Conformational and structural studies of n-propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations
van der Veken, Benjamin J.
Faculty of Sciences. Chemistry
Journal of Raman spectroscopy. - Dordrecht
, p. 1329-1336
University of Antwerp
The Raman and infrared spectra (4000 to 50 cm(-1)) of the gas, liquid or solution, and solid have been recorded of n-propylamine, CH3CH2CH2NH2. Variable temperature (-60 to -100 degrees C) studies of the Raman (1175 to 625 cm(-1)) and far infrared (600 to 10 cm(-1)) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans-trans (Tt) for trans-gauche (Tg) of 79 +/- 9 cm(-1) (0.9 +/- 0.1 kJ/mol); for Gg of 91 +/- 26 cm(-1) (1.08 +/- 0.3 kJ/mol); for Gg' of 135 +/- 21 cm(-1) (1.61 +/- 0.2 kJ/mol); for Gt of 143 +/- 11 cm(-1) (1.71 +/- 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 +/- 1% for Tg, 23 +/- 3% for Gg, 18 +/- 1% for Gg' and 18 +/- 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ from both second-order Moller-Plesset (MP2, full) and density functional theory calculations by the Becke, three-parameter, Lee-Yang-Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r(0) structural parameters were obtained from adjusted MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright c 2012 John Wiley & Sons, Ltd.