Natural bond orbital delocalization

Ab initio electron correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities were reported for oxadiazoles <1996JPC8752>. The various measures of delocalization in the five-membered heteroaromatic compounds were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by J. P. Bean from a principal component analysis of other measures of aromaticity <1998JOC2497>. 

A similar conclusion was drawn based on nucleus-independent chemical shift (NICS) calculations, that is, there is no significant aromatic character to a 1,2-dihydrodiazete. Natural bond orbital (NBO) analysis provides the means of investigating the extent of electron delocalization within a given structure and also indicates the bond order. In Lewis-type bonding orbitals electron density is depleted with increase in the occupancy in antibonding or non-Lewis orbitals. In the case of cis-14 or trans-14, there is less occupancy in non-Lewis orbitals and the bond order is more like localized bonds N-N (1.01), C-N (1.06) and C=C (1.76), which are similar to the nonaromatic substances. 

The synthesis, spectral characteristics, and the reactions of electrophiles with 1//- and (mesoionic) 2//-pyrrolotetrazoles were studied <01JCS(P1)720, 729>. The synthesis and crystal structure of a a-keto tetrazole-based dipeptide mimics has been reported <01TL5641>. Novel tetrazol-5-yl-isoquinolinium zwitterions have been synthesized and confirmed by X-ray diffraction analysis <01JHC199>. Application of natural bond orbital analysis to delocalization and aromaticity in C-substituted tetrazoles has been investigated <01JOC8737>. 

Natural bond orbital (NBO) analysis The NBO analysis transforms the canonical delocalized Hartree-Fock (HF) MOs and non-orthogonal atomic orbitals (AOs) into the sets of localized natural atomic orbitals (NAOs), hybrid orbitals (NHOs), and bond orbital (NBOs). Each of these localized basis sets is complete, orthonormal, and describes the wavefunction with the minimal amount of filled orbitals in the most rapidly convergent fashion. Filled NBOs describe the hypothetical, strictly localized Lewis structure. NPA charge assignments based on NBO analysis correlate well with empirical charge measures. ... 

Bean, Sadlej- Sosnowska < = ) application of natural bond orbital analysis to delocalization and aromaticity ... 

Bean, G. 1998. Application of natural bond orbital analysis and natural resonance theory to delocalization and aromaticity in five-memhered heteroaromatic compounds. J. Org. 

The natural bond orbital (NBO) method of Weinhold et al. [8, 57] provides a scheme appropriate to the analysis of Lewis acid/base interactions [8, 58] as it emphasizes the calculation of delocalization of electron density into unoccupied orbitals. 

Ab initio calculations (MP2/6-31G ) of the parent compound of 8 revealed that the most stable arrangement of the dimer adopts Dih symmetry (Fig. 5). Interestingly, the four Li ions and the two phosphorus centers constitute an octahedral skeleton with relatively short Li-Li and Li-P distances of 2.645 and 2.458 A, respectively. Charge analysis (22) undoubtedly supports the electrostatic bonding model for this system because of the high net charges of the natural atomic orbitals (NBO) at Li (+0.768) and P (-1.583), while NBO-Lewis resonance structures support stabilization through delocalization (Fig. 5). 

The natural localized orbitals introduced in Section 1.5 provide a useful alternative to the canonical delocalized MOs (CMOs) that are usually employed to analyze chemical bonding. The NAO and NBO basis sets may be regarded as intermediates in a succession of basis transformations that lead from starting AOs /, to the final canonical MOs , ... 

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