Natural Bond Orbital
In quantum chemistry, a natural bond orbital or NBO is a calculated bonding orbital with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include "Natural Atomic Orbitals" (NAO), "Natural Hybrid Orbitals" (NHO), "Natural Bonding Orbitals" (NBO) and "Natural (semi-)Localized Molecular Orbitals" (NLMO). These natural localized sets are intermediate between basis atomic orbitals (AO) and molecular orbitals (MO):
- Atomic orbital → NAO → NHO → NBO → NLMO → Molecular orbital
Natural (localized) orbitals are used in computational chemistry to calculate the distribution of electron density in atoms and in bonds between atoms. Natural localized orbitals have the "maximum-occupancy character" in localized 1-center and 2-center regions of the molecule. Natural bond orbitals (NBOs) include the highest possible percentage of the electron density, ideally close to 2.000, providing the most accurate possible “natural Lewis structure” of ψ. A high percentage of electron density (denoted %-ρL), often found to be >99% for common organic molecules, correspond with an accurate natural Lewis structure.
The concept of natural orbitals was first introduced by Per-Olov Löwdin in 1955, to describe the unique set of orthonormal 1-electron functions that are intrinsic to the N-electron wavefunction.[1]
Theory
Each bonding NBO σAB (the donor) can be written in terms of two directed valence hybrids (NHOs) hA, hB on atoms A and B, with corresponding polarization coefficients cA, cB:
- σAB = cA hΑ + cB hB
The bonds vary smoothly from covalent (cA = cB) to ionic (cA >> cB) limit.
Each valence bonding NBO σ must be paired with a corresponding valence antibonding NBO σ* (the acceptor) to complete the span of the valence space:
- σAB* = cA hΑ − cB hB
The bonding NBOs are of the "Lewis orbital"-type (occupation numbers near 2); antibonding NBOs are of the "non-Lewis orbital"-type (occupation numbers near 0). In an idealized Lewis structure, full Lewis orbitals (two electrons) are complemented by formally empty non-Lewis orbitals. Weak occupancies of the valence antibonds signal irreducible departures from an idealized localized Lewis structure, which means true "delocalization effects".[1]
See also
Weblinks
- IUPAC Gold Book definition: natural bond orbital (NBO)
References
- ↑ a b Frank Weinhold and Clark R. Landis: Natural bond orbitals and extensions of localized bonding concepts Chem. Educ. Res. Pract. Eur.; 2001, 2, p. 91-104