Molecular structures of 2-arylaminomethyl-1H-benzimidazole: Spectral, electrochemical, DFT and biological studies

Article abstract of DOI:10.1016/j.saa.2012.01.080

In the present work, structural studies on (1H-benzimidazol-2-ylmethyl)-N- (4-chloro-phenyl)-amine (L¹) and (1H-benzimidazol-2-ylmethyl)-N-(4- iodo-phenyl)-amine (L²) have been done extensively by a variety of physico-chemical techni

Full text of DOI:10.1016/j.saa.2012.01.080

Spectrochimica Acta Part A 91 (2012) 272–284
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Molecular structures of 2-arylaminomethyl-1H-benzimidazole: Spectral,
electrochemical, DFT and biological studies
Nour T. Abdel Ghani, Ahmed M. Mansour∗
Chemistry Department, Faculty of Science, Cairo University, Gamaa Street, Giza 12613, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, structural studies on (1H-benzimidazol-2-ylmethyl)-N-(4-chloro-phenyl)-amine
(L1) and (1H-benzimidazol-2-ylmethyl)-N-(4-iodo-phenyl)-amine (L2) have been done extensively by
a variety of physico-chemical techniques. Optimized geometrical structures, harmonic vibrational fre-
quencies, natural bonding orbital (NBO) analysis, and Frontier molecular orbitals (FMO) were obtained
by DFT/B3LYP method. TD-DFT calculations help to assign the electronic transitions. The polarizable
continuum model (PCM) fails to describe the experimental chemical shift associated with the NH pro-
tons as calculated by applying Gauge-invariant atomic orbital (GIAO) method, but a very good correlation
between the theoretical and experimental values was achieved by taking into account the specific solute-
solvent interactions. DFT calculations showed a good agreement between the theoretical and observed
results. These compounds exhibited a high biological activity through the inhibition of the metabolic
growth of the investigated bacteria.
Received 17 November 2011
Received in revised form 15 January 2012
Accepted 30 January 2012
Keywords:
Benzimidazole
NBO
TD-DFT
PCM
Antibacterial
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
As a continuation of our recent studies on synthesis, biological
activities, structural and theoretical investigations of some sub-
Benzimidazole derivatives have been proved to be an impor-
tant group of fungicides with systemic activity and are well known
for their pronounced ability to control a large number of fungal
diseases [1,2]. Recently, the emergence of resistance to the major
classes of antibacterial agents is recognized as a serious health
concern. Particularly, multidrugs-resistant strain of Gram-positive
bacterial pathogens is a problem of ever-increasing significance,
organisms including methicillin-resistant Staphylococcus aureus
and Staphylococcus epidermidis, as vancomycin-resistant Entero-
cocci [3]. In addition, substituted benzimidazoles, benzoxazoles
and some related heterocycles, which are the structural isosters
of nucleotides owing to the fused heterocyclic nuclei in their
structures, interact easily with biopolymers and possess potential
activity with lower toxicities in the chemotherapeutic approach
in man [4,5]. Furthermore, benzimidazoles were used as ligands
towards metal ions [6] with a variety of biological molecules includ-
ing ionheme systems, vitamin B12 and its derivatives and several
metallo-proteins. These different applications of benzimidazole
compounds have attracted many experimentalists and theorists
to investigate the structural characteristics of benzimidazole and
some of its derivatives.
stituted benzimidazole [6–8], the main aspect of this study are:
(i) synthesis and structural investigation of (1H-benzimidazol-2-
ylmethyl)-N-(4-chloro-phenyl)-amine (L1) and (1H-benzimidazol-
2-ylmethyl)-N-(4-iodo-phenyl)-amine (L2) (Fig. 1) derivatives by
FT-IR, 1H NMR, UV–vis, and elemental analysis. (ii) The molec-
ular geometries, absorption wavelengths, and vibrational spectra
of the title compounds were calculated by applying density func-
tional theory (DFT) computations. (iii) The solvent effect on 1H
NMR data was introduced by applying the polarizable continuum
model (PCM). (iv) HOMO, LUMO, and NBO analysis have been used
to give more information regarding charge transfer within the
molecule.
2. Experimental
2.1. Synthesis
All chemicals used in the preparation and investigation of
the present study were of reagent grade (Sigma). The title com-
pounds were prepared by condensation of equimolar quantities of
2-chloromethylbenzimidazole [7] with 4-chloroaniline (L1) and 4-
iodoaniline (L2) in ethanol in presence of small amount of sodium
iodide for about 24 h [8]. Then, the reaction mixtures were neutral-
ized and the solids were separated by cooling, and re-crystallized
from ethanol.
∗
Corresponding author. Tel.: +20 2 0122211253; fax: +20 2 35728843.
E-mail address: inorganic am@yahoo.com (A.M. Mansour).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.080

