A molecular beam Fourier transform microwave study of 2-methylpyridine and its complex with argon: structure, methyl internal rotation and 14N nuclear quadrupole coupling

Abstract

The rotational spectrum of 2-methylpyridine (α-picoline, CH3C5H4N) in the two lowest levels of methyl internal rotation (m=0, ±1) has been recorded in the frequency range from 2 to 15 GHz using a molecular beam Fourier transform microwave spectrometer. The high resolution and sensitivity of this spectroscopic technique allowed resolution of hyperfine structures due to l4N nuclear quadrupole coupling with high accuracy and detection of the spectra of the 15N- and all 13C-isotopomers. These investigations considerably extend the results from an earlier study on the normal species (Dreizler, H., Rudolph, H. D., and Mader, H., 1970, Z. Naturforsch., 25a, 25); improved rotational and centrifugal distortion constants as well as all components of the 14N quadrupole coupling tensor have been obtained. Analysis of the spectra of the isotopomers yielded the I4N quadrupole coupling constants χ cc and χ aa – χ bb (for the 13C species), the potential parameter V 3 for the barrier hindering the internal rotation of the methyl group, and, in particular, ro, rs r m (1) and r m (2) structural parameters for the molecule. In addition to the microwave studies on the monomer, we have also investigated the rotational spectrum of the weakly bound dimer of normal 2-methylpyridine with Ar. The results obtained for the quadrupole coupling constants and the hindering potential for the internal methyl rotation show that the corresponding parameters are not significantly, or only slightly, changed in the complex.