AH INITIO STUDIES OF FREE AND MONOHYDRATED CARBOXYLIC-ACIDS IN THE GAS-PHASE

Simple carboxylic acids, R-COOH (R = methyl, ethyl, vinyl, phenyl, and hydroxyphenyl), and the monohydrates of acetic and benzoic acids have been studied in the gas phase using ab initio quantum chemical method s. HF/6-31G and MP2/6-31G* geometry optimizations prove that calculat ions at the MP2 level reproduce well the available experimental gas-ph ase structures. In the most stable conformation of the free acids the acid hydrogen is syn to the carbonyl oxygen. Conformers with an anti a cid hydrogen are local energy minima for the aliphatic acids and for t he nonplanar benzoic acid; however, arrangements with an sp(2) hybridi zed P carbon and with a planar heavy atom skeleton correspond to trans ition structures. The anti acid conformers are higher in energy than t he syn form by 7-10 kcal/mol obtained in MP2/6-31G//HF/6-31G* calcula tions. Results obtained at the MP2/6-311++G*//MP2/6-31G* level for ac etic acid and benzoic acids show that the calculated energy separation decreased by 1-2 kcal/mol. The syn/anti rotation barrier for the benz oic acid carboxylic hydrogen was estimated at 12.6 kcal/mol at this le vel. Monohydration of the acetic acid and benzoic acids leads to small geometry changes for the carboxylic group. The syn monohydrates have a cyclic COOH...OH2 structure with a shorter Ow...H-O(ac) and a longer Ow-Hw...O=C hydrogen bond. This structure is optimal in the gas phase but is less likely in aqueous solution. There is a single Ow..H-O(ac) hydrogen bond for the anti monohydrate. The anti structures are highe r in energy than the syn by 7-9 kcal/mol obtained in MP2/6-311++G*// MP2/6-31G calculations. The energies of association are estimated as -9.5 kcal/mol and -8.2 kcal/mol for the syn and anti monohydrates, res pectively.