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
273
the measurements. A potential range of +2000 to −2000 mV, with
a scan rate of 100 mV s−1 was used.
2.3. Computational calculations
The gas phase geometries of (1H-benzimidazol-2-ylmethyl)-N-
(4-chloro-phenyl)-amine (L1) and (1H-benzimidazol-2-ylmethyl)-
N-(4-iodo-phenyl)-amine (L2) in the ground state were optimized
by DFT/B3LYP method [9] using 6-31G(d) basis set for L1 and SDD
for L2 using GAUSSIAN03 [10] program. The main reason for choos-
ing the SDD basis set is its inclusion of relativistic effect that is
essential for heavy elements. The optimized geometries were ver-
ified by performing a frequency calculation. Frontier molecular
orbitals (FMO) were performed at the same level of theory. Vibra-
tional modes were analyzed using GAUSSVIEW software [11]. Based
on the optimized geometries, the electronic spectra of the studied
compounds were obtained by using time-dependent density func-
tional theory (TD-DFT) [12] method. Net atomic charges had been
obtained by the natural bond orbital (NBO) analysis of Weinhold
and Carpenter [13]. The 1H NMR chemical shifts were computed at
the B3LYP/6-311 + G(2d,p) (L1) and SDD (L2) level of theory in the
gaseous state by applying Gauge-invariant atomic orbital (GIAO)
approach [14] and the values for the 1H, isotropic were referenced
to TMS, which was calculated at the same level of theory. The effect
of solvents (DMSO, and CHCl3) on the theoretical NMR parameters
was performed using the polarizable continuum model (PCM).
Fig. 1. Optimized structures of the benzimidazoles (L1,2).
2.4. Biological activity studies
Data for L1 (C14H12ClN3). Color: yellow. MS: M+ = 257 (calcd.
257.72). Anal. calcd. %C, 65.18; %H, 4.66; %Cl, 13.77; %N, 16.30.
Found: %C, 65.49; %H, 4.59; %Cl, 13.69; %N, 16.13. FT-IR: 3445
(␯(NH)sec), 3055 (␯CHass/An), 2894 (␯CH2ass), 2827 (␯CH2ss), 1597
(␯CCss/An), and 1309 cm−1 (␯(C N)sec). 1H NMR (DMSO): ı 12.21
(1H, s, benzimidazolic NH); ı 7.47, 7.46, 7.13, and 7.10 (4H, m,
benzimidazole ring (Bz)); ı 7.08, 7.06, 6.64, and 6.62 (4H, dd, ani-
line ring (An)), ı 6.45 (1H, t, NHsec), and ı 4.42 (2H, d, CH2). UV–vis
(ethanol): 204, 222, 250, 274, and 280 nm.
•
•
The antimicrobial activities of the test samples were deter-
mined by means of a modified Kirby-Bauer disc diffusion method
[15] under standard conditions using Mueller-Hinton agar medium
(tested for composition and pH), as described by NCCLS [16].
The antimicrobial activities were carried out using culture of
Bacillus subtilis, S. aureus, and Streptococcus faecalis as Gram-
positive bacteria and Escherichia coli, Pseudomonas aeruginosa,
and Neisseria gonorrhea as Gram-negative bacteria. The solution
of 100 mg/ml of each compound (studied compounds and stan-
dard drug Tetracycline) in DMSO was prepared for testing against
bacteria. Centrifuged pelletes of bacteria from a 24 h old culture
containing approximately 104–106 CFU (colony forming unit) per
ml were spread on the surface of Mueller-Hinton agar plates. Then
the wells were seeded with 10 ml of prepared inocula to have
106 CFU/ml. Petri plates were prepared by pouring 100 ml of seeded
nutrient agar. DMSO (0.1 ml) alone was used as control under the
same conditions for each microorganism, subtracting the diameter
of inhibition zone [17] resulting with DMSO, from that obtained
in each case. The antimicrobial activities could be determined as a
mean of three replicates.
Data for L2 (C14H12IN3). Color: yellow. MS: M+ = 349 (calcd.
349.17). Anal. calcd. %C, 48.16; %H, 3.46; %I, 36.34; %N, 12.03.
Found: %C, 47.97; %H, 3.69; %I, 35.98; %N, 11.91. FT-IR: 3440
(␯(NH)sec), 3052 (␯CHass/An), 2875 (␯CH2ass), 1691 (␯(C N)),
1588 (␯CCss/An), and 1308 cm−1 (␯(C N)sec). 1H NMR (DMSO):
ı 12.25 (1H, s, benzimidazolic NH); ı 7.49, 7.48, 7.14, and 7.12
(4H, m, benzimidazole ring (Bz)); ı 7.36, 7.33, 6.53, and 6.49 (4H,
dd, aniline ring (An)), ␦ 6.50 (1H, t, NHsec), and ı 4.46 (2H, d, CH2).
UV–vis (ethanol): 204, 222, 254, 273 and 280 nm
2.2. Instruments
Elemental microanalyses were performed at the Micro-
analytical Center, Cairo University. The analyses were repeated
twice to check the accuracy of the analyzed data. The mass spec-
tra were recorded with the aid of a SHIMADZU QP-2010 plus mass
spectrometer at 70 eV. Infrared spectra were recorded as KBr pellet
using FTIR-460 plus, JASCO, in 4000–200 cm−1 region. The 1H NMR
spectra were run at 300 MHz in DMSO-d6 using Varian-Mercury
VX-300 NMR. The UV–vis measurements were carried out using
automated spectrophotometer UV–vis SHIMADZU Lambda 4B using
1 cm matched quartz cells. Cyclic voltammetry measurements were
performed using a three-electrode configuration cell connected to
“EG&G” scanning potentiostat model 372. A Pt disc was used as
working electrode and a Pt wire as auxiliary electrode. The ref-
erence electrode was Ag/AgCl electrode adjusted to 0.00 V versus
SCE. Sample solutions (50 ml) of 10−3 M concentration in acetoni-
trile with 0.1 M NaClO4·H2O as supporting electrolyte were used for
3. Results and discussion
3.1. Vibrational analysis
The theoretical IR spectra of the investigated compounds (L1,2
)
were obtained at DFT/B3LYP level of theory using 6-31G(d) and SDD
basis sets, respectively (Fig. 2). All band assignments are presented
in Tables 1 and 2. It is well known that the calculated harmonic fre-
quencies by DFT method are usually higher than the corresponding
experimental quantities due to the electron correlation approxi-
mate treatment, anharmonicity effects and basis set deficiencies,
etc. [18]. To compensate these shortcomings, scale factors were
introduced and an explanation of this approach was discussed [19].
Two different methods were applied: (i) uniform scaling [19], the
scaling factor is 0.963 for B3LYP/6-31G(d) and 0.970 for B3LYP/SDD

274
N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
Table 1
Observed and calculated selected frequencies (cm−1) and assignments of the fundamental modes for (1H-benzimidazol-2-ylmethyl)-N-(4-chloro-phenyl)-amine (L1).
No.
Exp. frequency
Calculated un-scaled
frequency B3LYP
Scaled frequency
uniform scaling
Scaled frequency linear
regression scaling
Vibrational assignments
B3LYP/6-31G(d)
1
2
3
4
5
6
7
8
9
3649
3562
3221
3219
3219
3209
3204
3197
3188
3185
3000
2945
1683
1666
1642
1631
1595
1559
1544
1538
1497
1470
1449
1425
1369
1367
1337
1333
1290
1258
1248
1216
1207
1183
1165
1144
1136
1046
1042
674
3513
3430
3101
3099
3099
3090
3085
3078
3070
3067
2889
2836
1620
1604
1581
1570
1535
1501
1486
1481
1441
1415
1395
1372
1318
1316
1287
1283
1242
1211
1201
1171
1162
1139
1121
1101
1093
1007
1003
649
3515
3431
3102
3101
3101
3091
3086
3079
3071
3068
2890
2837
1621
1604
1581
1571
1536
1501
1487
1481
1442
1415
1395
1372
1318
1316
1287
1283
1242
1211
1202
1170
1162
1138
1121
1101
1093
1006
1003
648
␯NHss/Bz
␯NHss/An
␯CHss/An
␯CHass/An
␯CHss/Bz
␯CHass/Bz
␯CHass/An
␯CHass/Bz
␯CHass/Bz
␯CHass/An
3445
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
3055
2894
2827
ass
CH2
CH2
ss
␯CC/Bz + ␯CN/Bz + ␤NHsc/Bz
␯CC/An
1597
␯CC/Bz + ␤NH/Bz
␯CC/An + ␤NHsc/An
␯CC/Bz + ␤NH/Bz + ␤NH/An + ␦sCH2 + ␯CN/Bz
␤NH/An + ␦sCH2 + ␯CC/An
␤NH/An + ␦sCH2
␯CC/An + ␤NH/An + ␻CH2
␯CC/Bz (boat shape)
␦sCH2 + ␯C NH/Bz
␯CC/Bz + ␯CC/An + ␤NH/Bz + ␤NH/An
␤NH/Bz + ␯CC/Bz + ␤NH/An + ␻CH2
␯CC/An + ␤NH/Bz + ␻CH2
␯CC/An + ␯C NH/An
␤CH/An
1501
1431
1309
1270
␤CH/Bz + ␤CH/An + ␻CH2
␤NH/An + ␻CH2
␶CH2 + ␤NH/Bz + ␤CH/An
␶CH2
1184
1115
␤CHss,sc/An + ␤NH/Bz
␤CHss,sc/An + ␤NH/Bz + ␤CHss,sc/Bz
␤CHss,sc/Bz
␤CHss,sc/An + ␯CH2 NH/An + ␤NH/Bz
␤CHss,sc/Bz
1088
1019
␤CHss,sc/An + ␯CH2 NH/An
␤CH/Bz + ␳CH2
␳CH2
40
41
42
1024
1020
975
986
982
938
985
981
938
Rtorsion/Bz
Rtrigd/An
␥CH/Bz
917
931
896
896
862
830
829
758
729
729
43
806
744
942
931
907
896
906
896
␥CH/An
825
794
793
808
778
777
704
677
677
44
45
46
47
48
49
911
669
648
631
443
579
877
644
624
607
876
643
623
607
425
556
Rtorsion/Bz
␯C Cl
Rtrigd/An
Rtrigd/Bz
␥NH/Bz
657
613
438
499
426
␥NH/An
R: ring; ss: symmetric stretching; ass: asymmetric stretching; ␯: stretching; ␤: in-plane bending; ␥: out-of-plane bending; ␦s: scissoring; ␻, wagging; ␶: twisting; ␳: rocking;
trig: trigonal; trigd: trigonal deformation.
(ii) linear regression method [19], in this method, the plot of the cal-
culated frequencies versus their experimental values resulted in a
straight line, whose equation was used to correct the calculated
frequencies (␯calcd.) (Fig. 3).
capable of associating through hydrogen bond formation [20]. The
scaled values at 3513 and 3569 cm−1 are assigned to the stretch-
ing mode of the benzimidazolic NH group in the benzimidazoles
(L1,2). The presence of a sharp band at 3445 (L1) and 3440 cm−1 (L2)
indicated the existence of free secondary amino group (NHsec) and
these values deviate from the theoretically scaled modes, 3430 and
3471 cm−1, respectively. This discrepancy may be justified on the
basis that the calculations were performed on a single molecule in
the gaseous state, whereas packing molecules with intermolecular
interactions are treated in the experimental measurements.
Benzimidazole derivatives (L1,2) have a strong intermolecular
hydrogen bond [8], between the pyridine-type nitrogen, and the
benzimidazolic NH group, which makes the IR spectra show a
broad absorption band in the region 3500–2200 cm−1. It is not
surprising to find this effect since benzimidazole and imidazole
derivatives, possessing free imino hydrogen, are known to be

