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Magnetically induced current densities in the lowest lying singlet and triplet states of a series of phenylene molecules were examined at the B3LYP/lanl2DZ level of theory. Previous findings have shown that in the ground singlet state, phenylenes exhibit ring currents with opposed tropicity: diatropic currents within six-membered rings and paratropic currents within four-membered rings. However, for biphenylene in the first excited triplet state, there is a drastic change in aromaticity in comparison to the ground state. Specifically, this molecule in the triplet state sustains a strong diatropic global circulation. In the present work, we showed that higher members of the phenylene family do not exhibit such radical changes in aromaticity when excited to the lowest lying triplet states.

The electronic structure and aromaticity of the [12]infinitene molecule (1) and its formation via the Mallory reaction were studied using density functional theory (DFT). The examined reaction is based on a stepwise cyclization process. The nucleus-independent chemical shifts (NICS) were used to assess the aromatic character of the chemical species involved in the examined reactions. In addition, NICS-Scan, 2D and 3D multidimensional off-nucleus siso(r) magnetic shielding scans were also employed to examine the aromaticity of 1. It was found that the formation of 1 is an endothermal process, as a result of the opposed stabilizing effects of aromaticity and destabilizing effects of planarity distortions found in molecules included in the considered reaction.

The famous aromaticity index NICS (nucleus independent chemical shift) was introduced 25 years ago. The authors use this anniversary for a short and to some degree personal retrospective.

The Clar aromatic sextet theory predicts that the intensity of cyclic conjugation in chevron-type benzenoid hydrocarbons monotonically decreases along the central chain. This regularity has been tested by means of several independent theoretical methods (by the energy effects of the respective six-membered rings, as well as by their HOMA, NICS, and SCI values, calculated at the B3LYP/6-311G(d,p) level of DFT theory). Our results show that the predictions of Clar theory are correct only for the first few members of the chevron homologous series, and are violated at the higher members. This indicates that Clar theory is not universally applicable, even in the case of fully conjugated benzenoid molecules. [Projekat Ministarstva nauke Republike Srbije, br. 174033]

The commonly accepted opinion that the thermodynamic stability of isomeric benzenoid hydrocarbons (assessed by their total π-electron energy and various resonance energies) increases with increasing number of Kekulé structures is shown to be violated in numerous cases. The smallest examples of such anomalous behavior are two hexacyclic pericondensed benzenoids of formula C24H14 and several pairs of heptacyclic catacondensed benzenoids of formula C30H18.

The applicability of the Hall rule (linear relation between the total π-electron energy and the number of Kekulé structures) was investigated in the case of fluoranthene-type benzenoid hydrocarbons. It was found that the originnal Hall rule is not obeyed, but holds for sets of isomers with a fixed number of bay regions. For such groups of isomers, two apparently contradictory Hall-type rules were conceived, and it was found that both give almost identical numerical results.

The Estrada index EE is a recently proposed molecular structure-descriptor, used in the modeling of certain features of the 3D structure of organic molecules, in particular of the degree of folding of proteins and other long-chain biopolymers. The Estrada index is computed from the spectrum of the molecular graph. Therefore, finding its relation with the spectral radius r (= the greatest graph eigenvalue) is of interest, especially because the structure-dependency of r is relatively well understood. In this work, the basic characteristics of the relation between EE and r, which turned out to be much more complicated than initially anticipated, was determined.

The Zhang-Zhang polynomial ζ(x) is a recently conceived tool in the mathematical apparatus of theoretical chemistry. It combines (in a quantitative manner) the Kekulé- and Clar-structure-based features of benzenoid molecules. It is shown that the topological resonance energy (TRE) can be accurately approximated as TRE ≈ a√ζ(x)+ b where x≈0.5, by which fact a significant insight is gained into the structure-dependence of TRE of benzenoid molecules.

The Laplacian energy LE is a newly introduced molecular-graph-based analog of the total π-electron energy E. It is shown that LE and E have a similar structure-dependency only when molecules of different sizes are compared, when a good linear correlation between them exists. Within classes of isomers, LE and E are either not correlated at all or (as in the case of acyclic systems) are inversely proportional. The acyclic graphs and molecular graphs having the greatest and smallest LE values (determined in this work) differ significantly from those (previously known) having the greatest and smallest E values.

The dependence of the Dewar resonance energy (DRE) on the Kekulé structure count (K) was found to be significantly different from that earlier anticipated. Within classes of benzenoid isomers, the DRE increases either as Kλ for λ ≈ 0.3 or as (ln K) for λ ≈ 2. Both functional dependencies result in approximate expressions for DRE of nearly equal accuracy. Approximations of the form DRE ≈ a K + b and DRE ≈ a' ln K + b' are somewhat less accurate, but can still be used in usual practical applications of the Dewar resonance energy. .

Using a recently developed method for computing the effect of non-bonding molecular orbitals (NBMOs) on the total ¶-electron energy (E), it was found that the dependence of E on the number n0 of NBMOs is almost perfectly linear. We now show that this regularity remains valid for very large values of n0, in particular, to hold up to n0 = 20.