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QCMP044. POAV3: Rehybridization and p-Orbital
Overlap in Nonplanar Conjugated Organic
Molecules: p-Orbital Axis Vector (POAV) and
Three-Dimensional Hückel Molecular-Orbital
(3D-HMO) Theory
by R. C. Haddon, AT&T Bell Laboratories,
Murray Hill, New Jersey 07974
The concept of hybridization of atomic-
orbital basis functions to produce spatially
directed wave functions with the orientation
necessary for bond formation is fundamental
to the modern understanding of the molecular
and electronic structure of molecules. The
POAV theory has been developed to analyze a
bonding situation which has become
increasingly important in recent years, i.e.,
nonplanar conjugated organic molecules, which
are usually considered to possess formal sp2
hybridization. It has been shown1,2 that,
with a single assumption, it is possible to
obtain analytical solutions for the
hybridization in such compounds, which in
turn leads directly to the orientation of the
p-orbital axis vectors (POAV) and hence to a
measure of p-orbital alignment and overlap in
distorted ã-electron systems of known
geometry. The p-orbital axis vector (POAV)
analysis provides a vivid picture of the p-
bonding in nonplanar conjugated organic
molecules and the manner in which the s-
system has rehybridized and adjusted to
facilitate the maintenance of favorable p-
orbital overlap. The method is nonparametric
and merely requires the atomic coordinates of
the molecule or molecular fragment for its
implementation. The analysis is based on the
s,p hybrid orthogonality relationships and
the geometry of the s-skeleton. As such, the
method provides the most logical and natural
bridge between the s-p separability assumed
in planar conjugated systems and the
realities of p-bonding in nonplanar
situations. The analysis is not recommended
in circumstances where the s-bond angles are
method may be used with confidence (the
technique provides a lower bound to the p-
orbital alignment attained by most non-planar
conjugated organic molecules).
The 3D-HMO theory3 further quantifies the
POAV analysis by utilizing the local POAV
hybrids to calculate overlap integrals
between pairs of conjugated atoms. Suitably
scaled, the overlap integrals provide reduced
HMO resonance integrals which allow a unified
treatment of conjugated systems, both planar
and nonplanar.
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References:
1. POAV1: R. C. Haddon and L. T. Scott,
Pure and Appl. Chem., 58, 137 (1986).
2. POAV2: R. C. Haddon, J. Am. Chem. Soc.,
108, 2837 (1986); J. Phys. Chem., 91, 3719
(1987).
3. 3D-HMO: R. C. Haddon, J. Am. Chem. Soc.,
109, 1676 (1987).
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