3.6. Vibrational study
In this analysis, we have considered the two forms of CQ because the
energy differences between both forms in gas phase and solution are low
(1.83 and 3.67 kJ/mol) and, also due to the fact that in gas phase the
S(-) form is the most stable while in aqueous solution the R(+) form is
the most stable one. Hybrid B3LYP/6-311++G** calculations have optimized
both enantiomers S(-) and R(+) of CQ with C1symmetries and, as their structures have 53 atoms a total of 153
vibration modes are expected in the experimental spectra. In this case,
all vibration modes are active in both infrared and Raman spectra. The
experimental available IR spectrum was taken from that reported in the
solid phase in KBr pellet [1]. Comparisons of that experimental
spectrum with the corresponding theoretical for both forms in gas phase
are presented in Figure 6 while the corresponding predicted
Raman spectra can be seen in Figure 7 . The theoretical Raman
spectra predicted in activities were corrected to intensities by using
known equations [65,66]. The SQMFF methodology and the Molvib
program were used to calculate the harmonic force fields for both forms
of CQ in gas phase by using B3LYP/6-311++G** calculations together with
transferable scaling factors and the normal internal coordinates
[38-40]. In the assignments potential energy distribution (PED)
contributions ≥ 10 % were used. Table 5 shows calculated and
observed wavenumbers for the S(-) and R(+) forms of CQ in gas phase by
using B3LYP/6-311++G** calculations. Both Figures 6 and 7 show clear
differences in the intensities of some bands between the S(-) and R(+)
forms.