Structures of 4-substituted thioanisole radical cations studied by time-resolved resonance Raman spectroscopy during pulse radiolysis and theoretical calculations

Article abstract of DOI:10.1039/c6ra21460f

To understand dimerization-reactivity, structures of 4-substituted thioanisole radical cations (ArSCH₃⁺) in aqueous solution were studied by the nanosecond time-resolved resonance Raman (ns-TR³) spectroscopy during pulse radiolysis and density functional theory (DFT) calculations. The downshift of the CC stretching mode of the benzene ring and the upshift of the C-S stretching mode upon oxidation were observed with aromatic sulfides such as 4-hydroxymethylthioanisole (MTPM), suggesting formation of a semi-quinoidal structure. For the 4-methoxythioanisole radical cation (MTA⁺), weak C_Ph-O stretching was observed additionally. Based on the DFT calculations, semi-quinoidal structures including an S atom and benzene ring were indicated for MTPM⁺ and so on, in which the positive charge and spin are localized on the S-atom and C₄-position, respectively. On the other hand, the quinoidal structure including the S and O atoms and benzene ring was indicated for MTA⁺. The results of DFT calculation agreed well with TR³ spectra of ArSCH₃⁺. The reactivities of ArSCH₃⁺ toward dimerization were discussed in terms of the conjugation between S non-bonding electrons and π-electrons of the aromatic system.

Full text of DOI:10.1039/c6ra21460f

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Structures of 4-substituted thioanisole radical
cations studied by time-resolved resonance Raman
spectroscopy during pulse radiolysis and
theoretical calculations†
Cite this: RSC Adv., 2016, 6, 109334
*
*
Sachiko Tojo, Mamoru Fujitsuka and Tetsuro Majima
To understand dimerization-reactivity, structures of 4-substituted thioanisole radical cations (ArSCH3c+) in
aqueous solution were studied by the nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy
during pulse radiolysis and density functional theory (DFT) calculations. The downshift of the C]C
stretching mode of the benzene ring and the upshift of the C–S stretching mode upon oxidation were
observed with aromatic sulfides such as 4-hydroxymethylthioanisole (MTPM), suggesting formation of
a semi-quinoidal structure. For the 4-methoxythioanisole radical cation (MTAc+), weak CPh–O stretching
was observed additionally. Based on the DFT calculations, semi-quinoidal structures including an S atom
and benzene ring were indicated for MTPMc+ and so on, in which the positive charge and spin are
localized on the S-atom and C4-position, respectively. On the other hand, the quinoidal structure
including the S and O atoms and benzene ring was indicated for MTAc+. The results of DFT calculation
agreed well with TR3 spectra of ArSCH3c+. The reactivities of ArSCH3c+ toward dimerization were
discussed in terms of the conjugation between S non-bonding electrons and p-electrons of the
aromatic system.
Received 26th August 2016
Accepted 8th November 2016
DOI: 10.1039/c6ra21460f
www.rsc.org/advances
time-resolved vibrational spectroscopic techniques such as
nanosecond time-resolved resonance Raman (ns-TR3) spec-
Introduction
troscopy provide us structural information of short-lived inter-
mediates including radical cations.9–16 As pointed out, there is
only limited number of investigation on Raman spectra of
RSR0c+ and nothing for ArSRc+ until now.17,18 Thus, systematic
study on TR3 spectra of ArSRc+ will be useful for understanding
of their reactivities and so on.
In the present study, we studied vibrational spectra of 4-
substituted thioanisole (ArSCH3, Fig. 1) radical cations
(ArSCH3c+) using ns-TR3 spectra measurements during the
pulse radiolysis as well as the ns-TA. Furthermore, the charge
and spin distributions in ArSCH3c+ were estimated by the
density functional theory (DFT) calculation. Based on these,
Sulfur compounds play a central role in many vital systems. In
the living cell, they minimize oxidative damage of biological
tissues and are transformed into a variety of sulfur intermedi-
ates. To date, the oxidation reactions of various sulfur
compounds have been studied by nanosecond transient
absorption (ns-TA) measurement during the pulse radiolysis
technique and so on.1–6 One of the unique radical cations
derived from the oxidation reactions of aliphatic suldes is a s-
dimer radical cation, s-(RSR0)2c+ (R, R0 ¼ alkyl), generated from
the reaction of RSR0c+ with RSR0. The s-(RSR0)2c+ has a three-
electron S–S bond and shows a relatively strong absorption
peak around 400–600 nm due to the s / s* transition.7 In
contrast to s-(RSR0)2c+, the formation of s-dimer radical cation
of aryl sulde, s-(ArSR)2c+, depends on the electronic properties
of the substituents of aryl sulde radical cation (ArSRc+).8
Although these products can be detected by ns-TA, under-
standing on reactivities of sulde radical cations toward
dimerization has not been well established because of the
absence of their structural information. On the other hand, the
The Institute of Scientic and Industrial Research (SANKEN), Osaka University,
Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan. E-mail: fuji@sanken.osaka-u.ac.jp;
majima@sanken.osaka-u.ac.jp; Fax: +81-6-6879-8499
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra21460f
Fig. 1 4-Substituted thioanisoles used in this study.
