In search of aromatic seven-membered rings

A theoretical search for aromatic seven-membered rings has been carried out using density functional theory calculations with the B3LYP functional. The rings considered include the C7H7+ cation and its hetero-derivatives by replacing C, CH or CC units by B, Al, Ga, Si, Ge, N, P, As and BN group, the B82− dianion, the CB7− anion, the neutral S3N4 ring and its derivatives by substituting one or two N atoms by CH group, the S4N3+ cation and the all-metallic cycles M73− and M7T with M = Cu, Ag, Au and T = Y, Sc. Most molecules studied have planar structure in their electronic ground state. Vibrational spectra of some derivatives are plotted. Nucleus independent chemical shift (NICS) indices show that the parent molecule C7H7+ has a higher degree of aromaticity than its derivatives. Substitution of N in S3N4 by CH marginally influences the aromaticity in such a way that S3N4, S3N3(CH) and S3N2(CH)2 are similarly aromatic. The B82− dianion and both D7h and C2v isomers of CB7− possess comparable aromatic character. As for the all-metal clusters Cu73−, Ag73−, Au73−, Cu7Sc, Cu7Y, Ag7Y and Au7Y, the binary clusters become more aromatic than the pure metal anions, and are thus characterized by σ-aromaticity.