STRUCTURE OF THE DIMETHYLAMINE SULFUR-DIOXIDE COMPLEX

The microwave spectrum of the charge-transfer complex between dimethylamine and sulfur dioxide was studied with a pulsed molecular beam Fourier transform microwave spectrometer. The rotational constants (in MHz) of (CH3)2NH.SO2 are A = 4445.495 (3), B = 2063.031 (1), and C = 1752.470 (1). In addition to the normal isotopic form, the rotational spectra of the (CH3)2NH.34SO2, (CH3)2(15)NH.SO2, (CH3)2ND.SO2, and two (CH3)2NH.SO18O isotopic species were assigned. Stark effect measurements gave electric dipole components of mu-a = 4.025 (1), mu-c = 1.747 (2), and mu-total = 4.388 (1) D. The structure of this complex lacks any symmetry plane. The nitrogen lone pair points toward the S atom, nearly perpendicular to the plane of the sulfur dioxide, and one methyl group staggers the oxygen atoms. The nitrogen-to-sulfur distance of 2.34 (3) angstrom is about 0.08 angstrom longer than in the trimethylamine-SO2 complex which correlates with the relative strength of the complexes. From the dipole moment and the nitrogen nuclear quadrupole coupling constants, an upper limit is estimated for electron transfer from the nitrogen to the sulfur atom of 0.25 electron. Ab initio calculations also conclude that a methyl group staggers the two oxygens of SO2.