DFT studies of the structure and vibrational and NMR spectra of 1-(2-methylpropenyl)-2-methylbenzimidazole

Article abstract of DOI:10.1016/j.molstruc.2008.07.008

Synthesis of two new 1-(2-methylpropenyl)-2-methylbenzimidazoles by reaction of 2-methylbenzimidazole with 3-chloro-2-methylpropene using a strong base as a catalyst is described. Gas chromatography-mass spectrometry (GC-MS) allowed the characterization o

Full text of DOI:10.1016/j.molstruc.2008.07.008

Journal of Molecular Structure 917 (2009) 158–163
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
DFT studies of the structure and vibrational and NMR spectra
of 1-(2-methylpropenyl)-2-methylbenzimidazole
*
Ricardo Infante-Castillo , Samuel P. Hernández-Rivera
Department of Chemistry, University of Puerto Rico-Mayagüez, P.O. Box 9019, Mayagüez, PR 00681-9019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of two new 1-(2-methylpropenyl)-2-methylbenzimidazoles by reaction of 2-methylbenzimida-
zole with 3-chloro-2-methylpropene using a strong base as a catalyst is described. Gas chromatography–
mass spectrometry (GC–MS) allowed the characterization of the structural isomers: the slightly more
stable 1-(2-methyl-1-propenyl)-2-methylbenzimidazole (51%) and a less stable 1-(2-methyl-2-prope-
nyl)-2-methylbenzimidazole (49%). The results of theoretical calculations indicate that the difference
in the energy between the two structural isomers is 3.21 kcal molꢀ1 at the B3LYP/6-311+G** level.
The structures were confirmed by 1H and 13C Nuclear Magnetic Resonance, elemental analysis and spec-
troscopic methods such as FT-Raman, FT-IR and UV–VIS. The experimental results were supported by
performing DFT calculations for energies, geometries, vibrational frequencies and shieldings constants
using 6-311+G** basis sets and B3LYP functional. The theoretical data have satisfactorily reproduced
the experimental results. The consistency and efficiency of the GIAO method used to calculate absolute
shielding of the studied compounds were checked by the analysis of statistical parameters and
were found to be in excellent agreement with experimental values. This correlation has been used for
unambiguous NMR signal assignments for studied compounds.
Received 21 January 2008
Received in revised form 8 July 2008
Accepted 10 July 2008
Available online 19 July 2008
Keywords:
2-Methylbenzimidazole
2-Methylpropenyl
GIAO
DFT
Chemical shifts
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
confirmed by Nuclear Magnetic Resonance (1H and 13C) analysis
and their purity was checked by elemental analysis. The experimen-
Substituted benzimidazoles have drawn considerable attention
inmedicinalchemistryduetoawiderangeofbiologicalactivitiesex-
erted by this class of compounds [1–7]. Benzimidazole is also of
interest as a corrosion inhibitor for metal and alloys and other indus-
trial applications [8–9]. In previous recent works, the structural
properties of two 1-alkyl-2-methylbenzimidazoles compounds
[10] and the isomers of 1-alkenyl-2-propenyl-2-methylbenzimidaz-
oles [11] were studied. In the present work, as part of our investiga-
tions in the field of N-substituted benzimidazoles, we extend the
studies to the synthesis and the experimental and theoretical
structural characterization of the isomers of new 1-(2-methylprope-
nyl)-2-methylbenzimidazole derivatives. In the synthesis of these
derivatives a method that has been applied successfully for N-alkyl-
ation of indoles and pyrroles using KOH as base in DMSO [12] was
employed. Scheme 1 shows the structures of the investigated
compounds. GC–MS analyses of the purified product allowed
identification and characterization of two structural isomers: the
1-(2-methyl-1-propenyl)-2-methylbenzimidazole and the 1-(2-
methyl-2-propenyl)-2-methylbenzimidazole with 51% and 49%
abundance, respectively. The structures of the compounds were
tal FT-IR and FT-Raman spectra were obtained for the mixture of
structural isomers of 1-(2-methylpropenyl)-2-methylbenzimida-
zole. For the interpretation of the experimental results, the energies,
geometries, harmonic vibrations and shieldings constants of the
structural isomers were calculated by density functional theory
(DFT) [13] using 6-311+G** basis set and B3LYP functional [14,15].
