Microwave spectra of isotopic N-methylpyrroles, quadrupole coupling constants and substitution structure

The rotational spectra of seven D, C-13 and N-15 isotopomers of N-methylpyrrole nave been measured between 7 and 20 GHz with a pulsed nozzle Fourier transform microwave (FTMW) spectrometer. Rotational constants and nuclear quadrupole coupling constants have been fitted to the measured frequencies in the m = 0 A state of the methyl internal rotation with an iterative least-squares fit. The substitution structure of the pyrrole ring of N-methylpyrrole has been derived from the moments of inertia of the isotopomers. In addition, ab initio calculations have been performed for pyrrole and N-methylpyrrole. The calculated structures have been compared to the substitution structures from rotational spectroscopy. (C) 1997 Elsevier Science B.V.