D. Luo et al. / Journal of Molecular Structure 1198 (2019) 126857
5
ring and the indole ring, which can be explained by the resonance
effect. In addition, according to the above analysis, the vibrational
frequencies observed at 1113 cmꢀ1 in the IR spectrum are assigned
to the stretching vibrations of the ether, which is located in the
morpholine ring.
3.4.3. CeBr vibrations
The vibration belonging to the CeBr bond, through which the
halogen atom and the ring are linked, is indicated since it is possible
that the vibrations are mixed due to the lowering of the molecular
symmetry and the presence of heavy atoms [30e32]. In the organic
halogen
compounds,
the
bands
at
1360e1000 cmꢀ1
,
760e505 cmꢀ1, and 650e485 cmꢀ1 may be assigned to the CeF,
CeCl, and CeBr stretching vibrations, respectively [33]. In our dis-
cussion, a shift in the absorption frequencies of CeBr may occur
owing to the vibrational coupling with neighboring C groups. The
strong IR bands obtained at 710 cmꢀ1 and 604 cmꢀ1 for C30Br33 and
C2Br25, respectively, may be attributed to the stretching vibrations
of CeBr.
Fig. 4. Molecular electrostatic potential map of conformer 1e4.
3.4.4. C]C vibrations
The peaks at approximately 1600 cmꢀ1 and 1500 cmꢀ1 are the
characteristic absorption bands of the stretching vibrations of the
aromatic ring skeleton. In compound 1, one of the C]C stretching
vibrations of the aromatic ring overlaps with the vibrations of the
C]O stretching mode. The bands at 1560 cmꢀ1 and 1550 cmꢀ1 may
be assigned to the stretching vibrations of C]C. Moreover, the C]N
stretching vibrational bands, which are too weak to be identified
except in the presence of other substituent groups in the IR spec-
trum, usually coincides with the C]C vibrational bands in the re-
gion of 1690e1590 cmꢀ1. Consequently, it is not discussed herein.
Fig. 5. The highest occupied and lowest unoccupied molecular orbitals of conformer
1e4 obtained by the DFT/6311G (2 d, p) method.
3.4.5. eCH group vibrations
The aromatic CeH stretching vibrations in the substituted
benzene rings are generally indicated by the bands in the high
region of 3000e3100 cmꢀ1 [34,35]. As shown in Fig. S2, the weak
band at 3054 cmꢀ1 in the IR spectrum can be assigned to the CeH
stretching modes of the pyrimidine ring. For this compound, the
bands due to the CeH stretching modes of the benzene ring were
encountered during chemical process [27e29]. The hardness of
compound 1 is 2.1844 eV.
3.4. Vibrational analysis
observed at 3451 cmꢀ1
.
3.4.1. Carboxylate group (eCOOe) vibrations
For the primary ester groups, the stretching deformation vi-
bration is generally indicated by a sharp absorption peak in the
region around 1735 cmꢀ1 (VC¼O) and two or three strong correla-
tion absorption bands in the region between 1300 and 1050 cmꢀ1
(VCeOeC). The frequency band indicative of C]O shifts to the right
if the carbonyl group in the ester group is conjugated to the aro-
matic ring. Thus, the stretching vibration of C]O is indicated by a
very strong band at 1688 cmꢀ1, as showed in Supporting Informa-
tion. This band is attributed to the relatively strong conjugation
effects of the indole ring, and the absorption bands at 1688 cmꢀ1
were assigned to the asymmetrical stretching mode of C]O in
molecule 1.
3.4.6. eCH2 group vibrations
Compound 1 has four CH2 groups with sp3 hybridization. Here,
the bands indicative of the asymmetric stretching and scissoring
vibration of the methylene group are expected in the regions of
2940e2915 cmꢀ1 and 1385e1345 cmꢀ1, respectively [36]. In addi-
tion, the average peak at 2936 cmꢀ1 in the IR spectrum is attributed
to the asymmetric stretching vibration mode of CH2(C15), while the
strong bands at 1479 cmꢀ1 indicate the scissoring vibrations of
CH2(C17), CH2(C18), and CH2(C21).
3.4.7. eCH3 group vibrations
3.4.2. Ether (CeOeC) vibrations
Compound 1 has three CH3 groups. The bands, indicative of the
methyl (eCH3) symmetric stretching frequencies, appear in the
region of 2885e2860 cmꢀ1, whereas those of the asymmetric
stretching frequencies lie in the range of 2975e2950 cmꢀ1 [37]. In
Fig. S2, the bands at 2970 cmꢀ1 and 2876 cmꢀ1 are assigned to the
asymmetric stretching vibration and symmetric stretching vibra-
tion of CH3, respectively. Two strong and sharp peaks indicative of
the asymmetrical deformation vibration and symmetrical defor-
The CeOeC stretching vibrations are expected within the region
of 1270e1010 cmꢀ1. Both the asymmetric stretching vibration fre-
quency and the symmetric stretching peak intensity increase when
the oxygen atom of ether oxy is connected to the aromatic ring or
alkenyl group. There are two kinds of ethers positioned at different
chemical locations in the title molecule. One is an aromatic oxide
and its asymmetrical stretching vibration and symmetrical
stretching vibration frequencies are 1238 cmꢀ1 and 1036 cmꢀ1
respectively, since the ether group is connected to the pyrimidine
,
mation vibration are observed at 1445 cmꢀ1 and 1373 cmꢀ1
respectively.
,
Luo, Dali
