Branching, short-lived electron

Carbenes are defined as molecular species with formally divalent and two-coordinate carbon atoms bearing various substituents X and Y and a lone pair of electrons. While the simple representatives are of low stability (such as CH2) and may only appear as short-lived reaction intermediates or in adducts with electron donors, some cyclic systems can be readily isolated. This is particularly true for many of the A-heterocyclic carbenes (NHCs), which are now widely applied as ligands to metals ( Wanzlick-Arduengo carbenes ). Such carbenes based on imidazol and benzimidazol have become the working horses in this branch of organogold chemistry (Scheme 54). 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

As traditional representations of the world involve three branches picturing the male, female and neutral features of nature, particles are classified as Fermion half-integer spin matter (electrons, protons, neutrons, muons, neutrinos,. ..) or antimatter (positrons, antiprotons,. ..) and Boson full-integer spin particles (photons, mesons, gluons,. ..). Some of these are perfectly stable (protons, electrons, neutrinos, photons,. ..) or relatively long-lived (neutrons 16.9 min, muons 2.2 x lO sec) while others are very short-lived (mesons, hyperons, x > 10 sec) or unstable (resonances, x < 10" sec), and some have not been isolated (quarks,. ..). Nuclei of common matter are made of protons and neutrons, and these latter disintegrate to a proton, an electron and a neutrino when they are isolated. Nowadays one considers a proton as being compounded... 

Electron transfer within the bacterial photosynthetic reaction center (RC) has been shown by a variety of methods to only occur along one branch of the chromophores (the so-called L or A branch) (Kirmaier and Holten, 1987 Parson, 1987). Recent room temperature kinetic data support a model in which the electronically excited primary donor (a dimer of bacteriochlorophylls, P) decays with a time constant of 3.5 ps (in Rhodobacter (Rb,) sphaeroides) to populate a very short-lived intermediate containing P+ and a reduced bacteriochlorophyll (B") molecule (Holzapfel et al, 1989,1990). Subsequently the electron is transferred to a bacteriopheophytin (H) with a time constant of 0.9 ps. The existence of the state P+H is generally accepted but there is presently debate over the existence of the... 

Purple bacterial reaction centers (RCs) contain the special pair P860 (primary electron donor) and two cofactor pigment/protein branches denoted L and M which are formed by one accessory bacteriochlorophyll (B) and one bacteriopheophytin (H) each . The L-branch contains also a quinone which acts as the final electron acceptor in isolated RCs". The sequence and kinetics of the primary electron transfer processes in isolated purple bacterial reaction centers (RCs) are still discussed controversially. The model which seems to have the strongest experimental support at present is the so-called sequential two-step model. It involves a reduced accessory bacteriochlorophyll (Bj] ) in the L-branch as a short-lived intermediate which is formed in 3.5 ps and is reoxidized by... 

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