Ground-state reactions

Hydrogen Abstra.ction. These important reactions have been carried out using a variety of substrates. In general, the reactions involve the removal of hydrogen either direcdy as a hydrogen atom or indirectly by electron transfer followed by proton transfer. The products are derived from ground-state reactions. For example, chlorarul probably reacts with cycloheptatrienyl radicals to produce ether (50) (39). This chemistry contrasts with the ground-state reaction in which DDQ produces tropyhum quinolate in 91% yield (40). 

Figure 9.3. Cartoon of a classic double cone conical intersection, showing the excited state reaction path and two ground state reaction paths. See color insert.

An extensive review of the literature reveals that the only studies of vibrational effects in insertion chemistry have focused on reactions of 0(1D)175-177 and C(1D)177,178 with H2. Since there is no potential energy barrier to insertion in these systems, reaction proceeds readily even for unexcited reactants.179 Since the efficiency of vibrational excitation was 20% in both studies, due to the large cross-sections for ground state reactions, only small changes were observed in the experimental signal. From an analysis of the product distributions, it was concluded that while H2(v = 0) primarily reacted via an insertion mechanism, direct abstraction seemed to become important for = 1). For 0(1D), this is similar to behavior at elevated collision energies.180... 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

Fig. 16 (a) R (D + RX) and P (D,+ + R + X ) zero-order potential energy surfaces. Rc and Pc are the caged systems, (b) Projection of the steepest descent paths on the X-Y plane J, transition state of the photoinduced reaction j, transition state of the ground state reaction W, point where the photoinduced reaction path crosses the intersection between the R and P zero-order surfaces R ., caged reactant system, (c) Oscillatory descent from W to J on the upper first-order potential energy surface obtained from the R and P zero-order surfaces. 

The text assumes a certain familiarity with the derivation and use of correlation diagrams for ground state reactions and with fundamentals of their derivation for excited states, say on the level of Woodward and... 

However, often the minimum in Si or Ti which is reached at first is shallow and thermal energy will allow escape into other areas on the Si or Ti surface before return to So occurs (Fig. 3, path e). This is particularly true in the Ti state which has longer lifetimes due to the spin-forbidden nature of both its radiative and non-radiative modes of return to So-The rate of the escape should depend on temperature and is determined in the simplest case by the height and shape of the wall around the minimum, similarly as in ground state reactions (concepts such as activation energy and entropy should be applicable). In cases of intermediate complexity, non-unity transmission coefficients may become important, as discussed above. Finally, in unfavorable cases, vibronic coupling between two or more states has to be considered at all times and simple concepts familiar from ground-state chemistry are not applicable. Pres-... 

The use of correlation diagrams for the derivation of the location of barriers in Si and Ti is entirely analogous to their use for ground-state reactions. Fig. 8 shows that a barrier in Si (Ti) will result if, going along... 

E° for the quencher couple show regions of slope 1/2 and slope 1 as predicted by eq. 9 and 10. In fact, the theoretical equations appear to account for the observed variation of lnk q with Eot satisfactorily. Given the agreement with theory, it follows that if an excited state is thermally equilibrated, it can be viewed as a typical chemical reagent with its own characteristic properties, and that those properties can be accounted for by using equations and theoretical developments normally used for ground state reactions. 

Relative rates of some prototypical carbenes, obtained by Stem-Volmer methods, are listed in Table 2. Although many of these carbenes have triplet ground states, reaction with nucleophiles Y occurs prior to spin equilibration. Most often, ylide formation with solvent molecules was analysed in terms of Eq. 3. The pyridine-ylide served as the probe for 154. 

Most of the reactivity problems described in Sect. VI C have been treated recently by methods based on the electronic Overlap Population concept as introduced in the Electronic Population analysis of R. S. MuUiken. In fact the studies of the problems of Sect. VI C have established the general usefulness and apphcability of electronic overlap populations as reactivity measures for other excited- and ground-state reactions. The most important advantage of the electronic overlap population method of reactivity analysis is the possibihty provided for relating bond forming reactivity of two initially nonbonded atoms to the strength of their electronic interaction. 

As a final comment there is the question whether the cation observed with LFP is the same as the one produced in a ground-state reaction. With nanosecond LFP, the time interval from excitation to observation is likely sufficient to ensure that this is the case. Two pieces of evidence can be cited. (1) There is a good correspondence of ultraviolet-visible (UV-vis) spectra for the transient cations with ones obtained for solutions of ground-state cations under strongly acidic solutions. (2) Ratios of rate constant obtained directly by LFP agree with selectivities measured for the ground-state reactions. Diffusional separation of ion pairs is complete within 1-10 ns, so that a transient cation observed with nanosecond LFP is a free ion. At shorter times, that is, in picosecond LFP, ion pairs can be observed and their dynamics studied. ... 

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