The theoretical Raman and IR DFT spectra were obtained incorporat-
ing the contribution of each isomer in accordance the respective per-
centage in the isomeric mixture that was determined by gas
chromatography analysis. The wavenumber-linear scale (WLS)
method [16] was used to reproduce the experimental vibrational
data in order to reduce the overestimation associated with DFT cal-
culations. Vibrational frequency assignments were made with the
aid of the GaussView 03 program [17]. Calculations on proton and
carbon magnetic shielding values were carried out in solution and
using the gauge independent atomic orbital (GIAO) [18] approach.
The solvent effects on NMR data were introduced by the IEF-PCM
method [19] implemented in the GAUSSIAN 03 program [20]. Basic
experimental and theoretical information about a 1-(2-methylp-
ropenyl)-2-methylbenzimidazole and its structural isomer is pre-
sented and discussed. To the best of our knowledge to date, no
evidence of similar studies for the 2-methylbenzimidazole and this
alkenyl derivative has been reported in the open chemical literature.
* Corresponding author. Tel.:+1 787 832 4040; fax: +1 787 265 3849.
E-mail address: ricinfante@gmail.com (R. Infante-Castillo).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.07.008

R. Infante-Castillo, S.P. Hernández-Rivera / Journal of Molecular Structure 917 (2009) 158–163
159
penyl)-2-methylbenzimidazole are given in Table 1. The bond
lengths and bond angles determined at the DFT level of theory
for the structural isomers of the 1-(2-methylpropenyl)-2-methyl-
benzimidazole are listed in Table 2. The vibrational frequencies
and corresponding normal modes were then evaluated at the opti-
mized geometry using the same basis set. The vibrational modes
were analyzed using the GaussView 03 software [17]. Also, the
same basis set and computational method was used for the 1H
and 13C NMR shielding constants calculations by applying the GIAO
method. The effect of solvent on the theoretical NMR parameters
was included using the default model IEF-PCM provided by Gauss-
ian 03. Dimethylsulfoxide (DMSO), which has a dielectric constant
4
(a)
(b)
4
3
N
3
N
α
α
3
3
5
6
5
6
2
2
CH
CH
3
3
8
1
N
8
1
N
1α
1α
7
7
CH
CH 1C0 CH
3
12
C
C
2
2
12
9
10
9
H
CH
3
11
CH
3
11
Scheme 1. Atom numberings and studied compounds. (a) 1-(2-Methyl-1-propenyl)-
2-methylbenzimidazole and (b) 1-(2-methyl-2-propenyl)-2-methylbenzimidazole.
(e) of 46.7, was used as solvent.
2. Materials and methods
3. Results and discussion
2.1. General procedure for the synthesis
3.1. Synthesis of 1-(2-methylpropenyl)-2-methylbenzimidazole
The compound 2-methylbenzimidazole was obtained in solid
form from Sigma–Aldrich Chemical Co., and was used without fur-
ther purification. In the synthesis of 1-(2-methylpropenyl)-2-
methylbenzimidazole derivatives, 2-methylbenzimidazole and
alkenyl halide were added to powder KOH stirred in DMSO at room
temperature under nitrogen. The crude product was purified by
recrystallization with methylene chloride. The compounds in-
cluded in this study are shown in Scheme 1.
Potassium hydroxide KOH (2.13 g, 47.9 mmol) was dissolved in
20.0 mL of dimethylsulfoxide with stirring in a 250 mL round-bot-
tomed flask under
a N2 atmosphere. 2-Methylbenzimidazole
(4.07 g, 30.8 mmol) was added and the mixture was stirred for
2 h, upon which 3-chloro-2-methylpropene (3.0 mL, 30.7 mmol)
was transferred. The mixture was stirred at room temperature for
20 h under a N2 atmosphere. The reaction mixture was diluted with
dichloromethane and washed several times with water. The organic
layer was dried over anhydrous Na2SO4 and the solvent was re-
moved. Purified product was isolated and GC–MS method analysis
evidenced the presence of two isomers. 1-(2-Methylpropenyl)-2-
methylbenzimidazole was obtained as a brown solid: 93% yield
(2-propenyl isomer 49% and 1-propenyl isomer 51%); mp 49–
50 °C. The mass spectrum produced by electron impact (EI) ioniza-
tion showed signals at m/e: 186(M+Å), 171, 156, 145, 130, 118, 102,
91, 77, 63, 50 and 39. Elemental analysis confirmed the calculated
percent compositions of C12H14N2: C, 77.41%; H, 7.52%; N, 15.05%.
