Vibrational Spectra and Density Functional Calculations of Bridged [14]Annulenes with an Anthracene Perimeter

Abstract

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 (4 and 5 in Figure 1, respectively), 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.

Conjugated Polymers and Aromaticity

“…This mode corresponds to the Kekulé (BLA) coordinate. Note that this level of theory is deemed as reliable for investigating the structures and vibrational properties of aromatic systems …”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Both 8 and 9 have approximate bond equalized geometries, 1.38-1.41Å [235,236]. These structures have been calculated with different computational methods [237][238][239]. It may be seen from Table 12 that 6, 8 and 9 have bond equalized C 2v structures.…”

“…The infrared and Raman spectra of the two molecules have been measured and discussed in terms of ring and bridge modes [239], similarly to the bridged [10]annulenes case but with larger mixing between the two types of vibration. Figure 36.…”

“…With the 647.1 nm excitation all the Raman peaks above 800 cm −1 show decreased intensities with respect to those below 800 cm −1 . The peaks at 251, 420 and 615 cm −1 are assigned to modes of anthracene parentage, the lowest to an out-of-plane and the other two to in-plane modes, where the displacements of the bridgehead atoms enlarge the ring size [239]. Therefore, it has been assumed that the equilibrium geometry of the lowest excited state (L b ) has increased transannular distances.…”

Raman and infrared characterization of the vibrational properties of the antimalarial drug artemisinin

“…The B3-LYP exchange-correlation functional and the cc-pVDZ basis set were chosen, as recommended for this type of calculation. 32,33 The vibrational modes are classified in Tables 1 and 2 on the basis of their normal displacements and making extensive use of the Gaussian graphical option.…”