Ab initio calculations on the geometry and OH vibrational frequency shift of cyclic water trimer

Abstract

The equilibrium geometrical parameters R OO θ and χ of cyclic water trimer have been determined at the counterpoise corrected SCF+MP2 level in the ESPB basis within pseudo-C 3v symmetry. The final structure has short ( R OO=2.85 Å) and strongly bent (θ=20°) hydrogen bonds. The non-bonded OH bonds are directed by χ=48° out of the OOO plane. The interaction energy (Δ E) is −14.7 kcal mol −1 and the corresponding D 0=10.2 kcal mol −1. The likely error bars on these results are discussed. The second order polarization interactions in the trimer are markedly non-additive. The total non-additivity contributes −2.0 kcal mol −1 to the final Δ E, and it is responsible for a shortening of R OO by 0.07 Å. Its largest effect (about −70 cm −1) is in the H-bonded OH vibrational frequency shift Δν OH, which at the equilibrium geometry is calculated to be −230 cm −1. The shift is markedly sensitive to the angle χ, and vibrational averaging along this coordinate is expected to reduce Δν. The results therefore support Nelander's reassignment of the IR and Raman gas phase OH spectra, which implies Δν OH≈ −175 cm −1.