MICROWAVE-SPECTRUM AND STRUCTURE OF THE PYRIDINE SULFUR-DIOXIDE COMPLEX

The rotational spectrum of the charge-transfer complex between pyridine and sulfur dioxide has been studied by using a Fourier transform microwave spectrometer employing a Fabry-Perot cavity and pulsed supersonic nozzle as a molecular beam source. The spectroscopic constants (MHz) for pyridine-SO2 are A = 3534.946 (4), B = 759.923 (1), C = 647.636 (1), chi-aa = -4.087 (3), chi-bb = 1.347 (2), and chi-cc = 2.740 (2). In addition to the normal isotopic form, the rotational spectra of the (C5H5N.SO2)-S-34, (C5H5N.SO2)-O-18, C5D5N.SO, and (C5H5N.SO2)-N-15 isotopic species were assigned. Stark effect measurements gave electric dipole components of mu-a = 4.360 (5), mu-c = 1.311 (3), and mu-total = 4.552 (5) D. The dipole moment and moment of inertia data show that the complex belongs to the C(s) point group. The nitrogen to sulfur distance is 2.61 (3) angstrom, with the C2 axis of the pyridine ring approximately perpendicular to the sulfur dioxide plane. The pyridine plane is rotated 90-degrees to the C2 axis of the SO2; i.e., the complex is approximately L-shaped. Comparisons are made to the trimethylamine.SO2, dimethylamine.SO2, and HCN.SO2 complexes.