A theoretical calculation of the rotation-vibration energies for lithium hydroxide, LiOH

We have calculated ab initio the three-dimensional potential energy surface of the LiOH molecule at 166 nuclear geometries spanning energy ranges of about 2500 cm^-1 for the LiO stretch, 7500 cm^-1 for the OH stretch, and 4000 cm^-1 for the bend. The ab initio configuration interaction calculations were performed within the framework of the average coupled pair functional method [R. J. Gdanitz and R. Ahlrichs, Chem. Phys. Lett. 143, 413-420 (1988)]. The equilibrium structure is linear, but we found a very strong interaction between the LiO stretch and the bend such that at longer LiO distances the optimal structure is bent. The rotation and vibration energies for ⁷Li¹⁶OH, ⁷Li¹⁶OD, and ⁷Li¹⁸OH were calculated variationally using the Morse oscillatorigid bender internal dynamics Hamiltonian (MORBID) [P. Jensen, J. Mol. Spectrosc. 128, 478-501 (1988); J. Chem. Soc. Faraday Trans. 2 84, 1315-1340 (1988)]. For ⁷Li¹⁶OH we find that ν₁ = 3831 cm^-1, ν₂ = 289 cm^-1, and ν₃ = 923 cm^-1, and we obtain B_e = 1.183 cm^-1. The MORBID results are compared with rotation-vibration energies calculated with the nonrigid bender [P. Jensen and P. R. Bunker, J. Mol. Spectrosc. 118, 18-49 (1986)] and semirigid bender [P. R. Bunker and B. M. Landsberg, J. Mol. Spectrosc. 67, 374-385 (1977)] Hamiltonians.