High-energy transitions

No simple electrophilic substitution, for example nitrosation, nitration, sulfonation or halogenation of a C—H bond, has so far been recorded in the pteridine series. The strong 7T-electron deficiency of this nitrogen heterocycle opposes such electrophilic attack, which would require a high-energy transition state of low stability. 

A more complete analysis of interacting molecules would examine all of the involved MOs in a similar wty. A correlation diagram would be constructed to determine which reactant orbital is transformed into wfiich product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without intervention of high-energy transition states or intermediates can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment because it focuses attention not only on the reactants but also on the products. We will describe this method of analysis in more detail in Chapter 11. The qualitative approach that has been described here is a useful and simple wty to apply MO theory to reactivity problems, and we will employ it in subsequent chapters to problems in reactivity that are best described in MO terms. I... 

In summary, the NLE technique offers a concepmally new approach to observe NFS. Existing limits for time resolution could be overcome by a microfocused synchrotron beam (as planned for PETRA III) and by detectors with high spatial resolution and background from SAXS could be suppressed by employing high-energy transitions and crystalline sapphire as rotor material. 

Figure 1-3 shows a comparison of the calculated and experimental spectra. There are two high-energy transitions predicted by theory, around 190 and 240 nm. The first one is clearly visible in the experimental spectrum at the same wavelength, and roughly the same broadening. The second one was predicted to have intensity of 0.01 relative to the first peak, and is probably masked by the broad 200 nm band. Both are n - n transitions localized in the C=C bond in the ring and are likely to be present in most molecules found in the aromatic acid pathways, which hinders... 

The two bands which appear in the region of 1810—1880 cm-1 and 1510—1550 cm-1 can be regarded as characteristic of the IR spectra of the triafulvene system, as documented by the following examples of alkyl-substituted triafulvenes79,170 Mono-phenylsubstituted 4,4-diacyl triafulvenes5 are exceptions in that the high-energy transition is displaced to 1770-1780 cm-1. 

The formulas for the transition rates are summarized in Table 9.2 for the lowest five multipoles of each character. The transition rates always increase with a high power of the y-ray energy so that low-energy transitions, say below 100 keV, are much slower than high-energy transitions, say above 1 MeV. Table 9.2 also shows that in some cases, particularly in heavy nuclei, an / + 1 electric transition can compete favorably with an l magnetic transition. 

In these reactions the pentacoordinate species is a high-energy transition state, not an intermediate. 

R. Sadeghi, R.T. Skodje, High energy transition state resonances in the H+H2 reaction, J. Chem. Phys. 98 (1993) 9208 R. Sadeghi and R.T. Skodje, Spectral quantization of high energy transition state resonances in the H+H2 reaction, J. Chem. Phys. 99, 5126 (1993). 

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