ИСТИНА

TD-DFT method is used to compare calculated values of excitation energies of [n]-cyclacenes and BN ones. [n]-cyclacenes are a kind of macrocyclic belt-shaped molecules and structurally related to nanotubes (n, 0). According to analytical estimations within Huckel model π-electron spectrum of alternant BN systems is characterized by high value of HOMO-LUMO gap. Energies of π-MO of BN- and carbon cyclacenes of the same topology are linked by the equation xBN2= xCC2+ξ2, where xBN=(αB+αN-2ε)/2βBN, xCC=(αC-ε)/βCC, ξ= (αN-αB)/2βBN, αB, αN, αC, βBN and βCC are conventional Huckel parameters. TD-DFT estimates show that excitation energies in BN-systems are much more in comparison to carbon cyclacenes. This fact indirectly indicates that cyclacenes have free radical nature and explains unsuccessful attempts to synthesize them. TD-DFT (B3LYP, 6-31G*) calculations were carried out for BN-[n]-cyclacenes (n = 7 - 14). Excitation energies increase quite monotony in the range from 4.9 eV to 6.3 eV with the growth of the system. This trend quite agrees with the value of energy gap in hexagonal boron nitride (5.2 eV). In contrast to carbon cyclacenes boron nitride systems can be simulated in closed-shell approximation. TD-DFT studies and qualitative Huckel estimates predict that boron nitride would have no color in comparison to their carbon analogs. The authors acknowledge gracefully the financial support from RFBR (grant № 10-03-00665)