A MICROWAVE STUDY AND CENTRIFUGAL-DISTORTION ANALYSIS OF THE PYRROLE-CO COMPLEX

The rotational spectrum of the pyrrole...CO complex has been measured between 7.8 and 18.1 GHz with a pulsed nozzle Fourier transform microwave spectrometer. Ih addition to the parent species, seven C-13, N-15, and O-18 isotopic variants have been analyzed. Rotational constants, quartic centrifugal distortion constants, and, where necessary, quadrupole coupling constants have been fitted to the measured transition frequencies. The determination of r(o) and r(s) structures of the complex has been affected seriously by the van der Waals vibrations. The structure is compatible with CO lying above the pyrrole ring in the symmetry plane of pyrrole. In order to correct for the effects of the van der Waals vibrations, an anharmonic potential function for the five van der Waals modes has been adjusted from the centrifugal distortion constants and rotational constants. The frequencies of the van der Waals modes and the equilibrium dissociation energy of the complex have been estimated from the potential. An improved structure followed from the rotational constants corrected for the low-frequency zero-point vibrational contributions. However, a second possible orientation of CO above the pyrrole plane could not be ruled out.