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Letters in Organic Chemistry, 2016, 13, 33-43 33
A New Index for the Estimation of the Aromatic Character - IX
Maurizio D'Auria
*
Dipartimento di Scienze, Università della Basilicata, Viale dell'Ateneo Lucano 10, 85100 Potenza,
Italy; Corresponding author: Prof. Maurizio D'Auria, Dipartimento di Scienze, Università della
Basilicata, Viale dell'Ateneo Lucano 10, 85100 Potenza, Italy
Received June 19, 2015: Revised October 29, 2015: Accepted October 30, 2015
Abstract:
Background: Recently, the author of the article proposed a new index for the estimation of the aro-
matic character. The aromaticity index D, D usually obtained by using
DFT/B3LYP/6-311G+(d,p) theoretical method.
Method: The D index was tested using different functionals: LSDA, BVP86, B3LYP, CAM-B3LYP, B3PW91,
mPW1PW91, PBEPBE, HSE1PBE, HCTH, TPSSTPSS, and wB97XD.
Results: The results showed that the D values for monocyclic aromatic compounds are not largely sensible to the variation
of the functional. Large variations of D values were observed for polycyclic aromatic compounds. D values were corre-
lated to ERE (experimental resonance energy) and ASE (aromatic stabilization energy). ERE and ASE gave different cor-
relations with D.
Conclusion: The best correlation between D and ERE was obtained by using CAM-B3LYP. The best correlation between
D and ASE was obtained by using CAM-B3LYP.
Keywords: Aromatic stabilization energy, aromaticity, DFT calculations, experimental resonance energy, functionals, orbitals.
INTRODUCTION
The aromatic character of an organic compound is one of
the most important research theme from almost one hundred
and fifty years. The quantitative evaluation of the aromatic
character of a compound is an important challenge and sev-
eral approaches have been reported. The most diffuse ap-
proaches are based on structural properties [1-4], on the
magnetic properties of the suited compounds [5-7], and on
the energetic properties of the aromatic compounds [8-11].
In this field, we recently proposed a new useful index to es-
timate the aromatic character of the aromatic compounds
based on the energy of orbitals [12-18]. The new index, D,
is determined as
(1)
where is the sum of the energy
differences between the orbitals (
1
....
n
) in the parent
compound (benzene), is the same
value for a generic aromatic compound, and a represents the
number of cycles in the molecule. The energy related to the
*Address correspondence to this author at the Dipartimento di Scienze,
Università della Basilicata, Viale dell'Ateneo Lucano 10, 85100 Potenza,
Italy; Tel: +39 0971 205480; Fax: + 39 0971 205678;
E-mail: maurizio.dauria@unibas.it
presence of the aromatic ring in benzene is determined as 2,
and it is the difference between the energy of three isolated
double bond (6 + 6) and that of the occupied orbitals of
benzene (6 + 8) (Fig. 1). In a generic aromatic compound
such as pyrrole, the
1
orbital has an energy of + 2 +
0.037 a.u., the
2
orbital has an energy of + 0.011 a.u.
while the
3
orbital has energy + 0.041 a.u. (Fig. 1).
These differences induce a different aromatic energy and the
new index allows us to determine these differences.
This new approach seems to work well with generic aro-
matic compounds [12, 13, 17], for polycyclic aromatic com-
pounds [14], for pentaatomic [15] and hexaatomic heterocyc-
lic compounds [16]. Our calculations were performed at
DFT/B3LYP level of theory, by using 6-311G+(d,p) basis
set. More recently, we examined the possible use of different
basis set [18]. We showed that D values are not largely sen-
sible to the variation of the basis set. On the basis of these
results, it should be interesting to know whether the use of
different functionals shows the same behavior.
In this letter we want to report the results obtained calcu-
lating the D values of several compounds by using the same
basis set [6-311G+(d,p)] but in the presence of different
functionals.
In this study, we used the same compounds tested in the
previous article [18] where the change of the basis set was
estimated (Fig. 2).
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