STRUCTURES, BONDING, AND ABSORPTION-SPECTRA OF AMINE SULFUR-DIOXIDE CHARGE-TRANSFER COMPLEXES

The 1:1 and 2:1 charge-transfer (CT) molecular complexes between ammonia and the methyl-substituted amines, CH3NH2 (MA), (CH3)2NH (DMA), and (CH3)3NH (TMA), and SO2 have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP2/6-31G(d), and binding energies were determined at the MP3/6-31+G(2d,p) level. Correction for electron correlation is found to be necessary for an accurate description of the structures and stabilities of these amine.SO2 complexes. For TMA.SO2, the calculated N-S equilibrium distance is reduced dramatically, by 0.18 angstrom, in going from the Hartree-Fock level to the MP2 level of theory. The calculated enthalpies of complex formation (DELTA-H, 298 K) for NH3.SO2, MA.SO2, DMA.SO2, and TMA.SO2 are 4.5, 6.7, 7.9, and 10.7 kcal mol-1, respectively, and the corresponding N-S bond lengths are 2.79, 2.62, 2.46, and 2.36 angstrom, respectively. The computed formation enthalpy and geometry for TMA.SO2 are in good accord with experimental data. The trends of geometry and stability of these amine.SO2 complexes can be rationalized in terms of the donor strength of the amines (TMA > DMA > MA > NH3). The 2:1 adducts, (NH3)2.SO2, MA2.SO2, DMA2.SO2, and TMA2.SO2, are all predicted to be thermodynamically stable, with binding energies slightly less than the corresponding 1:1 complexes, and are predicted to be experimentally accessible species in the gas phase. The bonding characteristics of the amine.SO2 CT complexes have been investigated by charge density analysis. The N-S bond is characterized by a strong ionic interaction. The charge-transfer component plays an important role in determining the stabilities of these donor-acceptor complexes. The calculated amount of charge transfer from the amine to SO2 are 0.04, 0.08, 0.13, and 0.18 eV for NH3.SO2, MA.SO2, DMA.SO2, and TMA.SO2, respectively. The infrared and ultraviolet spectra of the 1:1 and 2:1 CT complexes were calculated for the first time. The computed vibrational frequencies and transition energies are consistent with experimental data.