Natural Bond Orbital

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In quantum chemistry , a natural bond orbital ( NBO , German “natural bond orbitals”) corresponds to a calculated binding orbital with a maximum electron density . The NBOs are part of a sequence of natural, localized orbitals. Other parts of this sequence are the Natural Atomic Orbitals (NAO, "natural atomic orbitals"), the Natural Hybrid Orbitals (NHO, "natural hybrid orbitals"), and the Natural (semi-) Localized Molecular Orbitals (NLMO, "natural (semi-) localized orbitals "). These can all be classified between atomic orbitals (AO) and molecular orbitals (MO):

AO → NAO → NHO → NBO → NLMO → MO

Natural (localized) orbitals are used in computational chemistry to analyze the distribution of electron density in atoms and in bonds . They correspond to the “maximum occupation character” in localized 1- and 2-center regions within the molecule. NBOs contain the highest proportion of electron density, ideally close to 2,000, and offer the most accurate possible “natural Lewis structure ” of ψ. The level of the percentage of electron density (given as% -ρ L ) corresponds to the accuracy of the natural Lewis structure. For ordinary organic molecules it is often over 99%.

The concept of natural orbitals was first time in 1955 by Per-Olov Löwdin introduced to the unique set of orthonormal to describe one electrical functions of the N -electron wave function intrinsically is.

theory

Each binding NBO σ AB (the donor) can be described as the sum of the product of two directed valence hybrid orbitals (NHOs) h A , h B of atoms A and B with the corresponding coefficients c A , c B :

σ AB = c A h Α + c B h B

The character of the bond varies from covalent ( c A = c B ) to ionic ( c A ≫ c B ).

Each binding valence NBO σ must be paired with a corresponding antibonding valence NBO σ * (the acceptor) in order to complete the entire span of the valence space:

σ AB * = c A h Α - c B h B

The binding NBOs are "Lewis orbital" -like and their occupancy number is about 2. The antibonding NBOs are "non-Lewisorbital" -like, their occupancy number is close to 0. In an idealized Lewis structure, fully occupied Lewis orbitals with two electrons are formal empty (unoccupied) non-Lewis orbitals added. Small occupations of the antibonding valence orbitals signal irreducible deviations from an ideally localized Lewis structure, which ultimately means delocalization of the electrons.

Lewis structures

Resonance structures of amides

With a computer program that can calculate NBOs, the optimal Lewis structure can be found. This in turn can be defined as the one with the highest electronic charge within Lewis orbitals (Lewis charge). A low electronic charge in Lewis orbitals indicates strong electron delocalization.

Both more or less contributing structures can exist in resonance structures. For amides , for example, NBO analyzes show that the structure with a double bond in the carbonyl group is the dominant Lewis structure. However, the covalent-ionic resonance structure is not required in NBO calculations due to the inclusion of bond polarities in the resonance structures. This is similar to other methods of modern valence bond theory .

See also

Individual evidence

  1. ^ A b Frank Weinhold, Clark R. Landis: Natural Bond Orbitals and Extensions of Localized Bonding Concepts . In: Chemistry Education Research and Practice . 2, No. 2, 2001, pp. 91-104. doi : 10.1039 / B1RP90011K .
  2. ^ Frank Weinhold, Clark R. Landis: Discovering Chemistry With Natural Bond Orbitals . John Wiley & Sons, New Jersey 2012, ISBN 978-1-118-22916-3 , pp. 132-133.

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