Pure rotational spectrum, quadrupole coupling constants and structure of the dimer of pyrrole

Rotational transitions of a pyrrole dimer have been identified and measured over the 8–18 GHz range using a pulsed-nozzle Fourier-transform microwave spectrometer. In addition to the parent species, nine N15 and D isotopomers have been analyzed. Apart from the N14 and D nuclear quadrupole splittings, the rotational transitions did not show any additional splittings arising from large amplitude motions. Rotational constants, centrifugal distortion constants and, for two isotopomers, N14 quadrupole coupling constants have been fitted to the measured frequencies of rotational transitions. The observed rotational constants are consistent with essentially a T-shaped structure for the dimer. From the rotational constants of all isotopomers, a partial structure of the dimer has been determined. Three structural parameters have been fitted to the differences in the planar moments of inertia between the isotopically substituted species and the parent species. The planes of the two pyrrole monomers form an angle of 55.4(4)° with the nitrogen side of one ring directed to the π electron system of the other ring establishing a weak hydrogen bond. The centrifugal distortion constants of the dimer of pyrrole have been used to estimate the frequency of the van der Waals stretching mode and the dissociation energy of the complex in a pseudodiatomic approximation.