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
275
Table 2
Observed and calculated selected frequencies (cm−1) and assignments of the fundamental modes for (1H-benzimidazol-2-ylmethyl)-N-(4-iodo-phenyl)-amine (L2).
No.
Exp. frequency
Calculated un-scaled
frequency
Scaled Frequency
Uniform scaling
Scaled frequency linear
regression scaling
Vibrational assignments
B3LYP/SDD
1
2
3
4
5
6
7
8
3680
3579
3239
3234
3230
3225
3210
3209
3198
3194
2989
2967
1675
1653
1635
1612
1580
1547
1533
1516
1513
1480
1446
1433
1427
1377
1340
1325
1290
1249
1248
1229
1193
1166
1140
1134
1049
676
3569
3471
3141
3136
3133
3128
3113
3112
3102
3098
2899
2877
1624
1603
1585
1563
1532
1500
1487
1470
1467
1435
1402
1390
1384
1335
1299
1285
1251
1211
1210
1192
1157
1131
1105
1099
1017
655
3543
3446
3118
3113
3109
3105
3090
3089
3079
3075
2877
2856
1612
1591
1573
1551
1520
1488
1475
1459
1456
1424
1391
1379
1373
1325
1289
1275
1241
1201
1201
1182
1148
1122
1096
1091
1009
650
␯NHss/Bz
␯NHss/An
␯CHss/Bz
␯CHss/An
␯CHass/An
␯CHass/Bz
␯CHass/Bz
␯CHass/An
␯CHass/Bz
␯CHass/An
3440
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
3052
2875
ass
CH2
CH2
ss
1691
1588
␯CC/Bz + ␯CN/Bz
␯CC/An
␯CC/Bz + ␤NHsc/Bz
␯CC/An + ␤NHsc/An
␯CC/Bz + ␤NH/Bz + ␤NH/An + ␦sCH2
␤NH/An + ␦sCH2
␤NH/An + ␦sCH2
␯CC/An + ␤NH/An + ␻CH2
␯CC/Bz + ␯CN/Bz
␯CC/Bz
␦sCH2 + ␯C NH/Bz
␯CC/Bz + ␯CC/An + ␤NH/Bz
␯CC/Bz + ␯CC/An + ␤NH/An + ␻CH2
␯CC/An + ␯CN/Bz
␤CH/An
␤CH/An + ␤CH/Bz + ␤NH/An
␤CC/Bz + ␤CH/Bz
␶CH2
1494
1425
1308
1268
1223
␤CH/Bz + ␤NH/Bz
␤CHss,sc/An + ␤NH/Bz
␤CHss,sc/Bz
1119
1018
␤CHss,sc/An + ␯CH2 NH
␤CHss,sc/Bz
␤CHss,sc/An + ␯CH2 NH
␳CH2
38
39
992
877
1034
1024
983
1002
993
953
994
985
945
Rtorsion/Bz
␥CH/Bz
897
870
862
799
775
768
786
762
756
40
41
42
1009
1004
995
976
851
978
973
965
946
970
966
957
939
Rtorsion/Bz
Rtrigd/An
␥CH/An
801
743
497
825
818
836
810
804
722
700
694
521
505
500
424
411
407
43
44
908
647
880
627
873
622
Rtrigd/Bz
Rtrigd/An
626
607
601
Para deformation of aniline ring
45
46
47
575
603
562
302
584
545
292
579
540
289
␥NH/Bz
␥NH/An
␯C-I
R: ring; ss: symmetric stretching; ass: asymmetric stretching; ␯: stretching; ␤: in-plane bending; ␥: out-of-plane bending; ␦s: scissoring; ␻: wagging; ␶: twisting; ␳: rocking;
trig: trigonal; trigd: trigonal deformation.
The band at 1691 cm−1 in the p-I derivative (L2) is assigned to
ꢀ(C N) [21] and is slightly deviated from the un-scaled calculated
mode, 1674 cm−1. It is possible to notice that in the latter range,
the scaling is not necessary, as already pointed by Agathabay et al.
[22] and Miranda et al. [23]. However, in the p-Cl derivative, this
band is overlapped with the aromatic C C bands in the same region
under the effect of the intermolecular hydrogen bond. The theoret-
ical value at 1683 cm−1 (L1) is assigned to ꢀ(C N). Theoretically,
the benzimidazoles L1,2 give rise to eight C H aromatic stretching
mode of vibration corresponding to the presence of eight aromatic
C
H bonds as shown in Tables 1 and 2.
For assignment of CH2 group frequencies theoretically, six fun-
damentals can be associated to each CH2 group [24]. The bands
observed at 2889 and 2836 cm−1 in L1 derivative are ascribed to CH2
asymmetric and symmetric stretching vibration. Infrared bands
established at 1535, 1481, 1201, and 1003 cm−1 are attributed to

