2-Benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives: Spectroscopic and theoretical study

Article abstract of DOI:10.1016/j.saa.2006.08.013

The spectroscopic behavior of 2-benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives in solution was studied in terms of electronic and nuclear magnetic resonance (¹H and ¹³C NMR) techniques. The experimental spectra were compared to the theoretical ones, obtained at DFT level, proving that the compounds adopt in solution a bird-like conformational distribution. Also, theoretically this conformational distribution resulted the most stable in gas phase. Infrared spectroscopy was used to study solid state behavior identifying experimentally the N-O stretching near to 1380, 1365 and 1225 cm^-1 and the vibrational benzimidazole skeleton near to 1610 and 1590 cm^-1. The vibrational spectrum was satisfactorily described by DFT calculations funding the N-O stretching as a coupled vibration near to 1470, 1350 and 1285 cm^-1. The fragmentation that takes place in mass spectrometry was assigned for all of the new derivatives.

Full text of DOI:10.1016/j.saa.2006.08.013

Spectrochimica Acta Part A 67 (2007) 540–549
2-Benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives:
Spectroscopic and theoretical study
∗
∗
´
Alicia Merlino, Mariana Boiani, Hugo Cerecetto , Mercedes Gonzalez
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias-Facultad de Qu´ımica, Universidad de la Repu´blica, Igua´ 4225, Montevideo 11400, Uruguay
Received 18 April 2006; received in revised form 10 August 2006; accepted 14 August 2006
Abstract
The spectroscopic behavior of 2-benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives in solution was studied in terms of electronic and
nuclear magnetic resonance (1H and 13C NMR) techniques. The experimental spectra were compared to the theoretical ones, obtained at DFT level,
proving that the compounds adopt in solution a bird-like conformational distribution. Also, theoretically this conformational distribution resulted
the most stable in gas phase. Infrared spectroscopy was used to study solid state behavior identifying experimentally the N–O stretching near to
1380, 1365 and 1225 cm−1 and the vibrational benzimidazole skeleton near to 1610 and 1590 cm−1. The vibrational spectrum was satisfactorily
described by DFT calculations funding the N–O stretching as a coupled vibration near to 1470, 1350 and 1285 cm−1. The fragmentation that takes
place in mass spectrometry was assigned for all of the new derivatives.
© 2006 Elsevier B.V. All rights reserved.
Keywords: 2-Benzyl-2-methyl-2H-benzimidazole 1,3-dioxide; DFT calculations; Electronic spectra; NMR spectra; IR spectra; Mass spectrometry
1. Introduction
work 2,2-dimethyl-2H-benzimidazole 1,3-dioxide derivatives
were identified as excellent anti-T. cruzi and anti-Leishmania
agents [12] (Fig. 1a). The synthesis of 2H-benzimidazole 1,3-
dioxide derivatives from benzofuroxans is quite old [17] and in
this description information on spectroscopy data was extremely
scanty. On the other hand, nothing is known about its molecu-
lar structure due to the lack of crystallographic data. Knowing
the benzimidazole 1,3-dioxide conformeric distribution, in solu-
tion, results very relevant in the identification of the bio-active
entities, thus allowing to perform further QSAR studies.
Having this in mind and taking into account our current
interest in the investigation of heterocyclic amine N-oxides, we
have developed new derivatives of 2-methyl-2H-benzimidazole
1,3-dioxide, 2-benzyl substituted (Fig. 1b), and studied experi-
mentally the electronic, NMR, and IR spectroscopies and EI-MS
behavior along with theoretical calculation using density func-
tional theory (DFT) approach.
Benzimidazole N-oxides present a very interesting chemistry
and a number of reviews covering its synthesis and reactivity
are found [1–3]. At difference with other heterocycles N-oxides
they are not obtained by direct N-oxidation of benzimidazoles
[2,4], a fact that has lead to the development of an extensive
and rich chemistry around this system. The chemistry of hete-
rocyclic amine N-oxides has raised widespread interest due to
the exceptionally high bioactivity of these compounds [3,5–8].
Despite little is known regarding the biological spectrum of
the benzimidazole N-oxide system a number of derivatives
have been described as antihelmintic [9], acaricides, antibac-
terial [10] and trypanocidal agents [11,12]. What is more, 1-
hydroxybenzimidazole derivatives have shown central nervous
system depressant effects [13] and N-alkoxybenzimidazoles has
been described as anti-HIV agents [14,15]. These compounds
have also found application in the field of organic chemistry
as synthetic intermediates [16]. As an effort to develop novel
trypanocidal agents we focused our current research on the
development of benzimidazole N-oxide [11,12]. In a previous
2. Experimental
2.1. Samples
All starting materials were commercially available research-
grade chemicals and used without further purification. All
solvents were dried and distilled prior to use. The typical
∗
Corresponding authors. Tel.: +598 2 525 8618/216; fax: +598 2 525 0749.
E-mail addresses: hcerecet@fq.edu.uy (H. Cerecetto),
megonzal@fq.edu.uy (M. Gonza´lez).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.08.013

