Multinuclear magnetic resonance studies of 2-aryl-1,3,4-thiadiazoles

Article abstract of DOI:10.1002/mrc.3850

The ¹H, ¹³C and ¹⁵N spectra of aryl-substituted 1,3,4-thiadiazoles were recorded. The results obtained were correlated with Hammett coefficients. The experimental results were compared with DFT-calculated chemical shifts.

Full text of DOI:10.1002/mrc.3850

MRC Letters
Received: 5 June 2012
Revised: 29 June 2012
Accepted: 3 July 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.3850
Multinuclear magnetic resonance studies of
2-aryl-1,3,4-thiadiazoles
Błażej Gierczyk,a* Michał Cegłowski,a Marcin Kaźmierczaka and Maciej Zalasa
1
The H, 13C and 15N spectra of aryl-substituted 1,3,4-thiadiazoles were recorded. The results obtained were correlated with
Hammett coefficients. The experimental results were compared with DFT-calculated chemical shifts. The results obtained were
compared with those for 1,3,4-oxadiazoles and 1,3,4-selenadiazoles. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
1
Keywords: NMR; H; 13C; 15N; 33S; 1,3,4-thiadiazoles; DFT calculations
600.3029, 150.9456 and 60.8511 MHz, respectively) or 5 mm
Introduction
BBO probehead (BB/1H; 90ꢁ pulse width for 33S, 7 ms; spectral
frequency: 46.0793 MHz). The 19F NMR experiments were made
1,3,4-Thiadiazoles make an interesting group of heterocyclic
molecules. The compounds containing 1,3,4-thiadiazole moiety
show interesting pharmacological activities as well as properties
useful from the technological and agricultural point of view.[1–5]
Their antibacterial,[6–11] antiviral,[12–15] anticancer,[11,13,16] antipar-
asitic,[17–19] anti-inflammatory,[14] anticholinergic[7] and antihista-
mine[7,20] activities have been widely studied. Some compounds
belonging to this class are potent radioprotective agents.[21,22]
1,3,4-Thiadiazoles were also used as lubricants,[23–26] dyes,[27–29]
corrosion and oxidation inhibitors,[24,30–33] liquid crystals[34,35]
and optoelectronic materials.[36–38] Moreover, their herbicidal,[39]
insecticidal,[40–42] fungicidal[10,43,44] and algicidal[45] properties
have been investigated. Besides the information on the wide
range of thiadiazoles applications and extensive studies on their
chemistry, no systematic studies on their NMR properties have
been published.
on a Varian VNMRS-400 spectrometer (Palo Alto, CA, USA)
equipped with auto-switchable 5 mm probehead (90ꢁ pulse width
for 19F, 31.5ms; spectral frequency 378.8194MHz) at the same
1
temperature. The H, 13C, 15N and 19F measurements were made
for 0.02 M solutions in CDCl3, the 33S NMR experiments were made
for 1 M solution. 1H, 13C and 19F NMR spectra were recorded using
one-pulse sequence and standard acquisition parameters, whereas
the 33S NMR experiments were made with aring2 sequence. The
1
15N chemical shifts were obtained from two-dimensional H–15N
gradient-selected HMBC experiments, performed using a standard
pulse sequence from the Bruker pulse library. The delay for evalua-
tion of multiple bond couplings was set to 31.25–62.5 ms, which
corresponds to 4–8 Hz coupling constant values. The 2D spectra
were recorded as 1024ꢂ 512 matrix and the spectral widths for
F2 and F1 were 6000 Hz for proton and 10 000 Hz for nitrogen.
The sine-bell window function was applied and the zero filling
to 2048 ꢂ 2048 matrix was used before Fourier transformation.
Total number of scans in each increment of 2D spectra needed
to obtain a satisfactory signal-to-noise factor was 32–128,
depending on the compound structure. The 1H and 13C NMR
spectra were referred to internal TMS, 19F to internal CFCl3, 15N
NMR spectra to external neat CH3NO2, whereas 33S to external
saturated solution of (NH4)2SO4 in D2O (0.00 ppm). The chemical
shift measurements accuracy for 1H, 13C and 19F was better than
0.01 ppm, whereas for 15N, better than 0.05 ppm.
