Ab initio studies of borazine and benzene cyclacenes and their fluoro-substituted derivatives

Abstract

We have performed unrestricted density functional theory calculations for the borazine and benzene cyclacene systems to obtain insights into their structural and electronic properties as a function of the size of the cyclacene. The frontier molecular orbital gap, ΔEgap, in borazine cyclacene increases steadily with the number of rings, n, and stabilizes at ∼6.7 eV for n > 9, in contrast to the benzene cyclacene system where the gap decreases and approaches 1.13 eV for n > 8. For borazine cyclacene, the top and bottom trannulenes in the rings are structurally different. A stronger delocalization of π electrons in the boron-apexed trannulene compared to the nitrogen-apexed trannulene was discovered. For the carbon cyclacene, we observed alternation in both ΔEgap and the bond distances as the number of electrons in the trannulene chains alternate between 4k and 4k + 2. The effect of fluorine substitution on boron atoms in the borazine cyclacene increases the bonding energy and ΔEgap, whereas substitution on nitrogen atoms results in the opposite. A special π-type interaction between the fluorine lone-pair p electrons and the B-N-B σ-bonding framework results in special enhancement in the stability of the F-B substituted systems. F substitution on carbon cyclacene increases the ΔEgap when n is even but decreases ΔEgap when n is odd.