The values obtained were C, 77.45%; H, 7.58; N, 15.03%.
2.2. Physical methods
GC–MS analyses were performed using a Hewlett–Packard 5890
series II Gas Chromatograph coupled with a Hewlett–Packard mod-
el 5970 mass selective detector. A Supelco SPB-5 capillary column
(length: 30 m ꢁ 0.25 mm i.d.) was used for the chromatographic
separation. The helium carrier gas was set to a flow rate of
0.9 mL/min. The oven temperature was initially at 70 °C (held for
10 min) and then increased to 250 °C at 10 °C/min. The percentage
composition of the compounds was computed from GC peak areas
using the area normalization method. The mass spectrometer was
operated in positive ion detection after electron impact ionization
mode with ionization energy of 70 eV. The ion source temperature
was maintained at 280 °C.
3.2. Theoretical and experimental spectroscopic data
UV–VIS spectrum of 1-(2-methylpropenyl)-2-methylbenzimi-
dazole is characterized by four bands in the deep UV (DUV) region:
283, 276, 253 and 228 nm. The absorption bands of parent com-
pound (280, 274 and 245 nm) were displaced to longer wavelength
and intensified by conjugation with the double bond of the
2-methylpropenyl group. The absorptions that result from transi-
tions within 2-methylbenzimidazole and derivatives chromophore
The room temperature Fourier transform infrared spectra of the title
compound were measured on a Perkin-Elmer System 2000 FT-IR spec-
trometer in the 4000–400 cmꢀ1 region with 4 cmꢀ1 resolution. Raman
spectra were recorded with a Bruker Optics RFS-100 Fourier transform
Raman spectrometer (excitation source: Nd:YAG, 1064 nm). The mea-
surement of the spectra was performed in the range of 100–
3600 cmꢀ1, Stokes region, with 1 cmꢀ1 spectral resolution.
are basically of the
ropenyl substituent in relation to its attachment on the
p
?
p
* type. To explore the effect of 2-methylp-
-system,
The 1H and 13C NMR spectra were recorded on a Varian Gemini
300 FT-NMR spectrometer. Internal lock was provided by a deuter-
ated DMSO solvent (d = 39.51 ppm) and both proton and carbon
signals were referenced to TMS. All NMR spectra were measured
at room temperature.
p
we compared the experimental UV–Vis absorptions with those ob-
tained by DFT configuration interaction approach (CIS). The excita-
tion energies were systematically underestimated in gas phase.
Theoretically, we observed major energetic shifts for the
p ?
p*
excitations. Table 3 summarizes the observed and calculated
absorption energies for benzimidazole and its N-substituted deriv-
atives. Concerning the 1-propenyl-2-methylbenzimidazole spec-
trum, the deviations between theoretical and experimental
results is of the same order of magnitude as those obtained for
1-(2-methylpropenyl)-2-methylbenzimidazole.
2.3. Computational methods
GAUSSIAN 03 [20] software package was used for all theoretical
calculations. Absolute, relative, and zero-point energies of 1-(2-
methyl-1-propenyl)-2-methylbenzimidazole 1-(2-methyl-2-pro-
Table 1
Absolute and relative energies for the studied compounds
Isomer
Absolute energy (Hartrees)
Zero point energy (kcal/mol)
Relativea energy (kcal/mol)
1-(2-Methyl-1-propenyl)-2-methylBZM
1-(2-Methyl-2-propenyl)-2-methylBZM
ꢀ575.367380
ꢀ575.361325
146.36
146.94
0.00
3.21
a
Zero-point-corrected energies relative to 1-(2-methyl-1-propenyl)-2-methylbenzimidazole isomer BZM, benzimidazole.

