Frequencies exchange reaction

The exchange reactions (6.20) and (6.21) have been among the basic objects of chemical-reaction theory for half a century. Clearly further investigation is needed, incorporating real crystal dynamics. It is worth noting that the adiabatic model, upon which the cited results are based, can prove to be insufficient because of the low frequency of the promoting vibrations. 

An exchange reaction between bis(trifluorometbyl)mercury and tetrame-thylleud gives trimethyl(mfluoromethyl)plumbane [23] (equation 17) This plum-bane can also be prepared via the reaction of tetramethyllead with tnfluoromethyl radicals produced in a radio-frequency discharge of C2F [24]... 

It can be difficult to estimate theoretically the bond lengths and vibrational frequencies for the activated complex and the energy barrier for its formation. It is of interest to assess how the uncertainty in these parameters affect the rate constant predicted from transition state theory (TST). For the exchange reaction... 

Figures 6 and 7 also point out the two principal techniques we have employed in monitoring these oxygen exchange reactions, namely, frequency shifts in the...

An additional concern arises in regard to any differences which may exist between the classical theory and the quantum-mechanical approach in the calculation of the Franck-Condon factors for symmetrical exchange reactions. In fact, the difference is not very large. For a frequency of 400 cm for metal-ligand totally symmetric vibrational modes, one can expect... 

Fig. 12.4 The (squared) frequency of the motion along the reaction coordinate q for a symmetric atom exchange reaction. L is the range of the chemical barrier region, and the frequency is shown in units of the mean frequency of the unperturbed solvent. The range of frequencies in the solvent is indicated as a solid bar. The negative values for the solvent correspond to unstable solvent modes.

The transmission coefficient k is approximately 1 for reactions in which there is substantial (>4kJ) electronic coupling between the reactants (adiabatic reactions). Ar is calculable if necessary but is usually approximated by Z, the effective collision frequency in solution, and assumed to be 10" M s. Thus it is possible in principle to calculate the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters, including molecular size, bond length, vibration frequency and solvent parameters (see inset). This represents a remarkable step. Not surprisingly, exchange reactions of the type... 

The exponential decay of the strongest carbonyl absorption is presented in Figure 6.16A. The natural logarithm of the relative absorption of the carbonyl frequency at 2020 cm versus time is presented in Figure 6.16B, indicative of first-order kinetics in the rhodium concentration and the pressure of FI2 and the H/D exchange rate was 1140 h Although the H/D exchange reaction was very... 

CN-stretching frequencies, 12 387 kinetic data for, 12 413 ligand substitution reactions, 12 406 Pentacyanocobalt(II) exchange reactions of, 10 201, 202 kinetic data for, 10 202... 

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

This approach yields a series of intrinsic energy barriers for the SN2 selfexchange reactions which vary widely depending on the vibrational frequencies assumed for the transition states. Nevertheless, the numbers derived for a given model can be used to establish some interesting conclusions. For example, chloride- and bromide-exchange reactions display an E 0... 

Fig. 3. Relation between frequency factors and heats of activation for the hydrogen-deuterium exchange reaction on zinc oxide catalysts (ref. 28a).

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