EPR spectra and spin density distribution of O, S-dimethyl 4,4-dithioterephthalate radical anions

Article abstract of DOI:10.1080/10426507.2011.615771

The preparation of O-methyl S-trideuteromethyl 4,4-dithioterephthalate and S-methyl O-trideuteromethyl 4,4-dithioterephthalate is described. The EPR spectra of the corresponding radical anions are measured. Comparison with the spectrum of O,S-dimethyl 4,4

Full text of DOI:10.1080/10426507.2011.615771

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Phosphorus, Sulfur, and Silicon and the
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EPR Spectra and Spin Density
Distribution of O,S-Dimethyl 4,4-
Dithioterephthalate Radical Anions
Jürgen Voss a , Dirk Buddensiek a & Jan Folkert Rosenboom a
a Department of Chemistry , University of Hamburg , Hamburg ,
Germany
Published online: 31 Jan 2012.
To cite this article: Jürgen Voss , Dirk Buddensiek & Jan Folkert Rosenboom (2012) EPR Spectra and
Spin Density Distribution of O,S-Dimethyl 4,4-Dithioterephthalate Radical Anions, Phosphorus, Sulfur,
and Silicon and the Related Elements, 187:3, 382-391, DOI: 10.1080/10426507.2011.615771
To link to this article: http://dx.doi.org/10.1080/10426507.2011.615771
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Phosphorus, Sulfur, and Silicon, 187:382–391, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.615771
EPR SPECTRA AND SPIN DENSITY DISTRIBUTION
OF O,S-DIMETHYL 4,4-DITHIOTEREPHTHALATE
RADICAL ANIONS
Ju¨rgen Voss, Dirk Buddensiek, and Jan Folkert Rosenboom
Department of Chemistry, University of Hamburg, Hamburg, Germany
GRAPHICAL ABSTRACT
Abstract The preparation of O-methyl S-trideuteromethyl 4,4-dithioterephthalate and S-
methyl O-trideuteromethyl 4,4-dithioterephthalate is described. The EPR spectra of the cor-
responding radical anions are measured. Comparison with the spectrum of O,S-dimethyl 4,4-
dithioterephthalate radical anions allows the unequivocal assignment of the proton hyperfine
structure (proton “hfs”) coupling constants in the above asymmetric species. Assignment of
the arene proton hfs coupling constants is achieved by PM6 and density functional theory MO
calculations of the spin density distribution and application of McConnell’s relationship aµH
=
−2.4·ρπµ. The spin density distribution in the asymmetric title compound is compared with
those in the radical anions of dimethyl terephthalate and the corresponding symmetric sulfur
analogs.
Keywords Trideuteromethanethiol; dimethyl 4,4-dithioterephthalates; in-situ electroreduc-
tion; radical anions; electron paramagnetic resonance (EPR) spectroscopy; molecular orbital
(MO) calculations
Received 19 July 2011; accepted 15 August 2011.
The Universita¨t Hamburg and the Fonds der Chemischen Industrie are gratefully acknowledged for financial
support. Jan Folkert Rosenboom thanks the Fritz-Prosiegel-Stiftung for a scholarship.
Address correspondence to Ju¨rgen Voss, Fachbereich Chemie, Universita¨t Hamburg, Martin-Luther-King-
Platz 6, D-20146 Hamburg, Germany. E-mail: voss@chemie.uni-hamburg.de
382

