transformation than uniting two systems or dissociating one (e.g. 2 ethene
cyclobutadiene); thus one could argue that the systems represented by closing/
opening are the better mutual references. Certainly, cyclobutadiene is regarded
from empirical evidence and more advanced theoretical studies as an electroni-
cally destabilized molecule [1], so the butadiene reference, which predicts a
destabilizing effect for four cyclicpelectrons, is in much better accord with the
general collection of experimental and computational work.
Note that in fact cyclobutadiene does not have degenerate, singly-occupied
molecular orbitals, as a Jahn–Teller type (actually a pseudo-Jahn–Teller) distortion
lowers its symmetry from square to rectangular and leads to a closed-shell paired-
electron molecule [4].
References
- Bally T (2006) Angew Chem Int Ed Engl 45:6616
- Krygowski TM, Cyran ̃ski MK, Czarnocki Z, H€afelinger G, Katritzky AR (2000) Tetrahedron
56:1783 - Dewar MJS (1975) The PMO theory of organic chemistry. Plenum, New York, p 90
- (a) Bersuker IB (2006) The Jahn–Teller effect. Cambridge University press, Cambridge. (b)
Bala ́zs R, Kolonits M, Marsden CJ, Heully J-L (1997) J Am Chem Soc 119:9042
Chapter 4, Harder Questions, Answers
Q6
What is the problem with unambiguously defining the charge on an atom in a
molecule?
Let us be ambitious and replace “unambiguously” by “uniquely”. The problem is
to define where an atom in a molecule begins and ends. If we can mathematically
specify the region of space over which the electronic charge distribution is to be
integrated, we can calculate the number of electrons which should be assigned to
each atomin the molecule. The algebraic sum of this electronic charge and the
nuclear charge would then give the net charge on the atom. This is the principle
behind the (quantum theory of)atoms in molecules(QTAIM, AIM) method of
Bader [1]. In the AIM method, an atom in a molecule is demarcated from the rest of
the molecule by a “zero-flux surface” defined in terms of the gradient of the electron
density. Bader and coworkers essentially regard their definition as unique, from
which it would seem to follow that in some sense it yields “the correct” definition of
atomic charges. Criticisms of the approach have engendered delightful polemics by
Bader and Matta [2].
616 Answers