Investigation of ν(NH) and ν(CN) stretching modes of propylamine (C3H7NH2) in a binary system C3H7NH2 + CH3OH via concentration dependent Raman study and ab initio calculations

Raman spectra of propylamine (C3H7NH2) and its binary mixtures, C3H7NH2 + CH3OH with varying mole fractions of the reference system, C3H7NH2, C were recorded in two widely apart wavenumber regions, 3100-3600 cm−1 and 1225-1325 cm−1. In the former region, the two Raman bands at ∼3305 and ∼3326 cm−1, obtained after the line shape analysis, which were assigned to symmetric ν(NH) and anti-symmetric ν(NH) stretching modes, respectively, show a downshift upon dilution. However, whereas the ν(NH) anti-symmetric mode shows a shift of 18.6 cm−1, the ν(NH) symmetric mode shows a much smaller shift (5.7 cm−1) between neat liquid and high dilution, C = 0.1. This aspect has been explained using the optimized geometries calculated employing ab initio theory (MP2 level) for the neat C3H7NH2 and its different hydrogen-bonded complexes. The linewidth versus concentration plot for the ν(NH) anti-symmetric stretching mode, however exhibits a distinct maxima at C = 0.4, which has been explained as a slight departure from the concentration fluctuation model. In the latter region, a symmetric peak is observed, which corresponds to ν(CN) stretching mode, which shows an upshift upon dilution and an almost linear concentration dependence. This has also been explained in terms of the parameters obtained from the optimized geometries of the different hydrogen-bonded complexes.