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.