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Relationship between Stereochemistry and Charge Density in Hydrogen Bonds with Oxygen Acceptors
journal contribution
posted on 2013-01-02, 00:00 authored by M. Ahmed, C. Jelsch, B. Guillot, C. Lecomte, S. DomagałaAn extensive survey of Cambridge Structural Database
was carried
out to study the directionality and stereochemistry of hydrogen bonds
with an oxygen acceptor including carbonyl, alcohols, phenols, ethers,
and esters groups. The results obtained through this survey are correlated
with the charge density of these different chemical groups. The electron
density of these different oxygen atom types shows striking dissimilarities
in the electron lone pair configuration. Esters and ethers with the
C–O–C oxygen atom located in an aromatic cycle display
merged lone pairs lobes, which is not the case when one of the bonded
carbon atoms has sp3 hybridization. The positions of the
lone pairs in the deformation electron density maps derived from theoretical
calculation and from experimental charge density generally agree with
the notable exception of phenols and C(sp3) esters. The
experimental studies show generally lone pairs lobes that are closer
to each other. Differences are found within COH groups: the two electron
lone pairs are slightly closer in phenol oxygen atoms compared with
alcohols in theoretical electron densities. In experimental charge
densities, the discrepancy is more drastic because the two lone pair
lobes appear merged in phenols; this might be due to a resonance effect
with the neighboring sp2 carbon atom. This difference in
the configuration of the two electron lone pairs affects the directionality
of hydrogen bonds. For phenols, the preferred donor hydrogen atom
position is close to the COH plane, while for alcohols, it is out
of plane with the direction O···Hdonor forming
an angle of around 30° to the COH plane. The number of H-bonds
occurring with the donor hydrogen atom pointing toward the middle
of the two lone pairs is small for carbonyl, contrary to alcohols
and phenols. Also H-bonds involving alcohol/phenol acceptors have
a stronger tendency to occur in directions close to the electron lone
pair plane than for carbonyl. As expected, the directional attraction
of hydrogen bond donors toward the lone pairs is much more pronounced
for short H···O distances. This study could have implications
in the design of force fields, in molecular recognition, in supramolecular
crystal engineering, and in drug design.