Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.

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Quantum chemistry-based dipole polarizable and nonpolarizable force fields have been developed for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Molecular dynamics simulations of TATB crystals were performed for hydrostatic pressures up to 10 GPa at 300 K and for temperatures between 200 and 400 K at atmospheric pressure. The predicted heat of sublimation and room-temperature volumetric hydrostatic compression curve were found to be in good agreement with available experimental data. The hydrostatic compression curves for individual unit cell parameters were found to be in reasonable agreement with those data. The pressure- and temperature-dependent second-order isothermal elastic tensor was determined for temperatures between 200 and 400 K at normal pressure and for pressures up to 10 GPa on the 300 K isotherm. Simulations indicate considerable anisotropy in the mechanical response, with modest softening and significant stiffening of the crystal with increased temperature and pressure, respectively. For most properties the polarizable potential was found to yield better agreement with available experimental properties.

Characteristic patterns of infrared bands in the ν(CO) region have been observed in the time-resolved infrared (TRIR) spectra of fac-rhenium tricarbonyl complexes that allow for identification of transient states that result following laser flash excitation. These patterns can be interpreted by combining experimental TRIR data with density functional theory (DFT) calculations. The DFT calculations are particularly valuable as they provide vibrational energy shifts between the ground and excited states and an analysis of the electronic interactions in terms of the orbitals involved in the excitation. TRIR and DFT results for four different transient excited states, intraligand π f π*, metal-to-ligand charge transfer (MLCT), intramolecular (dπ-Οπ) f π* excited states, and a redox-separated (RS state), are presented here. A unique example of competing excited states studied by TRIR is also presented. The complexes studied include fac-[Re I (CO) 3 (Me 2 dppz)(4-Etpy)] + , fac-[Re I (CO) 3 (bpy)(4-Etpy)] + , fac-[Re I (CO) 3 (4,4′-(CH 3 ) 2 bpy)(OQD)] , fac-[Re I (CO) 3 (Me 2 dppz)(py-PTZ)] + , and fac-[Re I (CO) 3 (dppz)(py-PTZ)] + (Me 2 dppz is dimethyl dipyrido[3,2-a:2′,3′-c]phenazine; dppz is dipyrido-[3,2-a:2′,3′-c]phenazine; 4Etpy is 4-ethylpyridine; bpy is 2,2′-bipyridine; 4,4′-(CH 3 ) 2 bpy is 4,4′-(CH 3 )-2,2′bipyridine; OQD is 1-methyl-6-oxyquinone; py-PTZ is 10-(4-picolyl)phenothiazine). In addition to the DFT studies on the lowest triplet states probed by TRIR spectroscopy, time-dependent DFT (TD-DFT) calculations were also performed to analyze several of the lowest singlet and triplet excited states for each of the complexes.