“Aromaticity” is another ill-defined term used in chemistry, just like “chemical bond” itself. At some point chemists (or more
precisely, physicist Michael Faraday) synthesized a compound that they could not
describe on the basis of the contemporary knowledge of chemical structure. The
compound called “benzene” had very low ratio between hydrogen and
carbon atoms - it was highly unsaturated. It also had a strong aroma, sometimes
recognized as similar to the smell of old gloves. Very soon the entire class of substances
was singled out: they all were highly unsaturated and fragrant, derived from essential
oils, resins etc. Due to the lack of the actual information about the chemical structure and
the origin of chemical properties, these substances were called “aromatic” – the easily
recognizable common feature was used for pure labeling purposes.
Model of the benzene molecule and diagram of the valence ?-orbitals of
benzene responsible for the
delocalized (aromatic) bonding.
The concept of antiaromaticity originally was
accepted and established in organic chemistry. As of year 2001, it was extended to the
realm of all-metal clusters and later main-group element and transition metal clusters. It
is relevant to the chemistry of nanosystems such as nanotubes, nanospheres, nanowheels,
a) structure of an isolated Al42- unit; it is a part of NaAl4– cluster observed in
b) Canonical Molecular Orbitals of Al42–(D4h, 1A1g) structure; HOMO
1a2u is π–aromatic orbital, HOMO–1 2a1g is radial σ–aromatic orbital and HOMO–2 1b2g is
tangential σ–aromatic orbital.
The development of methods of structural analysis together with evolution of the
theoretical insights into the world at atomic and molecular scale led to the elucidation of
the peculiarities of the electronic structure of aromatic compounds. In chemistry the
structure of molecules is represented using the concept of chemical bonds, where a pair
of electrons is associated with two atoms (see Lewis Model of Bonding). It turns out that in aromatic compounds it is impossible to make such
association. Certain amount of valence electrons has to be associated with all the atoms in
the molecule. In other words, these electrons cannot be localized and the structure of the
molecule cannot be uniquely represented using only pair-wise interactions between atoms
– 2-center 2-electron (2c-2e) bonds. So, from the bonding point of view aromaticity is
equivalent to the delocalization of certain amount of valence electrons.
There are two approaches to the description of the electronic structure of aromatic
systems. First one is based on the theory of resonances. If there is no unique
representation involving only 2c-2e bonds between atoms, one might think of several
representations being in “resonance” or “mixing” with each other. It is important
to keep in mind though, that there are no real resonance structures rapidly transforming
into each other. “Mixing” occurs between several non-unique representations of a single
real system. From the theoretical point of view, these resonance representations and their
relevance can be obtained through computations based on valence bond theory.
Resonance (Lewis) structures contributing to the description of aromatic Al42 –(according to Havenith, R. W. A., van Lenthe, J. H. Chem. Phys. Lett. 2004, 385, 198).
The second approach considers molecular orbitals (MOs), or eigenstates of some energy related
operator (see Molecular Orbital Theory and Self-Consistent Field
Approximation). Molecular orbital characterizes spatial distribution of a pair of electrons
with anti-parallel spins. So, by their very nature, MOs are delocalized over the entire
molecule. It turns out though that certain sets of MOs can be transformed into lone-pairs
(LPs) and 2c-2e bonds (see Natural Bonding Orbital Analysis
). In aromatic systems there are also sets of MOs that cannot be
transformed in this manner and should remain completely delocalized.
a) geometry of the most stable isomer of B5
+. Though it is supposed to have
five-fold rotational axis and possess D5h symmetry, the second-order Jahn-Teller effect
leads to the structural distortion resulting in C2v symmetry of the global minimum
structure; b) Canonical Molecular Orbitals of B5
+ (D5h, 1A1‚) structure; higher symmetry
is chosen to preserve degeneracy of CMOs; c) localized description of the electronic
structure of B5
+(C2v,1A1) cluster obtained by Adaptive Natural Density Partitioning;
color-coding shows relations between localized multi-centered bonds and CMOs from
part (b); it is clearly recognizable, that 5c-2e bonds of σ– and π–nature make the system
The delocalization of bonding has one important implication. Perfect delocalization
requires atoms (or fragments) of a chemical system to be equivalent in geometric sense,
i.e. meet certain symmetry requirements (see Symmetry). As the result, aromatic systems are
very often highly symmetric and, in its turn, high symmetry can be a hint that the system
is aromatic. High symmetry usually (but not always!) means presence of a multifold
rotation axis in case of planar systems or polyhedral cage-like structure of Td, Oh, and Ih
symmetry in case of 3D geometries (spherical aromaticity).
Examples of a) planar (2D) and b) spherical (3D) highly symmetric aromatic systems.
