The imidazole molecule is paired with NH3 in order to
examine the proton transfer properties of the former
by ab initio methods. The primary minimum on the surface is
ImH+···NH3 wherein the
inter-nitrogen distance
in the H bond is 2.89 Å. A second well appears in the surface,
corresponding to Im···+HNH3, and the
barrier
between the two minima is rapidly enlarged, when the latter distance is
elongated. When the NH3 is displaced
from the N lone pair direction of the imidazole in the plane of the
latter, the greater proton-attracting power
of the imidazole relative to NH3 is enhanced; the opposite
is observed when the NH3 is pulled out of the
imidazole plane. This distinction is explained simply on the basis
of the dipole and quadrupole moments of
imidazole. The ability of imidazole to act as a proton shuttle
from one molecule to another is examined by
placing one NH3 molecule on either side. Taking
H3NH+···ImH···NH3
as a starting point, the simultaneous
transfer of two protons to form
H3N···HIm···H+NH3
must overcome a large energy barrier. A stepwise
process, passing through the
H3N···HImH+···NH3
intermediate, is greatly favored energetically. If the
central
imidazole is permitted the freedom to translate between the two
NH3 molecules, it is possible for the latter
to be quite some distance apart. The imidazole will first approach
within about 2.65 Å of the donor
H3NH+
ion. The transfer to imidazole can then take place with little or
no energy barrier. The protonated imidazole
will then move close to the receptor NH3 before depositing
the proton with it. In most cases, the largest
energy barrier is associated not with the proton transfers, but with
the motion of the imidazole cation. The
barrier for this translation grows as the ultimate donor and acceptor
NH3 molecules are moved further apart.
Semiempirical Austin model I (AM1) calculations have been performed on a family of inclusion complexes of heptakis(2-O-hydroxypropyl)--cyclodextrin isomers derived from the 2-hydroxyl position (2HPCD) and from the 6-hydroxyl position (6HPCD) with alkylated phenol derivatives. From the stabilization energies and hydrogen bonding studies of the inclusion complexes of 2HPCD and 6HPCD with substituted phenols in head-first and tail-first positions, we found that the main driving forces for the formation of the inclusion complexes are from the van der Waals interactions.
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