Density functional theory calculations have been performed to explore the substituent effect on benzene’s structure and aromaticity upon excitation to the first triplet excited state (T1). Discussion is based on spin density analysis, HOMA (harmonic oscillator model of aromaticity), NICS (nucleus-independent chemical shift), ACID (anisotropy of the induced current density), and monohydrogenation free energies and shows that a large span of aromatic properties, from highly antiaromatic to strongly aromatic, could be achieved by varying the substituent. This opens up a possibility of controlling benzene’s physicochemical behavior in its excited state, while molecular motion, predicted for several derivatives, could be of interest for the development of photomechanical materials.

The influence of the relative boron and nitrogen positions on aromaticity of the three isomeric 1,2-, 1,3-, and 1,4-azaborines has been investigated by computing the extra cyclic resonance energy, NICS(0)πzz index and by visualizing the π-electron (de)shielding pattern as a response of the π system to a perpendicular magnetic field. The origin of the known stability trend, in which the 1,2-/1,3-isomer is the most/least stable, was examined by using an isomerization energy decomposition analysis. The 1,3-arrangement of B and N atoms creates a charge separation in the π-electron system, which was found to be responsible for the lowest stability of 1,3-azaborine. This charge separation can, in turn, be considered as a driving force for the strongest cyclic π-electron delocalization, making this same isomer the most aromatic. Despite the well-known fact that the BN bond attenuates electron delocalization due to large electronegativity difference between the atoms, the 1,4-B,N relationship reduces aromaticity to a greater extent by making the π-electron delocalization more one-directional (from N to B) than cyclic. Thus, 1,4-azaborine was found to be the least aromatic. Its lower stability with respect to the 1,2-isomer was explained by the larger exchange repulsion.

Nuclear magnetic resonance (NMR) spectroscopy is an important technique for structure determination. Within it, anisotropic effects of different functional groups and ring systems, depicted as familiar "anisotropy cones", are broadly used to deduce the stereochemistry, for chemical shift assignments and to explain shielding or deshielding of nuclei spatially close, or directly attached to the corresponding functional group, or ring. Progress in computational methods has enabled the quantification of anisotropic effects, an insight into their origin and to the source of (de)shielding of proximal nucleus. Some widely accepted traditional explanations, presented in NMR spectroscopy textbooks, have been questioned. The purpose of this review is to collect and discuss the research, mainly based on theoretical calculations, that provided new insight into the anisotropic effects, their origin and factors responsible for (de)shielding of proximal protons.

Based on the nucleus-independent chemical shift (NICS) concept, isotropic magnetic shielding values have been computed along the three Cartesian axes for ethene, cyclobutadiene, benzene, naphthalene, and benzocyclobutadiene, starting from the molecular/ring center up to 10 Å away. These through-space NMR spectroscopic shielding (TSNMRS) values, which reflect the anisotropic effects, have been broken down into contributions from localized- and canonical molecular orbitals (LMOs and CMOs); these contributions revealed that the proton NMR spectroscopic chemical shifts of nuclei that are spatially close to the C=C double bond or the aromatic ring should not be explained in terms of the conventionally accepted π-electron shielding/deshielding effects. In fact, these effects followed the predictions only for the antiaromatic cyclobutadiene ring.