An Ab Initio Study of the Structures and Enthalpies of the Hydrogen-Bonded Complexes of the Acids H2O, H2S, HCN, and HCI with the Anions OH-, SH-, CN-, and CI-

Ab initio calculations at second-order Moller-Plesset perturbation theory with the 6-31 + G(d,p) basis set have been performed to determine the equilibrium structures and energies of a series of negative-ion hydrogen-bonded complexes with H2O, H2S, HCN, and HCI as proton donors and OH-, SH-, CN-, and CI- as proton acceptors. The computed stabilization enthalpies of these complexes are in agreement to within the experimental error of 1 kcal mol(-1) with the gas-phase hydrogen bond enthalpies, except for HOH center dot center dot center dot OH-, in which case the difference is 1.8 kcal mol(-1). The structures of these complexes exhibit linear hydrogen bonds and directed lone pairs of electrons except for complexes with H2O as the proton donor, in which cases the hydrogen bonds deviate slightly from linearity. All of the complexes have equilibrium structures in which the hydrogen-bonded proton is nonsymmetrically bound, although the symmetric structures of HOH center dot center dot center dot OH- and CIH center dot center dot center dot CI- are only slightly less bound than the equilibrium structures. MP2/6-31 + G(d,p) hydrogen bond energies calculated at optimized MP2/6-31 + G(d,p) and at optimized HF/6-31G(d) geometries are similar. Using HF/6-31G(d) frequencies to evaluate zero-point end thermal vibrational energies does not introduce significant error into the computed hydrogen bond enthalpies of these complexes provided that the hydrogen-bonded proton is definitely nonsymmetrically bound at both Hartree-Fock and MP2.