Charge-transfer complexes between the sulfur molecules SO2, S2O, S3, SONH, and SOCl2 and the amine donors NH3 and NMe3 -: A theoretical study

The formation of adducts between the acceptor molecules SO2, S2O, S-3, and SOCl2 and the donor molecules ammonia and trimethylamine has been studied by high-level G3X(MP2) theory, in most cases for the first time. Minimum energy structures with a 1:1 composition and with sulfur-mtrogen bonds have been located for all complexes. The thermodynamic properties, vibrational spectra, and atomic charges of these molecules have been calculated. Of the 10 investigated complexes, only S02-NMe3 (2) and S20-NMe3 (4) are predicted to be thermodynamically stable in the gas phase under standard conditions (AG'298 < 0). However, all adducts except S3-NH3 (5), SOCl2-NH3 (7), and SOCI2-NMe3 (8) are stable thermodynamically at 77 K. In a polar continuum with a dielectric constant (c) of 40, complexes 2, 4, and S3-NMe3 (6) are stable even at 25 oC. However, if the adducts are formed in the polar continuum from the gaseous components, all adducts except S3-NH3 are formed not only exo-. thermally but also exergonically. Complexes 2, 4, and 8 are the only species known in the solid state so far. The charge transfer from donor to acceptor in the various complexes in the gas phase ranges from 0.08 to 0.39 electrostatic units and correlates inversely with the S-N bond length, which ranges from 198 to 295 pm. In general, the charge transfer weakens the bonds within the acceptor molecule, which therefore may be considered to become activated. The formation of thionyl imide (SONH) from SOC12 and NH3 is exergonic in the gas phase and the interaction between SONH and NH3 is very weak. In the case Of S02, the 1:2 complexes S02-2NH3 (1a) and S02-2NMe3 (2a), which contain two S-N bonds, are unstable with respect to their three-component molecules both in the gas phase and in a polar medium at 25 'C, but they are stable in the polar phase with respect to the gaseous components.