Charge transfer, exothermicity

Intense sodium D-line emission results from excited sodium atoms produced in a highly exothermic step (175). Many gas-phase reactions of the alkafl metals are chemiluminescent, in part because their low ioni2ation potentials favor electron transfer to produce intermediate charge-transfer complexes such as [Ck Na 2] (1 )- There appears to be an analogy with solution-phase electron-transfer chemiluminescence in such reactions. 

The problems of distinguishing H+ produced from H2 by electron impact from the product of dissociative charge transfer reactions between He + and H2 can be studied by determining the kinetic energy distribution in the product H+ (6). The reaction He+ + H2 is exothermic by 6.5 e.v. if the products are atoms or atomic ions. If the reaction is studied with HD substituted for H2, then the maximum kinetic energy that can be deposited in the D + is approximately 2.16 e.v. On the other hand, D + can be produced by electron impact with 5.5 e.v. kinetic energy. If a retarding potential is applied at the repeller in the ion-source of a mass spectrometer, then it is possible to obtain curves related to the kinetic... 

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

Overall heat transfer coefficient Mass feeding rate Ammonia feed mass fraction Formaldehyde charge fraction Exothermic heat of reaction... 

The observed H+(NH3)n and H (NH3)n(PA) clusters are thought to be formed in a two-step reaction sequence taking place after ionization of the PA(NH3) cluster. The first step is a charge transfer (CT) reaction between the resonantly ionized PA+ and the NH3 molecules in the cluster. The second step is an intracluster ion-molecule reaction (ICIMR) of the charged ammonia cluster leading to the formation of an (n - 1) protonated cluster ion this has been previously established for NH3 clusters33 and is sufficiently exothermic for fragmentation of the cluster. 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

The general requirement for charge transfer is that the transfer should occur from a molecule of higher ionization potential to a molecule of lower ionization potential that is, that the reaction be exothermic. In general, it is also accompanied by some vibrational excitation.43 In the condensed phase the ionization potential may be modified somewhat by collective effects but the gas phase value can be used to a first approximation to determine the reaction energetics. 

The fluorescence of 3-t (113-117) and 3-7 (118) is quenched by secondary and tertiary amines. Rate constants for quenching of It by tertiary amines increase with decreasing amine ionization or oxidation potential (Table 11), as expected for the formation of a charge-transfer stabilized exciplex in which the amine serves as the electron donor. Electron transfer quenching in nonpolar solvent is calculated to be exothermic for amines with E 2 < 1 34 V. Thus, it is not surprising that secondary and tertiary amines quench 3-t with rate constants which approach or even exceed the rate of diffusion. The inefficient quenching of It and 3-7 by primary amines is consistent with their higher oxidation potentials. 

Tetrasubsubstituted derivatives 6 are easily obtained from N P Meg, and n-electron calculations show that antipodal disubstitution is to be expected for electropositive substituents, as found for R = Me3Si, Me (7). Monocarbanions are not obtained, principally because the intra-ring exothermic electrostatic interactions are reduced less by charge transfer from the second ylidic group than the first if the added base is strong enough to remove the first proton, it will also remove the second. 

In our work on copolymerizations involving charge transfer intermediates, it has been noted that when a mixture of styrene and maleic anhydride is heated to 80 °C. in the presence of benzoyl peroxide, an extremely exothermic reaction occurs, and in a sealed system the temperature rises from 80° to 250°C. within three minutes, and the conversion is quantitative. 

The latter process results in an overall second-order annihilation of the radicals as observed in the complete decays of the transient absorption to the spectral baseline on the microsecond time scale (see Figures 12 and 13). Since dimerization of the 17-electron radicals is orders of magnitude slower than the highly exothermic back electron transfer, no net photochemical transformations are observed even after prolonged charge-transfer irradiation. 

For the Cu-containing system Galactose oxidase, we conclude that the unpaired spins initially are located on the Cu atom and on the axial tyrosine, whereas the equatorial, cysteine-linked tyrosine obtains the unpaired spin upon proton transfer from substrate to axial Tyr. The computed barrier for the rate determining step (H-atom transfer from substrate to Tyr-S-moiety) is in excellent agreement with experimental data, whereas the charge transfer between Cu(II) and substrate ketyl anion is less exothermic than estimated experimentally. A question is however raised regarding the experimental estimates, based on the computed data and the overall net exothermicity of the substrate reaction. 

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