Theoretical study of H2...I- van der Waals anion complex

The ab initio potential energy surface ( PES) for the weak interaction of hydrogen molecule with iodine anion is presented. The surface was obtained by the supermolecular method at the MP4(SDTQ) level of theory. Our calculations indicate the van der Waals (vdW) system for the linear configuration at r(H - H) = 0.752 angstrom and R = 3.76 angstrom with a well depth of D-e = 2096 mu E-h. The presented PES reveals also a transition state for the perpendicular arrangement at r(H - H) = 0.7416 angstrom and R = 4.63 angstrom with an interaction energy of - 113 mu E-h. The physical origin of stability of the vdW H2 center dot center dot center dot I- structure with respect to the H2 center dot center dot center dot X- (X = F, Cl, Br) one was analysed by the symmetry adapted perturbation theory (SAPT) based on the single determinant HF wave function. The separation of the interaction energy shows that the dispersion forces play a much more important role for the systems with Cl, Br and I than for H2 center dot center dot center dot F- and their importance slightly increases in the order Cl < Br < I. The global importance of the electrostatic and the induction energies decreases in the order F > Cl > Br > I.