TITLE:
Three-Center Configuration with Four,Three, and Two Electrons for Carbon,Boron, Hydrogen, and Halogen Exchange. A Model and Theoretical Study with Experimental Evidence
AUTHORS:
Henk M. Buck
KEYWORDS:
Three-Center Configuration, Frozen Transition State, Molecular Drone, Enigma, Classical and Nonclassical 2-Norbornyl Cation
JOURNAL NAME:
Open Journal of Physical Chemistry,
Vol.4 No.2,
April
21,
2014
ABSTRACT: The introduction of
specific sites in organic frames for accommodation of various modes of bonding has
been focused on reaction types which are described by using different
theoretical models with or without a definite experimental proof. In this study
three-center four-, three-, and two-electron systems based on carbon-, boron-,
hydrogen-, and halogen exchange are under consideration. Based on the number of
electrons in the transition state or transition complex it is shown that all
transfer or exchange reactions share the same ratio numbers expressed as the
quotient of the transitional bond distance under investigation and its normal
bond length. With X-ray data of model systems it was even possible to give the
ratio numbers for a three-center four-electron configuration experimental
support with additional ab initio data. Furthermore a novel model type of substitution in organic chemistry is
introduced through electrophilic insertion, informative for enzyme-substrate
interactions based on the lock-and-key model. Reactions based on
a three-center two-electron configuration mostly follow a nonlinear transition.
In this alignment there will be a pursuit of cyclization for stabilization via
homoaromaticity as homocyclopropenyl cation. The molecular dynamics of
such a process is discussed based on recent X-ray
crystallographic data of the symmetrically bridged, nonclassical geometry of
the 2-norbornyl cation. In the present paper the focus is aimed at the
transition intermediate of the (classical) 2-norbornyl cation involved in the
isomerization into the nonclassical geometry. This model description is
compared with a simple molecular rearrangement of the 1-propyl cation into the
corner-protonated cyclopropane using the ab
initio data. The exclusivity of the former isomerization compared with the
latter one could be unambiguously demonstrated by the invention that theintramolecularelectron shift can be
expressed in a linear relationship between the concerned electron-donating and
accepting bond lengths. Finally, the fluor transitions as divalent atoms in a
three-center two-electron configuration are described. The role of fluor in
comparison with the other halogens is striking. The attention was focused on an
excellent correspondence between the recent chemical and theoretical evidence
for a symmetrical fluoronium ionin
solution. Simple dialkylfluoroniumions in contrast to the other halonium
ions are not present in solution. Although the geometry of the fluoronium ion theoretically
can be described as a real minimum, the C-F-C angle of 120° is apparently the
borderline transition for dissociation in C+ and F-C.