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The structures of a large number of hydrates of sulfur dioxide SO2·nH2O, of the sulfonate ion [HSO3]–·nH2O, of the tautomeric hydrogensulfite anion [SO3H]–·nH2O, and of sulfurous acid H2SO3·nH2O with up to eight water molecules attached to these species have been optimized at the B3LYP631G2df,p level of theory DFT. The calculated vibrational frequencies allow the definite assignment of certain characteristic modes, and in this way a convincing interpretation of published spectra of aqueous SO2as well as of SO2adsorbed on very cold ice crystals has been achieved for the first time. Singlepoint calculations at the G3XMP2 level of theory were used to calculate the binding energies of the water molecules in SO2·nH2O as well as the relative stabilities of the isomeric anionic species [HSO3]–·nH2O and [SO3H]–·nH2O. Generally, the water molecules tend to stick together forming clusters, whereas the particular sulfurcontaining molecule remains at the surface of the water cluster, but it is always strongly hydrogenbonded. Only when there are more than six water molecules are the anions more or less completely surrounded by water molecules. DFT calculations erroneously predict that the gaseous hydrated sulfonate ions are more stable than the isomeric hydrogensulfite ions, even when hydrated with six water molecules. However, if these hydrated species are calculated as being embedded in a polar continuum simulating the aqueous phase, the hydrogensulfite ions are more stable than the sulfonate ions, in agreement with various spectroscopic observations on aqueous sulfite solutions. On the other hand, at the higher G3XMP2 level, the gaseous hydrated hydrogensulfite anions are more stable than the corresponding sulfonate ions only if the number of water molecules is larger than four, whereas for the weakly hydrated anions the order of relative energies is reversed. The possible implications of these results for the enzymatic oxidation of “sulfite ions” [HSO3]–and [SO3H]– by sulfite oxidase are discussed. The conversion of SO2·6H2O into its isomer H2SO3·5H2O is predicted to be exothermic ΔH°298= –56.1 kJ mol–1 and exergonic ΔG°298= –22.5 kJ mol–1. © WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

The formation of adducts between the acceptor molecules SO2, S2O, S3, 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–nitrogen 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 SO2·NMe3 (2) and S2O·NMe3 (4) are predicted to be thermodynamically stable in the gas phase under standard conditions (ΔG°298 < 0). However, all adducts except S3·NH3 (5), SOCl2·NH3 (7), and SOCl2·NMe3 (8) are stable thermodynamically at 77 K. In a polar continuum with a dielectric constant (ε) of 40, complexes 2, 4, and S3·NMe3 (6) are stable even at 25 °C. However, if the adducts are formed in the polar continuum from the gaseous components, all adducts except S3·NH3 are formed not only exothermally 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 SOCl2 and NH3 is exergonic in the gas phase and the interaction between SONH and NH3 is very weak. In the case of SO2, the 1:2 complexes SO2·2NH3 (1a) and SO2·2NMe3 (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.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The structures of a large number of isomers of the sulfur oxides SnO with n= 4–9 have been calculated at the G3X(MP2) level of theory. In most cases, homocyclic molecules with exocyclic oxygen atoms in an axial position are the global minimum structures. Perfect agreement is obtained with experimentally determined structures of S7O and S8O. The most stable S4O isomer as well as some less stable isomers of S5O and S6O are characterized by a strong π*–π* interaction between SO and SS groups, which results in relatively long SS bonds with internuclear distances of 244–262 pm. Heterocyclic isomers are less stable than the global minimum structures, and this energy difference approximately increases with the ring size: 17 (S4O), 40 (S5O), 32 (S6O), 28 (S7O), 45 (S8O), and 54 kJ mol−1(S9O). Owing to a favorable π*–π* interaction, preference for an axial (or endo) conformation is calculated for the global energy minima of S7O, S8O, and S9O. Vapor‐phase decomposition of SnO molecules to SO2and S8is strongly exothermic, whereas the formation of S2O and S8is exothermic if n<7, but slightly endothermic for S7O, S8O, and S9O. The calculated vibrational spectra of the most stable isomers of S6O, S7O, and S8O are in excellent agreement with the observed data.

