SELECTIVE NAKED-CLUSTER CRYOPHOTOCHEMISTRY AND SCF-XALPHA-SW CALCULATIONS FOR CU2 AND AG2

The concept of conducting photoselective transformations on small, well-defined, ligand-free transition metal clusters is explored for the first time by investigating the visible (370-390 nm) photochemistry of Ar- and Kr-entrapped Cu2 and Ag2. With prior knowledge of the 200-900-nm optical spectra of Cu1-3 and Ag1-4 clusters, generated either by metal atom deposition or photoclustering techniques, one can rationalize the observed Cu2 and Ag2 visible cryophotochemistry in terms of a highly selective matrix-induced photodissociation step for which a mechanism involving formation of a metal atom-matrix atom(s) exciplex is proposed. SCF-Xα-SW MO calculations for Cu2 and Ag2 confirm that the wavelengths used for the pho-toexcitations are essentially those of the electronic transitions from the main bonding to the main antibonding orbital (i.e., sσg→sσu), thus providing a description of the excited states which is useful in accounting for the observed photochemistry. The calculations represent the most thorough theoretical study of Cu2 and Ag2 to date. Complete potential curves are calculated and the observed rc, Dc, and v(M-M) are in good agreement with experiment. The bonding at the likely equilibrium distances is discussed; that in Ag2 is found to be almost purely s, while in Cu2 there appears to be a minor d contribution. The electronic spectra arc assigned in detail from transition-state calculations of both energies and oscillator strengths. The A←X, C←X, and E←X systems of Ag2 are attributed to sσ→*, sσ←πand dπ←sσ*transitions, respectively. The A→X and B→X systems of Cu2 are ascribed to dπ*→sσ* and sσexcitations, and the UV bands of Cu2 near 260, 235, and 220 nm are assigned to mainly dπ*→pπ, sσ→sπ, and dσ→sσ* processes, respectively. Copyright © 1979, American Chemical Society. All rights reserved.