3-Center, 4-Electron Hyperbond Search (3CHB)
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Reference
F. Weinhold and C. R. Landis, Discovering Chemistry with Natural Bond Orbitals (Wiley, 2012), Sec. 8.3.
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Notes
The 3CHB keyword initiates the search for atom triads (A,B,C) that are connected by 3-center, 4-electron "hyperbonds," as described by Pimentel and Rundle in terms of 3-c MOs or by Coulson in terms of covalent-ionic resonance (A-B :C vs. A: B-C bond patterns). The 3CHB search is automatically invoked for all NRT-related keywords. Such bonding corresponds to apparent "hypervalency" at the central atom but typically requires only normal-valent (Lewis-like) resonance structures, in accordance with Coulson's picture. 3-center hyperbonds are found to be ubiquitous in coordination metal compounds, due to the favorable electronegativity and centrosymmetric bonding hybrids of transition metals. The 3CHB output identifies participating NBOs and NHOs of each 3-center hyperbond for the chosen Lewis-like structure of the main NBO search. 3CHB results are illustrated for the tungsten pentacarbonyl molecule [W(CO)5], B3LYP/LANL2DZ level (see NBO 6.0 Manual, pp. B154-157 for additional discussion).
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Sample Input (G9X)
%chk=w_co5
#b3lyp/lanl2dz pop=nboread scf=qc scfcyc=200
W(CO)5, E=-634.4598757
0 1
W 0.000000 0.000000 0.244387
C 0.000000 2.041935 0.272300
O 0.000000 3.221160 0.270578
C 2.041935 0.000000 0.272300
O 3.221160 0.000000 0.270578
C 0.000000 0.000000 -1.698425
O 0.000000 0.000000 -2.885976
C 0.000000 -2.041935 0.272300
O 0.000000 -3.221160 0.270578
C -2.041935 0.000000 0.272300
O -3.221160 0.000000 0.270578
$nbo file=w_co5 3chb $end