AB-INITIO CALCULATIONS ON THE GEOMETRY AND OH VIBRATIONAL FREQUENCY-SHIFT OF CYCLIC WATER TRIMER

The equilibrium geometrical parameters R(OO), theta and chi of cyclic water trimer have been determined at the counterpoise corrected SCF + MP2 level in the ESPB basis within pseudo-C3v symmetry. The final structure has short (R(OO) = 2.85 angstrom) and strongly bent (theta = 20-degrees) hydrogen bonds. The non-bonded OH bonds are directed by chi = 48-degrees out of the OOO plane. The interaction energy (DELTAE) is -14.7 kcal mol-1 and the corresponding D0 = 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 DELTAE, and it is responsible for a shortening of R(oo) by 0.07 angstrom. Its largest effect (about -70 cm-1) i s in the H-bonded OH vibrational frequency shift DELTAv(OH), which at the equilibrium geometry is calculated to be -230 cm-1. The shift is markedly sensitive to the angle chi, and vibrational averaging along this coordinate is expected to reduce DELTAv. The results therefore support Nelander's reassignment of the IR and Raman gas phase OH spectra which implies DELTAv(OH) almost-equal-to -175 cm-1.