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relation between structures of ArSCH3c+ and charge distribu- 2500i) equipped with a grating (1800 g mmꢂ1) aer passing
tions in ArSCH3c+ is discussed.
through an edge lter to remove Rayleigh scattering. The scat-
tered light was detected with an ICCD detector (Princeton
Instruments, PI-MAX3) operated with 5 ns of gate width, typi-
Experimental
Materials
cally. All instruments were controlled using
a personal
computer via a controller (Unisoku). The Raman spectrum was
4-Hydroxymethylthioanisole (98%) (MTPM, Sigma Aldrich), 4-
(methylthio)phenylacetic acid (99%) (MTPA, Sigma Aldrich) and
4-methoxythioanisole (97%) (MTA, Tokyo Chemical Industry
Co., Ltd.) were recrystallized from n-hexane before use. 4-
Methylthioanisole (99%) (MTT, Tokyo Chemical Industry Co.,
Ltd.) and thioanisole (>99%) (MTB, Tokyo Chemical Industry
Co., Ltd.) were used without further purication.
obtained by accumulation of 50 events typically.
Theoretical calculations
Optimized structures, non-resonance Raman spectra, Merz-
Singh–Kollman charges, and Mulliken atomic spin densities of
the molecules in this study were estimated by density functional
theory (DFT) at the (U)B3LYP/6-311+G(d,p) level using the
Gaussian 09 package.19 Raman peak positions estimated at (U)
B3LYP/6-311+G(d,p) level were scaled by using a factor of 0.97.20
XYZ coordinates of optimized structures are indicated in Tables
S9–S29 in the (ESI†). The resonance Raman spectra of radical
cations by 532 nm excitation were calculated with the Becke88
Perdew86 exchange-correlation functional and the TZVP basis
set using NWChem 6.6 program employing available
patches.21,22 No factor was applied to resonance Raman peak
positions estimated by UBP86/TZVP.
Pulse radiolysis
Pulse radiolysis experiments were performed using an electron
pulse (27 MeV, 8 ns, 0.87 kGy per pulse) from a linear acceler-
ator at Osaka University. All experiments were performed with
aqueous solutions (pH 7) that had been saturated with puried
N2O gas for a minimum of 15 min by using a capillary
technique.
Transient absorption spectra
A 2 mL solution was placed in a quartz cell (5 ꢀ 10 ꢀ 40 mm)
that was sealed with a silicon rubber stopper. The kinetic
measurements were performed using a nanosecond photore-
action analyzer system (Unisoku, TSP-1000). The monitor light
was obtained from a pulsed 450 W Xe arc lamp (Ushio, UXL-451-
0), which was operated by a large current pulsed-power supply
that was synchronized with an electron pulse. The monitor light
was passed through an iris with a diameter of 0.2 cm and sent
into the sample solution at a perpendicular intersection to an
electron pulse. The monitor light passing through the sample
was focused on the entrance slit of a monochromator (Unisoku,
MD200) and detected with a photomultiplier tube (Hamamatsu
Photonics, R2949). The transient absorption spectra were
measured using a photodiode array (Hamamatsu Photonics,
S3904-1024F) with a gated image intensier (Hamamatsu
Photonics, C2925-01) as a detector.
