Bond energies from electron-transfer

SN P spontaneously releases N O both thermally and photochemically [61-65], but is quite stable in the dark and in aqueous in vitro physiological media [66]. This implies that absorption of heat and light energy induces electron transfer from the Fe2+ center to the N 0+ ligand, resulting in weakening of the Fe-N O bond and subsequent release of NO [65]. SNP also decomposes in an aqueous environment in the presence of biological reductants [65, 66] and some transition metal ions to produce nitric oxide. 

The +C(SH)3 cation and the radical dication derived from it have been the subject of high-level calculations.67 The ability of two adjacent sulfur atoms to stabilize cations, anions, and radicals makes these species usefiil for relating bond-breaking and electron-transfer energies.68,69 Electrophilicity parameters for the dithiocarbenium ions (27) have been worked out,70 and the stabilities of the cations (28), (29) and (30) have been estimated using PM3 calculations.71 Cation (31) can be captured by solvent or azide ion, or it may ring close to (32), which subsequently alkylates another (31) cation as shown.72... 

Proton transfer from the hydrogen peroxide molecule to His 74 is accompanied by O—H bond break and electron transfer to the oxygen atom. Then owing to the following O—H bond break and 0=0 bond formation this electron is transferred to the hydrogen atom with simultaneous hydride-ion transfer. The whole sequence of electron transfer in BRC is implemented without high-energy consumption. 

The thermodynanucs of gas-phase species adsorbing on a surface can be separated into the classes of physical adsorption (physisorption) and chemical adsorption (chemisorption). The difference between the two classes of adsorption is related to the heat of adsorption, A Hads. In physisorption, weak attractive forces (van der Waals) such as dipole-dipole and induced dipole interactions drive the adsorption of species from the gas phase. For chemisorbed systems, chemical bonds associated with electron transfer between adsorbate and substrate are formed upon adsorption. Each of these interactions can be described by a two-dimensional potential-energy diagram as in Figures 10 and 11. 

In the case of simnltaneons bond breaking and electron transfer, the contribntion of the eleetronic interaction to the expectation valne of the total energy ean be also obtained from an equation similar to Eq. (30) bnt now involving the density of states of the molecular orbitals p i. Now we have to consider an additional coordinate, namely the separation between the atoms of the molecule which changes during the reaction. An analytical expression... 

This tendency to add rather than to oxidize is probably a result of stabilization of the transition state for addition by contributions from bond making, whereas electron transfer requires pronounced bond and solvent reorganization with a correspondingly large free energy change to reach the transition state. 

Many radicals are produced by homolytic cleavage of bonds. The energy for this kind of bond breaking comes from thermal or photochemical energy or from electron-transfer reactions effected by either inorganic compounds or electrochemistry. These kinds of processes initiate reactions that proceed by a radical mechanism. Compounds that readily produce radicals are called initiators or free radical initiators. 

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