Three-centre bonds abundance

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

As a tribute to Pauling s contributions, I shall restate and summarize some of the implications for bonding theory that arise when the three-electron bond is incorporated as a mainstream component for VB descriptions of the electronic structures of electron-rich molecules. Attention will be focussed on increased-valence structures for molecular systems that involve four-electron three-centre and six-electron four-centre bonding units. However initially, consideration will be given to the one-electron bond, for which Pauling also provided some attention to both the theory and examples of systems that involve this type of bond in their VB structures. As indicated in ref. [8(a)], experimentally one-electron bonds and three-electron bonds are abundant and well-characterized for odd-electron systems. 

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