Rearrangements electronic barrier

Scheme 11 summarizes the various unimolecular transformations of the [ct, ct]-, [ct, 7t]-, and [%, 7i]-type dioxyl diradicals, which have been disclosed by our qualitative analysis of the orbital orientations in these electronic isomers. The answer to the original question as to the reasons for the persistence of dioxiranes rests on the appreciable electronic barrier towards rearrangement into the ester for the thermally produced [ct, ct] diradical instead, the [ct, ct] dioxyl species recloses to the dioxirane Consequently, these most highly strained cyclic peroxides can be prepared, isolated, handled, and utilized for synthetic purposes even under ambient conditions. On photochemical activation (n, CT excitation), however, the [ct, it] diradical results, which prefers P-scission into a carboxy-alkyl radical pair rather than rearranging into the ester product. For the latter process,... 

The conformational rearrangements of water, ammonia and ethane studied in this section exhibit a number of features common to all symmetry-allowed conformational processes dominated by the Hartree-Fock description. In all three cases, the Hartiee—Fock barrio is less than 3 kJ/mol from the true electronic barrier. For processes involving rebybiidization, diffuse functions improve the convergence significantly. Although the Hartree-Fock contribution is poorly reproduced at the double-zeta level, for larger aug-cc-pVXZ sets, the barriers are within 0.4 kJ/mol of the basis-set limit. Thus, even in modest basis sets, the Hartree-Fock model reproduces the electronic conformational barriers to within a few kilojoules per mole. 

If the probability for the system to jump to the upper PES is small, the reaction is an adiabatic one. The advantage of the adiabatic approach consists in the fact that its application does not lead to difficulties of fundamental character, e.g., to those related to the detailed balance principle. The activation factor is determined here by the energy (or, to be more precise, by the free energy) corresponding to the top of the potential barrier, and the transmission coefficient, k, characterizing the probability of the rearrangement of the electron state is determined by the minimum separation AE of the lower and upper PES. The quantity AE is the same for the forward and reverse transitions. 

The identification of unknown chemical compounds isolated in inert gas matrices is nowadays facilitated by comparison of the measured IR spectra with those computed at reliable levels of ab initio or density functional theory (DFT). Furthermore, the observed reactivity of matrix isolated species can in some instances be explained with the help of computed reaction energies and barriers for intramolecular rearrangements. Hence, electronic structure methods developed into a useful tool for the matrix isolation community. In this chapter, we will give an overview of the various theoretical methods and their limitations when employed in carbene chemistry. For a more detailed qualitative description of the merits and drawbacks of commonly used electronic structure methods, especially for open-shell systems, the reader is referred to the introductory guide of Bally and Borden.29... 

Steric factors are often responsible for skeletal isomerization in ion-radical states. The simple example in Scheme 6.31 illustrates the effect of steric congestion on activation energy of this kind of isomerization and depicts the transition of 2,2,3,3-tetramethylmethylenecyclopropane into 1,1,2,2-tetramethyltrimethylenemethane cation-radical. The rearrangement is brought about by one-electron oxidation of the substrate and represents an entirely barrierless process. Interestingly, methylenecy-clopropane (bearing no methyl groups) is protected from such a spontaneous collapse by a barrier of 7.4 k J mol l (Bally et al. 2005). 

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