The iron-sulfur-cluster-free hydrogenase Hmd (H(2)-forming methylenetetrahydromethanopterin dehydrogenase) from methanogenic archaea has recently been found to contain one iron associated tightly with an extractable cofactor of yet unknown structure. We report here that Hmd contains intrinsic CO bound to the Fe. Chemical analysis of Hmd revealed the presence of 2.4 +/- 0.2 mol of CO/mol of iron. Fourier transform infrared spectra of the native enzyme showed two bands of almost equal intensity at 2011 and 1944 cm(-)(1), interpreted as the stretching frequencies of two CO molecules bound to the same iron in an angle of 90 degrees . We also report on the effect of extrinsic (12)CO, (13)CO, (12)CN(-), and (13)CN(-) on the IR spectrum of Hmd.

Fast time-resolved infrared spectroscopy (TRIR) is used to probe the v(CO) spectrum of ClRe(CO)3(4,4'-bipyridyl)2 in the lowest MLCT (metal-to-ligand charge transfer) excited state. The three ground-state v(CO) bands all shift up in frequency, but not by the same amount. The energy-factored force field (EFFF) of the ground state is accurately solved by employing ,3CO-enrichment. Approximate methods yield information on the force field of the excited state; the principal EFFF force constants (¿co(ami) and fcco(equatoriai)) change by the same amount on excitation. This leads to a new interpretation of the resonance Raman spectrum of the compound. has been time-resolved resonance Raman (TR3) spectroscopy. Classic experiments have probed the structure of the excited states of a range of ruthenium complexes, and have shown, for instance, that in the lowest excited state of [Ru(bpy)3]2+ there is charge transfer from Ru to one of the bpy groups.7 Detailed isotopic studies8 have unravelled the details of the excited state structure. There is, however, a limitation of resonance Raman spectroscopy: only certain of the vibrational modes in the excited state will be resonance enhanced, partly because of the nature of the

Photochemical experiments with Fe(CO) 3 (η 4 -1,5-cyclooctadiene) (1) in a 13 CO matrix at 10 K, monitored by means of IR spectroscopy, indicate the generation of stereoselectively labeled Fe(CO) 2 ( 13 CO)-(η 4 -1,5-cyclooctadiene) (1-1a), with 13 CO in the apical position of the square-pyramidal coordination geometry. The spectral changes occurring upon annealing the matrix to 28 K reveal the thermally activated conversion into a mixture of the two possible stereoisotopomers, the species with 13 CO in a basal position (1-1b) becoming predominant. These findings characterize the carbonyl ligand site exchange in complex 1 as a chemical reaction involving a very small barrier. The variable-temperature IR spectra of 1 in hydrocarbon solution exhibit broadening and coalescence of bands in the ν(CO) region, which is interpreted in terms of a CO site exchange occurring in the picosecond time domain. The theoretical approach to the simulation of these spectral changes involves a transfer of transition dipole moment between the ν(CO) modes. On the basis of this approach, the rates of CO site exchange at the various temperatures could be evaluated by line shape simulation. They were found to range from 0.15 × 10 12 s -1 at 133 K to 1.54 × 10 12 s -1 at 293 K, yielding ∆H q ) 0.7 kcal‚mol -1 (Eyring plot) and E a ) 1.1 kcal‚mol -1 (Arrhenius plot) for the activation barrier of the underlying process.

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