3.6.1. Band Assignments
4000-2000 cm-1 region . In this region,
typical bands corresponding to stretching modes of CH2,
CH3, N-H and C-H groups of S(-) and R(+) forms of CQ are
expected [42-47,51-53,76,77]. The observed weak band at 3461
cm-1 can be assigned to the N3-H30 stretching modes of
both forms while the group of IR bands located between 3257 and 3025
cm-1 are attributed to aromatic C-H stretching modes
of both fused rings. The only aliphatic C3-H27 bonds corresponding to
chiral C7 atom of both forms are predicted by SQM calculations in
different positions, thus, in the S(-) form this modes is assigned to
the weak band at 2870 cm-1 because it is predicted at
2899 cm-1. In the R(+) form that stretching mode is
predicted at 2913 cm-1 and assigned to the IR of
medium intensity at 2936 cm-1. Due to the presence of
five CH2 groups in both forms of CQ, a total of ten
anti-symmetric and symmetric stretching modes are expected for the two
species, hence, these modes are predicted between 2957 and 2755
cm-1 and assigned to observed IR bands in this region.
On the other hand, nine anti-symmetric and symmetric stretching modes
are expected for both forms due to the three CH3 groups;
therefore, they are assigned as predicted by calculations between 2977
and 2900 cm-1. The symmetries of CH2and CH3 modes were no confirmed due to the absence of a
Raman spectrum.
1800-1000 cm-1 region . In this region, the
C-C and C-N stretching modes together with deformation, wagging and
rocking modes of CH2, CH3 and aromatic
and aliphatic C-H groups are expected [42-47,51-53,76,77]. Here, the
N4=C19 stretching mode in the R(+) forms is predicted to 1568
cm-1 with double bond character while in the S(-) form
as partial double bond character at 1262 cm-1, hence,
they are assigned to the strong and medium intensity bands at 1586 and
1258 cm-1, respectively. Also, in the R(+) form that
modes is predicted with higher PED contribution at 1257
cm-1. The C=C stretching modes of both forms are
assigned to the very strong, strong and medium intensities IR bands
between 1611 and 1489 cm-1, as observed in similar
compounds and as detailed in Table 5 [42-47,51-53,76,77]. The
CH2 deformations modes are normally found in the
1485-1410 cm-1 region [42-47,51-53,76,77]. In both
forms of CQ, the SQM calculations predict these modes between 1450 and
1420 cm-1, hence, these vibration modes are assigned
to the IR bands observed in this region. The two anti-symmetric and
symmetric CH3 deformation modes are assigned as
predicted by calculations between 1440 and 1342 cm-1.
The wagging and rocking CH2 modes in both forms are
predicted between 1408/1198 and 1392/1209 cm-1 while
the rocking CH3 modes in the 1099 and 838
cm-1 region. Hence, those vibration modes are assigned
in the regions predicted by SQM calculations. On the other hand, the
aromatic βC-H rocking modes are predicted by SQM calculations in the two
forms between 1403 and 1053 cm-1, as in similar
species and, for these reasons, they are assigned in that region
[42-47,51-53,76,77].
1000-40 cm-1 region. The CH2and CH3 twisting modes are expected in this region
together with C-C and N-C stretching and out-of-plane C-H deformation
modes and other different skeletal modes [42-47,51-53,76,77]. In
both forms, the CH2 and CH3 twisting
modes are assigned as predicted by SQM calculations to the bands
observed between 959/678 and 232/196 cm-1,
respectively [42-47,51-53,76,77]. The deformations and torsions
rings are predicted from 1117 up to 65 cm-1. Then, the
assignments of other vibration modes are specified in Table 5.