Rotational spectra, structure and barrier to internal rotation of N-methylpyrrole and its argon complexes

The rotational spectra of the lowest internal rotation states of N-methylpy rrole and its van der Waals complexes with one or two argon atoms have been measured with a pulsed beam Fourier-transform microwave spectrometer in th e frequency range of 8-24 GHz. All rotational transitions have been found t o be split into A-E doublets because of the internal rotation of the methyl group. Several C-13 and N-15 isotopomers have been observed in natural abu ndance. Rotational constants, centrifugal distortion constants, and N-14 nu clear quadrupole coupling constants have been determined from the accuratel y measured transition frequencies of the A states. The derived moments of i nertia of the isotopomers have led to a structure of N-methylpyrrole-argon where argon is located above the aromatic ring plane at a distance of 3.43 Angstrom and an angle of 5.5 degrees to the c(M) axis of the monomer. The s ubstitution structure of the aromatic ring has been shown to coincide almos t exactly with that of the N-methylpyrrole monomer. For N-methylpyrrole-(ar gon)(2), the two argon atoms have been found to lie on opposite sides of th e aromatic ring plane at a distance of 3.45 Angstrom and an angle of 4.9 de grees. The barriers to internal rotation have been evaluated from the A-E s plittings with the help of a program which considers one arbitrary large am plitude motion for N-methylpyrrole and its argon complexes. In an attempt t o determine the conformation of the methyl group with respect to the ring p lane, the rotational spectra have been measured for N-methylpyrrole with a partially deuterated methyl group and for its argon complexes. A staggered conformation with one of the C-H bonds perpendicular to the ring plane of p yrrole is consistent with the combined results of the monomer and the argon complexes.