Theoretical study of borazine and its fluoroderivatives:: Aromaticity and cation-π, anion-π interaction

A systematical estimation of aromaticity of borazine and its fluoroderivatives has been conducted using structural, energetic and magnetic criteria. Estimations based on Aromatic Stabilization Energy (ASE) and Magnetic Susceptibility Exaltation (MSE) calculations predict fluorinations on boron and nitrogen both decrease molecular aromaticity. This result is further reinforced by the topological analysis using Atoms in Molecules (AIM) method. AIM analysis indicates pi component of N-B bonds is reduced upon fluorination both on boron or nitrogen. Nucleus Independent Chemical Shift (NICS) values calculated at several points above ring center (NICS(0), NICS(0.5) and NICS(1)) fail to give the result consistent with that based on ASE and MSE criteria, while the NICS tensor components corresponding to the principal axis perpendicular to the ring plane calculated at points 2 angstrom above. ring center, NICSzz(2), have been found to be able to provide qualitatively correct results. The cation-pi and anion-pi interactions in complexes of borazine (1) and B-trifluoroborazine (4) with cations and anions have also been systematically investigated. The results show that both 1 and 4 are able to form cation-pi complexes with cations, while anion-pi interactions are also energetically favorable in complexes of 1 with F-, Cl-, and complexes of 4 with F-, Cl-, Br-. The bonding nature of cation-pi and anion-pi interactions has been further explored by means of AIM and MIPp methods. (c) 2004 Elsevier B.V. All rights reserved.