W0218

Determination of the Adiabatic Potential Energy Surfaces of Copper(II) Jahn-Teller Complexes Using Temperature Dependent Copper-Ligand Bond Lengths. Charles J. Simmons, Dept. of Chemistry, University of Hawaii at Hilo, Hilo, Hawaii 96720-8932 U.S.A., and Horst Stratemeier and Michael Hitchman, School of Chemistry, University of Tasmania, Box 252-75, Hobart, Tasmania 7001, Australia.

The Tutton salts form a series of isomorphous compounds crystallizing in the monoclinic space group P21/a (Z=2) having the general formula X2[M(H2O)6](YO4)2, where X represents a monovalent cation (K+, Rb+, Cs+, NH4+, ND4+), M a divalent metal cation and Y is either S or Se. These compounds contain discrete octahedral M(H2O)62+ complexes possessing Ci site symmetry with three unique M-O bonds. Of the first-row transition metal elements, only M(H2O)62+ complexes containing high-spin Cr2+ (5Eg) and Cu2+ (2Eg) ions are Jahn-Teller (JT) active, and, generally, two of the three observed M-O bond lengths are temperature dependent.

Recently, we have been investigating the structures of the mixed-crystal series K2[Cu(H2O)6] (S1-xSexO4)2 at different temperatures and dopage. The long and intermediate Cu-O bonds in the pure potassium salt switch with x __0.5. Thus, the adiabatic ground-state surfaces of the complexes can be modulated by varying the amount of selenate. We are able to calculate these surfaces using a vibronic coupling model developed by Riley et al. (Riley, M.J.; Hitchman, M.A.; Mohammed, A.W. J. Chem. Phys. 1987, 87, 3766). To apply this model to Cu(H2O)62+, approximate values of the parameters describing the JT coupling (h_, energy of the eg vibration in the absence of the JT effect; M, effective mass of ligand; A1, first-order vibronic coupling constant; A2, second-order vibronic coupling constant, _E, energy between split Eg energy surfaces) and the influence of strain (S_, tetragonal strain; S_, orthorhombic strain) are input as parameters. The program then calculates the energy states, vibronic wave functions and Cu-O bond lengths. The final potential energy surfaces and vibronic wave functions are determined from the values of the best-fit parameters A1, A2, S_, and S_, obtained by comparing the observed with the calculated Cu-O bond lengths.