A MICROSCOPIC INSIGHT INTO OSCILLATOR-STRENGTHS - THE CHARGE-TRANSFERBANDS FOR CUCL42-

The oscillator strengths f(1), f(2), and f(3) corresponding to the thr ee allowed 2e(u) --> 3b(1g), 3e(u) --> 3b(1g) and 1b(2u) --> 3b(1g) ch arge-transfer transitions of the D-4h CuCl42--complex together with th eir dependence on the metal-ligand distance R are investigated using t he self-consistent charge extended Huckel (SCCEH) and MSX alpha method s. The main conclusions of this work are the following: (1) f(1) and f (2) directly reflect the sigma-character displayed by 2e(u) and 3e(u) orbitals. When this character is suppressed, f(1) and f(2) decrease by an order of magnitude and then become comparable to f(3). This trend concurs with experimental findings. (2) The different sigma-character exhibited by 2e(u) and 3e(u) orbitals arises mainly from the 4p(Cu) ad mixture. A similar one involving the 3s(Cl) orbitals plays a minor rol e. When such admixtures are neglected, the two wave functions \2e(u)] and \3e(u)] can simply be written as \sigma]\pi] and \sigma] - \pi], r espectively. The inclusion of 4p(Cu), although small, produces an inte rference between \sigma] + \pi] and \sigma] - \pi], enhancing the sigm a-character in 2e(u) and destroying it in 3e(u). This explains well th e asymmetry between f(1) and f(2) observed experimentally. (3) The sig ma-character in 2e(u) and 3e(u), together with f(1) and f(2) are found to be nearly independent upon R. (4) Also, the separation, Delta, bet ween the energies of the 2e(u) --> 3b(1g) and 3e(u) --> 3b(1g) transit ions was explored. It is shown that Delta is determined mainly by the ligand-ligand interaction and, thus, is strongly dependent upon R. (C) 1997 John Wiley & Sons, Inc.