Comparison of the Vibration Frequencies of [6] Cyclacene (Zigzag, Armchair and Chiral) Monoring Molecules

Abstract

The (3N-6) vibration frequencies and IR-absorption intensities of [6] Cyclacene (zig
zag, armchair and chiral) monoring molecules were calculated applying Density Functional
Theory (DFT) of the type (B3LYP) and a Gaussian basis (6-311G) method. Comparison of the
results showed that, similar to the polyaromatic hydrocarbons, the following relations hold:
νsym. CH str. > ν asym. CH str.
and,
νsym. CC str. > ν asym. CC str. i. e.,
νsym. CC str.(axial.) > νasym. CC str.(axial) > νasym. CC str.(circumferential) in the zigzag
molecule, where axial are the vertical C-Ca bonds (annular bonds) in the rings and
circumferential C-Cc are the outer ring bonds. The relation for the C-C str. is reversed in the
armchair and the chiral molecules, since the C-Ca bonds are converted to C-Cc bonds in the
armchair and chiral cyclacenes. The results include the assignment of all puckering, breathing
and clock-anticlockwise bending vibrations. They allow a comparative view of the charge
density at the carbon atoms too.