Abstract
The thermodynamics of the hydrogen bonding of pivalic and 3-chloropivalic acids in gas phase was studied with FTIR spectroscopy and quantum chemistry (semi-empirical and ab initio) calculations. All theoretical methods suggest the existence of the only one stable conformer (Cs) of 3-chloropivalic acid. The heat of formation of the cyclic dimers of pivalic and 3-chloropivalic acids in gas phase was calculated from experimental data and found to be of 64.7±1.4kJ/mol and 61±3kJ/mol, respectively. The results obtained indicate that the substitution of one hydrogen in the CH3 group of pivalic acid by chlorine has no measurable influence on the heat of the cyclic dimerization. However, 3-chloropivalic acid does not form the high temperature orientationally disordered solid phase (plastic crystal) in contrast to pivalic acid. Different molecular shapes (different globularity) of the investigated acids could be the main reason for different orientational behaviour of the investigated acids.
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