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Molecular Modelling Analysis of the Formation of HX.H2O
Aggregates and (HX)n Oligomers where X = F, Cl, Br and I
Fazlul Huq
Discipline of Biomedical Science,
School
of Medical Sciences, Faculty of Medicine, The University of Sydney,
Australia.
Tel: +61 2 9351 9522, Fax: +61 2 9351 9520,
E-mail :
F.Huq@fhs.usyd.edu.au.
Abstract:
Molecular modelling analyses using molecular mechanics and
semi-empirical calculations were carried out to investigate formation
of HX.H2O aggregates (X = F, Cl, Br and I) and (HX)n
rings with n = 3 to 10, through hydrogen bonding. Although among HF,
HCl, HBr and HI, HF is polar, a solution of HF in water is a weak acid
whereas those of HCl, HBr and HI in water are strong acids. When the
HX distances in the optimized structures of HF.H2O, HCl.H2O,
HBr.H2O and HI.H2O are compared, it is found
that the HF bond length is shortest and HI bond length is longest (HF,
HCl, HBr and HI lengths are respectively 0.946, 1.285, 1.508 and
6.0679 Å), indicating that the degree of ionization would be highest
for HI(aq) and least for HF(aq). The calculated heats of formation of
HF..H2O, HCl.H2O, HBr.H2O and HI.H2O
in kcal mol-1 are respectively -121.66, -79.55, -55.79,
-31.40, indicating that HF.H2O is most stable and the
formation HI.H2O is least favoured energetically.
Key words:
HF, HCl, Weak acid, Polar, Hydrogen bond, Activity coefficient
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