276
N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
100
the given compound. The first is the polarizable continuum model
a
90
80
70
60
50
40
30
20
10
0
(PCM) [25] provided by GAUSSAIN03 with DMSO and chloroform as
solvents (UFF radii were used to obtain molecular cavity). Molec-
ular clusters, as microscopic model for the environment, have also
been used by placing two DMSO molecules near the NH groups in
the studied compounds. In these two models, the 1H NMR chemical
shifts were computed at the DFT/B3LYP/6-311 + G(2d,p) and SDD
levels of theory for the benzimidazoles (L1,2), respectively. Results
of linear regression fit between experimental and calculated chem-
ical shifts (1H) performed for the structures tested are also included
in Table 3.
p-Cl (L ) Exp.
3400
2900
2400
1900
1400
900
400
Wavenumber, cm-1
120
100
80
60
40
20
0
b
Experimentally, the benzimidazoles (L1,2) showed a broad sin-
glet signal at 12.21 and 12.25 ppm [6], respectively, due to the
benzimidazolic NH proton (Fig. 4). The triplet signal at 6.45 and
6.50 ppm is attributed to the secondary amino group, whereas the
doublet signal at 4.42 and 4.46 ppm is assigned to the CH2 protons
in L1,2. For benzimidazole L1, the protons of the aniline ring give
rise to four-line pattern at 6.64 and 6.62 ppm for protons in the
ortho-position with respect to secondary amino group, whereas
the protons at 7.08 and 7.06 ppm are assigned to aromatic protons
in the ortho-position with respect to chloro atom. The signals at
7.47, 7.46, 7.13, and 7.10 ppm are assigned to the aromatic protons
of the benzimidazole ring. Similar, L2 has the same assignments for
the aromatic protons as L1 at 7.36, 7.33, 6.53, 6.49 and 7.49, 7.48,
7.14, 7.12, respectively.
p-Cl (B3LYP/6-31G(d))
3400
2900
2400
1900
1400
900
400
Scaled Wavenumber, cm-1
Fig. 2. Experimental and theoretical (B3LYP/6-31G(d)) FT-IR spectra of benzimida-
zole (L1).
␦sCH2 (scissoring), ␻CH2 (wagging), ␶CH2 (twisting), and ␳CH2
(rocking) vibration modes, respectively. For benzimidazole (L2), the
IR bands observed at 2889, 2877, 1532, 1470, 1211, and 1017 cm−1
are unambiguously assigned to CHa2ss; CHs2s, ␦sCH2, ␻CH2, ␶CH2,
and ␳CH2, respectively. Other vibration modes are tabulated in
Tables 1 and 2. The RMS error of the frequencies between the
un-scaled and experimentally observed in benzimidazole (L1) was
found to be 86 cm−1. After scaling, the RMS error is found to be
The 1H chemical shifts of all protons in the investigated com-
pounds except the imidazolic and the secondary amino protons,
(H23 and H26) (L1) and (H22 and H25) (L2), are in a good agree-
ment with the theoretically computed values as calculated by the
PCM model (Table 3). The regression coefficient between the calcu-
lated and the experimental chemical shifts is improved in presence
of DMSO as a solvent rather than in the gaseous state. However,
the chemical shifts of the NH protons remain unacceptable apart
from the experimental values, being obviously that the chemical
shifts associated with these protons are not correctly described
by continuum model. Thus, it is clear that the PCM model fails in
reproducing the experimental findings for the hydrogen-bonded
protons and specific solute–solvent interactions are expected to
completely explain the 1H NMR spectra of the studied compounds
[26]. For this purpose, we optimized and then calculated the NMR
spectra of the p-Cl and p-I molecules with two DMSO molecules. As
shown in Table 3, the chemical shift of the benzimidazolic NH pro-
ton is in a very good agreement with the theoretical computed value
revealing that some sort of interaction occurs between the studied
compound and solvent molecules and this will be confirmed later
by studying the electronic spectra of these compounds in DMSO.
Therefore, the hydrogen bonding interaction of these compounds,
is better reproduced by the second model rather than using the
polarizable continuum model (PCM) solvation model.
13 cm−1
.
3.2. 1H NMR spectral analysis
In order to provide an unambiguous assignment of 1H NMR
spectra of the studied compounds, we undertook a series of NMR
calculations using GIAO approximation, and the results of these
calculations are shown in Table 3. Two models have been used for
assessing the influence exerted by solvent on the NMR spectra of
4000
3500
3000
2500
2000
3.3. Electronic absorption
3.3.1. Band assignment
The electronic spectra of the benzimidazoles (L1,2) displayed
five absorption bands in ethanol. The first two bands at 204 and
1500
y = 1.0378x, R2 = 0.9996
*
222 nm may be assigned to medium and low energy ␲–␲ tran-
1000
500
0
sitions within the phenyl rings of the aniline and benzimidazole
moieties, respectively [27]. In benzimidazole ring, three kinds of
transitions are possible: (i) n–␲*, (ii) ␲–␲*, and (iii) charge-transfer.
However, it is well established that the n–␲* transition is not
observed in the benzimidazole compounds, although the system
has a lone pair of electrons on the tertiary nitrogen atom [28].
Therefore, the bands at 250, 274, and 280 nm (L1) and 254, 273
and 280 (L2) may be assigned to ␲–␲* transitions in the benz-
imidazole ring. Moreover, the lowest energy bands in the studied
compounds appear doublet due to the presence of a tautomeric
0
500
1000
1500
2000
2500
3000
3500
4000
Experimental Wavenumber, cm-1
Fig. 3. The linear regression between the experimental and the theoretical DFT
predicted wavenumbers (cm−1) for the p-Cl benzimidazole derivative (L1) using
B3LYP/6-31G(d) method.