A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
541
Fig. 1. (a) Examples of 2,2-dimethyl-2H-benzimidazole 1,3-dioxide derivatives developed as trypanocidal agents and (b) 2-benzyl-2-methyl-2H-benzimidazole
1,3-dioxide derivatives used in the study.
Scheme 1. Synthetic procedures used to prepare the new 2H-benzimidazole 1,3-dioxides 12–15. Notes: (*) Reference [19]. (**) References [20,21].
work-up included washing with brine and drying the organic
layer with sodium sulphate. The synthetic procedure is shown in
Scheme 1 and some relevant experimental details are gathered
in Table 1. Complete synthetic procedures and spectroscopic
characterization of new compounds (12–15) are given as
supporting information. Benzofuroxan, 11, and derivative 1
(Fig. 1) were obtained as described in the literature [12,17,18].
solvents. TMS (tetramethylsilane) was used as internal refer-
ence. FTIRspectrawererecordedwitharesolutionof4 cm−1, on
aPerkin-Elmer1310apparatus, usingKBrwaferscontaining1%
of the sample. Mass spectra were recorded using a quadrupole
Hewlett Packard 5988A instrument, with electronic impact at
70 eV, with direct insertion probe, the ion source was set at
150 ◦Candthemassrangewas40–500 amu. Meltingpointswere
determined using an Electrothermal IA-9100 apparatus and are
uncorrected.
2.2. Apparatus
Electronic spectra were performed in a UV–vis spectropho-
tometer UV-1603 Shimadzu in methanol between 210 and
700 nm and at 303 K. NMR spectra were acquired on a Bruker
DRX-400 spectrometer at 303 K, approximately 10% (w/v),
using the standard sequences for the NOE, HMQC and HMBC
experiments. Samples were dissolved in the indicated deuterated
2.3. Computational methods
All DFT calculations were performed with the GAUSSIAN-
03 program package [22]. The B3LYP exchange-correlation
functional was used for all the calculations [23–26]. Molecules
were constructed using standard geometries and bond lengths.
Table 1
Relevant results of the synthesis of 2-benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives
Reference
–R2
Conditions
treaction (h)
Yielda (%)
ꢀE (msc − smsc)b
12
13
–H
–OCH3
Room temperature
Room temperature
72
48
62
76
−2.4
−2.3
Room temperature
Reflux
72
6
c
–
–
–N(CH3)2
14
15
–Cl
–NO2
Room temperature
Room temperature
30
35
71
72
−2.5
−2.6
a
Chromatographically isolated products. Yields were not optimized.
kcal mol−1, in vacuo.
Product was not isolated.
b
c