In our previous papers, we have presented 1H, 13C, 15N, 17O and
77Se NMR studies of substituted 1,3,4-oxa(selena)diazoles.[46–49]
This paper is a concise report on the NMR properties of their
thia analogs.
Experimental
Synthesis
The compounds studied were prepared from the corresponding
N-aryloyl-N0-formylhydrazides using Lawesson’s reagent as
thionation agent, according to the procedure described previ-
ously.[50] The structures of the compounds studied are presented
in Fig. 1.
DFT calculations
The initial structures were pre-optimized by the PM3 semi-empirical
method. The conformational search was performed by examining
NMR measurements
All spectra, except 19F NMR, were recorded at 298 ꢀ 0.1 K on a
Bruker Avance DRX 600 spectrometer (Billerica, MA, USA),
equipped with a 5 mm triple-resonance inverse probehead
(1H/31P/BB) with a self-shielded z-gradient coil (90ꢁ pulse width
for 1H, 9.0; for 13C, 15; for 15N, 11.3 ms; spectral frequencies:
*
Correspondence to: Błażej Gierczyk, Faculty of Chemistry, Adam Mickiewicz
University, Grunwaldzka 6, 60–780 Poznań, Poland. E-mail: hanuman@amu.
edu.pl
a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780,
Poznań, Poland
Magn. Reson. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

B. Gierczyk et al.
1
2
3
4
5
6
7
8
9
10
11
12
13
NMe2
NH2
OMe Me
CF3
H
Ph
F
Cl
Br
I
CN
NO2
R
Figure 1. The 2-aryl-1,3,4-thiadiazoles studied.
all torsion angles along exocyclic bonds using CAChe 5.0. One
conformer was found to be over 16.7 kJ/mol (4 kcal/mol) more
stable than other ones. The pre-optimized input structures were
then fully optimized by the DFT method at the B3LYP/6-311G**
(2d, 2p) level of theory with Gaussian 03.[51] The molecules were
allowed to relax concurrently and without any imposed symmetry
restrictions. The optimized structures were used for calculations of
NMR shifts. These calculations were also performed using DFT
method, at the same level of theory, which provided a good
correlation with experimental results. The computed spectral and
molecular parameters for iodo-substituted thiadiazole were not
presented, because the iodine atom was not parameterized in this
calculation basis set. The standards used for the calculations of the
chemical shifts were tetramethylsilane for 13C and ammonia for 15N.
The geometry and nuclear shielding constants of these molecules
were optimized at the same level of theory as the compounds
studied. The calculated shieldings are as follows: tetramethylsilane
(13C) 183.77ppm and ammonia (15N) 259.94 ppm. The 15N chemical
shifts were converted to nitromethane scale with equation:
Table 1. 1H, 15N and 19F NMR parameters of the compounds studied
Compound
1H, 15N and 19F chemical shift [ppm]
H-5 H-20 H-30
N-3
N-4
Other atoms
1
2
8.99 7.84 6.76 ꢃ26.89 ꢃ6.14
9.01 7.80 6.63 ꢃ24.51 ꢃ5.90
3.04 (NCH3)
ꢃ327.99 (NCH3)
~ 4.1 (NH)
ꢃ320.71 (NH)
3.84 (OCH3)
2.40 (CH3)
3
4
5
6
7
9.07 7.91 6.97 ꢃ22.29 ꢃ5.84
9.10 7.87 7.27 ꢃ20.00 ꢃ5.73
9.23 8.13 7.76 ꢃ12.29 ꢃ3.27
9.17 8.05 7.49 ꢃ17.96 ꢃ5.43
9.08 8.01 7.65 ꢃ17.41 ꢃ5.43
ꢃ63.53 (CF3)
7.52 (H-40)
7.58 (H-200)
7.43 (H-300)
d
CH3NO2 = dNH3 ꢃ 399.3ppm (the value of the 15N chemical shift of
gaseous ammonia[52]).