DIMETHYL DITHIOTEREPHTHALATE RADICAL ANIONS
383
INTRODUCTION
We have comprehensively studied the influence of electron-withdrawing substituents
such as thiolo (CO-SR), thiono (CS-OR), and dithiocarboxylate (CS-SR) groups on the spin
density distribution ρπµ in the radical anions1 of aromatic π-electron systems, for example, in
the benzene,2 naphthalene,3 and the nonalternating azulene4 series. The effect of one single
or two identical electron-withdrawing groups on ρπµ can be determined rather easily via
EPR spectroscopic measurements of the proton hyperfine structure (proton “hfs”) coupling
constants aHµ and application of the McConnell relationship aµH = Q·ρπµ. The necessary
assignment of aHµ to a specific center µ is unequivocal in most of these cases.
The interaction of two different electron-withdrawing substituents with an aromatic
π-electron system, on the other hand, is more difficult to explore because the assignment of
the hfs coupling constants is not as straightforward. Since we were, however, particularly
interested in the competition between substituents such as CO-OR and CS-SR, we have
performed further experiments and MO theoretical calculations on the radical anions of
dimethyl terephthalate (6) and its various sulfur analogs 3 and 7–10, and thus present our
results here. In order to unequivocally assign the methyl proton hfs coupling constants of
the asymmetric radical anion 3•−, we synthesized its deuterium-labeled derivatives 4 and 5
and recorded the EPR spectra of their in-situ electro-generated radical anions. In addition,
we performed new MO calculations by the use of modern semi-empirical and density
functional theory (DFT) methods in order to get a better understanding of the complex spin
density distribution in the bifunctional radical anions 3•− to 10•− and its dependence on the
torsion angle ꢀ between the plane of the benzene ring and the planar functional groups.
RESULTS AND DISCUSSION
We prepared the desired O-trideuteromethyl dithioester 4 from 4-cyanobenzoyl chlo-
ride via trideuteromethyl 4-cyanobenzoate (2), and the S-trideuteromethyl dithioester 5
from methyl 4-cyanobenzoate (1) according to known procedures for the preparation of
3. Trideuteromethanethiol that was required for the preparation of 5 was prepared from
trideuteroiodomethane via S-trideuteromethylisothiouronium iodide.
1. R2SH, HCl
2. pyridine, H2S
3
2
R2 S
S
O
O
O
4
1
N
OR1
1
R
6
5
1: R1 = CH3
2: R1 = CD3
3: R1 = CH3, R2 = CH3
4: R1 = CD3, R2 = CH3
5: R1 = CH3, R2 = CD3
The three dithioesters 3, 4, and 5 were transformed into persistent radical anions
by in-situ electroreduction in dry dimethyl sulfoxide (DMSO) in the cavity of the EPR
spectrometer as described earlier.2b,5 The observed EPR spectra are shown in Figures 1–3.
They exhibit a better resolution and a higher signal-to-noise ratio as compared with
our earlier results.2b The proton hfs coupling constants could, therefore, be determined from
the spectra more precisely. They were verified by simulation and are compiled in Table 1.

384
J. VOSS ET AL.
Table 1 Proton hfs coupling constants aHµ (mT) and g-factors of 3•− to 5•−
Radical anion
aH2,6
aH3,5
aHOMe
aHSMe
g-factor
3•−
4•−
5•−
0.046
0.058
0.052
0.300
0.287
0.279
0.046
0.007a
0.052
0.127
0.123
0.021a
2.00790
2.00793
2.00788
aMe = CD3.
Figure 1 Experimental (left) and simulated (right) EPR spectrum of 3•−
.
Figure 2 Experimental (left) and simulated (right) EPR spectrum of 4•−
.

DIMETHYL DITHIOTEREPHTHALATE RADICAL ANIONS
385
Figure 3 Experimental (left) and simulated (right) EPR spectrum of 5•−
.
The largest observed coupling constant turned out to be aH = 0.300 mT rather than the
previously reported2b value 0.238 mT.
It is obvious from the data that the splitting of 0.127 mT in 3•− is due to the S-methyl
protons since it nearly disappears in 5•−. Simulation with aSDCD3 = 0.020 mT (0.127:6.5)
and aHOCH3 = a2H,6 = 0.052 mT results in the EPR spectrum of 5•− consisting of only three
resolved broad lines due to aH3,5 = 0.279 mT as shown in Figure 3. The smaller splitting of
0.046 mT in the spectrum of 3•− thus belongs to the O-methyl protons. Accordingly, it is
missing in the spectrum of 4•−. This result, aSHMe > aOHMe, is not unexpected. It corresponds
with the observed coupling constants aHSMe = 0.108 mT in dimethyl tetrathioterephthalate
9•− versus aOHMe = 0.078 mT in dimethyl terephthalate radical anions 6•− 2b and is also
observed for the pair methyl dithiobenzoate (aHSMe = 0.108 mT)2a and methyl benzoate
(aHOMe = 0.094 mT).6
The assignment of the coupling constants of the aromatic protons to the 2,6- or the 3,5-
positions of 3•− to 5•− was not quite as straightforward. An experimental assignment was
not possible since the dithioester with specifically deuterium labeled arene ring positions
was not at our disposal. The corresponding coupling constants could, however, be calculated
from the spin densities by use of the McConnell relationship aHµ = Q·ρµπ (as discussed later).
We could measure the g-factors (2.00790
0.00003) of the three radical anions
3•−, 4•−, and 5•− (see the Table) more precisely as previously.2b They agree with each
other within the limits of experimental error (see Table 1). The high value is due to the spin
orbit-coupling (heavy-atom effect) of the thiocarbonyl sulfur atom. This effect is even more
pronounced for the dimethyl tetrathioterephthalate radical anion 9•− with g = 2.0095.2b
It is, in a semiquantitative sense, indicative of a high spin density ρπ in the dithioester
substituent,2a,3 although a correct calculation of ρπ from g is not easily possible since the
electron excitation energy ꢁE(n→π∗) and the symmetry of the g-tensor are not known in
the present case.

