Reactions with Formation of Biradicals

The axial bonds in VIII are longer than the equatorial ones, as in the analogous 03 structure of the transition state structure for the Sn2 reaction (see Sect. 5.1.1.2), though much less so than in the latter. The structures IX and X whose formation could retain the configuration of the central atom have much higher energy than VIII, as is apparent from the data of Table 8.2. 

The data of this table and the results of some other ab initio calculations [6,26] show that the axial abstraction of Eq. (8.3) is the most preferable channel for the reaction of methane with hydrogen isotopes. The substitution reaction of Eq. (8.5) must proceed exclusively from the vibrationally excited states, as has been verified experimentally. 

The highest bonding MO of cyclopropane is shown in Fig. 8.2a. As the cycle is being opened, the two-fold occupancy of this MO is disturbed and it splits 

Clearly, the rupture of the bond in XI and the transition to the biradical form XIa proceeds continuously, rather than in a jumpwise fashion. The sharpest changes during this continuous process occur in the region of instability of the HF solutions. It is for this reason that the analysis of stability of the HF values is a convenient technique for determining the part of the configurational space in which the PES contains biradical structures. The appearance of triplet instability may be regarded as an indication of the transformation of structures with closed electron shell into those with two separated radical centers or biradicals. 

Quantitatively the degree y of the biradical character is given (in %) by the equation suggested in Ref. [4]. 

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