Chemistry Reference
In-Depth Information
cluster atoms like Li
12
or Cu
14
(the so called ferromagnetic bonding, since achieving
a plateau of about 19 kcal/mol where the van der Waals bonding is excluded) as based
on covalent-ionic resonance interaction in two atoms (by a triplet-state electronic pair,
i.e. for anti-bonding state in molecular orbital theory terms); we thus remain with
the message that by mimicking the covalent bonding (as in H
2
) the
ionic-covalent
bonds
(as in F
2
)
may be viewed as a realization of distance interaction of parallel
spins in triplet (valence) states
.
(iii) The “polyphonic culture” of chemical bonding is completed by the molecular
orbital (MO) approach initiated by the seminal work of Heitler and London (
1927
)
introducing the
mathematical term of resonance
through overlapping and exchange
integrals of bonding of atomic orbitals in molecule, having H
2
as a paradigmatic
covalent system; it opened the way of physical, mathematical and computationally
tracking the molecular structure by mean-field and beyond approaches of many-body
(fermionic) systems by the celebrated method of Hartree-Fock (Löwdin
1955
) then
augmented to include correlations by delocalization symmetry adapted, towards the
(less intuitive) Complete Active Space of Self Consistent Field (CAS-SCF) calcula-
tion (Jensen
2007
) and multi-reference (by multi-configuration) systems including
static (“left-right”) correlation based on near-degeneracy correlation, along the dy-
namic (“in-out”) correlation based on local fluctuation in charge due to chemical
environment—viewed merely as fluctuation in Hilbert space rather than in time
(Truhlar
2007
); the lesson is paradoxically:
dealing with the molecular orbitals' de-
localization in modeling of size increasingly systems
is like “building a larger and
larger pyramid from smaller and smaller stones” (Malrieu et al.
2007
)
so loosing
the chemical bonding intuition
; this way, we face with high demand in searching for
alternative quantum chemistry theory and allied computation schemes for electroni-
cally delocalized and correlated structures by necessary complex-intuitive picture of
chemical bonding.
There is therefore understandable why “Coulson's dream” is populated by
emerged unicorns like resonance, conjugation, hyperconjugation, frontier orbitals,
bonding-antibonding, donor-acceptor bond,
-bonding, aromaticity, and many oth-
ers. On the other side, Hoffman (
2008
) firmly advice: “push the chemical bonding
concept to its limits. Be aware of the different experimental and theoretical measures
out there. Accept that (at the limits) a bond will be a bond by some criteria, maybe
not others, respect chemical tradition, relax, and
...
have fun with the fuzzy richness
of the idea.”
Along this line, our project proceeds with the recent frontier idea of chemical
bonding, till now a non observable quantum object simply because wave-functions
do not possess an observable nature (Scerri
2000
). What is traditionally missing in the
molecular orbital approach so far is the proper focus on the quantum (quasi)particle
associated to the chemical bond wave function. Basal quantum mechanics adopts the
dual-picture
by associating to each particle (say an electron) its dual wave-function,
and vice-versa, i.e. any field (say electromagnetic) may be quantized by particles in
the second quantization.
The chemical bonding scenario remained till now incomplete from the quantum
perspective: atomic electrons (eventually in valence states) were represented by their
π