386
J. VOSS ET AL.
MO CALCULATIONS
In order to achieve a better understanding of the spin density distribution and to be able
to assign the hfs coupling constants of the arene protons by use of McConnell’s relationship
aHµ = –2.4·ρπµ, we performed semiempirical PM6 type and DFT MO calculations7 on 3•−
.
In addition, we included the results of new PM6-MO calculations on the spin density
distribution in the related radical anions 6•− to 10•−. The results are compiled in Table 2.
3
2
1
1
4
Me
Y
X
X
Y
4
1
4
Me
6
5
3: X1 = Y1 = O; X4 = Y4 = S
6: X1 = X4 = Y1 = Y4 = O
7: X1 = Y1 = Y4 = O; X4 = S
8: X1 = X4 = S; Y1 = Y4 = O
9: X1 = X4 = Y1 = Y4 = S
10: X1 = X4 = O; Y1 = Y4 = S
The PM6 results obtained for 3•− do not deviate much from the more sophisticated
ab-initio-DFT [B3Lyp1 ROHF 6–31G (p,d)] values. We have, therefore, only used PM6
calculations for 6•− to 10•− because of the much longer calculation times required for DFT
calculations.
Although the quantitative agreement with the experimental data for 3•− is not excel-
lent, the assignment of the larger, average coupling constant aH3,5 = 0.289 mT to the arene
protons 3-H and 5-H neighboring the CS-SMe group and of the smaller one aH2,6 = 0.052
mT to the protons 2-H and 6-H neighboring the CO-OMe group is unequivocal.
Not unexpectedly, the spin density within the dithioester radical anions 3•− and
the thionoester radical anions 7•− is shifted toward the thiocarbonyl group. This effect is
Table 2 Spin densities ρπµ in the radical anions 3•− and 6•− to 10•−
Comp.
ρ1π
ρ2π
ρ3π
ρ4π
ρ5π
ρ6π
ρπC(O)
ρOπ
ρπC(S)
ρSπ
3•−
Expa
PM6
—
0.115
0.104
—
0.207
—
0.165
—
0.133
—
0.022
0.033
0.021
0.065
0.076
0.021
0.051
0.050
0.068
0.045
0.055
0.056
0.071
0.120
0.069
0.067
0.065
0.076
0.110
0.088
0.050
0.056
0.045
0.045
0.056
0.075
—
0.091
0.068
—
0.207
—
0.135
—
0.133
—
0.120
0.076
0.061
0.065
0.077
0.110
0.085
0.050
0.068
0.045
0.045
0.056
0.071
0.022
0.024
0.027
0.065
0.077
0.021
0.040
0.050
0.056
0.045
0.055
0.056
0.075
—
0.024
0.043
—
0.074
—
0.041
—
—
—
—
—
0.093
—
0.019
0.042
—
0.050
—
0.031
—
—
—
—
—
0.066c
—
0.230
0.241
—
—
—
0.179
—
0.117
—
0.122
—
—
—
0.254
0.261
—
—
—
0.130
—
0.094
—
0.148
—
—
DFT
6•−
7•−
8•−
9•−
10•−
Expa,b
PM6
Expa,b
PM6
Expa,b
PM6
Expa,b
PM6
0.097
—
0.180
0.097
—
0.180
Expa,b
PM6
aρπ = aH / –2.4; average values of aµH from 3•−, 4•−, and 5•− were used.
µ
µ
bFrom Ref. 2b.
cρπ
.
S

DIMETHYL DITHIOTEREPHTHALATE RADICAL ANIONS
387
Figure 4 Calculated (PM6) spin density distribution in 3•−
.
not only reflected in the hfs coupling constants of the arene protons but is also evident
from the calculated values ρC = 0.241 (3•−)/0.179 (7•−) and ρS = 0.261 (3•−)/0.130
CS
(7•−) compared with ρC = 0.043 (3•−)/0.041 (7•−) and ρ◦ CS= 0.042 (3•−)/0.031
CO
CO
(7•−) (see Table 2). Unfortunately, we could not experimentally prove this result, which
is also demonstrated in Figure 4, because significant 13C or 33S satellite lines were not
observed in the EPR spectra. The pronounced spin-withdrawing effect of thiocarbonyl
groups is due to the high polarizability of sulfur and is generally observed in radical anions
exhibiting corresponding functionalities such as thioamides, thiono, and dithioesters.5b The
spin density distribution of the bis-thiolester radical anion 10•−, on the other hand, is not
significantly different compared with that of the ester radical anion 6•−
.
Interestingly, the calculations yield different spin densities at the respective two
chemically equivalent ortho-positions. This is due to the fact that an optimized, nearly
coplanar conformation of the radical anion with the lowest potential energy is calculated.
The dependence of the calculated (PM6) free enthalpy ꢁH from the torsion angle ꢀ between
the planes of the arene ring and the dithiocarboxylate group of 3•− is shown in Figure 5.
Figure 5 Dependence of the free enthalpy ꢁH on the torsion angle ꢀ in 3•−
.