160
R. Infante-Castillo, S.P. Hernández-Rivera / Journal of Molecular Structure 917 (2009) 158–163
Table 2
Optimized parameters of 1-(2-methylpropenyl)-2-methylbenzimidazole and isomers obtained by B3LYP/6-311+G** calculations
Parameters
2-MethylBZM
1-(2-Methyl-1-propenyl) 2MBZM
1-(2-Methyl-2-propenyl) 2MBZM
Bond lengths (Å)
N1AC2
C2AN3
X-raya
1.335
1.339
1.389
1.395
1.383
1.379
1.382
1.395
1.360
1.389
Calculated
1.3827
1.3083
1.3891
1.4130
1.3852
1.3975
1.3901
1.4068
1.3918
1.3933
1.4918
Calculated
1.3914
1.3088
1.3868
1.4120
1.3930
1.3976
1.3903
1.4068
1.3922
1.3935
1.4912
1.3377
1.5035
1.5055
1.0836
1.0841
1.0841
1.0834
1.0925
1.0865
1.0942
1.0934
1.4243
Calculated
1.3884
1.3103
1.3853
1.4128
1.3892
1.3977
1.3899
1.4067
1.3920
1.3951
1.4920
1.5188
1.5051
1.3340
1.0836
1.0840
1.0840
1.0839
1.0924
1.0931
1.0936
1.0845/1.0852
1.4617
N3AC3
a
C3 AC1
a
a
C1 AN1
a
C3 AC4
a
C4AC5
C5AC6
C6AC7
C7AC1
C2AC8
a
C9AC10
C
C
10AC11
10AC12
C4AH4
C5AH5
C6AH6
C7AH7
C8AH8
C9AH9
1.0834
1.0839
1.0839
1.0841
1.0917
C
C
11AH11
12AH12
N1AC9
Bond angles (°)
N1AC2AN3
112.7
106.3
107.6
107.2
106.1
118.2
121.9
120.7
117.6
122.1
112.4
105.6
110.1
110.1
107.3
118.1
121.3
121.4
116.7
122.5
123.0
105.6
110.2
105.1
106.2
118.1
121.3
121.5
116.9
122.4
132.6
127.1
124.3
120.0
123.8
120.3
119.6
119.3
121.4
110.5
123.8
120.6
111.1
111.1
115.1
112.9
105.6
110.1
105.0
106.4
118.1
121.3
121.5
117.0
122.1
132.8
126.6
114.3
116.9
120.2
120.2
119.6
119.3
120.7
110.5
120.2
109.8
111.1
121.6/121.7
107.9
C2AN3AC3
a
N3AC3 AC1
a
a
C3 AC1 AN1
a
a
C1 AN1AC2
a
C3 AC4AC5
a
C4AC5AC6
C5AC6AC7
C6AC7AC1
a
C7AC1 AC3
a
a
C7AC1 AN1
a
C1 AN1AC9
a
N1AC9AC10
C9AC10AC11
C9AC10AC12
C3 AC4AH4
a
C4AC5AH5
C5AC6AH6
C6AC7AH7
C2AC8AH8
C9AC10AH12
C
C
C
10AC9AH9
10AC11AH11
10AC12AH12
N1AC9AH9
a
Ref. [21]; BZM, benzimidazole; for numbering of atoms refer to Scheme 1.
Table 3
Observed and calculated absorption spectra of benzimidazole and its 1-propenyl derivatives
Compound
Experimental k’s max (nm)
Calculateda k’s max (nm)
Benzimidazole
2-Methylbenzimidazole
278, 272, 267, 248
280, 274, 245
250, 224, 204, 198
251, 238, 202
1-Propenyl-2-methylbenzimidazole
E-1-Propenyl-2-methylbenzimidazole
Z-1-Propenyl-2-methylbenzimidazole
2-Propenyl-2-methylbenzimidazole
1-(2-Methylpropenyl)-2-methylbenzimidazole
1-(2-Methyl-1-propenyl)-2-methylbenzimidazole
1-(2-Methyl-2-propenyl)-2-methylbenzimidazole
283, 276, 252, 230
269, 256, 240, 231
261, 254,242, 230
256, 254, 244, 231
283, 276, 253, 228
256, 250, 244, 233
256, 244, 242, 230
Calculated with CIS/DFT 6-311+G**
.
a
The molecular structures with atom numbering taken from
geometries optimizations are shown in Scheme 1. Absolute, relative
and zero-point energies obtained by the DFT structure optimization
for both isomers are presented in Table 1. The calculations yielded a
difference of 3.21 kcal molꢀ1 between the two structural isomers.
