Study on the Aromaticity of Heterobenzenes C5H5X (X＝N, P, As, Sb, Bi) by Nucleus Independent Chemical Shifts (NICS) and Isomerization Stabilization Energies (ISE)

Abstract

In different research fields nucleus independent chemical shift (NICS) has been used frequently as a convenient tool for obtaining information about induced dia-paratropic and paratropic ring currents, especially for the purpose of assign- ing aromaticity and anti-aromaticity to molecules, obviously, aromaticities and magnetic properties of molecules can be measured by NICS. This paper presents the investigation on some important features of benzene and heterobenzenes C5H5X (X=CH, N, P, As, Sb, Bi) based on the analyzing maximum value of NICS above the cycle planar about 0.8~0.9 A. Com- pared with density functional theory (DFT), the calculated results by ab initio (HF) are more accurate and acceptable for the NICS of heterobenzenes. There are two primary reasons to explain the phenomenon. First, the correlation coefficient of the proton chemical shifts of 1 H NMR between calculated and experimental data by HF method is larger than adopted DFT methods. Second, because the ghost position plays a critical role in judging aromaticity, only involving HF method satisfy the results through comparing NICS(1) with the aromaticity of benzene, pyridine and other heterobenzenes in normal tempera- ture and pressure. Furthermore, according to the calculation of natural localized molecular orbitals (NLMO), we can get a conclusion that the π bonds are main contributions to the zz tensor of NICS(max), and the order is benzene>pyridine> phosphabenzene>arsabenzene>stibabenzene>bimabenzene. σ bonds all show σ aromaticity. However, other bonds' anti-aromaticity is revealed. In order to be further studied for aromaticity and chemical shifts of 1 H NMR about the hetero- benzenes, the magnitudes induced are depicted by the outer magnetic field. For pyridine, the magnitudes of red area coincide with the proton chemical shifts (α-H>γ-H>β-H); others are in accord with the order (α-H>β-H>γ-H). Finally, the induced magnitudes by the outer magnetic field show the same global aromaticity characteristics as NICS(max)zz and π bonds con- tributing to NICS(max)zz. These results all represent that the order of global aromaticity is benzene> pyridine> phosphabenzene>arsabenzene>stibabenzene>bimabenzene. Especially, the π molecular orbital excluding the X atom pre- sents a surprising high π aromaticity. It is very useful for us to design new materials in the magnetic theory. Keywords heterobenzenes; aromaticity; nucleus independent chemical shifts (NICS); isomerization stabilization energies; natural localized molecular orbitals