542
A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
Fig. 2. (a) Conformer distribution is obtained through rotation of the methylene “bridge” (example compound 12). (b) Left: most stable conformer (msc, see text)
obtained in gas phase for derivative 12 (bird-like conformer) and right: second most stable conformer (smsc) in gas phase for 12.
Table 2
The initial conformations were obtained from systematic search
Electronic spectra data, in methanol, for studied benzimidazole 1,3-dioxide
derivatives
using molecular mechanics force field (MMFF) [23]. The dif-
ferent conformeric situations resulted by bond rotation around
a
Reference
λmax (nm)
E (eV)
log εb
the methylene group (Fig. 2a). The most stable conformer (msc)
results, in gas phase, approximately 2.5 kcal mol−1 more sta-
ble than the second most stable conformer (smsc). The two
most stable conformers, for each derivative (Fig. 2b), were
fully optimized. Geometry optimizations, single point vibra-
tional frequency [26] and TDDFT [27] calculations were carried
out using the 6-31G(d) basis set. Structures obtained have C1
symmetry [27]. Calculations on proton and carbon magnetic
shielding values were carried out using the gauge including
atomic orbital (GIAO) approach based on the 6-311 + G(2d,
p) basis set [28–31]. Solvent effects on calculated UV–vis and
NMR spectra were introduced by the IEF-PCM method [29–33]
implemented in the GAUSSIAN program. To compare isotropic
shieldings with the experimentally observed chemical shifts, the
NMR parameters for TMS (tetramethylsilane) were used as the
reference molecule.
519
248
2.39
5.01
3.92
4.47
12
520
244
226
2.39
5.09
5.50
3.66
4.29
4.43
13
520
248
2.39
5.01
3.71
4.26
14
15
520
248
2.39
5.01
3.59
4.47
511
248
2.43
5.01
3.89
4.40
1
a
At 30 ␮M.
ε: molar absorptivity (L mol−1 cm−1).
b
(log ε < 4) band centered at 520 nm. Derivative 13 also shows an
intense band located at 226 nm. In Table 2 is gathered these λ
and the corresponding molar absorptivity for each new deriva-
tive and for compound 1. Parent compound 1 showed the same
band in the ultraviolet region and at 511 nm in the visible one
(Fig. 3). There are no differences between the electronic spectra
of new compounds and 2,2-dimethyl analogue 1 (Fig. 3), except
3. Results and discussion
3.1. Electronic spectra
All the derivatives, 12–15, showed in methanol two bands, a
strong band (log ε > 4) located near to 248 nm and a weak broad
Fig. 3. Left: superimposed electronic spectra for the new derivatives 12–14 and right: electronic spectrum for derivative 15. Inset: spectrum for compound 1.

A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
543
Table 3
Computed vertical excitation energy (eV), oscillator strength (f) and fraction composition for the optical transitions with f > 0.05 of compounds 12–15 and 1 in MeOH
in terms of single molecular orbital excitationsa
States
Composition
Excitation energy (eV)
Wavelength (nm)
f
Derivative 12 msc
H → L (54%)
11A
2.24
5.25
554
236
0.07
0.68
H → L + 3 (12%)
H-5 → L (34%)
111A
H → L + 4 (48%)
Derivative 12 smsc
11A
H → L (54%)
2.23
5.25
555
236
0.08
0.92
H-5 → L (26%)
H-4 → L (22%)
H → L + 3 (34%)
H → L + 4 (35%)
111A
Derivative 13 msc
H → L (54%)
11A
2.24
5.25
554
236
0.07
0.65
H → L + 2 (10%)
H-5 → L (33%)
H-1 → L + 2 (14%)
H → L + 3 (47%)
131A
171A
H-9 → L (19%)
H-2 → L + 1 (24%)
H-1 → L + 1 (11%)
H-1 → L + 2 (17%)
H-l → L + 4 (56%)
5.77
215
0.20
Derivative 14 msc
H → L (54%)
11A
2.25
5.28
550
235
H → L + 3 (12%)
H-4 → L (33%)
111A
H → L + 4 (50%)
Derivative 15 msc
H → L (53%)
21A
2.26
4.37
549
284
0.07
0.25
H → L + 1 (13%)
H-8 → L + l (16%)
131A
211A
H-3 → L + 1 (64%)
H-8 → L (28%)
H-4 → L (30%)
H → L + 4 (46%)
H-8 → L (64%)
H-4 → L (12%)
H → L + 4 (20%)
5.28
5.35
235
232
0.60
0.14
221A
Parent compound 1
H → L (54%)
11A
2.27
5.29
547
234
0.09
0.78
H → L + 1 (13%)
H-3 → L (36%)
111A
H → L + 2 (48%)
a
H, HOMO (highest occupied molecular orbital); L, LUMO (lowest unoccupied molecular orbital); msc, most stable conformer; smsc: second most stable
conformer.
for the absence of a shoulder near to 260 nm for the dimethyl
derivative.
To estimate the excitation energies and oscillator strength
obtained in the theoretical conformational searching in gas
phase, the most stable conformers (msc) and the second most
stable conformers (smsc).
TDDFT calculations have been performed. Table 3 summaries
the calculated electronic spectra for each of the conformers,
The electronic spectra are characteristic of 1,3-diene sys-
tems 1,4-disubstituted by ␲-electron rich moieties, confirming