7.36 (H-400)
8
9.12 8.01 7.19 ꢃ17.43 ꢃ4.18 ꢃ108.55 (Ar-F)
9
9.18 7.91 7.44 ꢃ16.56 ꢃ5.27
9.16 7.85 7.61 ꢃ16.23 ꢃ4.55
9.14 7.84 7.72 ꢃ15.76 ꢃ3.99
—
—
—
10
11
12
13
Results and Discussion
The 1H, 13C and 15N NMR experimental data were summarized in
Tables 1 and 2. The calculated chemical shifts were presented in
Table 3.
9.27 8.15 7.67 ꢃ11.97 ꢃ3.52 ꢃ121.70 (CN)
9.30 8.20 8.36 ꢃ10.15 ꢃ4.64
ꢃ11.20 (NO2)
Table 2. 13C and 33S NMR parameters of the compounds studied
Compound
13C and 33S chemical shift [ppm]
C-2
C-5
C-10
C-20
C-30
C-40
Other C atoms
S-1
1
2
3
4
5
6
7
168.77
168.66
167.83
168.31
166.69
168.04
167.82
149.64
150.00
150.47
150.91
151.94
151.27
151.09
118.70
118.73
121.99
126.70
132.75
130.94
128.20
129.35
128.87
129.42
127.88
128.32
126.64
126.80
113.32
115.51
114.32
129.74
126.10
128.73
128.34
147.62
149.60
161.73
141.64
132.71
131,85
143.68
40.33
—
55.21
ꢃ59
ꢃ55
21.32
123.48
—
139.38 (C-100)
127.52 (C-200)
128.75 (C-300)
127.89 (C-400)
8
167.07
166.95
167.06
167.36
166.09
165.97
151.15
151.54
151.38
151.35
152.39
152.77
125.92
127.80
128.36
129.03
134.49
135.10
130.12
129.06
129.30
129.42
128.50
128.96
116.38
129.26
132.28
138.35
132.73
124.45
164.41
137.16
125.60
97.84
—
—
9
10
11
12
13
—
—
114.83
149.19
118.33
—
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Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2012)

NMR studies of 1,3,4-thiadiazoles
Table 3. Calculated chemical shifts
to that estimated for the compounds studied here. This indicates
that the result given in literature for 1,3,4-thiadiazole is incorrect
and needs re-determination.
Compound
Chemical shifts [ppm]
C-2
C-5
C-10
N-3
N-4
A good correlation (r2 0.94) between the chemical shift of C5–H
proton and global Hammett constants[54] was observed. The
dissection of the substituent parameter onto its resonance and
inductive components indicates the equal contribution of both
effects, contrary to 1,3,4-selenadiazoles for which resonance
effects are predominant and 1,3,4-oxadiazoles, for which induc-
tive effect is dominant (Table 4). This is the effect of increasing
of aromaticity in the series of 1,3,4-chalcogendiazole with the
increasing atomic number of chalcogen atom.
1
177.45
177.09
176.98
177.53
176.36
177.60
—
152.87
153.29
154.10
154.82
156.51
155.18
—
125.76
127.58
129.92
134.97
140.57
137.65
—
ꢃ6.49
ꢃ4.73
ꢃ1.30
2.77
19.64
19.69
20.22
20.39
21.63
21.09
—
2
3
4
5
10.86
5.04
6
7
—
8
176.22
176.37
176.41
—
155.32
155.61
155.64
—
133.62
135.70
136.44
—
3.79
20.96
21.07
21.23
—
The 1H NMR signals of aryl substituents show typical chemical
shifts values. The value of Z2 parameter for 1,3,4-thiadiazo-2-yl
substituent, calculated from the values obtained for the studied
series of compounds is 0.76 ꢀ 0.07. This is a value intermediate
between those determined for oxa and selena analogs
(0.69 and 0.84, respectively). The values of Z3 = 0.19 ꢀ 0.04
and Z4 = 0.26 are the same as for 1,3,4-oxadiazo-2-yl and 1,3,4-
selenadiazo-2-yl substituents.
