Computational Studies of Sulphur Trioxide (SO3) and its Protonated Analogues

Abstract

This article reports quantum chemical calculations on sulphur trioxide (SO3) and its protonated analogues (HSO3+ and HOSO2+) by employing six different computational methods. This covers the simple Hatree Fock (HF) method, coupled cluster method, the Gaussian-4 (G4) compound method amongst others with two different basis sets (6-311++G** and cc-pVDZ). Optimized geometries, bond distance, rotational spectroscopy, dipole moment, proton affinity, vibrational spectroscopy and vibrational zero-point energy are among the parameters that have been effectively and successfully computed for the three molecular species under investigation. From the result obtained, there is a perfect agreement between the different parameters investigated and the available experimental values. The high accuracy of the calculated results gives a good image of the protonated analogues (HSO3+ and HOSO2+) which are deficient in experimental data. The optimized geometry reveals that two of  the three molecular species (i.e. SO3 and HOSO2+) have a trigonal geometry in all the computational methods employed while HSO3+ analogue was discovered to have a square planar geometry for MP2/6-311++G**,  MP2/cc-pVDZ,  CCSD/cc-pVDZ and G4  methods. We found that the proton affinities (PA) of SO3  was between 139.6 – 151.1 kcal/mol with B3LYP/6-311++G** method predicting the best result and that S-protonation was by far the most favoured site for proton attachment. Thus, the present quantum chemical studies have helped in bridging the gap existing in the literature regarding this species since they are less studied experimentally due to their unstable nature.  Keywords: Protonation, Silicon trioxide, Quantum chemistry, Spectroscopy