On the structure of the 3B 1 excited state of water

Abstract

Ab initio calculations using several large basis sets at the unrestricted (U) and projected (P) Hartree-Fock (HF) and second-order Møller-Plesset (MP2) levels have been performed on the lowest triplet excited state ( 3B 1) of C 2v water. It was found that the unrestricted SCF calculations are able to produce geometries, dipole moments and dipole polarizabilities in agreement with available complete active space (CAS) SCF calculations, if sufficiently extended basis sets are used. The smaller basis sets used in this work (6-31 + G(d,p), +VP S(2d) S, +VP S(3d) S and atomic natural orbital (ANO)) predicted a linear equilibrium geometry instead of the CAS SCF bent one. Adiabatic excitation energies, however, are not in good agreement with those at the CAS SCF level even for large basis sets, unless dynamic correlation effects (at the MP2 level) are taken into account. Optimum geometries at this level are also in better agreement with these CAS SCF ones. However, analysis of the force-constant matrix showed the presence of a large negative eigenvalue. The associated eigenvector shows that the optimum C 2v structure is a transition state towards dissociation in OH( 2Π) + H. A potential energy surface was then built for the C 2v structure varying both the bond length and the bond angle at the PHF, UMP2 and PMP2 levels with the 6-311++G(2d,2p) basis set. No other minimum than the present one and the previously reported asymptotic limit O( 3 P) + H 2( 1Σ +) were found. At the PHF level, the linear D ∝ h structure behaves like a minimum, but it turns out as a transition state at the more accurate PMP2 level. All the critical points on the UMP2 surface were directly located with several of the larger basis sets and their relative energies are reported. Generally speaking, PMP2 and UMP2 calculations are in agreement, implying that spin-contamination is not imported in determining the characteristics of this surface. Moreover the vertical and adiabatic excitation energies from the ground 1A 1 state are in agreement with the CAS SCF calculations. However, the results for the critical points on the surface are not in quantitative agreement with a previous CIPSI study. The results found imply also that the 3B 1 state is dissociative, as shown by a potential energy surface built around the bent C 2v structure but relaxing the geometry constraint.