Geometrical consequences of fluorination and intermolecular hydrogen bond formation in fluoroformic acid and trifluoroacetic acid dimers from ab initio MO calculations

The molecular structures of fluoroformic and trifluoroacetic acids and their dimers have been determined by ab initio molecular orbital calculations including electron correlation at the MP2 level with standard 6-31G(*) and 6-31+G(*) basis sets. Fluorination introduces geometrical changes as expected for a strong electron withdrawing substituent. Considerable intermolecular interactions were detected in the dimers accompanied by pronounced structural changes compared with the monomers, similarly to those observed for formic acid and acetic acid monomers/dimers, and described as consequences of resonance-assisted hydrogen bonding. The hydrogen bonding in the fluoroformic acid dimer appears to be somewhat stronger than in the trifluoroacetic acid dimer. (C) 1997 Elsevier Science B.V.