HOMO-LUMO Transition Research Articles

Ligand effects on the electronic structure, spectra, and electrochemistry of tetracobalt carbonyl clusters

Self-consistent field X..cap alpha.. calculations using the discrete variational method (SCF-X..cap alpha..-DV) have been performed for Co/sup 4/(CO)/sub 12/ (I) and the C/sub 3nu/ symmetry model clusters (eta-C/sub 6/H/sub 6/)Co/sub 4/(CO)/sub 9/((PH/sub 2/)/sub 3/CH) (III), and (eta-C/sub 6/H/sub 6/)Co/sub 4/(CO)/sub 6/((PH/sub 2/)/sub 3/CH) (IV) to explore the effect of phosphine and arene ligands on cluster electronic structures. Density of states plots have been used to simplify analysis of bonding trends in these complex clusters. The cluster core charge varies from 3.11, to 2.58, to 2.41, to 1.80 e along the I-IV series. Charge on the apical Co is least positive in the arene-capped cluster II while basal cobalts are least positive in IV. Ionization potential calculations support qualitative Hueckel studies that suggested (eta-C/sub 6/H/sub 6/)M fragments bind more strongly to other metal centers than isolobal M(CO)/sub 3/ fragments. Electronic absorption spectra of I, (eta-MeC/sub 6/H/sub 5/)Co/sub 4/(CO)/sub 9/(tripod) (VI) (tripod = (PPh/sub 2/)/sub 3/CH), (eta-MeC/sub 6/H/sub 5/)Co/sub 4/(CO)/sub 6/(tripod) (VII), and (eta-C/sub 6/Me/sub 6/)Co/sub 4/(CO)/sub 6/(tripod) (VIII) exhibit low-energy features between 1000 and 400 nm. Arene-capped clusters exhibit an extra low-energy absorption and calculations for the models II and IV assign this to a HOMO-LUMO transition that resembles intramolecular chargemore » transfer from the apical to basal cobalt atoms.« less

A critical analysis of the nature of the MOs and electronic transitions in fluorene, indene and some isoelectronic benzene derivatives

The frontier MOs and the electronic transitions of benzaldehyde (I), benzonitrile (II), styrene (III), indene (IV) and fluorene (V) have been calculated by the INDO/S-CI method and analyzed in order to establish their nature. The HOMO and LUMO of I and II are essentially benzenic orbitals whereas those of III, IV and V closely resemble the corresponding MOs of cyclopentadiene (VI) and cis-butadiene. As a consequence the HOMO—LUMO transition in III—V is found to possess both butadienic and benzenic character. The calculated spectra of I—IV show the same trend, in complete agreement with the experimental findings.

Theory of ultraviolet spectra for Ni2 and Ni3 and hypothesis for argon matrix frequency shifts for Ni atoms

Atomic orbital 3d and 4s ionization energies and the 4p orbital exponent are assigned using gas phase uv Ni atom spectra. These parameters are used in a molecular orbital theory, producing an accurate description of the three observed electronic transitions in Ni2 and the two observed transitions in Ni3, and they show the latter has a linear geometry. It is found that HOMO–LUMO transitions are not necessarily the strongest in intensity. It is suggested that antibonding interactions with the Ar matrix are responsible for the Ni atom frequency shifts.