Quantitative probing of subtle interactions among H-bonds in alpha hydroxy carboxylic acid complexes

Hydrogen (H) bonds are of fundamental importance in a wide range of molecular sciences. While the study of two-center H-bonding A⋯H is well advanced, much remains to be learned in a quantitative and definitive manner for complexes with multiple H-bonds. Exemplary cases are in the category of alpha hydroxy carboxylic acids, i.e., the complexes of glycolic acid with water and formic acid. In glycolic acid, an intramolecular H-bond forms between the carboxyl group and the alpha OH group. The alpha OH stretching frequency may be affected upon complexing with water or formic acid. Direct study of glycolic acid complexes is unfortunately very difficult. However, an aromatic analogue, 9-hydroxy-9-fluorene carboxylic acid (9HFCA), permits detailed and accurate gas phase spectroscopic studies. Since computational analysis establishes that H-bonding is very similar from glycolic acid complexes to 9HFCA complexes, direct studies on 9HFCA complexes by laser spectroscopy also deduce new and subtle information for glycolic acid complexes in terms of molecular structures, binding strength, and collective effects of multiple H-bonds associated with anti-cooperativity and cooperativity.