STRUCTURE OF THE TRIMETHYLAMINE-SULFUR DIOXIDE COMPLEX

The microwave spectrum of the charge-transfer complex between trimethylamine and sulfur dioxide has been studied with a pulsed molecular beam Fourier transform microwave spectrometer. In addition to the normal isotopic form, the rotational spectra of the (CH3)3N.(SO2)-S-34, ((CH3)3N)-N-15.SO2, (CH3)3N.(SOO)-O-18, (CH3)3N.(SO2)-O-18, and (CH3(CH3)2N)-C-13.SO2 isotopic species were assigned. Stark effect measurements gave electric dipole components of mu-a = 4,676 (5), mu-c = 1.081 (4), and mu-total = 4.800 (5) D. The dipole moment and moment of inertia data show that the complex belongs to the C(s) point group. The crystal structure of the charge-transfer complex has been reexamined at -70-degrees-C by X-ray crystallography and is in good agreement with an earlier room-temperature determination. The structure of this complex in both the gas and solid phase is consistent with the nitrogen lone pair pointing toward the sulfur atom, with the SO2 plane tilted by approximately 75-degrees from the C3 axis of the trimethylamine. The methyl groups are staggered with respect to the oxygen atoms. In the gas phase, the nitrogen to sulfur distance is 2.26 (3) angstrom; this distance shortens to 2.05 (1) angstrom in the crystal. From the dipole moment and the nitrogen nuclear quadrupole coupling constants, an upper limit was estimated for the transfer of charge from the nitrogen to the sulfur atom as 0.2 to 0.3 electron.