The aromaticity and electronic properties of monosubstituted benzene, borazine and diazadiborine rings: an ab initio MP2 study

Aromatic nature of neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives

The aromatically relevant parameters of boron-rich inorganic benzenes-neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives (B4N2R6)-have been computationally estimated and evaluated from geometric, electronic, magnetic, and energetic points of view. The majority of the criteria (ASE, NICSzz, ELF, and PDI) indicate that the aromaticity of the neutral B4N2 benzene analogue stabilized by Lewis bases lies in between those of benzene and borazine. On the other hand, the aromaticity of the dianionic B4N2 benzene analogue 4' is controversial. The pronounced aromatic nature of 4' is supported by ELFπ, PDI, and NICSπzz, but ASE, the FiPC-NICS plot, and ACID oppose this. These data confirm that even with the same B4N2-skeletal framework of a 6π-system, the aromatic feature varies depending on the overall charge of the B4N2 systems.

An electronic properties investigation to interpret the substituent constants of monosubstituted benzene derivatives

In the present work, the π and π^∗-electronic nature of substitution constants (σ) of the twenty-two monosubstituted benzene derivatives (MSBDs) are estimated in terms of the para-delocalization index (PDI) and total non-Lewis structure (TNLS), respectively. Since these compounds are aromatic, the other descriptors of aromaticity such as nuclear independent chemical shifts and aromatic stabilization energy have been examined. Because of no considerable variation for the π and π^∗-electron delocalization in the ring systems, a very weak correlation has been demonstrated between all aromaticity indices. Also, none of these descriptors has a linear correlation with the values of σ for both electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) in a simultaneous relationship. We now propose the usage of the dipole moments values of molecules, considering their directions, by multiplying the PDI and TNLS values as probes to interpret σ values for benzene and MSBDs since the dipole moment can affect the π and π^∗-electron delocalization. It is interesting to note that there is a remarkable linear correlation between our suggested probes, ±μPDI and ±μTNLS, and σ_p, σ_m, and σ⁺/σ^- for EDGs and EWGs in simultaneous linear relationships. Also, these probes have a relative relationship with Kirkwood and Westheimer equation. Additionally, the regression coefficient between ±μPDI and ±μTNLS is 0.989.

Does aromaticity account for an enhanced thermodynamic stability? The case of monosubstituted azaborines and the stereoelectronic chameleonism of the NH2 group

This work was initiated by the increasing interest in BN/CC isosterism and by the long-lasting interest in the concepts of aromaticity and substituent effects. We have theoretically examined the aromaticity and stability of monosubstituted BN isosters of benzene, the three isomeric azaborines. The results provide insight into the effect of substitution on two basic molecular properties, which are influenced, here, by the substituent effects and by the B/N relationship in the ring. The results, along with other examples in the literature, also warn chemists that the general belief that aromaticity accounts for enhanced thermodynamic stability is not always true. The stability of cyclic, conjugated compounds depends on several effects, and only one of them is aromaticity. In addition, our calculations predict a switching of electronic properties of the NH2 group from the usual p-electron donor to a π-electron acceptor when it is moved from the B/C atoms to the nitrogen atom in all isomers, or C6 in 1,3-azaborine. This is the result of the conformational change that places the NLP in the plane of the ring and the NH bonds in a favourable spatial position to act as acceptors of π-electron density.

Method and basis set dependence of the NICS indexes of aromaticity for benzene

The role of theory level in prediction of benzene magnetic indexes of aromaticity is analysed and compared with calculated nuclear magnetic shieldings of ³ He used as NMR probe. Three closely related nucleus-independent chemical shift (NICS) based indexes were calculated for benzene at SCF-HF, MP2, and DFT levels of theory and the impact of basis set on these quantities was studied. The changes of benzene NICS(0), NICS(1), and NICS(1)zz parameters calculated using SCF-HF, MP2 and several density functionals were within 1 to 3 ppm. Similar deviations between magnetic indexes of aromaticity were observed for values calculated with selected basis sets. Only very small effect of polar solvent on benzene aromaticity was predicted. The ³ He nuclear magnetic isotropic shielding (σ) and its zz-components (σ_zz ) of helium atom approaching the centre of benzene ring from above produced similar curves versus benzene-He distance to NICS parameters calculated for similarly moving Bq ghost atom. We also propose an experimental verification of NICS calculations by designing the ³ He NMR measurement for benzene saturated with helium gas or in low temperature matrices.