Photosynthetic organisms have developed antenna systems to enlarge their cross section for capturing sunlight. These systems involve in some cases aggregates of bacteriochlorophylls (BChls). The structure of one such aggregate, a tightly coupled circular hexadecamer of BChls and a loosely coupled octamer of BChls, has recently been solved. In this paper we investigate the electronic excitations in these two aggregates as well as in monomeric BChl and BChl dimers by means of INDO/S-CI calculations. The results provide a detailed description of the properties of electronic states in the BChl aggregates (energies, dipole and transition dipole moments) that are relevant for the biological function of BChls, to absorb light and transfer energy on the subpicosecond time scale. The lower-energy excitations of the BChl hexadecamer are of exciton type, i.e., experiencing a strong coupling that leads to excitation delocalization over the entire aggregate. An effective Hamiltonian is provided which reproduces these exciton states and which can be readily generalized to other aggregates.

Structures are calculated for magnesium phthalocyanine (MgPc) and its radical anion doublet (MgPc-), using both ab-initio (6-31G**) and semiempirical (INDO/1) self-consistent field approaches. The anion is first-order Jahn-Teller distorted, and the various distortions that are possible are examined. The electronic absorption spectra of both molecular species and the effect that varying the degree of distortion has on the computed anion spectrum are discussed. These results suggest that the four-orbital model often applied to porphyrin systems in interpreting the low-energy spectrum is incomplete for the anion case. We further conclude that the Jahn-Teller distortions calculated by either ab-initio or semiempirical models may be too great.

A consistent embedding hierarchy is applied to the calculation of binding enthalpies for organophosphate molecules to a silica surface and compared to experiment. The interaction of four probe molecules, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diisopropyl fluorophosphate (DFP), and sarin, with the silica surface is examined. Quantum chemical methods are employed to compute binding enthalpies and vibrational spectra for all interactions between probe molecules and silanol sites on the silica surface. Comparison with experimentally measured infrared shifts indicates that the theoretically modeled adsorption sites are similar to those found in experiment. The calculated binding enthalpies agree well with experiment for sarin, ΔH ads,443K = −22.0 kcal/mol (calculated) vs −18.8 ± 5.5 kcal/mol (measured, 433 K < T expt < 453 K), and DIMP, ΔH ads,463K = −26.9 kcal/mol (calculated) vs −29.3 ± 0.9 kcal/mol (measured, 453 K < T expt < 473 K). Agreement with experiment is less good for DMMP, ΔH ads,463K = −19.7 kcal/mol (calculated) vs −26.1 ± 1.5 kcal/mol (measured, 453 K < T expt < 473 K), and DFP, ΔH ads,423K = −20.4 kcal/mol (calculated) vs −27.5 ± 3.1 kcal/mol (measured, 413 K < T expt < 433 K).

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An intermediate neglect of differential overlap method for examining the electronic structure of actinide complexes is developed. It is characterized by a basis set obtained from relativistic Dirac–Fock atomic calculations, the inclusion of all one-center two-electron integrals, and a parameter set based on molecular geometry and ionization spectra. The model is successful in reproducing the geometries of many small test molecules, especially the hexahalides and tetrahalides of the early actinides. We also investigate the bonding in actinocenes and the photoelectron spectra of pentavalent uranium amide/imide complexes as two diverse examples in which this model can be used to help in understanding and prediction.