Aromaticity of benzenoid hydrocarbons with inserted –B=B– and –BH–BH– groups: a comparison

Abstract

Structures of selected polycyclic conjugated hydrocarbons with -B=B-and -BH-BH-moieties inserted in different places were calculated at the B3LYP/6-311??G** level and their aromatic properties evaluated. HOMA, NICS(0), NICS(1)zz, K and PDI indices were used for studying their aromatic properties. Both optimized planar (as in parent hydrocarbons) and non-planar structures were taken into account. It is shown that insertion of both types of boron groups disturbs and decreases the aromaticity of the corresponding hydrocarbons. The decreasing effect of the -BH-BH-group is much stronger. What is quite intriguing is that it appears that non-planar structures of the studied compounds have a little higher aromaticity than the strictly planar ones. Mutual correlations between results obtained by different aromaticity indices are calculated and thoroughly discussed.

Quantitative Assessment of Aromaticity and Antiaromaticity Utilizing Vibrational Spectroscopy

“…Accordingly, the peripheral delocalization is weak, which is typical of a nonaromatic molecule (0.274). Pleiadiene (29) can be considered as a butadieno-bridged naphthalene, which gets some local aromaticity because of the latter: peripheral AI: 0.582, ring B: 0.112, but each of rings A: 0.584. By this, the local aromaticity is comparable to that of acenaphthylene, but larger than that of azulene.…”

“…During the last 80 years since the formulation of the Hückel rules, the concepts of aromaticity and antiaromaticity have been probed with regard to almost every molecular property. A multitude of methods and procedures has been developed to define and measure the degree of aromaticity in polyaromatic hydrocarbons (PAHs), their heteroatomic analogues, − to estimate the influence of substituent effects on aromaticity, − and to determine the degree of aromaticity in nonplanar compounds. − The performance of the different approaches has been tested and compared. − …”