Correlation bond parallel

The general agreement between experimental values obtained for p at the BCP of covalent bonds and those derived theoretically for the corresjjonding isolated molecules supports the widely cited error estimate of 0.05 eA . The fact that V pbcp is a far less robust index than Pbcp> as revealed by discrepancies between theory and experiment, has been attributed mainly to the different BCP locations and bond-parallel curvatures obtained by the two methods. Basis set, correlation, and bulk effects are commonly invoked as sources of the discrepancies [74, 78]. 

An alternative stream came from the valence bond (VB) theory. Ovchinnikov judged the ground-state spin for the alternant diradicals by half the difference between the number of starred and unstarred ir-sites, i.e., S = (n -n)l2 [72]. It is the simplest way to predict the spin preference of ground states just on the basis of the molecular graph theory, and in many cases its results are parallel to those obtained from the NBMO analysis and from the sophisticated MO or DFT (density functional theory) calculations. However, this simple VB rule cannot be applied to the non-alternate diradicals. The exact solutions of semi-empirical VB, Hubbard, and PPP models shed light on the nature of spin correlation [37, 73-77]. 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

Although it is important to bear all these considerations in mind when analysing results for a given system, it is not usually difficult to identify reactions which involve primarily bond formation or cleavage, and those where both processes occur in parallel. We will retain this familiar classification for the discussion of structure-reactivity correlations, and discuss first bond-making processes. 

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