9
5.83
10
11
12
13
6.26
—
176.09
175.75
156.88
157.53
140.56
143.44
12.93
15.64
22.15
22.89
The 1H NMR parameters
The 13C NMR parameters
The observed chemical shifts of C5–H proton (8.99–9.30 ppm;
Table 1) take values intermediate between those of 1,3,4-
oxadiazoles (8.32–9.30 ppm)[49] and those of 1,3,4-selenadiazoles
(9.77–10.05 ppm)[47]. The value reported for unsubstituted 1,3,4-
thiadiazole (7.55 ppm)[53] is distinctly smaller than that observed
for 2-aryl-substituted ones. For 1,3,4-oxa and 1,3,4-selena
analogs, such differences were not observed. Moreover, for C2–H
signal of thiazole, the observed chemical shift (8.88 ppm)[53] is close
Chemical shifts of both 1,3,4-thiadiazole heterocyclic ring carbon
atoms are very close to that of oxadiazole analogs. The ranges of
variation of the chemical shifts of these atoms are similar:
2.80 ppm for C-2 and 3.13 ppm for C-5, respectively. The values
obtained for C–H of 2-aryl substituted thiadiazoles (average
151.2 ppm) are very close to this reported for unsubstituted ring
(152.7 ppm).[53] The changes observed for C-2 and C-5 have
Table 4. Regression statistics for experimental data versus Hammett substituent parameters, calculated chemical shifts and bond lengths
Probe atom
Equation
r ꢀ sr
i ꢀ si
n
r2
C5-H
C5-H
d
d
exp(1H) = rPsp + i
0.19 ꢀ 0.02
rR: 0.20 ꢀ 0.02
rI: 0.18 ꢀ 0.04
ꢃ1.8 ꢀ 0.2
rR: ꢃ1.2 ꢀ 0.1
rI: ꢃ3.0 ꢀ 0.2
1.4 ꢀ 0.2
9.129 ꢀ 0.007
9.14 ꢀ 0.02
13
13
0.939
0.940
exp(1H) = rIsI + rRsR + i
(SD = 0.005; f = 0.05)
0.903
C-2
C-2
dexp
dexp
(
(
13C) = rPsp + i
13C) = rIsI + rRsR + i
167.54 ꢀ 0.08
168.04 ꢀ 0.08
13
13
0.982
(SD = 0.12; f = 0.10)
0.848
C-2
C-5
C-5
dexp
dexp
dexp
(
(
(
13C) = r dcalc
(
13C) + i
ꢃ81 ꢀ 35
151.12 ꢀ 0.04
157.20 ꢀ 0.08
11
13
13
13C) = rPsp + i
1.83 ꢀ 0.08
rR: 1.9 ꢀ 0.1
rI: 1.6 ꢀ 0.2
0.66 ꢀ 0.02
11 ꢀ 1
0.979
13C) = rIsI + rRsR + i
0.981
(SD = 0.12; f = 0.13)
0.992
C-5
C-10
C-10
dexp
dexp
dexp
(
(
(
13C) = r dcalc
(
13C) + i
48 ꢀ 3
11
13
13
13C) = rPsp + i
127.0 ꢀ 0.5
129.6 ꢀ 0.7
0.902
13C) = rIsI + rRsR + i
rR: 14 ꢀ 1
0.965
rI: 4 ꢀ 2
(SD = 0.95; f = 0.11)
0.986
C-10
N-3
N-3
dexp
dexp
dexp
(
13C) = r dcalc
(
13C) + i
1.03 ꢀ 0.05
10.2 ꢀ 0.4
ꢃ12 ꢀ 6
11
13
13
(
15N) = rPsp + i
15N) = rIsI + rRsR + i
ꢃ18.2 ꢀ 0.2
ꢃ17.7 ꢀ 0.4
0.985
(
rR: 10.7 ꢀ 0.6
rI: 9.2 ꢀ 0.9
0.75 ꢀ 0.03
1.6 ꢀ 0.4
0.987
(SD = 0.53; f = 0.11)
0.986
N-3
N-4
N-4
dexp
dexp
dexp
(
(
(
15N) = r dcalc
(
15N) + i
ꢃ21.3 ꢀ 0.2
ꢃ5.0 ꢀ 0.1
ꢃ5.3 ꢀ 0.4
11
13
13
15N) = rPsp + i
0.636
15N) = rIsI + rRsR + i
rR: 1.3 ꢀ 0.6
rI: 2.3 ꢀ 0.9
0.7 ꢀ 0.2
0.658
(SD = 0.53; f = 0.50)
0.553
N-4
dexp
(
15N) = r dcalc
(
15N) + i
ꢃ20 ꢀ 5
11
sp, sI and sR – global, inductive and resonance Hammett values; dexp and dcalc – experimental and calculated chemical shifts of the corresponding
nuclei; L – bond length; r – regression coefficient for the single parameter; i – intercept; sr and si – standard deviations; n – number of compounds
taken for statistical analysis; r – correlation coefficient; SD – standard deviation of regression; f = SD/RMS, where RMS – root mean square of SCS
Magn. Reson. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
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B. Gierczyk et al.