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
277
Table 3
Experimental and calculated 1H NMR chemical shifts for the title compounds (L1–2).
L1
L2
Atom
ı Calcd.
ı Exp.
Atom
ı Calcd.
ı Exp.
Gaseous
CHCl3
DMSO
L1-DMSOa
Gaseous
DMSO
CHCl3
L2-DMSOa
H19
H20
H21
H22
H23
H24
H25
H26
H27
H28
H29
H30
R2
8.15
7.61
7.59
7.63
8.16
4.42
4.42
5.62
7.02
7.51
7.38
6.33
0.9326
8.09
7.58
7.62
7.81
8.76
4.66
4.46
5.62
7.16
7.40
7.60
6.73
0.9706
8.08
7.60
7.66
7.92
9.01
4.76
4.51
5.62
7.21
7.42
7.64
6.84
0.9759
7.87
7.49
7.57
7.72
13.03
4.59
4.20
7.33
7.67
7.37
7.48
6.76
0.9711
7.47
7.10
7.13
7.46
12.21
4.42
4.42
6.45
7.08
6.64
7.06
6.62
H18
H19
H20
H21
H22
H23
H24
H25
H26
H27
H28
H29
R2
8.25
7.75
7.75
7.51
7.56
4.14
4.14
5.88
6.98
7.50
7.48
6.37
0.8777
8.24
7.89
7.95
7.96
8.57
4.44
4.44
5.90
7.24
7.60
7.64
6.84
0.9152
8.26
7.85
7.89
7.81
8.26
4.34
4.34
5.92
7.17
7.57
7.58
6.68
0.9052
7.78
7.48
7.51
7.56
12.22
4.38
3.76
7.14
7.99
7.31
7.39
6.69
0.9161
7.15
7.14
7.49
7.48
12.25
4.46
4.46
6.50
7.36
7.33
6.53
6.49
a
These values were obtained after optimization of L1,2·2DMSO in the gaseous state.
structure [29]. This phenomenon is supported by comparing our
spectra with the spectrum of 1-methyl-2-phenyl-benzimidazole
[28], where this fine structure is lost. To explain the effect of the
substituent on the absorption spectra of the benzimidazoles (L1,2),
the unsubstituted derivative [30], which has five absorption bands
in ethanol at 204, 222, 242, 274 and 280 nm was taken as a refer-
ence. The introduction of electron-attracting substituents (Cl or I)
causes a red shift of the band at 242 nm on going from p-Cl to p-I
derivative.
electronic transitions between HOMO and LUMO + 2 (7%)/LUMO + 3
(4%), HOMO-1 and LUMO (55%)/LUMO + 3 (3%) in addition to
HOMO-2 and LUMO (22%)/LUMO + 3 (3%) are attributed to 233 nm
(5.31 eV). The highest energy transitions at 211 (5.87 eV) and
214 nm (5.79 eV) are assigned to HOMO-3 → LUMO (96%) and
HOMO-1 → LUMO + 1 (96%), respectively. The excitation energy at
209 nm (5.92 eV) is assigned to HOMO-2 → LUMO + 1 (24%), and
HOMO → LUMO + 5 (71%).
On the other hand, the theoretical UV–vis spectrum of the benz-
imidazole (L2) is characterized by five electronic transitions at 233,
240, 249, 253, and 283 nm. The lowest excitation energy at 283 nm
(4.37 eV) characterizes the transitions from HOMO → LUMO + 2
(93%) and HOMO-5 → LUMO + 4 (4%), while the electronic tran-
sition at 253 nm (4.90 eV) is due to HOMO → LUMO + 3 (95%)
and HOMO → LUMO + 4 (2%). The electronic transitions between
HOMO and LUMO + 4 (3%), HOMO-1 and LUMO + 3 (20%)/LUMO + 4
(2%) in addition to HOMO-2 → LUMO (67%) are attributed to
249 nm (4.99 eV). The excitation energy at 240 nm (5.17 eV)
is assigned mainly to HOMO → LUMO + 4 (67%). The highest
energy transition at 233 (5.30 eV) is due to HOMO-1 → LUMO
(73%), HOMO → LUMO + 4 (12%) and HOMO-2 → LUMO + 3 (8%)
transitions.
The absorption spectra of the benzimidazoles (L1,2) were
observed in solvents of different polarities and hydrogen bond for-
mation tendency, cyclohexane, DMSO, DMF, dioxane, acetonitrile,
2-propanol, and ethanol (Fig. 5). The two bands at 274 and 280 nm
in the cylcohexane (reference solvent) are slightly blue shifted in
ethanol and 2-propanol (hydrogen-bonding solvents) suggests that
the benzimidazole derivatives are acting as a proton acceptor [31]
at the pyridine-type nitrogen. However, the latter bands are red-
shifted in DMF and DMSO (hydrogen bond acceptor solvents) due
to an almost proton transfer from the solute molecules to these
solvents. The values of the observed transition energies (Eobs) and
oscillator strengths (fobs) of all the bands in the electronic spectra
of the title compounds in different solvents were calculated [27]
and tabulated in (Supplementary material, Table S1). Application
of dielectric relation [27] to the band at 250 (L1) and 254 nm (L2)
did not give a linear relation indicating that the spectral shifts are
not governed solely by the dielectric effect of the solvents. Thus, the
so-called specific solute-solvent interaction is the main controlling
factor in determining the band position.
The lowest 10 singlet-to-singlet spin-allowed excitation states
were taken into account for the calculation of the electronic absorp-
tion spectra by using TD-DFT method. The calculated energy of
excitation states and transition oscillator strength (f) (only f > 0.005
is listed) of the title compounds is taken into consideration. The
absorption spectrum was simulated using GAUSSSUM software
[32] based on the obtained TD-DFT results. Each excited state
was interpolated by a Gaussian convolution with the full-width
at half-maximum (FWHM) of 3000 cm−1. The theoretical UV–vis
spectrum of the benzimidazole (L1) is characterized by nine elec-
tronic transitions at 209, 211, 214, 227, 233, 235, 245, 270, and
297 nm. The lowest energy electronic transition at 297 nm (4.17 eV)
characterizes the transition from HOMO to LUMO, while that at
270 nm (4.59 eV) is assigned mainly to HOMO → LUMO + 1 (91%).
Moreover, the excitation energy at 245 nm (5.05 eV) is contributed
from the transitions between HOMO-1 and LUMO (21%)/LUMO + 2
(7%)/LUMO + 3 (10%) in addition to HOMO-2 → LUMO (54%) transi-
tion. The electronic transitions at 235 nm (5.26 eV) are connected
with HOMO → LUMO + 2 (68%), and HOMO → LUMO + 3 (28%). The
3.3.2. Electronic absorption spectra in solutions of varying pH
The absorption spectra of 3 × 10−5 M solutions of L1,2 com-
pounds in 20% (v/v) ethanolic solutions of varying pH values were
scanned in the UV–vis range. p-Cl derivative (L1) is neutral over
the entire pH range 4.5–8.2 and possesses a clear isosbestic point
in the acidic medium at 213 nm as shown in Fig. 6. The bands
located at 250, 274 and 280 nm in ethanol are blue shifted to
243, 269 and 275 nm at pH 2 (H2L2+). With increasing the pH,
these bands remain in their new positions with the appearance
of shoulder at 279 nm at pH 5. Between pH 6 and 12, these bands
are red shifted to 247, 273, and 279 nm, accompanied by a varia-
tion in the absorbance values according to the presented species
(L− and L2−). p-I derivative (L2) has the same behavior as p-Cl
compound with a clear isosbestic point in the acidic medium at
213 nm. The pH-absorbance changes [33] were utilized to calcu-
late the acid dissociation constants. Four pKa values were reported
for the investigated compounds. The first pKa value 2.49 0.02 for
the p-Cl derivative (L1) is assigned to the deprotonation of the pro-
tonated secondary amino group, while the second pKa 4.42 0.03
may be attributed to the deprotonation of the protonated pyridine
type nitrogen [34]. The third pKa 8.43 0.02 is accounted for the
ionization of the NHsec group, while the fourth pKa 11.42 0.01
is attributed to ionization of the imidazolic proton [33]. On the
other hand, the pKa values at 2.71 0.01, 5.06 0.04, 7.84 0.03