544
A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
Fig. 4. Molecular orbitals HOMO and LUMO of the two conformers studied of derivative 12.
the contribution of the N-oxide oxygens to the ␲-delocalizated
system [34]. It is an n␲ band with an internal charge-transfer
character, a transition from an n orbital that is highly localized
on the oxygen atom to one ␲ delocalized on the ring (Fig. 4)
[35].
DFT-calculated spectra of both conformers were in total
agreement with the experimental ones. According to these
results electronic spectroscopy does not allow to establish the
preferential conformer in solution. Theoretically both stud-
ied conformeric situations present the same absorption maxi-
mums showing that the benzylic substituent position did not
affect the ␲-electronic processes in the diene systems (see
Fig. 4 and Table 3). However, the absence of the 2-benzylic
substituent shifts the visible maximum to lower frequencies
(compare calculated values for derivative 12 and parent com-
pound 1, 554 versus 547 nm, or experimental values, 519 versus
511 nm). The calculated values describe correctly the exper-
imental spectra, the corresponding functional sub-estimated
the experimental values in the visible region (between −3.0
and −5.6%) and overestimated the ultraviolet ones (between
+5.6 and +7.0%).
*
3.2. NMR spectra
The experimental 1H and 13C NMR spectra of benzimidazole
1,3-dioxide derivatives resulted well resolved and together with
correlation experiments, HMQC and HMBC, allow to assign
all the protons and carbons of the molecules (Fig. 5). Parent
compounds 1 and 2 permit to assign unequivocally protons and
carbons 4–7. Table 4 gathered these assignments.
Fig. 5. Series of HETCOR-NMR experiments (left: HMQC and right: HMBC) in CDCl3 for derivative 13 that allow to assign the nuclear displacements.

A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
545
Table 4
1H and 13C NMR chemical shifts (ppm) and proton-coupling constants (Hz) of derivatives 1, 12–15 in CDCl3
Reference
1H NMR
ꢀ
ꢀ
ꢀ
ꢀ
δH = δH
δH = δH
J4,5 = J6,7
J4,6 = J5,7
δCH
δCH
3
δH = δH
δH = δH
J2 ,3 = J5 ,6
ꢀ
ꢀ
ꢀ
ꢀ
5
4
7
5
6
2
2
6
3
12
13
14
15
1
6.96
6.97
6.97
6.97
7.22
6.67
6.66
6.70
6.71
6.90
7.2
7.2
7.2
7.2
7.2
2.8
2.8
2.8
2.8
2.8
3.44
3.39
3.40
3.54
–
1.84
1.81
1.83
1.87
1.72
7.15
–
7.07
7.09
7.35
–
6.68
7.13
8.02
–
8.8
8.4
8.4
–
Reference
13C NMR
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
δC-2
δC-3a = δC-7a
δC-4 = δC-7
δC-5 = δC-6
δCH
δCH
δC-2 = δC-6
δC-3 = δC-5
δC-4
2
3
12
13
14
15
1
100.33
100.54
100.07
104.28
97.40
138.22
138.24
138.22
138.20
136.91
115.37
115.39
115.35
115.30
116.14
130.97
131.11
131.23
131.25
131.20
44.11
43.43
43.11
43.00
–
23.35
23.31
23.54
23.76
24.59
129.78
130.91
131.31
131.62
–
129.19
113.99
128.80
123.76
–
132.07
159.56
134.28
148.04
–
Table 5
Selected calculated 1H and 13C NMR chemical shifts (ppm) of derivatives 1, 12–15 in CDCl3
Reference
–R2
Results
δH = δH
δH = δH
δCH
δCH
δH = δH
δH = δH
ꢀ
ꢀ
ꢀ
ꢀ
5
4
7
5
6
2
3
2
6
3
Experimental
Theoretical, msc
Theoretical, smsc
6.96
7.05
7.26
6.67
6.50
6.82
3.44
3.40
3.33
1.84
1.98
1.61
7.15
7.40
7.56
12
–H
Experimental
Theoretical, msc
Theoretical, smsc
6.97
6.92
7.24
6.66
6.40
6.79
3.39
4.14
3.29
1.81
1.71
1.58
7.07
7.71
7.47
6.68
7.02
6.79
–OCH3
–Cl
13
14
15
Experimental
Theoretical, msc
Theoretical, smsc
6.97
7.03
7.32
6.70
6.56
6.80
3.40
3.43
3.34
1.83
1.99
1.60
7.09
7.42
7.46
7.13
7.58
7.57
Experimental
Theoretical, msc
Theoretical, smsc
6.97
7.06
7.32
6.71
6.56
6.84
3.54
3.53
3.48
1.87
2.01
1.62
7.35
7.76
7.63
8.02
8.53
8.69
–NO2
–
Experimental
Theoretical
7.22
7.27
6.90
6.71
–
–
1.72
1.82
–
–
–
–
1
–R2
Results
δC-2
δC-3a = δC-7a
δC-4 = δC-7
δC-5 = δC-6
δCH
δCH
ꢀ
δC-4
Reference
2
3
Experimental
Theoretical, msc
Theoretical, smsc
100.33
109.49
107.34
138.22
145.66
143.85
115.37
121.90
122.49
130.97
134.32
135.25
44.11
46.99
49.10
23.35
27.51
25.42
132.07
132.18
132.91
12
13
14
–H
Experimental
Theoretical, msc
Theoretical, smsc
100.54
118.95
108.16
138.24
145.88
144.27
115.39
121.82
122.29
131.11
131.21
135.24
43.43
50.91
48.51
23.31
31.82
24.96
159.56
168.84
167.82
–OCH3
Experimental
Theoretical, msc
Theoretical, smsc
100.07
108.44
107.13
138.22
146.04
144.49
115.35
122.15
122.17
131.23
135.64
135.33
43.11
46.94
47.68
23.54
27.54
25.59
134.28
149.83
149.90
–Cl
Experimental
Theoretical, msc
Theoretical, smsc
104.28
108.15
105.98
138.20
146.31
144.98
115.30
122.14
122.39
131.25
136.12
136.02
43.00
46.99
47.70
23.76
28.05
25.28
148.04
154.76
154.17
–NO2
–
15
1
Experimental
Theoretical
97.40
105.22
136.91
144.11
116.14
122.53
131.20
135.10
–
–
24.59
26.96
–
–

