VERY STRONG HYDROGEN-BONDS IN NEUTRAL MOLECULES - THE PHOSPHINIC ACIDDIMERS

Ab initio molecular orbital and density functional theories have been used to study the structures and binding energies of the dimers of pho sphinic acid (PA) and its dimethyl derivative (DMPA). For the first co mpound we have located all possible minima of the potential energy sur face, while for the second only the most stable dimer was considered. The geometries were fully optimized at the MP2(full)/6-31+G(d,p) and B 3LYP/6-31+G(d,p) levels of theory. The harmonic vibrational frequencie s were evaluated at the same levels, while the final energies were obt ained using a B3LYP/6-311+G(3 df,2p) approach. Both phosphinic acid an d its dimethyl derivative form cyclic dimers in the gas phase, where t he two monomers are held together by hydrogen bonds (HBs) which are si gnificantly stronger than those found for their carboxylic analogs. Th e estimated dimerization enthalpies for PA (23.2) and DMPA (23.2 kcal/ mol) are the highest reported so far for neutral homodimers in the gas phase and almost twice those measured for formic and acetic acid dime rs. For the particular case of DMPA this estimated value is in very go od agreement with the experimental one (23.9+/-6 kcal/mol). As a conse quence of the large strength of the HBs, the activation barriers assoc iated with the concerted double proton transfer are also sizably small er than those predicted for their carboxylic analogs. These barriers b ecome negligibly small when zero point energy (ZPE) corrections are ta ken into account and therefore tunneling must be very efficient. The c alculated harmonic vibrational frequencies for the most stable DMPA di mer are consistent with its experimental infrared (IR) spectrum in the gas phase, which shows a characteristic ABC structure of the nu(OH) b and, typically associated with strongly hydrogen bonded complexes. Int ernal cooperative effects are not negligible in the case of phosphinic acid dimers, and the P = O ... H hydrogen bonds in the global minimum are about 1.0 kcal/mol stronger than those found in other stable dime rs where only one of these linkages exists. (C) 1998 American Institut e of Physics.