278
N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
CH2
Fig. 4. (a) 1H NMR spectrum of p-I derivative (L2) in DMSO (b) 1H NMR spectrum in DMSO/D2O; (c) Aromatic region in DMSO (d) Aromatic region in DMSO/D2O.
and 11.50 0.01 for the benzimidazole (L2) are assigned to the lat-
ter processes as p-Cl derivative. It was found that the ionization of
the secondary amino group is directly influenced to some extent by
both the type and nature of the substituents. Thus, the substituents
of electron withdrawing characters increase the acidic behavior of
the NHsec group.
structure parameters of the benzimidazoles (L1,2) as calculated
by DFT/B3LYP level of theory using 6-31G (d) and SDD basis set,
respectively. For L1 derivative, a significant difference (0.067 A) is
˚
found between the bond lengths N15 C7 and N16 C7. Similar, a
2
˚
difference of 0.064 A is found between the latter bonds in the L
compound. This confirms that the hydrogen atom is fixed at one
of the two nitrogen atoms through the intermolecular hydrogen
bonding. Moreover, these bond lengths have a shorter distance than
any amine compound due to the participation of the lone electron
pair of the nitrogen atom in resonance of the benzimidazole ring. It
is found that, the phenyl ring appears little distorted and its angles
are slightly out of the perfect hexagonal structure. This is due to
the attachment of the imidazole moiety to the benzene ring in the
4. DFT studies
4.1. Geometry optimization
Full geometry optimizations of the title compounds were per-
formed at the DFT level of theory [9]. Table 4 presents the optimized

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
279
Acetonitrile
Cyclohexane
Dioxane
2
DMSO
Propanol
Ethyl Alcohol
DMF
1.5
1
0.5
0
190
210
230
250
270
290
310
330
Wavelength, nm
Fig. 5. Electronic absorption spectra of (3 × 10−5 M) of p-Cl (L1) derivative in different solvents.
place of hydrogen atoms. The breakdown of hexagonal symmetry
of the benzene ring in the p-Cl derivative is obvious from the elon-
gation of C4 C5 (1.415 A) and C1 C2 (1.409 A) comparing to the
remaining C1 C3, C3 C4, C2 C6 and C5 C6 bonds, which almost
have the same bond lengths as will be confirmed latter by the NBO
analysis.
structural feature related directly to the ability to bind the metal
ions. Several calculated thermodynamic parameters are presented
in Table 4.
˚
˚
4.2. Natural bond orbital (NBO) analysis
Substitution in the para position of the aniline ring with halo-
gen atom (X = Cl, and I) leads to some changes in the bond lengths
and bond angles of the aniline moiety as a result of the charge
redistribution. Due to the ␴-withdrawing character of the halo-
gen atom, the optimized C-C bond lengths adjacent to the halogen
atom are slightly shorter than the corresponding ones in the unsub-
stituted derivative [30] at the same level of theory. In addition,
the angle (C11 C12 C13) in L1 or (C15 C16 C17) in L2 near to
the position of halogen is longer than that found in case of the
unsubstituted compound [30]. The dipole moment of the unsubsti-
tuted derivative [30] increases dramatically from 3.615 D to 5.388
and 6.321 D for the chloro-, and iodo-derivatives, respectively. The
high electronegativity of the halogen atom induces polarization
both in ␴- and ␲-frameworks of the aniline moiety. The stud-
ied compounds (L1,2) show accumulation of the negative charge
density on the pyridine-type nitrogen, which is a very important
The natural bond orbital (NBO) calculation [13] of L1,2 com-
pounds was performed using NBO 3.1 program implemented in
the GAUSSIAN03 package at the DFT/B3LYP level. The ideal Lewis
structure picture is constructed from Lewis ␴-type (donor) NBOs
that are complemented by the non-Lewis ␴-type (acceptor) NBOs.
The filled NBOs of the natural Lewis structure are well adapted
to describe covalency effects in molecules. The anti-bonds repre-
sent empty valence-shell capacity and spanning portions of the
atomic valence space that are formally unsaturated by covalent
electrons. Weak occupancies of the valence anti-bonds signal irre-
ducible departure from an idealized localized Lewis picture, i.e.
true “delocalization effects”. Therefore, NBO analysis provides an
efficient method for studying intra- and intermolecular bonding
and provides a convenient basis for investigating charge transfer
or conjugative interaction in molecular systems.
Fig. 6. Electronic absorption spectra of (3 × 10−5 M) of p-Cl derivative (L1) in solutions of varying pH.

