NIR vibrational overtone spectra of N-methylaniline, N,N-dimethylaniline and N,N-diethylaniline--a conformational structural analysis using local mode model.

In aniline, the lone pair electrons on the nitrogen atom in the amino group interact with the pi electrons of the ring and the mechanical frequency of the ring CH oscillator shows a red shift from that of benzene. This happens because of the parallel orientation of the pi electrons in the two molecules. This effect is observed in N-methylaniline also. But in N,N-dimethylaniline, the addition of two alkyl group to the amino group changes the parallel orientation of pi electrons of the nitrogen. This changes the electron donating nature of the amino substituent and as a result, a blue shift is observed in the mechanical frequency of the ring CH oscillator from that of benzene. The same effect is observed in the ring CH oscillator frequency of N,N-diethylaniline also.