388
J. VOSS ET AL.
Global minima occur at ꢀ1 = 0◦ and ꢀ2 = 180◦, that is, for the coplanar configurations (s-
trans is shown in Figure 4) with maximum resonance stabilization, together with a tiny local
minimum (ꢁꢁH = 4 kJ mol−1) at ꢀ3 = 90◦, which is due to minimum steric hindrance
between the ortho-protons and the functional group. The calculated average barrier height
of ꢁꢁH = 21 kJ mol−1 between the two coplanar conformations with ꢀ1 and ꢀ2 obviously
is low enough as to allow free rotation of the functional group at room temperature and to
render the respective coupling constants indistinguishable on the EPR timescale.
Also, the spin density distribution and, as a consequence, the ring proton hfs coupling
constants of 3•− strongly depend on the torsion angle ꢀ between the benzene ring and the
dithioester group, which is illustrated in Figure 6. The best agreement of the MO (PM6)
theoretical values ρπµ with the experimental values ρπµ = aH/2.4 is observed for torsion
angles ꢀ1 = 0◦ and ꢀ2 = 180◦. This coplanar arrangement, which also represents the
Figure 6 Dependence of the spin density distribution on the torsion angle ꢀ in 3•−
.

DIMETHYL DITHIOTEREPHTHALATE RADICAL ANIONS
389
energetically favored conformation of 3•− (see above), can however not be taken as a fixed
conformation because free rotation occurs at room temperature. The spin densities ρSCS and
ρCCS in the thiocarbonyl group of 3•−, on the other hand, exhibit maxima at a fixed torsion
angle ꢀ = 90◦ where virtually no resonance interaction between the arene ring and the
functional group should exist.
CONCLUSION
Deuterium labeling of the OCH3 and groups of 3•− allows an unequivocal assignment
of the respective proton hfs coupling constants with aH(SCH3) > aH(OCH3).
MO calculations show that the spin density ρπ3,5 in the ortho-positions next to the
dithioester substituent is higher than ρπ2,6 and thus aH3,5 > a2H,6 in 3•−
.
The spin density distribution in 3•− as determined by MO calculations is com-
pared with the one in the radical anions of the corresponding ester (6•−), the asymmetric
monothionoester (7•−), the symmetric bisthionoester (8•−), the tetrathioester (9•−), and the
bis-thiolester (10•−). In agreement with the observed coupling constants and high g-factors,
the spin densities ρSCS and ρCCS within the CS-SMe group are much higher compared with
ρ◦CO and ρCCO within the CO-OMe substituents.
EXPERIMENTAL
General procedure. Melting points (mp, uncorrected) were determined on a Leitz-
Heiztisch-Mikroskop. Column chromatography was performed on Kieselgel 60 (Merck,
Darmstadt, Germany), 0.063–0.200 mm (70–230 mesh). Eluents [dioxan, petroleum ether
(PE)] were distilled prior to use. Solvents were purified and dried by standard laboratory
procedures.8 IR spectra were measured as KBr pellets on a Perkin-Elmer 399 spectrometer.
1H NMR spectra were measured in CDCl3 on a Varian T 60 spectrometer. Chemical shifts δ
are related to SiMe4 (δ = 0.00 ppm) as internal standard. The radical anions were generated
◦
by internal electrolysis at 25 C in dry and purified8 DMSO with tetrapropylammonium
bromide as a supporting electrolyte at −0.70 to −0.90 V versus the internal Ag/AgBr
reference electrode.9 The EPR spectra were measured as described5 in a quartz flat cell on
a Bruker ER 420 S spectrometer. They were simulated by use of the Symfonia program
(Bruker).
Preparations
Methyl 4-cyanobenzoate (1) and 4-cyanobenzoyl chloride (Sigma-Aldrich, Buchs,
Switzerland) as well as iodomethane-d3 and methanol-d4 (Merck) are commercially
available.
Trideuteromethanethiol. Iodomethane-d3 (16.1 g, 111 mmol) and thiourea (9.3
g, 122 mmol) were refluxed in EtOH (ethanol; 5 mL) for 6 h. S-Trideuteromethyl-
isothiouronium iodide (24.2 g, 99%) crystallization from the solution upon cooling. It was
filtered off, dried, and hydrolyzed with 5N NaOH (50 mL). The produced thiol was expelled
from the solution by a stream of N2 and trapped at –15 ◦C to yield trideuteromethanethiol
as a colorless liquid (1.74 g, 31%).
Trideuteromethyl 4-Cyanobenzoate (2). 4-Cyanobenzoyl chloride (1.50 g, 9.1
mmol) was added portion-wise under stirring and cooling with ice to a solution of CD3OD