280
N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
Table 4
◦
1,2
˚
Selected bond lengths (A), angles ( ) and charge for L compounds.
L1
L2
Bond lengths (A)
Bond angles (◦)
Charge
Bond angles (◦)
Charge
˚
˚
Bond lengths (A)
C1C2
C1C3
C3C4
C4C5
C5C6
C4C7
C7C8
C8C9
C9C10
C10C11
C11C12
C12C13
C13C14
C9C14
N15C7
N16C7
N17C9
C12Cl18
H23N15
H26N17
1.409
1.391
1.399
1.415
1.395
2.147
1.502
2.474
1.410
1.389
1.396
1.391
1.395
1.407
1.376
1.309
1.385
1.764
1.010
1.014
C3C1C2
C4C3C1
C5C4C3
C6C2C1
121.410
117.974
119.841
121.546
105.265
124.367
109.087
119.565
121.113
119.737
120.296
119.943
118.115
113.044
107.101
110.130
104.459
121.719
C1 = −0.143
C2 = −0.148
C3 = −0.178
C4 = 0.239
C1C2
C2C3
C3C4
C4C5
C5C6
C1C6
C11C12
C13C14
C14C15
C15C16
C16C17
C17C18
I19C16
N22C12
N21C11
N20C11
N22C13
H23N20
H26N22
1.407
C1C2C3
C2C3C4
C3C4C5
C4C5C6
H7C2C1
H8C3C2
H9C4C3
H10C5C4
C1N21C11
N21C11C12
C11C12N22
N22C13C14
C13C14C15
C14C15C16
C15C16C17
C16C17C18
I19C16C15
N20C11C12
117.858
121.336
121.547
116.719
120.454
119.577
119.169
121.016
105.806
123.415
108.619
148.335
121.035
119.929
120.056
120.098
119.773
123.415
C1 = −214
1.404
1.421
1.406
1.404
1.428
1.506
1.422
1.401
1.409
1.404
1.407
2.145
1.451
1.330
1.394
1.386
1.011
1.016
C2 = −194
C3 = −0.333
C4 = 0.027
C7N16C4
C8C7N16
N17C8C7
C10C9N17
C11C10C9
C12C11C10
C13C12C11
C14C13C12
C14C9C10
N16C7N15
C7N15C5
N16C4C5
N15C5C4
H19C3C1
C5 = 0.354
C5 = 0.253
C6 = −0.174
C6 = −0.359
C7 = 0.515
C7 = 0.149
C8 = −0.171
C9 = 0.372
C8 = −0.406
C9 = 0.458
C10 = −0.186
C11 = −0.135
C12 = −0.073
C13 = −0.141
C14 = −0.180
N15 = −0.747
N16 = −0.576
N17 = −0.682
Cl = −0.045
H23 = 0.333
H26 = 0.352
C10 = −0.406
C11 = -0.123
C12 = −0.359
C13 = −0.150
C14 = −0.332
N15 = −0.444
N16 = −0.126
N17 = −0.429
I = 0.066
H23 = 0.228
H26 = 0.347
E (a.u.)
−719.181
−716.280
Zero-point E (kcal mol−1
)
147.655
147.887
Rotational constants (GHz)
1.932, 0.112, 0.106
1.957, 0.064, 0.062
Entropy (cal mol−1 K−1
Translational
Rotational
Vibrational
Total dipole moment (D)
)
42.532
33.901
48.777
5.388
43.444
34.987
50.495
6.321
Hyperconjugation may be given as a stabilizing effect that arises
from an overlap between an occupied orbital with another neigh-
boring electron deficient orbital when these orbitals are properly
oriented. The hyperconjugative interaction energy was deduced
from the second-order perturbation approach of Fock Matrix in
NBO basis between donor–acceptor orbitals [35]. The larger the sec-
ond order interaction energy (E2) value, the more intensive is the
interaction between electron donors and electron acceptors, i.e. the
more donating tendency from electron donors to electron accep-
tors and the greater the extent of conjugation of the whole system.
These interactions can be identified by finding an increase in the
electron density (ED) of the anti-bonding orbital that weakens the
respective bonds. The results of NBO analysis for the p-Cl derivative
show that the ␲(C2 C6) and ␲(C7 N16) participate as donor and
the ␲*(C4 C5) anti-bond as acceptor, [␲(C2 C6) → ␲*(C4 C5)]
and [␲(C7 N16) → ␲*(C4 C5)] with charge transfer energy values
280 and 340 J mol−1, respectively. These interactions weaken the
C4 C5 bond with elongation of its bond length as previously men-
tioned. The ␴(C4 C5) is formed from sp2.08 hybrid on C4 (which is a
mixture of 32.43% s, and 67.52% p atomic orbitals) and sp2.02 on C5
(66.90% p contribution) as shown in Table 5. Thus, the increasing of
p characters on carbons C4 and C5 bond orbitals result in lengthen-
ing of the ␴(C4 C5) bond than the other C C bond lengths in the
benzene ring of the benzimidazole moiety. In addition, the intra-
molecular hyperconjugative interaction of the ␲ electrons occurs
from C1 C3 to ␲*(C2 C6) (ED ≈ 0.335e and E2 = 280 J mol−1) leads
to strong stabilization of the benzene ring of the benzimidazole
moiety. The lone pair interaction of LP N16 (sp2.12) with anti-
4.3. Frontier molecular orbitals
The frontier molecular orbitals play an important role in the
electric and optical properties [36]. The frontier orbital gap helps
to characterize the chemical reactivity and kinetic stability of the
molecule. A molecule with a small frontier orbital gap is more polar-
izable, is generally associated with a high chemical reactivity, low
kinetic stability, and is termed as soft molecule. Fig. 7 shows the dis-
tributions and energy levels of the HOMO and LUMO orbitals for the
studied compounds. The values of the energy separation between
HOMO and LUMO are 4.71 and 4.38 eV for the benzimidazoles (L1,2),
respectively.
4.4. Electrochemical studies
The redox behavior of the benzimidazoles (L1,2) was studied by
cyclic voltammetry (CV) in acetonitrile in presence of NaClO4·H2O
as a supporting electrolyte. These compounds behave similarly and
are oxidized in a two irreversible one-electron processes following
the ECEC mechanism as shown in Fig. 8. The studied compounds
form the cation-radical through a one-electron loss, which is fol-
lowed by a deprotonation process from the imidazolic NH group to
give the corresponding radical (Scheme 1). This radical undergoes
the second electron transfer followed by the removal of another
proton from the secondary amino group to form the di-radical
[37]. In order to elucidate the effect of the introduction of the sub-
stituent (Cl and I) on the para-position of the aniline moiety, the
CV of the unsubstituted derivative [30] has been also conducted.
The unsubstituted derivative exhibited the oxidation processes at
0.93 and 1.27 V, while the p-Cl (L1) derivative showed these pro-
cesses at 0.98 and 1.44 V. Thus, the presence of chlorine substituent
in the para-position of the aniline moiety shifts the second oxida-
tion process towards higher potential. On the other hand, the p-I
(L2) derivative showed only the first oxidation process at 1.04 V.
bonding ␴*(C7 N15) has a large energy difference (∼810 J mol−1
)
which is an evidence for charge transfer from nitrogen atom to the
␴*(C7 N15) and induces partial ␲ character. As shown in Table 5,
p-I (L2) derivative has the same behavior as the other deriva-
tive, where the strongest interaction is LP(1)N17 → ␲*(C9 C14)
(ED ≈ 0.427e and E2 = 44.7 kJ mol−1).

