Ab initio study of the intermolecular potential of the water–carbon monoxide complex

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Abstract
The combination of the supermolecular Mo/ller–Plesset scheme with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface (PES) of the H2O...CO complex. We located three low‐energy configurations on the potential energy surface corresponding to two isomeric H‐bonded complexes OC...HOH (C structure), CO...HOH (O structure), and a T‐shaped structure with CO bonded to the O atom of H2O. The absolute minimum corresponds to the C configuration OC...HOH, involving a nonlinear C...H–O bond. The tilt from the linearity is 11 deg, in agreement with the value derived from the experimental data. The computed binding energies on the fourth‐order perturbation 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 surface 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 of 11 deg from the linear H bond is due to the balance of the electrostatic and exchange repulsion terms; the repulsive Heitler–London term is minimal when the angle between the C2V axis of the water molecule and 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 sign of the dipole moment of the CO molecule. The tunneling motion of the water molecule around its c inertial axis was studied and the barrier to exchange of the bound and the free hydrogen atom was determined as 280 cm−1 (1289.470 μhartree).