Vibration Frequencies and Normal Coordinates of Perylene with Different Molecular Charges A Mindo/3-Forces Treatment

Abstract

The MINDO/3-FORCES method was applied for the calculation of the
vibration frequencies and normal coordinates of the neutral aromatic perylene
molecule and its radical cation and anion. The resulting frequency values were quite
near the experimental values when available. Comparison of the results showed that
for the C-H and C-C stretching vibrations the sym. mode shows higher frequency
values than the asym. As for the δCH and δCC deformation the asym. vibrations
where of higher frequency values than the sym. Comparing the frequencies of the
three species with each other it was found that;

νCH str. (C20H12)˙ + > νCH str. (C20H12) > ν CH str. (C20H12)˙ ¯
and,
νsym CC str. > νsym CC str.˙ ¯ > νsym CC str. ˙ +
νasym CC str.˙ + > νasym CC str. > νasym CC str. ˙ ¯
These relations are discussed in terms of the change of the σ electron densities at the
C atoms of the C-H bonds and the changes in the C-C bond orders of the related
bonds.