Branched-chain excitation

Generalizations of the three-state STIRAP to multistate chains [189, 205-209] to adiabatic momentum transfer [189, 206, 210, 211] and fo branched-chain excitation [212-216] have been made. The extensions of fhe fhree-state... 

In Table III are given calculated values of the resonance energy for straight conjugated chains containing two, three, and four double bonds and a branched chain with three double bonds (the list could be easily extended). In each case the secular equation was solved as a quadratic, all first-excited structures being given the same... 

A contribution to the synthesis of branched-chain aldonolactones. Helvetica Chimica Acta, 71, 981-987 (c) Mattay, J., Gersdorf, J., and Buchkremer, K. (1987) Photoreaction of biacetyls with electron-rich olefins. An excited mechanism. Chemische Berichte, 120, 307-318. 

In addition to showing branched-chain ignition, with (first and second) p-Ta limits similar to those of the H2 + O2 system, the CO -I- O2 reaction supports a response known as glow. This is a spontaneous chemiluminscent state, with a weak, pale-blue emission arising from electronically-excited... 

The system CO-O2 displays two explosion limits. The proposed mechanism involves chain branching by excited CO2 formed via the reaction CO + O CO2. The mechanism can be written as... 

PHa + Cla PHaCl+Cl. This reaction was proposed to be a chain-propagating step of the branched-chain reaction of PH3 with Clg. Branching may occur by formation of excited PHgCl [8]. 

Figure 5. 1. Schematic illustration of a multicentered non-branching chain reaction. R, is an active reaction center (atom, free radical, excited particle, etc.). Arrows between active reaction centers, chain carriers, R, and R , denote the reactions resulting in mutual transformations ky is the effective rate constant for the formation of the R reaction center with participation of the R reaction center A, is the...

A pressure variation can lead to a change in the relative importance of the different channels in multichannel reactions, for example, in reactions of biradicals with unsaturated hydrocarbons [115], to an increase in the yield of radical recombination products, and to a deactivation of excited molecules. Lastly, the pressure is a critical factor for branched-chain reactions. Some of the authors [79] also discussed the possibility of the appearance of fundamentally new reaction channels associated with the manifestation of the cage effect when the resulting short-lived molecular complex has time to interact with other agents before decaying. 

Using the first-principles molecular-dynamics simulation, Munejiri, Shimojo and Hoshino studied the structure of liquid sulfur at 400 K, below the polymerization temperature [79]. They found that some of the Ss ring molecules homolytically open up on excitation of one electron from the HOMO to the LUMO. The chain-like diradicals S " thus generated partly recombine intramolecularly with formation of a branched Sy=S species rather than cyclo-Ss- Furthermore, the authors showed that photo-induced polymerization occurs in liquid sulfur when the Ss chains or Sy=S species are close to each other at their end. The mechanism of polymerization of sulfur remains a challenging problem for further theoretical work. 

The dynamics of polysilane dendrimer excited states were also studied by Watanabe as mentioned above in a comparison with polysilynes,360,364 and suggested that a configuration coordinate model is applicable to the photophysics of branched silicon chains. Calculations showed a distorted geometry of the excited state localized at a branching point. 

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

Bond-order changes upon photoexcitation obtained from Sandorfy SCF-CI calculations have been employed to predict the ease of photoextrusion of a silylene from polysilane chains or rings236. Smaller bond-order changes with longer polysilane chains suggested that they are less efficient silylene sources. Similar conclusions were reached for branched and cyclic polysilanes. This approach focuses on a to a excitation without participation by Si 4s orbitals in descriptions of excited states237. 

In recent years polysilane dendrimers have been synthesized in several laboratories. Two examples are shown in schemes 5.7128 and 5.8.129 The dendrimers have electronic absorption spectra similar to those of the longest silicon chains present, but with intensity about ten times as great. Like the network polysilanes, these dendrimers display weak visible emission spectra, perhaps resulting from localized excitations at the branching points. The field of polysilane dendrimers in the subject of a recent authoritative review.130... 

Semenoff (Zoc. cit.) proposed excited CO2, formed from O + CO the chain carrier with the branching reaction being... 

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