (antiperiplanar; methyl group trans to the acetylenic group) c onformers, respectively. The methyl torsional fundamental (249.8 cm(-1 )) has only been observed for the gauche conformer. Infrared spectra ( 3500-400 cm(-1)) of 1-pentyne dissolved in liquid xenon have been reco rded from which variable temperature (-60 to -100 degrees C) studies h ave been carried out. From these data, the enthalpy difference has bee n determined to be 113 +/- 26 cm(-1) (323 +/- 74 cal/mol) with the tra ns conformer more stable than the gauche form. The Raman spectrum (350 0-40 cm(-1)) has also been recorded of the liquid to aid in the assign ment of the fundamentals. Ab initio electronic structure calculations of energies, conformational geometries, vibrational frequencies, and p otential energy functions have been carried out to complement and assi st in the interpretation of the spectra. In particular, the transition s among torsional energy levels for both the symmetric (methyl) and as ymmetric (ethyl) motions have been calculated. The results are compare d to the corresponding quantities for some similar molecules.