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
281
Table 5
The most important interactions between “filled” (donors) Lewis-type NBOs and “empty” (acceptors) non-Lewis NBOs.
Donora
Lewis-type
NBOs (A B)
Occupancy
Hybride
AO (%)f
Acceptorb
non Lewis
NBOs
NBOs
E2 (kJ mol−1
)
E(j) − E(i)c
×
F(i,
103(kJ mol−1
)
j)d × 103(kJ
mol−1
)
L1
sp (C1)
sp (C3)
s(0.00)p(99.96)
s(0.00)p(99.95)
␲(C1 C3)
␲*(C2 C6)
␲*(C4 C5)
␲*(C1 C3)
␲*(C4 C5)
␲*(C12 C13)
1.712
1.724
1.599
1.878
1.640
0.335
0.473
0.308
0.473
0.404
20.29
20.06
18.48
17.73
25.28
0.28
0.28
0.29
0.34
0.27
0.068
0.070
0.067
0.077
0.075
sp (C2)
sp (C6)
s(0.00)p(99.96)
s(0.00)p(99.96)
␲(C2 C6)
␲(C4 C5)
␲(C7 N16)
␲(C9 C14)
sp (C4)
sp (C5)
s(0.00)p(99.97)
s(0.00)p(99.97)
sp (C7)
sp (N16)
s(0.00)p(99.90)
s(0.00)p(99.70)
sp (C9)
sp (C14)
s(0.01)p(99.95)
s(0.00)p(99.97)
LP(1)N15
LP(1)N15
LP(1)N16
LP(1)N17
LP(1)Cl
1.627
1.627
1.915
1.783
1.993
sp
sp
s(0.00)p(99.98)
s(0.00)p(99.98)
s(32.00)p(67.85)
s(5.06)p(94.91)d(0.03)
s(82.23)p(17.75)
␲*(C4 C5)
0.473
0.341
0.042
0.417
0.405
31.59
50.00
10.43
37.10
10.98
0.30
0.29
0.81
0.29
0.33
0.090
0.107
0.083
0.098
0.059
␲*(C7 N16)
␴*(C7 N15)
␲*(C9 C14)
␲*(C12 C13)
sp2.12
sp18.74d0.01
sp0.22
L2
sp (C4)
sp (C5)
s(0.00)p(100.00)
s(0.00)p(100.00)
␲(C4 C5)
␲*(C1 C3)
1.602
1.602
1.971
1.713
0.304
0.332
0.403
0.320
19.56
19.61
27.69
23.20
0.29
0.28
0.27
0.29
0.069
0.068
0.079
0.074
sp (C4)
sp (C5)
s(0.00)p(100.00)
s(0.00)p(100.00)
␲(C4 C5)
␲*(C2 C6)
sp (C9)
sp (C14)
s(0.00)p(100.00)
s(0.00)p(100.00)
␲(C9 C14)
␲(C12 C13)
␲*(C12 C13)
␲*(C10 C11)
sp (C12)
sp (C13)
s(0.00)p(100.00)
s(0.00)p(100.00)
LP(1)N15
LP(1)N16
LP(1)N17
LP(1)I
1.638
1.919
1.760
1.993
sp
s(0.00)p(100.00)
s(33.59)p(66.41)
s(0.00)p(100.00)
s(89.33)p(10.67)
␲*(C4 C5)
0.476
0.044
0.427
0.403
30.75
10.35
44.70
5.97
0.31
0.74
0.27
0.28
0.089
0.079
0.103
0.040
sp1.98
sp
␴*(C7 N15)
␲*(C9 C14)
␲*(C12 C13)
sp0.12
a
LP(n)A is a valence lone pair orbital (n) on A atom.
b
c
d
e
f
(*) denotes anti-bonding, and Ry corresponds to the Rydberg NBO orbital.
Energy difference between donor and acceptor i and j NBO orbitals.
F(i, j) is the Fock matrix element between i and j NBO orbitals.
Hybrid on A atom in the A B bond or otherwise, as indicated.
Percentage contribution of atomic orbitals in NBO hybrid.
The reduction of the investigated compounds (L1,2) is done eas-
ily through the acceptance of two electrons in one irreversible
two-electron-wave producing the corresponding dianion of the
imine moiety, which is basic enough to accept two protons not
only from the residual water, but also from the supporting elec-
trolyte.
4.5. Biological activity
The antimicrobial activities of the studied compounds were car-
ried out using cultures of B. subtilis, S. aureus, and S. faecalis as
Gram-positive bacteria and E. coli, P. aeruginosa, and N. gonorrhea as
Gram-negative bacteria and compared with those of the unsubsti-
tuted derivative, (1H-benzimidazol-2-ylmethyl)-N-phenyl amine
[30]. In the earlier studies [30], it was found that, the unsub-
stituted derivative is more toxic than the standard tetracycline
against the bacterium S. aureus with a minimum inhibitory con-
centration (58 ␮g/ml) as compared with tetracycline (82 ␮g/ml). As
shown in Fig. 9, the title compounds have the capacity of inhibit-
ing the metabolic growth of the investigated bacteria to different
extents especially the P. aeruginosa, but less than the unsubstituted
derivative. The toxicity of these compounds may be arising from
the benzimidazole ring, which may play an important role in the
antibacterial activity [6].
In classifying the antibacterial activity as Gram-positive or
Gram-negative, it would be expected that a much greater number of
drugs would be active against Gram-positive than Gram-negative
bacteria [39]. However, in this study, the title compounds are
active against both types of the bacteria, which may indicate
broad-spectrum properties. It is seen that the activity of the title
compounds is affected by the nature of the substituents on the
It might be tempting now to correlate the values Eox derived
from CV with HOMO–LUMO energy differences obtained from DFT
calculations. The UV–vis absorption of a molecule corresponds
indeed to excitation of an electron from the HOMO into the LUMO,
whereas electro-oxidation corresponds only to the removal of an
electron from the HOMO. A correlation might be possible if reduc-
tion of this molecule (i.e. donation of an electron into the LUMO)
may be possible; in this case, the potential difference between
oxidation and reduction potentials may be correlated with the
HOMO–LUMO energy gap [38]. The potential difference between
the first oxidation peaks in the unsubstituted derivative and that of
the p-Cl derivative (L1) is 0.06 V, which is in a good agreement with
the shift, associated with the HOMO–LUMO gaps on going from
the unsubstituted derivative to the p-Cl compound (0.06 eV) [6].
The DFT data have assigned that the LUMO of the p-Cl (L1) and p-I
(L2) derivatives are constituted mainly by C N group of the ben-
zimidazole ring and thus the reduction is considered as electron
accommodation at C N dominated orbitals.

282
N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
Fig. 7. Molecular orbital surfaces and energy levels of (a) p-Cl (L1) and (b) p-I (L2) derivatives.
aniline ring, this in relation to the lipophilicity of the compounds
and their membrane permeability, a key factor in determining
their entry inside the cells. The introduction of substituent (Cl
or I) in the para-position of the aniline ring decreases markedly
the antibacterial activity. The antibacterial effect declines on going
from unsubstituted to p-Cl and finally p-I derivative. On the other
hand, the variation in the effectiveness of the studied compounds
against different organisms depends either on the impermeability
8.0e-5
6.0e-5
4.0e-5
p-Cl (L1)
6e-5
4e-5
2e-5
0
2.0e-5
0.0
-2.0e-5
-4.0e-5
-6.0e-5
-8.0e-5
-1.0e-4
-1.2e-4
6e-5
4e-5
2e-5
0
uns.
-2e-5
-4e-5
p-I (L2)
-2e-5
-4e-5
-6e-5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential, V
Fig. 8. CV of the benzimidazoles, p-Cl (L1), p-I (L2) and the unsubstituted derivative (uns.).

N.T. Abdel Ghani, A.M. Mansour / Spectrochimica Acta Part A 91 (2012) 272–284
283
H
N
H
N
H
N
H
N
-H+
-e
Cl
Cl
H
N
N
N
N
Cl
N
-e
H
N
-H+
N
Cl
N
Cl
N
N
N
Scheme 1. Proposed oxidation mechanism for benzimidazole (L1).
35
30
25
20
15
10
p-Cl
p-I
Unsubstituted
Tetracycline
5
Tetracycline
Unsubstituted
0
p-I
p-Cl
P. Aeruginosa
(G-)
E. coil (G-)
N. Gonorrhoeae
(G-)
S. Aureus (G+)
S. Faecalis (G+)
B. Subtilis (G+)
Fig. 9. Antibacterial activities of the benzimidazoles (L1,2) and tetracycline against B. subtilis, S. aureus, and S. faecalis as Gram-positive, P. aeruginosa, E. coli, and N. Gonorrhea
as Gram-negative bacteria. All experiments were carried out in triplicate and the mean results are given. DMSO was used as control and its inhibition zone was subtracting
from that of each case.
of the cells of the microbes or on differences in ribosome of micro-
bial cells.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.01.080.
5. Conclusion
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