Results and discussion
Radical cation of 4-hydroxymethylthioanisole (MTPMc+) was
generated during the pulse radiolysis of MTPM in N2O-
saturated aqueous solution containing NaBr. Initial products
of the radiolysis are eaqꢂ, Hc, cOH, H2, H3O+, and H2O2 (eqn (1))
ꢂ
under the present condition.23 The eaq is quantitatively con-
verted to cOH in N2O-saturated aqueous solution (eqn (2)). In
the presence of bromide ion (Brꢂ), cOH reacts with Brꢂ to give
Brc (eqn (3)), followed by a reaction of Brc with Brꢂ to give Br2cꢂ
(eqn (4)).23 One-electron oxidation of MTPM occurs via a reac-
tion with Br2cꢂ to generate MTPMc+ (eqn (5)).24 At high
concentration of MTPM (>5 mM), MTPMc+ reacts with MTPM to
give MTPM dimer radical cation (MTPM2c+) in the ns time scale
(eqn (6)).25
H2O ⇝ eaqꢂ, cOH, Hc, H2, H3O+, H2O2
(1)
(2)
(3)
(4)
(5)
(6)
Time resolved resonance Raman spectra
ꢂ
eaq + N2O + H2O / N2 + OHꢂ + cOH
The sample solution was circulated through a quartz tube with 4
mm internal diameter using a roller pump. The Raman scat-
tering was probed by 532 nm (80 mJ) or 355 nm (60 mJ) Nd-YAG
laser pulse (Brilliant, Quantel; 5 ns fwhm) in a collinear
conguration aer controlled delay time using a digital delay
generator (Stanford Research Systems, DG535), tuned in reso-
nance with the optical absorption of transient species. The cell
was adjusted to achieve the best overlap of focused electron
beam with the focused laser beam crossing each other at 45ꢁ
inside the sample cell at the scattering center. The Raman shis
cOH + Brꢂ / OHꢂ + Brc
Brc + Brꢂ / Br2cꢂ
Br2cꢂ + MTPM / 2Brꢂ + MTPMc+
MTPMc+ + MTPM / MTPM2c+
The ns-TA spectra during the pulse radiolysis of MTPM were
were calibrated by the known toluene solvent. Scattered light measured by changing the MTPM concentration as shown in
from the sample to the perpendicular direction with respect to Fig. 2. In ns-TA spectrum of MTPM (0.5 mM), the absorption
the laser pulse was collected using appropriate lenses and peak at 550 nm attributable to MTPMc+ was clearly observed.
focused onto the optical ber, which guided the scattered light With increasing the MTPM concentration from 0.5 to 5 mM, the
onto a slit of a polychromator (Princeton Instruments, Acton SP- peak around 550 nm showed blue-shi by approximately 20 nm
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Fig. 3 (a) ns-TR3 spectrum observed at 500 ns after an 8 ns electron
pulse during the pulse radiolysis of MTPM (0.5 mM) in N2O-saturated
aqueous solution containing NaBr (100 mM). Probe: 532 nm. (b and c)
Raman peaks of MTPMdc+ and MTPMec+, respectively (see Table S1 in
the ESI†), calculated by TDDFT at UBP86/TZVP level.
Fig. 2 Transient absorption spectra observed at 500 ns after an
electron pulse during the pulse radiolysis of MTPM (0.5 mM (black) or
5 mM (red)) in N2O-saturated aqueous solution (pH 7) containing NaBr
(100 mM). The optical densities of two spectra are normalized at
580 nm. Insets show time profiles of the transient absorption at
550 nm (black) and at 540 and 780 nm (red), assigned to the formation
of MTPMc+ and s- and p-MTPMc+, respectively.
peaks of Br2cꢂ were not observed with the 532 nm probe
(Fig. S3†). The ns-TR3 spectrum showed an intense peak at 1101
cmꢂ1 with weak peaks at 524, 657, 721, 803, 998, 1183, 1294 and
1475 cmꢂ1 (Fig. 3a). Delay time dependence of intensity of the
peak at 1101 cmꢂ1 was consistent with the kinetic trace of DO.D.
at 550 nm as shown in Fig. S4.† In addition, peak positions were
not changed with time and the ns-TR3 spectrum was indepen-
dent of the concentration of MTPM as shown in Fig. S2.† Thus,
these peaks can be attributable to MTPMc+. Assignments of the
peaks were carried out based on time-dependent density func-
tional (TD-DFT) calculation developed by Aquino and Schatz
recently for resonance Raman spectra of open shell systems.22
In the present study, BP86 exchange-correlation functional and
TZVP basis set were applied aer their study. Possible
conformers were also calculated for radical cations. The Raman
peaks of MTPMc+ were assigned based on the peak assignments
of thioanisole radical cation (MTBc+) using TD-DFT, because
straightforward assignments are possible for MTBc+ due to
simple structure as summarized in Table S7 (see also ESI†).
Interestingly, the C]C stretching vibration of benzene ring was
observed at 1599 cmꢂ1 for neutral MTPM, but it was not seen in
MTPMc+ (Fig. 3a). This observation is reproduced by the TD-DFT
calculation. The C]C stretching of MTPMc+ was calculated to
be 1577 or 1581 cmꢂ1 with very weak intensity (Fig. 3b and c).
