Chapter 12
Rules of Aromaticity
Ferran Feixas, Eduard Matito, Jordi Poater and Miquel Solà
Abstract The concept of aromaticity is elusive; it is not directly observable.
Somewhat surprisingly, given the fuzzy character of this concept, there exist a
number of very simple mathematical rules that can account for the aromaticity of a
large number of organic and inorganic molecules. Among them we can mention
Hückel’s, Baird’s, Wade-Mingos’, and Hirsch’s rules. In this chapter we summarize
recent advances carried out in our group in the study of these aromaticity rules.
12.1 Introduction
The field of aromaticity is in constant evolution and the variety of molecules that
present properties related to aromaticity is growing exponentially. Over the last two
decades there has been a remarkable expansion in the number of different types of
aromatic systems and in our understanding of aromaticity. In 2001 Boldyrev,
Wang, and coworkers [1] detected a series of bimetallic clusters containing Al 4
2− ,
the first all-metal aromatic cluster known, face-capped by an M
+ cation (M = Li,
Na, Cu). Six years later the same group identified Ta 3 O 3
− [2], the first discovered
metallic cluster with δ-aromaticity. From these and many other studies [3–5], it is
now recognized that the aromaticity concept can be applied to the entire periodic
table. It is also widely accepted that there is not a unique type of aromaticity (the
classical π-aromaticity) but chemical compounds can also have σ-, δ-, and even ϕaromaticity, together with combinations of these different types (multifold aromaticity) [3–5]. From a theoretical point of view, the last two decades also brought
several important advances. More powerful tools to quantify aromaticity have been
F. Feixas Á E. Matito Á M. Solà (&)
Institut de Química Computational I Catàlisi and Departament de Química,
Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia, Spain
e-mail: miquel.sola@udg.edu
J. Poater
Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling,
Vrije Universiteit, De Boelelaan 1083, NL-1081HV Amsterdam, The Netherlands
© Springer International Publishing Switzerland 2016
R. Chauvin et al. (eds.), Applications of Topological Methods
in Molecular Chemistry, Challenges and Advances in Computational
Chemistry and Physics 22, DOI 10.1007/978-3-319-29022-5_12
321
Rules of Aromaticity
Ferran Feixas, Eduard Matito, Jordi Poater and Miquel Solà
Abstract The concept of aromaticity is elusive; it is not directly observable.
Somewhat surprisingly, given the fuzzy character of this concept, there exist a
number of very simple mathematical rules that can account for the aromaticity of a
large number of organic and inorganic molecules. Among them we can mention
Hückel’s, Baird’s, Wade-Mingos’, and Hirsch’s rules. In this chapter we summarize
recent advances carried out in our group in the study of these aromaticity rules.
12.1 Introduction
The field of aromaticity is in constant evolution and the variety of molecules that
present properties related to aromaticity is growing exponentially. Over the last two
decades there has been a remarkable expansion in the number of different types of
aromatic systems and in our understanding of aromaticity. In 2001 Boldyrev,
Wang, and coworkers [1] detected a series of bimetallic clusters containing Al 4
2− ,
the first all-metal aromatic cluster known, face-capped by an M
+ cation (M = Li,
Na, Cu). Six years later the same group identified Ta 3 O 3
− [2], the first discovered
metallic cluster with δ-aromaticity. From these and many other studies [3–5], it is
now recognized that the aromaticity concept can be applied to the entire periodic
table. It is also widely accepted that there is not a unique type of aromaticity (the
classical π-aromaticity) but chemical compounds can also have σ-, δ-, and even ϕaromaticity, together with combinations of these different types (multifold aromaticity) [3–5]. From a theoretical point of view, the last two decades also brought
several important advances. More powerful tools to quantify aromaticity have been
F. Feixas Á E. Matito Á M. Solà (&)
Institut de Química Computational I Catàlisi and Departament de Química,
Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia, Spain
e-mail: miquel.sola@udg.edu
J. Poater
Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling,
Vrije Universiteit, De Boelelaan 1083, NL-1081HV Amsterdam, The Netherlands
© Springer International Publishing Switzerland 2016
R. Chauvin et al. (eds.), Applications of Topological Methods
in Molecular Chemistry, Challenges and Advances in Computational
Chemistry and Physics 22, DOI 10.1007/978-3-319-29022-5_12
321