Infrared spectroscopy of SO2 clusters in rare gas matrices revisited: Assignment of species in Ar matrix

The Structures, Molecular Orbital Properties and Vibrational Spectra of the Homo- and Heterodimers of Sulphur Dioxide and Ozone. An Ab Initio Study

The structures of a number of dimers of sulphur dioxide and ozone were optimized by means of a series of ab initio calculations. The dimer species were classified as either genuine energy minima or transition states of first or higher order, and the most probable structures consistent with the experimental data were confirmed. The molecular orbitals engaged in the interactions resulting in adduct formation were identified and relations between the orbitals of the dimers of the valence isoelectronic monomer species were examined. The vibrational spectra of the most probable structures were computed and compared with those reported in the literature, particularly with spectra observed in cryogenic matrices. The calculations were extended to predict the properties of a number of possible heterodimers formed between sulphur dioxide and ozone.

Development of an experimental set-up for low-temperature spectroscopic studies of matrix-isolated molecules and molecular ices using synchrotron radiation

An experimental set-up for studying photophysics and photochemistry of molecules in an inert gas medium (matrix-isolated) and in the ice phase at low temperatures has been developed and commissioned at the Photophysics beamline, Indus-1 synchrotron radiation source. This end-station uses an in-house-developed closed-cycle cryostat for achieving cryo-temperatures (∼10 K). Synchrotron radiation from the Photophysics beamline is used as the source of UV-VUV photons and the system is equipped with a Fourier transform infrared spectrometer for characterization of the molecular species formed at low temperature. Various individual components of the end-station like closed-cycle cryostat, experimental chamber, gas mixing and deposition systems are tested to ascertain that the desired performance criteria are satisfied. The performance of the composite system after integration with the Photophysics beamline is evaluated by recording IR and UV-VUV photoabsorption spectra of sulfur dioxide at low temperatures (10 K), both in the ice phase as well as isolated in argon matrices. Results obtained are in good agreement with earlier literature, thus validating the satisfactory performance of the system. As an off-shoot of the study, the VUV absorption spectrum of matrix-isolated SO₂ in argon matrix up to 10.2 eV is reported here for the first time. This experimental end-station will provide new opportunities to study photon-induced reactions in molecules of environmental, astrochemical and industrial importance. Details of the design, development and initial experimental results obtained are presented.

Theoretical study of formic acid-sulfur dioxide dimers

We report the first theoretical study of noncovalent and covalent interactions in formic acid (FA)-SO(2) complexes. Using ab initio and DFT model chemistries, five stable noncovalent complexes were identified, as well as a covalent adduct, formic sulfurous anhydride HOSO(2)CHO. syn-FA is predicted to form two nonplanar bidentate complexes with SO(2): the more stable one contains a normal hydrogen bond donated by OH, and the less stable one contains a blue-shifted hydrogen bond donated by CH. Both are stabilized by charge transfer from FA to SO(2). anti-FA forms three planar complexes of nearly equal energy containing OH-to-SO(2) hydrogen bonds. Formic sulfurous anhydride forms via an endothermic concerted cycloaddition. Natural bond orbital analysis showed that the bidentate SO(2)-FA complexes are stabilized by n → π* donation from FA to SO(2), and back-donation from SO(2) n and π* orbitals into FA σ(OH)* or σ(CH)* orbitals. The bidentate formic acid-SO(2) complex that contains an O-H···O hydrogen bond is more stable than the similar nitric acid-SO(2) complex. The latter contains a stronger hydrogen bond but shows no O→S charge transfer interaction.