Resonance mechanisms

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

The triplet-triplet energy transfer, which is doubly forbidden by a resonance mechanism, is allowed by an exchange mechanism ... 

All of the examples of singlet energy transfer we have considered take place via the long-range resonance mechanism. When the oscillator strength of the acceptor is very small (for example, n-> n transitions) so that the Fdrster critical distance R0 approaches or is less than the collision diameter of the donor-acceptor pair, then all evidence indicates that the transfer takes place at a diffusion-controlled rate. Consequently, the transfer mechanism should involve exchange as well as Coulomb interaction. Good examples of this type of transfer have been provided by Dubois and co-workers.(47-49)... 

Figure 7. The Fermi resonance mechanism within the adiabatic and exchange approximations. F, fast mode S, slow mode B, bending mode.

More convincing proof for a particle-enhanced energy transfer mechanism comes from a study of the concentration dependence of the transfer. Bulk Forster transfer leads to a linear dependence on acceptor concentration with constant donor-to-acceptor ratio. The resonance mechanism would be expected to saturate at (relatively) high concentrations and fall off linearly at very low concentrations. 

The resonance mechanism shown in Fig. 8.15 accounts for the greater stability of dihydropyridine pro-prodrugs vs. their pyridinium metabolites in base-catalyzed and enzymatic reactions of hydrolysis, but it also suggests a decreased stability of the former in acid-catalyzed hydrolysis. Indeed, the carbonyl O-atom is deduced from Fig. 8.15 to be more nucleophilic in dihydropyridine (A) than in pyridinium derivatives (B). The stability of dihydropyridine pro-prodrugs under the acidic conditions of the stomach and small intestine should, therefore, be examined further. 

We will be concerned with rate differences of different conjugated systems and with how electron transfer takes place. Specifically, we will try to distinguish the resonance mechanisms from chemical mechanisms in which electron transfer takes place to the ligand rather than through it. 

Finally—and I think the subject is becoming ripe for development on this level —let us turn to the question of the mechanisms by which electron transfer takes place. One important distinction is whether the electron transfers by a resonance mechanism or by a chemical one. Different observations can be made depending on this difference in mechanism. Perhaps one of the most significant is based on the fact that if there is resonance transfer, preparation for receiving the electron will be made at the Co(III) center, but if the electron transfers to the ligand, this kind of preparation at the metal ion center is not required. An experimental approach to distinguish between the two cases may be this when Co (111) receives the electron directly, there may be a strong discrimination between the isotopes of... 

Aside from the A effect which is an interesting feature for designing resonant mechanical devices, MSMM show a GMR (AR) effect (Quandt and Ludwig 1999,2000). Thus, as already introduced in section 3.3, the bending of a magnetostrictive bimorph... 

This is because the low efficiency of such a transfer is compensated by long lifetime. If other modes of triplet deactivation are less competitive, that is, similarly forbidden, intermolccular transfer may occur with reasonable rates. The slow rate of energy transfer is not incompatible with a large value of R0 for transfer by a resonance mechanism (Table 6.8). 

Figure 2. The basic bond resonance mechanism between horizontal and vertical bonds in an elementary plaquette of four sites.

The dynamics of a reaction that proceeds directly over the transition state is expected to be qualitatively different from that of a resonance-mediated reaction. In particular, one expects that the branching ratios into the product rovibrational states will be very different between the direct and the resonant mechanisms. For example, if a given Feshbach resonance corresponds to trapping on the v = 1 vibrationally adiabatic curve, then one might expect that the population of the v = l vibrational state of the product molecule may be greatly enhanced by the resonant mechanism. Similarly, the rotational product distribution resulting from the fragmentation of a resonance molecule may show a quite distinct pattern from that of a direct reaction. Indeed, Liu and coworkers [94], and Nesbitt and coworkers [95] have noted distinct rotational patterns in the F+HD resonant reaction. 

The most clear demonstration of the predictions of a resonant mechanism of vibrational excitation was provided by Hanh et al. [9]. The authors find a decrease in the conductance associated with the onset of activation of an 0-0 stretch mode, for O2 on Ag(llO). Such reversed behavior follows predictions made by Persson et al. [10] for those systems with narrow molecular resonances around the Fermi level (Ep). The theoretical fundaments of these and related issues will be discussed later in this chapter. 

Of course the above treatment is not definitive, but it does suggest that the antibonding a 2s MO mechanism can compare favourably with the pivotal resonance mechanism as the primary VB formulation for electron conduction in metallic lithium. 

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