Single step bond dissociation

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

With19 D(Ni-Ni) = 55 kcal.mole-1 and assuming the average Ni-CO bond dissociation energy in Ni(CO)2 is the same (35.2 kcal.mole-1)20 as in Ni(CO)4, reaction (8) would be endothermic by approximately 85 kcal.mole-1. It is therefore unlikely that this reaction occurs by a single homogeneous step. It might proceed by reaction (9)... 

There are other examples of such fast bond dissociations in the liquid phase, not necessarily hydrogen bonds. Thus the first step in the photo-chromic isomerization of spiropyrans (the ps events are shown in section 8.1.) is complete within 100 fs, that is within a single vibration of the bond. 

Decomposition reactions, A -+ B+C, provide two extreme cases. The uni-molecular decomposition that involves rupture of a single bond, A-B - A+B, usually has an activation energy almost equal to the bond dissociation energy. Excitation is absent, although either A or B may be unstable relative to other products and may isomerize in elementary steps with little or no activation energy. Decompositions which involve considerable bond rearrangement (bond shortening is the simplest example) may produce excited molecules. 

So far, two different mechanisms of single strand break formation based on adiabatically stable anions have been proposed. The first mechanism, suggested by the Leszczynski group, assumes the formation of stable anions of 3 - and 5 -phosphates of thymidine and cytidine in which the cleavage of the C-O bond take place via the SN2-type process. The second reaction sequence, proposed by us, starts from the electron induced BFPT process followed by the second electron attachment to the pyrimidine nucleobase radical, intramolecular proton transfer, and the C-O bond dissociation. In both mechanisms the bottleneck step is associated with very low kinetic barrier which enables the SSB formation to be completed in a time period much shorter than that required for the assay of damage. 

In this calculation, the potential energy surface (PES) for Cs -Os bond dissociation of 5 -dTMPH in vertical and adiabatic states was calculated using both compact (6-31G ) and diffuse (6-31++G ) basis sets. At each step on the PES, the singly occupied molecular orbital (SOMO)... 

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