Frequencies and Normal Modes of Vibration of Benz[a]anthracene Radical Ions

Abstract

MINDO/3-FORCES calculations were carried out for the radical ions of benz[a]anthracene. Both ions exhibit Cs symmetry with C-C bond alternation in all four rings. The obtained equilibrium geometry was applied for the calculation of all 3N − 6 vibration frequencies of each ion, and for the analysis of their normal coordinates. The so calculated frequencies of the radical cation were close to the experimental frequencies and those of the ab initio calculations. They fall in the ranges νCHstr. (3034 - 3087 cm−1), νCCstr. (1237 - 1609 cm−1), δCH (1142 - 1216 cm−1). Interesting correlations could be obtained for the frequencies of similar vibrations, e. g. νsymCHstr. >νasymCHstr. Exception is the frequency of vibration of CHα in ring A for the radical cation and the same bond in ring D for the radical anion. The vibration frequencies for the CH bonds depend on the σ -electron densities of the corresponding carbon atoms, i. e. νCH.+str. >νCHstr. >νCH.−str., where σ −ρĊ + >σ −ρC >σ −ρĊ − . For the C-C stretching vibrations the relation ν(C-C)str. >ν(C-C).−str. >ν(C-C).+str. holds, with the exception of the Cβ -Cβ bond, for which the relation ν(C-C)str. >ν(C-C).+str. >ν(C-C).−str. is found. As for the in-plane and out of-plane deformations, the following general correlations δ (CH) >δ (CH).− >δ (CH).+ and γ (CC) >γ (CC).− >γ (CC).+.