Theoretical study of the solvent effect on the aromaticity of benzene: a NICS analysis

Reactivity properties of the HOSO and HSO2 isomers in liquid medium: a sequential Monte Carlo/quantum mechanics study

CONTEXT

The rationalization of acid rain formation steps is fundamental for mitigating its effects. It is believed the hydroxysulfinyl radical is an intermediate species for the production of atmospheric sulfuric acid. Two stable configurations HOSO and HSO₂ have been reported for such a radical in the gas phase. This work aims at studying these isomers in the aqueous medium. The electrical and reactivity quantities - electronic chemical potential ([Formula: see text]), chemical hardness ([Formula: see text]), and electrophilicity ([Formula: see text]) - are here calculated and compared. Considering first solvation shells (15 H₂O for HSO₂ and 23 H₂O for HOSO), an increase of 25% in the dipole moment of HSO₂ was obtained, while the dipole moment of HOSO decreases in 11%. Both solvated isomers grow softer ([Formula: see text] decreases) contrasted to the gas phase.

METHODS

HOSO and HSO₂ are studied through a sequential Monte Carlo/quantum mechanics approach. Lennard-Jones plus the Coulomb potentials were used to represent intermolecular potential interaction in the frame of the DICE package. Molecular structure calculations were performed at CASPT2/aug - cc - pV(T + d)Z level of theory using the MOLPRO suite of programs.

Method and basis set dependence of the NICS indexes of aromaticity for benzene

The role of theory level in prediction of benzene magnetic indexes of aromaticity is analysed and compared with calculated nuclear magnetic shieldings of ³ He used as NMR probe. Three closely related nucleus-independent chemical shift (NICS) based indexes were calculated for benzene at SCF-HF, MP2, and DFT levels of theory and the impact of basis set on these quantities was studied. The changes of benzene NICS(0), NICS(1), and NICS(1)zz parameters calculated using SCF-HF, MP2 and several density functionals were within 1 to 3 ppm. Similar deviations between magnetic indexes of aromaticity were observed for values calculated with selected basis sets. Only very small effect of polar solvent on benzene aromaticity was predicted. The ³ He nuclear magnetic isotropic shielding (σ) and its zz-components (σ_zz ) of helium atom approaching the centre of benzene ring from above produced similar curves versus benzene-He distance to NICS parameters calculated for similarly moving Bq ghost atom. We also propose an experimental verification of NICS calculations by designing the ³ He NMR measurement for benzene saturated with helium gas or in low temperature matrices.