Structure of the dimethylamine-sulfur dioxide complex

Abstract

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 (CH{sub 3}){sub 2}NH{center dot}SO{sub 2} are A = 4,445.495 (3), B = 2,063.031 (1), and C = 1,752.470 (1). In addition to the normal isotopic form, the rotational spectra of the (CH{sub 3}){sub 2}NH {center dot} {sup 34}SO{sub 2}, (CH{sub 3}){sub 2}{sup 15}NH {center dot} SO{sub 2}, (CH{sub 3}){sub 2}ND {center dot} SO{sub 2}, and two (CH{sub 3}){sub 2}NH {center dot} SO{sup 18}O isotopic species were assigned. Stark effect measurements gave electric dipole components of {mu}{sub a} = 4.025 (1), {mu}{sub c} = 1.7474 (2), and {mu}{sub 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-SO{sub 2} 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 formore » 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 SO{sub 2}.« less