546
A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
Fig. 6. Theoretical and experimental (NOE analysis) hydrogen atomic distances CH2–CH3 for derivative 13. (a) Left: phenyl protons used as distance’s reference
ꢀ
ꢀ
and right: mixing times (ms) vs. H3 -NOE intensity when H2 was irradiated. Slope was correlated with the theoretical distance between these protons. (b) Left:
theoretical distances between CH2- and CH3-protons in both main conformers (msc and smsc) from DFT calculations and right: experimental distance from NOE
experiments (mixing time vs. CH3-NOE intensity when CH2 was irradiated).
The oxygens of the N-oxide moieties affect the protons in
the peri-position, protons 4 and 7, being protons 5 and 6 the
most shielded [36]. Carbon 3a, or 7a, appears at higher field
(∼138 ppm) than a typical iminic carbon (149 ppm [34]) due
to the mesomeric shielding effect of the oxygen of N-oxide
moiety. The most relevant feature observed in the NMR spec-
troscopy is the 4–7 proton displacement differences between
benzyl derivatives 12–15 and the parent compound 1, ∼6.7 and
6.9 versus ∼6.9 and 7.2, respectively. Clearly, it is the result
of the presence of the 2-benzylic substituent that in the most
stable conformer, bird-like conformer, affects diamagnetically
the nuclear magnetic resonance of 4–7 protons (Fig. 2b). In
this sense, it is also possible to observe the electronic effect of
the p-substituent in the 2-benzylic moiety, i.e. the effect of the
mesomeric electron-donor OCH3 produces more shielded signal
for 5- and 6-protons than the mesomeric electron-withdrawing
NO2 (compare 6.66 ppm for derivative 13 with 6.72 ppm for
derivative 15). The δH of protons 5 and 6 resulted linearly
dependent on σp physicochemical descriptor of p-substituent
(r2 = 0.8687, ra2dj = 0.8031, s = 0.0106, F = 13.23) [37]. When
we theoretically analyzed the other conformer, the second most
stable, clear differences were observed in the 1H and 13C NMR
displacements (Table 5). Protons 4–7 and phenyl ones appeared
more deshielded in the smsc than in the msc, clearly it is the
result of a mutually diamagnetic effect present in msc (i.e. com-
pare 7.05 and 7.26, 6.50 and 6.82 and 7.40 and 7.56 for derivative
12). The same effects were observed in calculated carbons 4–7.
Besides, the theoretical values of msc signals are very close
to the experimental values showing that in solution the prefer-
ential distribution is the bird-like conformation. Comparison of
calculated and experimental NMR shielding of both conform-
ers allows a clear assignation of the conformation in solution.
In theoretic terms, the electronic effects of the p-substituent in
the 2-benzylic moiety on the 4–7 proton displacements result
in agreements with the experimental effects (i.e. compare the
theoretical displacements for δH in msc derivatives 12, 13, and
5
15, Table 5).
In order to confirm these results nuclear Overhauser effect
(NOE) analysis were performed. The average distance between
the hydrogens of the 2-methylenic group and the 2-methyl
hydrogens was determined using different mixing times in the
NOE studies. These distances were determined taking the aro-
matic protons in the phenyl substituents as distance’s reference;
see Fig. 6 for derivative 13. Clearly, the experimental distance
between 2-CH2–benzylic and 2-CH3 is close to the theoretical
one in the bird-like conformer (msc).

