Nonclassical energy forms

What do we mean by classical and nonclassical energy forms In classical processes, energy is added to the system by heat transfer by electromagnetic radiation in the ultraviolet (UV), visible, or infrared (IR) range or in the form of electrical energy. On the other hand, microwave radiation, ultrasound, and the direct application of mechanical energy are among the nonclassical forms. 

Decades of extensive research followed, fueling controversy on the subject. In 1983, Olah, Saunders, and Schleyer were able to identify the intermediate as indeed the methylene-bridged nonclassical carbonium form of the norbomyl cation. This was demonstrated through extensive spectroscopic analysis, including H- and C-NMR, Raman, ESCA, and further physical and kinetic studies. Molecular mechanics and thermodynamic cycles exhibited a stabilization energy of 6 1 kcal/mol for the a bridging feature of the nonclassical ion. 

In [a] the reaction takes place without any intermediate and in [b] a shortlived intermediate is involved. In some systems the intermediate becomes more stable than the rearranging ion, e.g. a nonclassical ion is formed. Such a situation yields the profile [c]. In case [d] a stable symmetrical ion is formed directly from the substrate, e.g. by anchimeric assistance. There is also the possibility of having a case where the first vibrational level is of higher energy than the barrier between the wells. Such a case has not yet been demonstrated. 

In spite of the elimination of formic acid in a couple of steps changing the oxidation number of the rhodium metal center from -nl to -i-3 and vice versa, the reaction could take place by an alternative mechanistic pathway via cr-meta-thesis between the coordinated formate unit and the nonclassical bound hydrogen molecule [48, 49]. Initial rate measurements of a complex of the type 13 show that kinetic data are consistent with a mechanism involving a rate-limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species, first with CO2 and then with H2. The calculations provide a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. From these results s-bond metathesis seems to be an alternative low-energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen [48 a]. 

How far carbocation chemistry has evolved from the old solvolysis days is demonstrated by two recent stmcture determinations the IR spectrum of the nonclassical CHs cation has been measured by solvating this unusual species with molecular hydrogen in the gas phase. This slows down the ultrafast fluxional process which so far prevented the recording of vibrational spectra. The cluster ions CH5" (H2)n (n = 1, 2, 3), after mass selection by an ion trap, were then subjected to IR laser spectroscopy/quadrupol mass spectrometry which ultimately yielded the IR absorption. [40] And the benzene cation, formed by removal of one electron from the parent hydrocarbon was shown to possess Deh symmetry by rotation resolved ZEKE-photoelec-tron spectroscopy (Zero Tinetic Energy-PES). [41]... 

As discussed in Section 7.4, conformational control in deamination of open-chain amines is difficult to evaluate because the activation energies of conformational changes are often smaller than those of the steps in deamination reactions. Alicyclic amines are more suitable for such mechanistic investigations. In addition, the con-formers of such amines can be locked if they contain bulky substituents (tert-hvXyX) or if the amines are based on bi- and polycyclic hydrocarbons (decalinamines, cholestaneamines, norbornylamines, etc.). We shall therefore concentrate first on the deamination of the epimeric 4-( er butyl)cyclohexylamines. Then, we will discuss the structural problems of cyclic carbocations formed in deamination of norbor-nylamine, cyclopropylmethylamine, and cyclobutylamine, i. e., compounds that are at the center of interest in the debate on classical versus nonclassical carbocations. 

The exo stereoisomer reacts hundreds of times faster than the endo. This is explained by the exo forming a stabilized nonclassical carbocation in the rate-determining step. A nonclassical ion (note the 5-coordinate carbon) is a potential-energy minimum (lower left) rather than a transition state between two rapidly interconverting classical ions (lower right). 

FIGURE 3.15. Union of an odd AH (b) with methyl to form a nonclassical, even AH (a) involves no first-order change in n energy. The nonclassical AH is therefore less stable than classical isomers (c, d). 

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