opposite signs, i.e. the more electron-withdrawing groups cause
shielding of C-5 and deshielding of C-2 nuclei. The sensitivity of
the aryl-substituted carbon atom to the electronic properties of
the substituent is higher than that observed for 1,3,4-oxadiazoles
(2.39 ppm), but slightly smaller than that for 1,3,4-selenadiazoles
(2.84 ppm). The C-5 carbon atom of the compounds studied
shows a narrower chemical shift range than the analogous atom
of selenadiazoles (3.95 ppm), but it is more sensitive to the
substituent character than in oxadiazoles (1.89 ppm). This increasing
sensitivity of the chemical shifts of the C-5 of thiadiazoles studied to
the changes in the substituent in aryl group in comparison with
the shifts of C-5 atom in 1,3,4-oxadiazoles is caused by the higher
aromaticity of thiadiazole ring than the oxadiazole one. The
increasing of the aromaticity results in more effective coupling of
p-electrons system of both C═N bonds with phenyl ring electrons.
The chemical shifts of both carbon atoms of thiadiazole unit
show correlation with global Hammett constants, although for
C-2, the r 2 values are slightly smaller. Analysis of the contributions
inductive and resonance effects to the chemical shifts variation
(dual substituent parameter treatment) has shown that on C-2
the former dominates. For C-5, these contributions are of similar
importance (Table 4). In both cases, the ‘goodness of fit’ of dual
substituent parameter correlation, judged by f parameter (f = SD/
RMS), indicates the good correlation (f = 0.10 and 0.13 for C-2
and C-5, respectively). As for selenium and oxygen containing
analogs,[47,49] the opposite effects of the substituent electronic
character on C-2 and C-5 chemical shifts may be explained by
the redistribution of the C═N bond electron density caused by
its polarization and, in consequence, an increase in the diamag-
netic effects on C-5. This mechanism is more effective in highly delo-
calized electron systems of aromatic thiadiazole and selenadiazole
rings, in comparison with non-aromatic oxadiazoles.[49]
The C-10 chemical shift range of the compounds studied is
16.40 ppm, which is smaller than for 1,3,4-oxadiazoles (18.45 ppm).[49]
The analysis of the correlation of 13C d values with Hammett constants
shows the smaller contribution of resonance effects in comparison
with that of oxadiazoles, caused by the coupling with p-system of
heterocyclic ring (the resonance effects are not stopped on C-10 atom,
but are transferred via extended coupled electron systems on the
second part of the molecule). The value of f parameter (0.11) indicates
good correlation.
The Z factors, calculated for 1,3,4-thiadiazo-2-yl substituents
from the values obtained for the studied substances are
Z1 = 1.2 ꢀ 0.6, Z2 = ꢃ0.7 ꢀ 0.6, Z3 = 0.6 ꢀ 0.3 and Z4 = 2.3 ꢀ 1.2.
These values are intermediate between those for oxadiazol-2-yl
and selenadiazo-2-yl group.
data,[47,49] the chemical shift of nitrogen atom in thiadiazole
ring is less sensitive to electronic properties of substituent than
that of selenadiazole, but more sensitive than that of
oxadiazoles. This is a result of the higher aromaticity of 1,3,4-
thiadiazole than that of the oxy analogs (and lower than that of
selenadiazoles). It causes the stronger p-electron systems coupling
and more effective transfer of the electronic effects from phenyl to
chalcogenediazoyl ring. Both nitrogen atoms show the chemical
shift–Hammett constant relationship, but for N-4 atom, the r2
parameter is much smaller than for N-3. The origin of this effect is
unclear if compared with results presented earlier for oxadiazoles
and selenadiazoles – for these compounds the differences in r2
values obtained for N-3 and N-4 are less than 0.1, whereas for
thiadiazoles, the difference is 0.35.
