33 research outputs found
Time-Resolved Infrared Studies of a Trimethylphosphine Model Derivative of [FeFe]-Hydrogenase
Model compounds that structurally
mimic the hydrogen-producing
active site of [FeFe]-hydrogenases have been studied to explore potential
ground-state electronic structure effects on reaction mechanisms compared
to hexacarbonyl derivatives. The time-dependent behavior of Fe<sub>2</sub>(ÎĽ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>4</sub>(PMe)<sub>2</sub> (<b>A</b>) in room temperature <i>n</i>-heptane and acetonitrile solutions was examined using
various ultrafast UV and visible excitation pulses with broadband
IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground-
and excited-state electronic and CO-stretching mode vibrational properties
of the possible isomers of <b>A</b> were also examined using
density functional theory (DFT) computations. In <i>n</i>-heptane, 355 and 532 nm excitation resulted in short-lived (135
± 74 ps) bands assigned to excited-state, CO-loss photoproducts.
These bands decay away, forming new long-lived absorptions that are
likely a mixture of isomers of both three-CO and four-CO ground-state
isomers. These new bands grow in with a time scale of 214 ± 119
ps and persist for more than 100 ns. In acetonitrile, similar results
are seen with a 532 nm pump, but the 355 nm data lack evidence of
the longer-lived bands. In either solvent, the 266 nm pump data seem
to also lack longer-lived bands, but the intensities are significantly
lower in this data, making firm conclusions more difficult. We suggest
that these wavelength-dependent excitation dynamics significantly
alter potential mechanisms and efficiencies for light-driven catalysis
The Computational Complexity of Orientation Search in Cryo-Electron Microscopy
In this paper we study the problem of determining threedimensional orientations for noisy projections of randomly oriented identical particles. The problem is of central importance in the tomographic reconstruction of the density map of macromolecular complexes from electron microscope images and it has been studied intensively for more than 30 years