Natural Bond Analysis

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... 

Natural population analysis is carried out in terms of localized electron-pair bonding units. Here are the charges computed by natural population analysis (the essential output is extracted) ... 

Natural Atomic Orbital and Natural Bond Orbital Analysis... 

The concept of natural orbitals may be used for distributing electrons into atomic and molecular orbitals, and thereby for deriving atomic charges and molecular bonds. The idea in the Natural Atomic Orbital (NAO) and Natural Bond Orbital (NBO) analysis developed by F. Weinholt and co-workers " is to use the one-electron density matrix for defining the shape of the atomic orbitals in the molecular environment, and derive molecular bonds from electron density between atoms. 

NATURAL ATOMIC ORBITAL AND NATURAL BOND ORBITAL ANALYSIS... 

Reed, A. E., Weinhold, F., Curtiss, L. A., Pochatko, D. J., 1986, Natural Bond Orbital Analysis of Molecular... 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

Paddon-Row MN, Shephard MJ (1997) Through-bond orbital coupling, the parity rule, and the design of superbridges which exhibit greatly enhanced electronic coupling a natural bond orbital analysis. J Am Chem Soc 119 5355-5365... 

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>. 

The program NBO 5.0 was developed by E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M. Morales, and F. Weinhold, Theoretical Chemistry Institute, University of Wisconsin, Madison (2001) F. Weinhold, NBO 5.0 Program Manual Natural Bond Orbital Analysis Programs (Madison, WI, Theoretical Chemistry Institute, 2001). 

Table 3.40 displays the calculated 0b charges from natural population analysis (NPA) for all unique B atoms. This table also includes comparison of the NPA charges with corresponding zeroth-order (ZO) estimates given by Lipscomb, which are based essentially on the formal charges for conjectured three-center bond and. s /vx-code assignments for each species (to be discussed below). 

Because we are both computational chemistry researchers, we have naturally directed the book also to specialists in this field, particularly those wishing to incorporate natural bond orbital (NBO) and natural resonance theory (NRT) analysis into their methodological and conceptual toolbox. Researchers will find here a... 

---