As expected, the model shows that the lowest-energy for 1-prope-
nyl isomer is in accordance with the stabilizing effect generated by
the higher degree of substitution on a double bond system.

R. Infante-Castillo, S.P. Hernández-Rivera / Journal of Molecular Structure 917 (2009) 158–163
161
Table 4
Table 2 presents the optimized structure parameters of 1-(2-
methylpropenyl)-2-methylbenzimidazole molecules calculated
by B3LYP level with the 6-311+G** basis. These structures were
compared with that of 2-methylbenzimidazole, for which the
crystal structure has been solved [21]. The bicycle system of 2-
methylbenzimidazole is planar and the methyl group at atom
C8 deviates from the plane by 0.054°. As a result of partial proton-
ation of both nitrogen atoms the CAN bond lengths area approx-
imately equal in imidazole ring. The optimized geometry of 2-
methylbenzimidazole in the ground state corresponds to Cs sym-
metry and the calculated bond lengths and bond angles with the
computational method yielded a 0.04 Å and 2.9° discrepancy rel-
ative to the X-ray values.
Substitution of hydrogen atom by a 2-methylpropenyl group in
N1 causes small changes in imidazole ring interatomic distances.
The benzimidazole nitrogen bonds have a shorter distance than
amine, which is due to the lone pair electron of nitrogen involved
in benzimidazole rings. The optimized bond lengths of CAN
in imidazole ring fall in the range from 1.3083 to 1.389 Å for
2-mehylbenzimidazole and 1.3088 to 1.395 Å for 1-(2-methylp-
ropenyl)-2-methylbenzimidazole. By comparing these values with
experimental values it is observed that the B3LYP overestimates
them. The CAN bond distance is found overestimated by
0.04 0.01 Å. The results show essentially non-planar benzimid-
azole and 2-methylpropenyl groups (heavy atoms) and an angle
between the two planes (torsion angle C2AN1AC9AC10) of 117°.
The 6-31G basis set with d and p polarization functions predict a
planar geometry for the benzimidazole skeleton and no-planar
methyl groups in the 2-methylpropenyl fragment. It is the
inclusion of polarization functions which determines the non
planarity of the methyl groups.
For the structural isomers of 1-(2-methylpropenyl)-2-methyl-
benzimidazole, it can be seen that the corresponding bond lengths
and bond angles of the benzimidazole ring are almost equal except
that some corresponding imidazole ring parameters have small
differences with respect to benzimidazole. From compound
1-propenyl-2-methylbenzimidazole to 1-(2-methylpropenyl)-2-
methylbenzimidazole, the C@C double bond lengths exhibit an
increasing tendency. The influence of propenyl and 2-methylp-
ropenyl substitution on the imidazole ring angles is also notable,
particularly in the N1AC2AN3 bond angle. In the present study,
the calculated C@C double bond length (C9AC10 and C10AC12) for
1-propenyl to 2-propenyl isomer is shorter by 0.004 Å. The largest
bond length difference (0.04 Å) can be attributed to a N1AC9 bond
distance that corresponds to the bond between the 2-methylp-
ropenyl group and the 2-methylbenzimidazole ring. There are
slight differences in a few of the bond angles. The largest difference
involves N1AC9AH9 and the bond angles in double-bond isomers.
The isomer with most highly substituted double bond has a larger
angular separation between the methyl groups.
The two isomers of 1-(2-methylpropenyl)-2-methylbenzimida-
zole have C1 symmetry with 28 atoms and 78 fundamental normal
modes of vibration. The assignments of vibrational frequencies of
the mixture of isomers of 1-(2-methylpropenyl)-2-methylbenzimi-
dazole (1-propenyl and 2-propenyl) based on normal mode analy-
sis are presented in Table 4, which lists the experimental and the
calculated scaled frequencies. The experimental and calculated
FT-IR and FT-Raman spectra for the isomeric mixture of 1-(2-meth-
ylpropenyl)-2-methylbenzimidazole are presented in a common
frequency scale in Figs. 1 and 2, respectively. The mode number
assignments are made from comparisons of experimental spectra
and the theoretical calculations. Very weak signals were not
labeled. The reported experimental FT-IR intensities for the isomer
mix are comparable with the ones obtained theoretically. The
problem of the overestimation common in DFT vibrational
frequencies, due to neglecting of anharmonicity effects, was solved
Observed and calculated selected frequencies (cm-1) and assignments of the
fundamental modes for isomers mix of 1-(2-methylpropenyl)-2-
methylbenzimidazole
Mode no.
