A simple diagnostic is developed for the purpose of determining when a third state must be considered to calculate the electronic coupling element for a given pair of diabatic states within the context of the generalized Mulliken-Hush approach (Chem. Phys. Lett. 1996, 275, 15-19). The diagnostic is formulated on the basis of Löwdin partitioning theory. In addition, an effective 2-state GMH expression is derived for the coupling as it is modified by the presence of the third state. Results are presented for (i) a model system involving charge transfer from ethylene to methaniminium cation, (ii) a pair of donor-acceptor-substituted acridinium ions, and (iii) (dimethylamino)benzonitrile, and the diagnostic is shown to be a useful indicator of the importance of multistate effects. The effective 2-state GMH expression is also shown to yield excellent agreement with the exact 3-state GMH results in most cases. For cases involving more than three interacting states a similar diagnostic is presented and several approximations to the full n-state GMH result are explored.

We present calculations for various properties of the ground and excited states of several arylamine-substituted acridinium ion systems that have been studied experimentally. Using ab initio and semiempirical quantum mechanical methods together with the generalized Mulliken-Hush (GMH) model, we examine the excitation energies, dipole moment shifts, and electronic coupling elements for the vertical charge shift (CSh) processes in these systems. We also examine solvent effects on these properties using a dielectric continuum reaction field model. The results are in generally good agreement with available experimental results and indicate that there is strong electronic coupling in these systems over a wide range of torsional angles. Nevetheless, the initial and final cationic states remain reasonably well-localized over this range, and thus TICT state formation is unlikely in these systems. Finally, a version of the GMH model based on Koopmans' Theorem is developed and found to yield coupling elements generally within a factor of 2 of the many-electron GMH for a sample acridinium system, but with overestimated adiabatic and diabatic dipole moment differences.

The excitation energy to the 2 1 A g state of trans-1,3-butadiene is examined using a variety of ab initio electronic structure techniques. While analogous states have been shown to be the lowest singlet excited states for all longer polyenes, for butadiene the position of the 2 1 A g state relative to the HOMO f LUMO excitation (1 1 B u ) has been difficult to establish theoretically. We employ a variety of methods (CASSCF, CASPT2, MRSDCI, QDVPT) to examine both the vertical and adiabatic excitation energies for this state. At the groundstate geometry, the vertical excitation energies obtained by CASPT2 and Davidson-corrected MRSDCI for the 2 1 A g state differ by approximately 0.15 eV, but all of the methods predict that the 2 1 A g state has a lower 0-0 excitation energy than the 1 1 B u state. Possible reasons for the discrepancies between the various methods for the vertical excitation energy are discussed.

No abstract

Using electron nuclear double resonance (ENDOR) of GaP crystals containing shallow S donors it was shown that the ENDOR spectra previously detected optically i n the two infrared photoluminescence bands assigned to oxygen in oxygen-doped GaP are indeed due to S donors which must have been present simultaneously in those crystals. Furthermore it is shown that the resuits of t h e published ENDOR investigations of both S and Te donors in GaP must be revised. Finally the measured ligand hyperfine interaction of S and Te donors are discussed within the frame of effective mass theory.ENOOR of chalcogen donors in GaP Experimental results ENDOR measurementsin the GaP : S crystai we measured the known isotropic unstructured EPR line at g = 1.9975 with a half-width of 8 mT at 6 K. It is known that the half-widths depend strongly on the donor concentration (for example 5 mT for n = 5 x 1OI7 and 13 mT for n = 2 x 10"