AB-INITIO STUDY OF THE INTERMOLECULAR POTENTIAL OF THE WATER-CARBON MONOXIDE COMPLEX

The combination of the supermolecular Moller-Plesset scheme with the p erturbation theory of intermolecular forces is applied in the analysis of the potential energy surface (PES) of the H2O...CO complex. We loc ated three low-energy configurations on the potential energy surface c orresponding to two isomeric H-bonded complexes OC...HOH (C structure) , CO...HOH (O structure), and a T-shaped structure with CO bonded to t he O atom of H2O. The absolute minimum corresponds to the C configurat ion OC...HOH, involving a nonlinear C...H-O bond. The tilt from the li nearity is 11 deg, in agreement with the value derived from the experi mental data. The computed binding energies on the fourth-order perturb ation theory level are 651 cm(-1) for the C configuration, 301 cm(-1) for T, and 256 cm(-1) for O. The anisotropy of the potential energy su rface is analyzed using the perturbation theory. The absolute minimum results from the attractive electrostatic contribution and dispersion energy, which overcome considerable exchange repulsion. A small tilt o f 11 deg from the linear H bond is due to the balance of the electrost atic and exchange repulsion terms; the repulsive Heitler-London term i s minimal when the angle between the C-2V axis of the water molecule a nd the intermolecular axis is equal to 63.0 deg. The bonding in the T configuration is due largely to the dispersion energy which overcomes strong exchange repulsion. The third O configuration is more stable on the SCF level than on the MP2 level, because of the reversal of the s ign of the dipole moment of the CO molecule. The tunneling motion of t he water molecule around its c inertial axis was studied and the barri er to exchange of the bound and the free hydrogen atom was determined as 280 cm(-1) (1289.470 mu hartree).