1-(2-Methylpropenyl)-2-methylbenzimidazole
FT-IR
FT-Raman
Calc.a
(cmꢀ1
Assignmentb,c
(cmꢀ1
(cmꢀ1
)
(cmꢀ1
)
)
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
3046
3061
3004
2926
1657
3051
3006
2897
1677
1619
1591
1537
1457
1399
1380
1341
1293
1270
1245
1167
1148
1110
1053
1019
933
m
asCY(Bz)
msYAC@
2974
1681
1618
m
m
m
m
m
sHACMet)
C@C
C@C(Bz)
C@C/Bz
1588
1519
1449
1525
1456
1401
C@N(Im),m C@C(Bz)
bCYMet), b CYDZ
c
c
c
CYMet)
CYMet)
CY2
1378
1328
1281
1256
1230
1334
bCYDZ
bCY2
bCYDZ
bCYDZ
bCYDZ
bCYDZ
,
m
CAN(Im)
CAN(Im)
, c CY2
1234
1170
,
m
1145
1104
1055
1017
932
c
c
c
CYMet)
CYMet)
H2C@
1010
887
855
776
884
825
786
740
bdef(BZ + Im), bCY2
814
721
c
c
c
CYDZ, rg
H2C@
CYDZ
breathing
670
618
500
678
619
502
bdef(BZ + Im)
bdef(BZ + Im)
bdef(BZ)
,
,
c
c
H2C@
H2C@
Calculated with B3LYP/6-311+G**; scaled according to correlation equation
a
mobs.
/
m
calc. = 1.0087(9)ꢀ0.0000163 (
m
calc./cmꢀ1) [16].
b
Vibrational modes: , stretching, b, in-plane bending; c, out-of-plane bending;
m
rg, ring; def, deformation; superscript s, symmetric; superscript as, asymmetric; Bz,
benzene; Im, imidazole; Met, methyl.
c
Estimated graphical representation.
using the wavenumber-linear scaling method (WLS) performed by
Yoshida et al. with the following relationship:
mobs:
=
mcalc: ¼ 1:0087ð9Þ ꢀ 0:0000163ð6Þð
m
calc:=cmꢀ1
Þ
ð1Þ
Combining the results of experimental FT-IR and FT-Raman data
with the theoretical calculations, vibrational frequency assign-
ments were made with a high degree of confidence. The calculated
and scaled frequencies are very close to the corresponding experi-
mental values.
In the higher frequency region, the mCAH asymmetric mode of
vibration associated with the benzene ring (1) is observed at
3046 and 3061 cmꢀ1 in the FT-IR and FT-Raman spectra, respec-
tively. A weak Raman band at 3006 cmꢀ1 (2) is assigned to a
sp2CAH stretching mode. The CAH alkyl stretching vibrations
(mCAH) of the title compound have been assigned to the signals ob-
served at 2974 cmꢀ1 in the FT-IR spectrum and at 2926 cmꢀ1 in the
FT-Raman spectrum (3). The weak IR and Raman bands at 1681
and 1657 cmꢀ1, respectively, are assigned to a stretching mode of
the C@C double bond (4). In the FT-IR and FT-Raman spectra of
1-propenyl-2-methylbenzimidazole, the C@C stretching bands ap-
pear at 1658 and 1662 cmꢀ1, respectively [11]. The aromatic ring
stretching modes were observed in the region between 1618 and
1588 cmꢀ1 (5, 6). Complete analogies can also be found in the case
of 1-propenyl-2-methylbenzimidazole. The mC@N mode was found
at 1525 and 1519 cmꢀ1 (8). In addition, the mCAN stretching in the
imidazole ring was identified at 1281 cmꢀ1 (12) in combination
with other in-plane and out-of-plane bending modes. A group of
four bands (modes 9, 10, 18 and 19) have been assigned as out-
of-plane bending modes (cCAY) of the methyl groups. In the spectra

162
R. Infante-Castillo, S.P. Hernández-Rivera / Journal of Molecular Structure 917 (2009) 158–163
The CAC ring breathing mode is readily assigned to the band at
814 and 855 cmꢀ1 in the IR and Raman spectrum, respectively. The
remaining modes (25, 26 and 27) in the 700–500 cmꢀ1 region can
be associated with the deformations of the benzene and imidazole
rings. These correspond to a mix of vibration modes such as ring
(a)
(b)
breathing, CAH out-of-plane modes (
cCAH) and ring skeletal in
plane and out-of-plane deformations (bCACAC and
c
CACAC). The
same assignment can be done for 1-propenyl-2-methylbenzimida-
zole in modes below 700 cmꢀ1. All assignments and vibrational fre-
quencies are in agreement with typical values recently reported for
the benzimidazole ring [22,23].
