Association of nonvolatile globular carboxylic acids in gas phase as studied by means of FTIR spectrometry

The thermodynamics of the hydrogen bonding of pivalic and 3-chloropivalic a cids in gas phase was studied with FTIR spectroscopy and quantum chemistry (semi-empirical and ab initio) calculations. All theoretical methods sugges t the existence of the only one stable conformer (C-s) of 3-chloropivalic a cid. The heat of formation of the cyclic dimers of pivalic and 3-chloropiva lic acids in gas phase was calculated from experimental data and found to b e of 64.7 +/- 1.4 kJ/mol and 61 +/- 3 kJ/mol, respectively. The results obt ained indicate that the substitution of one hydrogen in the CH3 group of pi valic acid by chlorine has no measurable influence on the heat of the cycli c dimerization. However, 3-chloropivalic acid does not form the high temper ature 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 beh aviour of the investigated acids. (C) 1999 Elsevier Science B.V. All rights reserved.