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A [[(bpy)Re([CO.sub.3])L].sup.+] complex is studied by steady state and nanosecond time-resolved resonance Raman spectroscopy. An evidence is provided to support the photochemical mechanisms along with detailed information on structure and bonding that was available previously from UV-visible spectroscopy.

The isomerization dynamics of α-pyrone dissolved in CH3CN have been probed by femtosecond 267 nm pump/broadband infrared (IR) probe spectroscopy. A novel experimental setup allowed the populations of the parent molecule and ring-opened photoproducts to be monitored over pump/probe time delays ranging between 2 ps and 100 μs within a single experiment, and at 5 different temperatures between 0 and 40 °C. The photochemically prepared α-pyrone(S1) molecules decay rapidly (<10 ps) through internal conversion to the S0 potential energy surface, with an initial quantum yield for parent molecule re-formation of ∼60%. Probing the antisymmetric ketene stretch region (2100-2150 cm-1) confirms the presence of at least two ring-opened photoproducts, which are assumed to have an E-configuration with respect to the central CC double bond. These ketenes are observed to undergo two distinct, thermally driven, isomerization processes which occur on the nanosecond and microsecond time scales, respectively. The former reaction is ascribed to thermalization of the initially prepared E-isomer populations, while the slower (microsecond) process involves rotation around the central CC double bond leading to formation of Z-isomers. Subsequent rapid Z → Z isomerizations (occurring on a nanosecond time scale) result in ring-closure and a second, longer time recovery of parent molecule population. By determining rates as a function of the sample temperature, barrier heights of 0.23(3) eV and 0.43(2) eV are obtained for the E → E and E → Z transformations, respectively. [ABSTRACT FROM AUTHOR]

The photochemistry of (η6-anisole)Cr(CO)3 and (η6-thioanisole)-Cr(CO)3 was investigated by picosecond time-resolved infrared spectroscopy in n-heptane solution at 298 K. Two independent excited states are populated following 400 nm excitation of each of these complexes. An excited state with some metal-to-CO charge-transfer character is responsible for the CO-loss process, which is slow compared to CO-loss from Cr(CO)6. Observed first order rate constants of 1.8 x 1010s-1 and 2.5 x 1010s-1 were obtained for the anisole and thioanisole complexes, respectively. The second excited state has metal-to-arene charge transfer character and results in a haptotropic shift of the thioanisole ligand. DFT calculations characterized the excited states involved and the nature of the haptotropic shift intermediate observed for the thioanisole species. [ABSTRACT FROM AUTHOR]

The photochemistry of (η6-methylbenzoate)Cr(CO)3, (η6 - naphthalene)Cr(CO)3, and (η6-phenanthrene)Cr(CO)3 in n-heptane solution was investigated by picosecond time-resolved infrared spectroscopy (TRIR). The observation of two transient IR features in the organic carbonyl region at 1681 and 1724 cm-1 following 400 nm excitation of (η6-methylbenzoate)Cr(CO)3 confirms formation of two excited states which are classified as metal-to-arene charge transfer (MACT) and metal-to-CO charge transfer (MCCT), respectively. Time-dependent density functional theory calculations have been used to support these assignments. Population of the MCCT excited state results in a slow (150 ps) expulsion of one CO ligand. Excitation of (η6-naphthalene)Cr(CO)3 or (η6-phenanthrene)Cr(CO)3 at either 400 or 345 nm produced two excited states: the MCCT state results in CO loss, while the MACT excited state results in a change to the coordination mode of the polyaromatic ligands before relaxing to the parent complex. A comparison of the infrared absorptions observed following the population of the MACT excited state with those calculated for nonplanar polyaromatic intermediates provides a model for the reduced hapticity species. [ABSTRACT FROM AUTHOR]

The photoisomerization dynamics of N-methyl-2-pyridone (NMP) dissolved in CH3CN have been interrogated by time-resolved electronic and vibrational absorption spectroscopy. Irradiation at two different wavelengths (330 or 267 nm) prepares NMP(S1) molecules with very different levels of vibrational excitation, which rapidly relax to low vibrational levels of the S1 state. Internal conversion with an associated time constant of 110(4) ps, leading to reformation of NMP(S0) molecules, is identified as the dominant (>90%) decay pathway. Much of the remaining fraction undergoes a photoinitiated rearrangement to yield two ketenes (revealed by their characteristic antisymmetric C═C═O stretching modes at 2110 and 2120 cm(-1)), which are in equilibrium. The rate of ketene formation is found to be pump-wavelength dependent, consistent with ab initio electronic structure calculations which predict a barrier on the S1 potential energy surface en route to a prefulvenic conical intersection, by which isomerization is deduced to occur. Two kinetic models-differentiated by whether product branching occurs in the S1 or S0 electronic states-are presented and used with equal success in the analysis of the experimental data, highlighting the difficulties associated with deducing unambiguous mechanistic information from kinetic data alone.

The electronic structure of (η⁶-benzene)Cr(CO)₃ has been calculated using density functional theory and a molecular orbital interaction diagram constructed based on the Cr(CO)₃ and benzene fragments. The highest occupied molecular orbitals are mainly metal based. The nature of the lowest energy excited states were determined by time-dependent density functional theory, and the lowest energy excited state was found to have significant metal to carbonyl charge transfer character. The photochemistry of (η⁶-benzene)Cr(CO)₃ was investigated by time-resolved infrared spectroscopy with picosecond time resolution. The low energy excited state was detected following irradiation at 400 nm, and this exhibited ν(CO) bands at lower energy than the equivalent ν(CO) bands of (η⁶-benzene)Cr(CO)₃, consistent with metal to carbonyl charge transfer character, and is formed with excess vibrational energy, relaxing to the v = 0 vibrational state within 3 ps. The resulting "cold" excited state decays to form the CO-loss species (η⁶-benzene)Cr(CO)₂ in approximately 70% yield and to reform (η⁶-benzene)Cr(CO)₃ within 150 ps. The rates of relaxation from the vibrationally hot state to the cold excited state and its subsequent reaction to yield (η⁶-benzene)Cr(CO)₂ were measured over a range of temperatures from 274 to 320 K, and the activation parameters for both processes were obtained from Eyring plots. The vibrational relaxation exhibits a negative activation enthalpy ΔH(‡) (-10 (±4) kJ mol⁻¹) and a negative activation entropy ΔS(‡) (-50 (±16) J mol⁻¹ K⁻¹). A significant barrier (ΔH(‡) = +12 (±4) kJ mol⁻¹) was obtained for the formation of (η⁶-benzene)Cr(CO)₂ with a ΔS(‡) value close to zero. These data are used to propose a model for the CO-loss process to yield (η⁶-benzene)Cr(CO)₂ and to explain why low temperature irradiation of (η⁶-benzene)Cr(CO)₃ with light of wavelengths greater than 400 nm produced relatively minor amounts of (η⁶-benzene)Cr(CO)₂.