Lewis structure natural

For hydrogen fluoride, which is well described by a single Lewis structure (cf. Example 1.6), the localized natural Lewis-structure model wavefunction gives... 

In this case the energy of the natural Lewis-structure model... 

Figure 2.4 Components of the Li-F potential-energy curve E R) = E (R) + E(NL>(R), showing the localized natural-Lewis-structure model energy E(L> (circles, left-hand scale) and delocalized non-Lewis correction ,(NL) (squares, right-hand scale). The classical electrostatic estimate E (dotted line) is shown for comparison.

Table 3.1 compares key bonding and ionicity descriptors for the NBO Lewis-structure representations of the five AF diatomic fluorides in this series. From this table it can be seen that the single-term natural Lewis-structure description is practically exact for all species (with %p 99.9% for A = F, Cl, Br, H, and only a... 

As described in Sections 1.4 and 1.5, a general fia— 2b donor-acceptor interaction between Lewis ( 2a) and non-Lewis (fib ) NBOs leads to perturbative corrections to the zeroth-order natural Lewis-structure wavefunction... 

Let us first inquire whether basic criteria for the validity of low-order perturbation theory are actually satisfied in the present case. As described in Section 1.4, the perturbative starting point is an idealized natural Lewis-structure wavefunction (t//,l )) of doubly occupied NBOs. The accuracy of this Lewis-type starting point may be assessed in terms of the percentage accuracy of the variational energy (E) or density (p(l ). as shown for each molecule in Table 3.20. 

It must be emphasized that the formulation of three-center T-bonds provides a qualitative (not merely incremental) improvement in the accuracy of the natural Lewis-structure description of diborane. Because a three-center orbital is intrinsically more mathematically flexible than a two-center orbital, the description of any molecule is seemingly improved by employing three-center in place of two-center NBOs. However, for most non-boron molecules this improvement would be quite negligible (e.g., less than 0.1% for ethane, whose two-center Lewis-structure... 

Figure 4.23 Variations of metal-metal bond length with nominal formal bond order (the number of bonds in the natural Lewis structure) in duodectet-rule-conforming dinuclear hydrides H MMH (see Table 4.15).

Table 4.36. The NBO descriptors o/ M2H2 species (see Fig. 4.57) namely the accuracy of the natural Lewis structure (%/+). metal charge (Qu), metal hybrid (hM), percentage polarization toward metal (%m), and occupancies (Occ.) of two-center bonds and antibonds...

Since Lewis-type NBOs typically have (slightly) less than double occupancy, they are not yet true molecular orbitals in the Hartree-Fock sense. Equivalently, one may say that a determinant of doubly-occupied NBOs, the natural Lewis structure wavefunction "Pl = det (cTAB) (crcD) I must have higher energy than the Hartree-Fock MO wavefunction... 

What would the world be like if the Lewis structure picture were exact, and resonance effects were absent Such a world without resonance is the essential defining characteristic of the Natural Lewis Structure wavefunction a well-defined starting point for the systematic NBO-based exploration of chemical behavior. 

Figure 8.6 Torsional variations of non-Lewis error (pnl) for single-bonded (dashed line) versus double-bonded (solid line) natural Lewis structures of CH2CH2 (circles) and BH2NH2 (crosses), showing the NLS crossing point for each species (marked by a box) with crossing angles 66° and 75° for BH2NH2 and CH2CH2, respectively.

Table 11.2 Calculated bond length (Re), NRT bond order ( no) atomic charge at N (2n) and a/p natural lewis structure (NLS) representations (and percentage accuracy) for low-lying states of nitric oxide (CIS/6-311 +G level) (cf. Fig. 11.1a for state labels).

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