New spectral assignment of n-propanol in the C-H stretching region

The C-H stretching vibration serves as an important probe for characterizing molecular structures and properties of hydrocarbons. In this work, we present a detailed study on gas-phase Raman spectrum of n-propanol in the C-H stretching region using stimulated photoacoustic Raman spectroscopy. A complete assignment was carried out with the aid of quantum chemistry calculations and depolarization ratio measurement as well as isotope substitutions, i.e. CH3CD2CD2OH, CD3CH2CD2OH and CD3CD2CH2OH. It is shown that the spectra of three C-H groups of n-propanol overlap each other because of Fermi resonance coupling and different molecular conformations, leading to complex features that were not determined previously. In addition, the comparisons between the spectra of three isotopologues reveal that the C-H vibrations at different sites of carbon chain exhibit different sensitivity to conformational change of n-propanol. The CH3 stretching vibration at terminated gamma-carbon is not sensitive whereas the CH2 stretching vibrations at both alpha-carbon and beta-carbon atoms are sensitive. Furthermore, Raman spectra of liquid propanol recorded by conventional spontaneous Raman technique are reassigned on the basis of gas-phase analysis. Copyright (C) 2016 John Wiley & Sons, Ltd.