Also, as long as electronic structure is considered, the perfect delocalization and high
symmetry can be preserved only if MOs are occupied according to certain rules (see
Hückel Aromaticity and Mobius
aromaticity). These rules are based
on the requirement that all degenerate (having the same energy) MOs involved into aromatic bonding
should be completely occupied. If this requirement is not met, the first-order Jahn-
Teller effect leads to the structural
distortions and formation of islands of aromaticity in the system, which is now globally
First-order Jahn-Teller distortion of a) doubly (σ– and π–) aromatic B5+ (C2v)
cluster into b) B5
- C2v system with partially filled degenerate orbital upon addition of an
extra electron pair and further into c) σ–antiaromatic π–aromatic B5- C2v cluster.
Canonical Molecular Orbitals at (a) and (b) correspond to D5h symmetry so that the
degeneracy of orbitals is preserved for illustrative purposes. Structure (a) B5
+ initially has
C2v symmetry due to the second–order Jahn–Teller distortion. Evolution of the bonding
pattern and formation of islands of σ–aromaticity in (c) is revealed using AdNDP
Based on the nature of the overlap in the MOs forming aromatic system in planar
molecules, we distinguish between σ– aromaticity, π–aromaticity, δ–aromaticity (these
three types were actually spotted in real systems) and φ–aromaticity (predicted but not
encountered so far).
Various types of completely bonding (aromatic) MOs in a generic three–atomic
Can certain molecule or cluster possess several types of aromaticity? The answer is yes.
While aromaticity/antiaromaticity in organic systems is usually of π–type, the key feature
of bonding in subnano- and nano-clusters is multifold nature of aromaticity
. Think of all
possible combinations of σ–and δ– aromatic/antiaromatic bonding. They all can be found
in planar or quasi-planar clusters of main group elements! For example, B5+
is σ– and π– aromatic, B5-
is σ–antiaromatic and ?-aromatic, and B62-
is σ– and π– antiaromatic and so on
. Now, throw in δ– aromaticity/antiaromaticity, which can be present in transitional metal clusters
. The number
of possible combinations will be even bigger! In spherical
clusters where σ/π/δ separation is not possible there are still different types of
aromaticity: radial and tangential, according to the location of the overlap regions of MOs
responsible for the delocalized bonding.
Examples of MOs with a) radial overlap and b) tangential overlap in a generic system of Oh symmetry.
In old days a substance would be assigned as aromatic if it had an aroma. Many of the
substances considered as aromatic by contemporary chemists are not fragrant at all.
Aromatic nature of chemical bonding can be established by analysis of
electronic structure. To identify certain cluster as aromatic/antiaromatic it is necessary to
outline the sets of the MOs that can’t be transformed into the classical Lewis bonding
objects, such as lone pairs and 2c-2e bonds. These orbitals are compared with prototypical
aromatic systems on the basis of overlap type and separated into subsets, corresponding
to different types of aromatic bonding (e.g. σ, π, δ). Than aromaticity/antiaromaticity
is established on the basis of counting rules, specific for the delocalized bonding of each
type. In this way it was possible to develop comprehensive model of chemical bonding for
entire family of planar and quasi-planar boron clusters.
Family of planar and quasi-planar boron clusters (according to Zubarev, D. Yu.,Boldyrev, A. I. J. Comput. Chem. 2007, 28, 251.)
Are there any observable properties that can be used for distinguishing aromatic systems
from other (non-aromatic)? The additional stabilization energy (compared to the virtual structures with localized bonding) is a good
measure of aromaticity called resonance energy. Delocalized bonding is energetically
more efficient than localized and it is more important for the stability of a molecule, so
aromatic molecules should be inert in the reactions involving the delocalized bonds.
Delocalization of the electrons over the entire molecule should lead to certain type of
behavior in the presence of external magnetic field. Nuclear Independent Chemical
Shift (NICS) indexes and induced ring currents rely on this phenomenon when trying
to estimate degree of the aromaticity of the system. While useful for labeling purposes,
these measures of aromaticity depend on many factors and peculiarities of electronic
structure and very often contradict each other. On the other hand, assignment of
aromaticity on the basis of analysis of chemical bonding can easily establish connection
between structure of a certain system and its chemical behavior, but this procedure does
not rely on any observable features. Co-existence of various approaches to the definition
of aromaticity (on the basis of electronic structure, resonance energy, magnetic
properties, and chemical behavior) makes this topic ill-defined and somewhat
controversial. Nevertheless, success of this concept in organic chemistry makes it very useful.
Contributors: Alex Boldyrev (inspiration),
Dmitry Zubarev (design) and
Boris Averkiev (photo model)
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