Reaction mechanism electron jump

As the translational energy of the impacting ion increases, the G-S cross section will rapidly fall off until at energies above 10 e.v., the electron jump model for the reaction will predominate. That mechanism does not seem to depend strongly on translational energy. 

The reactions of Ba, Ca, Sr with F2 are found [354] to have large reaction cross sections ( 115—160 A2), indicating an electron jump mechanism. Visible chemiluminescence, which represents a minor (<0.5%) reaction channel, has been observed from several electronic states of MF. For Ca + F2, the vibrational population for CaF( 2E+) is strongly inverted. Emission is also observed [354, 357] from the dihalides, BaF2, CaF2 and SrF2, but whether these arise from the radiative two body recombination process... 

The formation of the alkaline earth cyanide is the major pathway in the reaction M + BrCN. The other channel (giving MBr + CN) is observed for the reactions of Ba and Sr. The ratio of the cross section is o(BaCN)/ a(BaBr) 25-100 and a(SrCN)/o(SrBr) 250-1000 [363]. It was not possible to measure internal state distribution for the alkaline earth salts, but for the CN product of the minor channel, the vibrational distribution was found to be N(d = 1)/N(p = 0) <. 0.2 and Txot = 1250K for Ba + BrCN and TIot = 750 K for Sr + BrCN. The reaction dynamics appear to be consistent with an electron jump mechanism which would favour the breakup of the M+(BrCN) ion pair to give MCN + Br. 

The vibrational state distribution for the ground state CaO product from the reaction Ca + C02 shows a similar monotonic decrease with increasing vibrational quantum number to that from Ca + 02 and, again, only 10% of the reaction energy appears as CaO internal excitation [383]. No information exists about the amount of CO excitation. There is also evidence that low-lying excited states of CaO are produced in the reaction which is again assumed to proceed via an electron-jump mechanism. 

CH3C1 (90, 34 and 8 A2, respectively). These values suggest a similarity between these reactions and the corresponding ground state alkali atom reactions. The ionisation potential of Hg (3P2) is 4.974eV which is similar to those for the alkali atoms and so an electron jump mechanism is proposed for these chemiluminescent reactions of Hg (3P2 ) In contrast, the reaction of another spin-orbit state of metastable mercury with bromine, Hg (3P0) + Br2, has a much smaller chemiluminescent cross section [3 A2 compared with 150 A2 for Hg (3P2) + Br2] [406], which cannot be reconciled with an electron jump, suggesting the existence of a barrier to reaction of Hg (3P0) which is not present in the case of Hg(3P2). 

The chemiluminescent reaction A1 + 03 yields AlO (A, B) with preferential population of high vibrational levels in both states [415, 416]. In contrast, a Boltzmann vibrational state distribution is observed [415] for the A10 (B) product from A1 + N20, suggesting a different reaction mechanism in this case. An electron-jump mechanism operates for A1 + 03 giving the observed preferential population of AlO (A) [417] with a high degree of vibrational excitation. For A1 + N20, reaction takes place at shorter range, allowing production of the B state of AlO. ... 

The reactions of Sn with Cl2 and Br2 show forward scattering of the SnX product, with about 30—45% of the reaction energy appearing as translation of the products in the case of Sn + Cl2 [424]. The exact contribution to the reaction of the various spin-orbit states of tin, Sn(3P012), is unknown. Similarities between the results for Sn + Cl2 and those for Li + Cl2 [297] suggest an electron-jump mechanism, although the ionic—covalent curve crossing radius is quite small for Sn + Cl2 (— 2.9 A). 

Both M and MX are scattered strongly forward relative to the incident M2 beam, and it appears that these reactions occur by an electron jump mechanism similar to that for the M + X2 series of reactions (see Section II.C.l). 

Although at pH 8 the electron distribution favours the formation of flavin semiquinone and reduced iron-sulfur center, the magnetic moments of the two redox centers do not interact. At pH 10, however, 2-electron-reduced TMADH exhibits the EPR spectrum diagnostic of the spin-mteracting state. In a more detailed analysis using the pH-jump technique, the interconversion of three states of TMADH [state 1, dihy-droflavin-oxidised 4Fe-4S center (formed at pH 6) state 2, flavin semi-quinone-reduced 4Fe-4S center (formed at pH 8) state 3, spin interacting state (formed at pH 10)] were studied in both H2O and D2O (Rohlfs et al., 1995). The kinetics were found to be consistent with a reaction mechanism that involves sequential protonation/deprotonation and electron transfer events (Figure 6). Normal solvent kinetic isotope effects were observed and proton inventory analysis revealed that at least one proton is involved in the reaction between pH 6 and 8 and at least two protons are involved between pH 8 and 10. At least three protonation/... 

At low free energy of the electron transfer reaction, the mechanism changes. Instead of prompt electron jumps in each collision (see above), an encounter complex appears which delays the electron transition and controls the energetics of the process. This results in the formation of only one uniform and metastable donor radical cation. 

It was found that the reaction cross section for the dimers of CO2 was between four and eight times larger than that of the monomers.This effect suggests a different mechanism for the dimeric process versus the monomeric one, probably because of the positive electron affinity of the dimers. The reaction in this case occurs via an electron jump mechanism like reaction (4.4). It was also established that the product BaO in both dimeric reactions is much colder rotationally than in the monomeric case (Fig. 11). This phenomenon seems to be quite general, both in reactions of vdW molecules and in their dissociation. It results from the multitude of channels available for dividing the total angular momentum in the reaction complex. 

The kinetics of the electrochemical reactions at arrangements of chemically modified electrodes has been interpreted by a charge and mass transfer electrochemical mechanism. Charge transfer can be, in general, described by an electron jump and a molecular diffusion step. At electrodes modified by complexes, the rate of electron tunneling lV r)) can be described by the equation... 

Electron-jump in reactions of alkali atoms is another example of non-adiabatic transitions. Several aspects of this mechanism have been explored in connection with experimental measurements (Herschbach, 1966 Kinsey, 1971). The role of vibrational motion in the electron-jump model has been investigated (Kendall and Grice, 1972) for alkali-dihalide reactions. It was assumed that the transition is sudden, and that reaction probabilities are proportional to the overlap (Franck-Condon) integral between vibrational wavefunctions of the dihalide X2 and vibrational or continuum wave-functions of the negative ion X2. Related calculations have been carried out by Grice and Herschbach (1973). Further developments on the electron-jump mechanism may be expected from analytical extensions of the Landau-Zener-Stueckelberg formula (Nikitin and Ovchinnikova, 1972 Delos and Thorson, 1972), and from computational studies with realistic atom-atom potentials (Evans and Lane, 1973 Redmon and Micha, 1974). 

Besides the above potentials, several potentials have been proposed to describe reactions such as K + Br2 which proceed by means of an electron-jump mechanism.239-242 Of these surfaces, the one with the greatest physical content is that of Karplus and Godfrey240 which incorporates an ionic surface based on the Rittner model and a reasonable covalent surface (for large K-Br2 separations). The jump from the covalent to the ionic surface is accomplished in a continuous manner by means of an energy dependent switching function. None of these surfaces are completely satisfactory for the description of K + Br2 type reactions.169... 

The electron jump mechanism, (see 1), has long been invoked71,2 in a highly simplified manner to explain the stripping dynamics of alkali atom-halogen molecule reactions M + X2. Electron transfer occurs in the entrance valley of the covalent M + X2 potential surface near the intersection with the ionic M+ + X2 potential surface. The M - X2 internuclear distance R at the intersection Rc is roughly estimated... 

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