390
J. VOSS ET AL.
(0.32 g, 8.9 mmol) in dry pyridine (3.0 mL). The mixture was heated to 80 ◦C for 30 min,
poured into ice/water, and acidified with HCl. The slightly brown crystals of 2 (1.18 g,
81%) were filtered off, dried, and used without further purification.
O,S-Dimethyl 4,4-Dithioterephthalate {Methyl 4-[(Methylthio)thiocarbon
yl]benzoate} (3). The dithioester 3 was prepared from 1 and unlabeled methanethiol
as described for 5. Red leaflets, mp 87 ◦C–89 ◦C (lit.10: 90 ◦C). 1H NMR: δ 2.75 (s, 3 H,
SCH3), 3.90 (s, 3 H, OCH3), 8.00 (s, 4 H, ArH). Anal. Calcd. for C10H10O2S2 (226.3): C
53.07, H 4.45, S 28.34. Found: C 53.03, H 4.42, S 28.48.
S-Methyl O-Trideuteromethyl 4,4-Dithioterephthalate {Trideuteromethyl
4-[(methylthio)thiocarbonyl]benzoate} (4). The dithioester 4 was prepared from 2
(1.1 g, 6.7 mmol) and unlabeled methanethiol (0.75 mL) as described for 5. Red leaflets
(0.35 g, 23%), mp 86 ◦C (lit.10 for 3: 90 ◦C) 1H NMR: δ 2.75 (s, 3 H, SCH3), 8.00 (s, 4 H,
ArH). Anal. Calcd. for C10H7D3O2S2 (229.5): C 52.38, H 3.08, D 2.62. Found: C 52.79, H
2.97, D 2.54.
O-Methyl S-Trideuteromethyl 4,4-Dithioterephthalate {Methyl 4-
[(Trideutero-methylthio)thiocarbonyl]benzoate}
(5). Trideuteromethanethiol
(1.74 g, 2 mL, 34 mmol) was added to a solution of 1 (7.0 g, 43 mmol) in dry toluene
(15 mL) at 0 ◦C. A stream of dry gaseous HCl was introduced into the solution at –10 ◦C.
Losses of toluene were supplemented during the procedure. The solvent was removed by
vacuum evaporation. The residue was dispersed in dry pyridine and a stream of H2S was
introduced into the suspension at 0 ◦C for 4 h. The red colored reaction mixture was poured
into ice and concd. aqu. HCl, and the mixture was extracted with CHCl3. The extract
was dried and evaporated under vacuum to yield an oily red residue, which crystallized
overnight. Column chromatography (dioxane/PE 15:85) gave red leaflets (1.3 g, 17%),
mp 87 ◦C (lit.10 for 3: 90 ◦C). 1H NMR: δ 3.95 (s, 3 H, OCH3), 8.00 (s, 4 H, ArH). Anal.
Calcd. for C10H7D3O2S2 (229.3): C 52.38, H 3.08, D 2.62, S 27.97. Found: C 52.35, H
3.02, D 2.58, S 27.97.
Calculations
Semiempirical PM6-type MO calculations were performed by use of the program
package Mopac 2009.7 DFT-based geometry optimizations and spin density calcula-
tions were performed by the B3LYP method11 with 6–31G(p,d) basis sets. The Fire-
fly-program12 was used for the DFT calculations. The Qcpe programs Draw and Jmol
(Open Project; http://sourceforge.net/projects/jmol/) were used for the graphical presenta-
tion of the results. A conventional PC (Pentium Dual Core CPU E5200, 2.5 GHz; 3.25 GB
RAM) was applied for the calculations.
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DIMETHYL DITHIOTEREPHTHALATE RADICAL ANIONS
391
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