A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
547
Fig. 7. FTIR spectrum of compound 13.
3.3. Infrared spectra
The experimental IR spectra of compounds 1, 12–15 resulted
well resolved and allowed to assign the vibrational modes
(Fig. 7). The observed and calculated wavenumbers are gath-
ered in Table 6.
ThevibrationalspectrumwassatisfactorilydescribedbyDFT
calculations showing the N–O stretching as a coupled vibration
near to 1380, 1365 and 1225 cm−1. According to the theo-
retical results and in disagree with previously reported N–O
stretching [17], we found that the band near to 1225 cm−1
was associated with the vibration of N–O moiety [30]. The
bands at 1584–1588 and 1605–1613 cm−1 were assigned to
the skeleton’s vibration of benzimidazole ring. The skeleton’s
vibrations of phenyl appear at 1514–1516 cm−1. For compound
12 the peaks at 707, 739 and 752 cm−1 were attributed to the
δoop(C–H) mode of phenyl ring. The corresponding δoop(C–H)
forcompounds13–15wereobservednearto806–808 cm−1. The
δ(CH3) mode appear at 1445–1452 cm−1 and the δ(CH2) mode
at 1481–1493 cm−1. The discrepancies between the calculated
and experimental values were in general smaller than 30 cm−1
.
The υ(N–O) modes were sub-estimated no more than 2% and
overestimated no more than 7%.
3.4. Fragmentation in mass spectrometry
Mass spectrometry with electronic impact at 70 eV yielded a
clear pattern of fragmentation. The proposed pattern of fragmen-
tation is presented in Fig. 8 and Table 7 gathered the informa-
tion related to these ions. Fragmentation under electron impact
of derivatives 12–15 indicates that deoxygenation and double
deoxygenation appear as indicative of N-oxide structure. An
OH• loss, Mc Lafferty-type rearrangement, is due to the pres-
enceof␣-alkylsubstituent. Thefragmentationviathe2-benzylic
substituent takes place as the main process in all cases, gener-
ating the benzylic cation as base peak in the cases of stable ion
(compounds 12–14). For derivative 15 where benzylic carboca-
tion is the least stable the base peak corresponds to ion II (see
Fig. 8). From ion II subsequent fragmentations were observed,
such as lost of 14 and 29 amu.

548
A. Merlino et al. / Spectrochimica Acta Part A 67 (2007) 540–549
Fig. 8. Proposed fragmentation in EI-MS for the 2-benzyl-2H-benzimidazole 1,3-dioxide family.
Table 7
Relevant ions observed in EI-MS for compounds 1 and 12–15
Reference
Ion m/z (abundance, %)
M•+
(I)a
(II)a
(III)a
(IV)a
m/z = 149
m/z = 134
Base peak
12
13
14
15
1
254(20)
284(5)
288(13)
299(36)
178(58)
238(1)
–
272(2)
283(95)
162(7)
163(3)
163(2)
163(5)
163(100)
163(18)
–
221(1)
–
–
–
(6)
148 (2)
(2)
(75)
(1)
–
m/z = 91
252(2)
–
267(7)
146(1)
135 (2)
(1)
(2)
m/z = 121
m/z = 125
m/z = 163, III
m/z = 43
145(3)
133(1)
a
According to Fig. 7.
4. Conclusions
Acknowledgment
We have completely spectroscopically characterized by
electronic, NMR, IR and MS, new 2-benzyl-2-methyl-2H-
benzimidazole 1,3-dioxide derivatives using experimental and
theoretical methods. The functional used in the calculus ade-
quately described the spectra. Density functional theory cal-
culations were able to predict the N–O stretching (N-oxide
moiety) accurately. From the data herein presented, it may be
concludedthatthe2-benzylderivativesadoptinsolutionapartic-
ular conformeric distribution. NMR spectroscopy, experimental
and theoretical approaches, confirms a preferential conformer
while, according to the theoretical study, electronic spectroscopy
does not allow to predict the preferential spatial distribution in
solution. These approximations allow to know the molecular
structure of these dioxide derivatives without any crystallo-
graphic data.
This work was financially supported by the Universidad de
´
la Republica.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2006.08.013.
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