The complex formation between the Li+ cation and the sulfur homocycle S8 has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Starting with various isomers of S8, the formation of LiS8 heterocycles and clusters is preferred over complexes with a monodentate ligand. The binding energies of the cation in the 23 complexes investigated range from –95 to –217 kJ·mol–1. The global minimum structure of [LiS8]+ is of C4v symmetry with the S8 homocycle in the well-known crown conformation and four Li–S bonds of length 254.2 pm (binding energy: –156.5 kJ·mol–1). The S–S bonds of the various ligands are slightly weakened by the complex formation and a more or less strong bond length alternation is induced. Relatively unstable isomers of S8 (chair, tub, exo–endo ring, branched rings, triplet chain) are partly stabilized and partly destabilized by complex formation with Li+. The interaction between the cation and the S8 ligands is mainly due to ion–dipole attraction with little to moderate charge transfer (0.04–0.27 electrostatic units). In the four most stable isomers of [LiS8]+, the number of sulfur–sulfur bonds is at a maximum and the coordination number of Li+ is either 4 or 3. Complexes of the type [Li(S4)2]+ are much less stable than isomers with an eight-atomic ligand. The Li–S bond lengths in all of these complex cations (230–273 pm) depend on the coordination number of Li and on the atomic charge of the donating sulfur atom(s). In contrast to [LiS8]+, the complexes of composition [MS8]+ with M = Ca, V, and Cu are more stable as [M(S4)2]+ than with an eight-atomic crown-shaped ligand. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

All chemically reasonable [2+2] and [2+3] dipolar addition reactions between two S2O molecules forming heterocyclic intermediates of composition S4O2, as well as the decomposition of these species to generate the observed products S3 and SO2, have been studied by gas-phase ab initio MO calculations at the G3X(MP2) level of theory. The reaction enthalpies, Gibbs energies and, partly, activation enthalpies of 17 reactions have been calculated. The [2+2] cycloadditions of S2O are all endothermic and are characterized by high barriers. The reaction of lowest activation enthalpy (ΔH#298 = 11 kJ·mol−1) is the exothermic [2+3] cycloaddition of two S2O molecules resulting in the five-membered heterocycle oxatetrathiolane 2-oxide S4O2 (7), which is predicted to decompose at 298 K exothermally and without any activation enthalpy to SO2 plus S3. The S3 molecule is predicted to add exothermally to S2O to give the five-membered homocycle S5O which has, in fact, been prepared in this way before. The bimolecular decomposition of S5O to SO2, S4 and S5 is also exothermic and may explain the formation of the red S4 molecule on condensation of S2O at temperatures below −130 °C. The implications of these results for the understanding of the chemistry on Jupiter’s moon Io are discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Eight hypothetical persulfurane molecules of the type X2SY4 (X, Y = H, F, Me) have been studied by ab initio MO calculations at the G3(MP2) level of theory. Depending on the substituents either the cis or the trans isomers of these octahedral molecules are most stable. While H4SF2 is most stable as the cis isomer, the global minimum structures of H2SF4, Me2SH4, and Me2SF4 are the trans isomers. The enthalpy differences between the corresponding isomers are in the range 7−53 kJ·mol⁻¹. The transition states for the intramolecular cis &lrhar2; trans isomerization have been located in all cases. Starting from the global minimum geometries the activation enthalpies (ΔH^#298) for this isomerization are in the range 199−263 kJ·mol⁻¹, indicating rather rigid structures. Therefore, it should be possible to isolate both isomers at low to moderate temperatures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Ab initio MO calculations at the G3(MP2) level of theory show that sulfur tetraamide S(NH2)4 is a local minimum energy structure but thermodynamically unstable with respect to both sulfur diamide S(NH2)2 plus hydrazine and sulfurimidediamide S(NH)(NH2)2 plus ammonia. The activation enthalpy of the latter reaction is 63 kJ·mol⁻¹ at 298 K. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)