2 Theoretical studies of hydrogen bonding
Theoretical procedures yielding accurate structures and stabilization energies of extended hydrogen-bonded complexes having more than 24 atoms are discussed. Accurate geometries of different types of hydrogen-bonded complexes are obtained by gradient optimization performed at correlated ab initio levels using the basis set of TZ + 2P quality. Stabilization energy of these complexes is constructed as a sum of the complete basis set limit of the MP2 stabilization energy and a correction term defined as the difference between MP2 and CCSD(T) stabilization energies. Accurate values of the latter term were obtained with medium basis sets. The results presented for the different hydrogen-bonded complexes support the fact that this correction term possesses a rather insignificant value. Comparing the geometrical and stabilization energy data with experimental results verifies the reliability of the procedure described. It can be concluded that when experimental data on extended hydrogen-bonded complexes either are missing or are not reliable, the theoretical data determined at the level described can be used with confidence.