Vibrational spectra and density functional calculations of bridged [14]annulenes with an anthracene perimeter

Vibrational spectra of two representative bridged [14]annulenes with an anthracene perimeter, 1,6:8,13-ethane- 1,3-diylidene[14]annulene and 1,6:8,13-propane-1,3-diylidene[14]annulene are presented and discussed on the basis of density functional calculations with the B3LYP functional and 6-31G** and cc-pVDZ basis sets. Infrared and Raman spectra of polycrystalline samples have been measured at room temperature. The Raman spectra have been obtained exciting at 647.1 and 1064 nm, that is, in preand off-resonance excitation conditions, respectively. Calculated structures of 4 and 5 are aromatic according to geometric criteria of aromaticity. Observed vibrational frequencies and infrared and Raman intensities of 4 and 5 are well reproduced by the present calculation using scaling factors of linearly condensed aromatic hydrocarbons taken from the literature. A correlation between vibrational modes of anthracene and ring modes of 4 and 5 is attempted in agreement with the aromatic nature of the [14]annulene ring. The effect of reduced symmetry and bridge structure on infrared and Raman intensities is discussed.