4
20
18
19
The NMR chemical shifts of two possible structures were calcu-
lated by B3LYP/6-311+G** hybrid functional using GIAO approxi-
mation. The 1H and 13C shieldings were converted into the
predicted chemical shifts using dTMS values, calculated at
the same level of theory (dC = 191.77 ppm and dH = 31.76 ppm).
The comparison of calculated chemical shifts in 1-(2-methylprope-
nyl)-2-methylbenzimidazole structures with the experimental
data is given in Table 5. The calculations reported here were per-
formed in DMSO solution, rather than in the gas phase, using
IEF-PCM model. These are in agreement with experimental chem-
ical shifts obtained in DMSO solutions. Results of linear regression
fits between experimental and calculated chemical shifts (1H and
13C) performed for the two structures tested are included in Table
5. The regression coefficients between the calculated and experi-
mental chemical shifts range from a low of 0.9958 for 13C NMR
data of (2-propenyl) structure to a high of 0.9980 for 13C NMR data
of (1-propenyl) structure in the worst case (see Table 5).
15
14
22
5
1
3
7
11
89
24
3200 2900 2600 2300 2000 1700 1400 1100
800
500
Wavenumbers (cm )
Fig. 1. Theoretical and experimental FT-IR spectra from 500 to 3200 cmꢀ1 for
isomers mix of 1-(2-methylpropenyl)-2-methylbenzimidazole: (a) calculated and
scaled and (b) experimental. Labeled peaks are explained in Table 4.
of 1-(2-methylpropenyl)-2-methylbenzimidazole, the CAH in-
plane modes (bCAY of benzene appear as a series of bands (modes
14,15,16,17) in the spectral range 1100–1240 cmꢀ1, while in 1-
propenyl-2-methylbenzimidazole these modes appear in the
1006–1284 cmꢀ1 region. In the FT-IR spectrum, the presence of
out-of-plane CAH bending mode in benzene ring (24) was
observed at 721 cmꢀ1. This band is shifted in the FT-IR spectrum
of 1-propenyl-2-methylbenzimidazole to 711 cmꢀ1. A medium
intensity Raman band located at 1256 cmꢀ1 is assigned to an in-
plane bending mode of CH2 in vinyl group (13); the corresponding
out-of-plane IR (20) and Raman (23) band was observed at 932 and
776 cmꢀ1, respectively.
Table 5
The experimental and theoretical 1H and 13C NMR chemical shifts d(ppm) from TMS
for studied compounds
Atom
1-(2-Methyl-1-propenyl)
2-methylBZM
1-(2-Methyl-2-propenyl)
2-methylBZM
d(exp.)
d(calc.)
d(exp.)
d(calc.)
C1
C2
C3
C4
C5
C6
C7
C8
C9
135
151
140
121
121
121
110
14
142
156
150
125
125
125
112
16
135
151
140
121
121
121
109
13
142
156
150
125
125
125
110
15
52
154
21
117
0.96
ꢀ0.3
0.996
ꢀ4.5
3.2
a
a
7
18
13
118
138
21
124
149
24
48
142
19
14
6
C10
C11
C12
aa
8
18
19
118
12
0.95
ꢀ0.6
0.998
ꢀ4.9
2.3
25 23
26
(b)
16
10
11
bb
4
27
22
R2c
Av.d
Sre
21
17
H4
H5,6
H7
H8
H9
7.6
7.2
7.2
2.6
6.5
1.7
2.0
7.9
7.3
7.1
2.4
6.5
1.6
2.1
7.6
7.2
7.2
2.6
4.4
1.4
4.9
4.8
7.9
7.4
7.4
2.4
4.6
1.5
5.1
5.2
0.95
0.1
0.996
ꢀ0.2
0.2
H11
H12
H12
aa
(a )
0.96
ꢀ0.1
0.997
ꢀ0.01
0.1
bb
R2
c
Av.d
Sre
450
650
850
1050
1250
1450
1650
1850
For numbering of atoms refer to Scheme 1.
