Logo of Science Foundation Ireland  Logo of the Higher Education Authority, Ireland7 Capacities
Ireland's High-Performance Computing Centre | ICHEC
Home | News | Infrastructure | Outreach | Services | Research | Support | Education & Training | Consultancy | About Us | Login


Title:Excited State Dynamics and Activation Parameters of Remarkably Slow Photoinduced CO Loss from (η6-Benzene)Cr(CO)3 in n-Heptane Solution: A DFT and Picosecond-Time-Resolved Infrared Study
Authors:Ian P. Clark, Michael W. George, Gregory M. Greetham, Emma C. Harvey, Conor Long, Jennifer C. Manton, and Mary T. Pryce, 2010
Abstract: 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 νCO bands at lower energy than the equivalent νCO 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.
ICHEC Project:Estimating the potential energy surfaces around pi-coordinated arene ligands in (eta-Arene)Cr(CO)3 complexes
Publication:The Journal of Physical Chemistry A, Volume 114, Issue 43 pp 11425–11431
URL: http://pubs.acs.org/doi/abs/10.1021/jp106290j
Status: Published

return to publications list