On the formation of proton-shared and contact ion pair forms during the dissociation of moderately strong acids: an Ab initio molecular dynamics investigation.

Acid ionization and dissociation are phenomena that play a fundamental role in chemistry and biology, but their microscopic details are largely unknown. We use ab initio molecular dynamics to identify and characterize various structures that are formed along the pathway of dissociation of trifluoroacetic acid (pK(a) = 0.5). The present results demonstrate that solutions of moderately strong (-1 <or= pK(a) <or= 1) carboxylic acids contain significant quantities of at least two forms whose bonding patterns are intermediate between those of covalent and dissociated structures, R-COOH...OH(2) and R-COO(-)(aq) + H(3)O(+)(aq), respectively. The first is a proton-shared structure of the form R-COO(delta-).H.(delta+)OH(2) which can be distinguished from the covalent form by its characteristic "continuous" absorption below 1700 cm(-1). The second is a contact ion pair whose average structure can be written as R-COO(-).H(3)O(+). We demonstrate that the transformation of proton-shared into contact ion pair forms coincides with the disappearance of the carbonyl vibration mode centered around 1720 cm(-1) and the appearance of the asymmetric and symmetric carboxylate [Formula: see text] vibration modes at 1620 and 1380 cm(-1), respectively. The aforementioned transformation is accompanied by significant changes in the first solvation shell of the acid molecule which we discuss.