Large amplitude bending motion in CsOH, studied through ab initio-based three-dimensional potential energy functions

The large-amplitude bending motion in CsOH, a 'classical' molecule whose microwave spectrum was first recorded in 1967, has been studied ab initio. The three-dimensional potential energy surface has been calculated at the RCCSD(T)_DK3/[QZP + g ANO-RCC (Cs, O, H)] level of theory and employed in MORBID calculations of the rotation-vibration energies and intensities. The ground electronic state is (1)Sigma(+) with the equilibrium structure r(e)(Cs-O) = 2.3930 angstrom, r(e)(O-H)= 0.9587 angstrom, and angle(e)(Cs-O-H) = 180.0 degrees. The O-H moiety is bound to C-s by an ionic bond and the molecule can be described as Cs delta+(OH)(delta-). Hence, the bending potential is shallow and gives rise to large-amplitude bending motion. The ro-vibrationally averaged structural parameters, determined as expectation values over MORBID wavefunctions, are < r(Cs-O)>(0) = 2.3987 angstrom, < r(O-H)>(0) = 0.9754 angstrom, and <angle(Cs-O-H)>(0) = 163 degrees. Although the averaged structure in the vibrational ground state is far from being linear, the Yamada-Winnewisser quasi-linearity parameter for CsOH is gamma(0) approximate to -1.0, the value characteristic for a linear molecule. (c) 2010 Elsevier Inc. All rights reserved.