Vibrational spectra of indole and assignments on the basis of ab initio force fields

New high‐resolution Fourier transform spectra for indole vapour and its N ‐deuterated analogue and Fourier transform Raman spectra for solid indole are reported. Observed frequencies for normal modes are compared with those calculated from normal coordinate analyses carried out on the basis of ab initio force fields. The latter were calculated using the 3–21G basis set and either the theoretical optimized geometry or that given by microwave spectroscopy. The force field obtained with the optimized geometry was shown to provide and excellent approach for calculating not only frequencies but also IR and Raman intensities. Except for the NH and CH stretching modes, only one scale factor for all the in‐plane modes and another for all the out‐of‐plane modes are needed in order to bring closer the calculated and observed frequencies for indole and for eleven deuterated analogues. Assignments for each normal mode were obtained from potential energy distributions among internal coordinates. Those for normal modes corresponding, in particular, to bands observed also for the tryptophar residue are discussed in detail.