HYDROGEN-BOND COOPERATIVITY - A QUANTITATIVE STUDY USING MATRIX-ISOLATION FT-IR SPECTROSCOPY

Matrix-isolation FT-IR spectroscopy is a suitable experimental method for evaluating hydrogen-bond cooperativities in complexes containing more than one single H bond. It is demonstrated that the cooperativity factor A(e), which represents the fortification of a H bond A-H...A-H by a further A-H...B interaction in a 1:2 complex A-H...A-H...B, increases with increasing proton affinity of the base B. On the other hand, the cooperativity factor A(b), representing the inverse fortification, is more or less constant for constant A-H. The A(b) values in the series HF, HCl, and H2O clearly depend on the induction attraction constant 2mu2alpha, which is an experimental proof for the dominating role of the induction energy term in the three-body effect. Evaluation of these cooperativity factors for homotrimers A-H...A-H...A-H allows one to differentiate between open, linear structures, and closed, cyclic trimers. The procedure can also be applied to differentiate between open and closed dimers. An illustrative case study for self-association and H-bonding of 3,4,5-trichlorophenol with water is presented.