Effect of overcrowding on mass spectra of polycyclic aromatic hydrocarbons; co-operative double cyclization of two methyl substituents at overcrowded positions

Abstract

Mass spectra of 1,10-(2) and 3,12-dimethyltetrabenzo[de,hi,op,st]pentacene (3), and 14-methyl-5-(2-methyl-1-naphthyl)-(5) and 12-methyl-5-(4-methyl-1-naphthyl)-benzo[a]perylene (6) were investigated to elucidate the effect of methyl substitution at overcrowded positions. Compound (2), showed a very strong [M– 6]ⁱ⁺ peak, whereas the spectra of (3), (5), and (6), which were similar to each other, showed intense [M– 32]ⁱ⁺ peaks. Thus, only in the case of the overcrowded compound (2) did a cyclization reaction occur. In the case of the analogous structure (5) release from overcrowding is achieved by twisting of the ring planes. This reaction also occurred on heating (2) in quinoline.