Natural bond order

Natural bond order analysis (NBO) is the name of a whole set of analysis techniques. One of these is the natural population analysis (NPA) for obtaining... 

NBO (natural bond order) the name of a set of population analysis techniques NDO (neglect of differential overlap) the fundamental assumption behind many semiempirical methods... 

The mean bond order computed from the resulting resonance weights wa is called the natural bond order. We find a good linear correlation between V3 and the difference in natural bond orders, Ob-03 (Figure 11). The best-fit slope is 950 cnf /bond. 

The methyl group responds to the difference in the three-dimensional electron density distribution about the two nearest ring CC bonds, and the natural bond orders most simply quantifies the key difference in a unified manner across many molecules. At one extreme, 2-methylpropene has essentially localized single and double bonds (03-0b = 1) and a 1010 cm-1 barrier. At the other extreme, when the geometry of the ring has good local C2v symmetry, as in the S0 state of toluene, m-fluorotoluene, p-fluorotoluene, 3,5-difluorotoluene, and 2,6-difluorotoluene, 03 - Ob and the barrier is invariably very small, even for nominal threefold cases. We interpret this equality of bond orders as indicative of essentially equal contributions of the two dominant resonance structures at all a. 

Figure 11. Ab initio potential parameter V3 vs. the difference in natural bond order, Ob-Oa, between the two ring CC bonds adjacent to the point of methyl substitution.

From the optimal NRT resonance weights, one can also evaluate the natural bond order bAB between atoms A and B as the resonance-weighted average... 

F. Weinhold and C. R. Landis, in Valency and Bonding A Natural Bond Order Donor-Acceptor Perspective, Cambridge University Press, Cambridge, 2005. 

Table 7-4. Change of the natural bond order (NBO) charges and of the dipole moment of pNA in the ICT state. The label (Un)relax/(Un)relax means that we do (not) have allowe for solute geometry relaxation/solvent dielectric relaxation. Charges are in a.u. and dipole moments in Debye...

Several alternatives to the Mulliken population have been presented that attempt to provide more rigorous estimates of the charges on atoms in molecules or clusters although not all have been applied in chemisorption and catalysis. We quote the Natural Bond Order analysis, and the elegant topological analysis of the electron density or of the electron localization function, ELF, introduced by Becke and Edgecombe. " ELF analysis has... 

Beryllium-silicon bonds are a recent development. They are likewise prepared by the reaction of the metal halide with alkali metal silyls, but unlike the Mg analogues, both of the well-characterized Be examples are base-free. While [(te/T-Bu3Si)BeCl] is oligomeric, the molecular structure of (tert-Bu3Si)2Be features a perfectly linear Si-Be-Si unit. CpBe(SiMe3), prepared from CpBeCl and trimethylsilyllithium in pentane, exhibits an intriguing thermal chemistry. The identity of the primary thermolysis products, Scheme 14, implies the participation of silylene intermediates, a contention supported by DFT/NBO (density functional theory/natural bond order) analysis. ... 

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