The Si-H bond activation reactions by group VIIB, d 6 organometallic compounds η 5 -CpM(CO) 3 (M ) Mn, Re; Cp ) C 5 H 5 ) were studied in neat triethylsilane under ambient conditions. Utilizing femtosecond and nanosecond pump-probe spectroscopic methods, the spectral evolution of the CO stretching bands was monitored from 300 fs to tens of microseconds following UV photolysis. The reactive intermediates observed on the ultrafast time scale were also studied using ab initio quantum chemical modeling. It was found that photolysis of the manganese tricarbonyl resulted in dicarbonyls in their singlet or triplet electronic states, whereas photolysis of the rhenium complex led only to the singlet dicarbonyl. The branching ratio of the two manganese intermediates was measured and was related to different electronic excited states. For both the Mn and Re complexes, the reactions were found to be divided into two pathways of distinct time scales by the initial solvation of the dicarbonyls through the Si-H bond or an ethyl group of the solvent molecule. The time scale for the Si-H bond-breaking process was, for the first time, experimentally derived to be 4.4 ps, compared to 230 ns for breaking an alkane C-H bond. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the reaction mechanisms for these reactions.

A time-resolved step-scan FTIR spectrometer with a time resolution of 20 ns was developed and used to investigate the KL to L transition in the photocycle of bacteriorhodopsin in the time range from -60 to 940 ns. Broadband FTIR absorbance difference spectra with a spectral range of 850-2050 cm -1 and a spectral resolution of 4 cm -1 have been obtained. Our data show that there are two sets of BR photoproduct difference bands exhibiting different kinetics. The intensity changes of bands attributed to structural changes of the carboxyl groups Asp-96 and Asp-115 as well as of bands assigned to CdC and CsC stretching modes of the chromophore show single exponential behavior with a time constant of about 400 ns, implying that these two processes are coupled. The time-dependent intensity changes of bands attributed to structural changes of the chromophore region near the Schiff base exhibit slower kinetics with a time constant of about 2 µs. We interpret our data in terms of a process during the KL to L transition where structural changes of the -ionone ring end of the chromophore and of Asp-115 occur faster than changes at the Schiff base region of the chromophore. Under our physiological sample conditions, a perturbation of Asp-115 occurs in the first 20 ns in contrast to results from hydrated films where this process is blocked or occurs more slowly. This fast protein response indicates that there is direct coupling between the carboxylic acid residues and the chromophore. Comparison of our data with low-temperature and microsecond time-resolved infrared spectra of the L intermediate in hydrated films of BR indicates that a different L structure is produced when the water activity is low.