Raman Shift (cm-1)
Slope.
Intercept.
Correlation coefficient.
a
b
c
Fig. 2. Theoretical and experimental FT-Raman spectra from 450 to 1850 cmꢀ1 for
isomers mix of 1-(2-methylpropenyl)-2-methylbenzimidazole: (a) calculated and
scaled and (b) experimental. Labeled peaks are explained in Table 4.
d
Average differences between experimental and calculated chemical shifts.
Standard error of the estimate; BZM, benzimidazole.
e

R. Infante-Castillo, S.P. Hernández-Rivera / Journal of Molecular Structure 917 (2009) 158–163
163
The experimental and calculated 1H and 13C chemical shifts
9
8
7
6
5
4
3
2
1
0
show no significant difference in the benzimidazole ring between
the investigated structures. 1H NMR resonances at 6.5 and 4.9/
4.8 ppm were observed and assigned to vinyl protons (H9 and
H12) of structures 1-propenyl and 2-propenyl of 1-(2-methylprope-
nyl)-2-methylbenzimidazole, respectively. In the 1H NMR spec-
trum of 1-propenyl-2-methylbenzimidazole, the chemical shift
associated to vinyl protons of E and Z structures are observed at
6.95/6.05 and 6.75/5.95 ppm. The vinyl protons in 1-(2-prope-
nyl)-2-methylbenzimidazole were found at 490 and 5.15 ppm.
The resonances at 118 and 138 ppm correspond to vinyl carbons
(C9 and C10) for structure (a) according to Scheme 1. In structure
(b), these signals were found at 118 and 142 ppm for carbons C10
and C12. The resonances at 117, 134 and 118 ppm correspond to
the vinyl carbon in E, Z and 2-propenyl isomers of 1-(propenyl)-
2-methylbenzimidazole. Based on the 1H and 13C chemical shifts,
data presented in Table 5 and the fits (Figs. 3 and 4), one can de-
duce that the selected DFT method combined with the basis set
represent a good compromise between accuracy and computa-
tional time, and yielded proton and carbon chemical shifts in
agreement with experimental values.
0
1
2
3
4
5
6
7
8
Calculated chemical shifts (ppm)
Fig. 4. The linear regression between experimental and theoretical DFT predicted
1H NMR chemical shifts for investigated structures using 6-311+G** basis set; s
structure (a), d structure (b).
4. Conclusions
been used for definitive signal assignments to the corresponding
structural isomers.
In this work, the 1-(2-methylpropenyl)-2-methylbenzimidazole
and its isomers have been synthesized and characterized by ele-
mental analysis, 1H/13C NMR, UV, IR and Raman spectroscopy.
The energies, geometric parameters, vibrational frequencies and
13C/1H chemical shift values of title compounds were calculated
by using B3LYP with the standard 6-311+G** basis sets. For the iso-
mers studied, the 1-propenyl structure is thermodynamically fa-
vored by 3.2 kcal/mol. Comparison between the calculated
geometric parameters and the X-ray structure by the 2-methyl-
benzimidazole ring does not show major discrepancies except for
some small differences in the imidazole ring. The experimental
infrared and Raman spectra for the isomeric mixture of 1-(2-meth-
ylpropenyl)-2-methylbenzimidazole have been assigned and were
supported by the calculated (scaled) vibrational spectra. The typi-
cal vibrations in the benzimidazole ring and the stretching mC@C
mode of the propenyl group have been very accurately predicted
by the selected computational method. Correlations between the
proton and carbon-13 experimental chemical shifts and the GIAO
NMR calculations are in good agreement and these values have
Acknowledgment
We thank the Chemical Imaging Center of the Chemistry
Department of University of Puerto Rico-Mayagüez for the finan-
cial support of this project.
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Fig. 3. The linear regression between experimental and theoretical DFT predicted
13C NMR chemical shifts for the investigated structures using 6-311+G** basis set; s
structure (a), d structure (b).