The 33S parameters
The 33S NMR signal of 2-phenyl-1,3,4-thiadiazole (6) was observed
at ꢃ55 ppm as a very broad line of Δn1/2 ~ 3500 Hz. Because of the
low intensity and great line width, the accuracy of chemical shift
measurement was lower than 10 ppm. The spectrum of 2-
(4-methoxyphenyl)-1,3,4-thiadiazole (3) shows a signal at ꢃ59 ppm.
Because the observed substituent-induced shifts were less than
the accuracy of measurement, the spectra of the other com-
pounds were not recorded. The value obtained is similar to that
reported for thiophenes (ꢃ112 ppm for 2-methylthiophene)
or 2-methylthiazol (ꢃ72 ppm).[55,56]
Theoretical calculations
The calculated chemical shifts for the compounds studied are
collected in Table 3. The computed bond lengths, charges
(Mülliken and natural bond orbital ones) and Cartesian coordi-
nates are presented in the Supporting Information. As follows
from a comparison of the experimental and calculated values,
the 13C chemical shifts are slightly overestimated by the
calculation methods, but an excellent correlation between
these values was found (r2 = 0.85–0.99; see Table 4 for details).
The highest difference between dcalc and dexp is observed for
C-2 (about 10 ppm), whereas the smallest for C-5 (less than
5 ppm). The calculated 15N chemical shifts are by 20–27 ppm
high-frequency shifted. If the values computed for N-3 correlate
well with the experimental ones, then the correlation obtained
for N-4 is very poor (r2 < 0.6). The reasons for such a difference
are not clear, such effects were not observed for oxa and
selena analogs.[46,47,49]
Conclusions
The 15N NMR parameters
The NMR results obtained for 2-aryl-1,3,4-thiadiazoles are similar
to those published previously for oxa and selena analogs. The
chemical shifts values are intermediate between those for the
corresponding O and Se containing 1,3,4-chalcogendiazoles.
The DFT calculations reproduce the 13C chemical shifts values
very well, but the observed systematic error is slightly smaller
than that for 1,3,4-selenadiazoles. The differences between calcu-
lated and measured d values of nitrogen atoms are much higher
than those for carbons. Contrary to the results obtained for
oxygen and selenium containing analogs, N-4 chemical shifts
show very poor correlation with Hammett constants as well as
calculated d values, but the reasons for such differences are not
known. The 33S NMR spectra of two representative 2-aryl-1,3,
The observed 15N signals are by about 68 ppm high-frequency
shifted in comparison with those obtained for substituted 1,3,4-
oxadiazoles (e.g. ꢃ86.77 and ꢃ73.44 vs ꢃ17.96 and ꢃ5.43 ppm
for N-3 and N-4 of 2-phenyl-substituted 1,3,4-oxa and 1,3,4-
thiadiazole, respectively).[49] The chemical shifts of N-4 are very
close to the value reported for unsubstituted 1,3,4-thiadiazole
(ꢃ7.9 ppm).[53] The 15N NMR data obtained for the molecules
studied are very similar to those presented previously for selena
analogs, the signal of the selenium containing compounds is less
than 5 ppm shifted.[47] The ranges of the chemical shifts of N-3
and N-4 in the molecules studied are 16.74 and 2.87 ppm, respec-
tively. As follows from a comparison with previously published
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Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2012)

NMR studies of 1,3,4-thiadiazoles
[29] M. R. De Giorgi, A. Cerniani. Melliand Textilberichte 1992, 73,
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[30] M. Palomar-Pardavé, M. Romero-Romo, H. Herrera-Hernández,
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[31] R. Solmaz. Corros. Sci. 2010, 52, 3321–3330.
[32] F. E. Heakal, A. S. Fouda, M. S. Radwan. Int. J. Electrochem. Sci. 2011, 6,
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