Computational studies of Sulphur trioxide and its protonated analogues

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.

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