Thus, downshied C]C stretching mode is expected for radical
cation. The C–S stretch vibration of MTPMc+ at 1101 cmꢂ1 was
upshied compared with MTPM.
together with appearance of a new peak around 800 nm, indi-
cating the formation of s- and p-MTPM2c+ by dimerization of
MTPMc+ and MTPM. The absorption band at 530 nm has been
assigned to s-MTPM2c+, which possesses a three electron S–S
bond between MTPMc+ and MTPM. It has been reported that
some ArSCH3c+ dimerize with ArSCH3 to give p-(ArSCH3)2c+,
showing a strong absorption peak around 800–1000 nm due to
the charge resonance (CR) interaction. The 800 nm band of
MTPM2c+ is similar to CR-bands of p-dimer radical cations of
alkylbenzenes, indicating the generation of p-MTPM2c+.25
The Raman spectrum of neutral MTPM in 1,2-dichloro-
ethane (DCE) solution was indicated in Fig. S1 in the ESI.†
Strong and medium Raman peaks were observed at 1599 and
1096 cmꢂ1, respectively, together with a weak peak at 1192
cmꢂ1. The observed Raman peaks were assigned based on the
previously reported spectra of thioanisole26 and the DFT calcu-
lations at the UB3LYP/6-311+G(d,p) level. Because substituents
of MTPM can rotate with respect to the benzene ring, several
conformers are possible. Thus, Raman spectra of the
conformers were calculated theoretically. In Table S1 in the
ESI,† calculated sum of electronic and zero-point energies were
summarized for the conformers of neutral MTPM. As shown in
Table S1,† energy difference is on the order of 10 meV, thus, all
conformers possibly exist in solution at room temperature.
Thus, the observed Raman spectrum is considered to be a sum
of these conformers (Fig. S1 in the ESI†). Assignments of the
observed peaks are summarized in Table S6 in the ESI.† Simi-
larly, conformers of other suldes are studied as summarized in
Tables S2–S5,† and peak assignments of non-resonance Raman
of their neutral states are also indicated in Table S6.†
The geometry of MTPMc+ optimized by DFT calculation is
compared with that of neutral MTPM (Fig. 4). As discussed
above, some conformers are possible for suldes. For neutral
MTPM, three are calculated as stable conformers by theoretical
calculation (Table S1†). Among them, conformer indicated in
Fig. 4a (MTPMc) showed the lowest energy. The SCH3 group is
almost perpendicular to the aromatic ring (dihedral angle: f ¼
89.8ꢁ), while the SCH3 groups of other two conformers are
coplanar to the benzene ring. For MTPMc+, two conformers
showed almost the same energy. In both conformers, the SCH3
The ns-TR3 spectra observed at various delay time aer an 8
ns electron pulse during the pulse radiolysis of MTPM (0.5 mM)
in N2O-saturated aqueous solution containing NaBr (100 mM)
were indicated in Fig. S2.† In Fig. 3a, the spectrum at 500 ns was
indicated as a representative. At this delay time, the Raman
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In addition, the bond lengths calculated by the DFT calcula-
tion indicated the semi-quinoidal structure of MTPAc+
(Table S8†).
In the cases of 4-methylthioanisole (MTT) and MTB, the ns-
TA peaks of MTT radical cation (MTTc+) and MTBc+ in aqueous
solutions were observed at 550 and 540 nm, respectively
(Fig. S8†). The ns-TA of s- and p-(ArSCH3)2c+ could not be
observed because of poor solubility in aqueous solution, but the
formations of s- and p-(ArSCH3)2c+ were conrmed in benzo-
nitrile27 by increasing the concentration to 50 mM (Fig. S9†).
The peaks of ns-TR3 spectra of MTTc+ and MTBc+ (Fig. S10 and
S5,† respectively) and theoretical calculation (Table S8†) sup-
ported semi-quinoidal structures in the oxidized state.
Fig. 4 Optimized structures of representative conformers of (a) MTPM
and (b) MTPMc+ by the DFT calculation at the UB3LYP/6-311+G(d,p)
level. Numbers are bond lengths (A˚). Yellow and red colors show S and
O atoms, respectively.
For 4-methoxythioanisole (MTA), the absorption peak
attributable to MTA radical cation (MTAc+) appeared at 580 nm
(Fig. S11†). It is notable that the spectral shape of ns-TA was
independent of the MTA concentration (1 and 50 mM) in
benzonitrile, indicating absence of MTA dimer radical cation
(MTA2c+) formation. The Raman spectrum of neutral MTA
showed peaks at 1096, 1570, and 1596 cmꢂ1 (Fig. S12†), which
agreed with the previously reported spectra and DFT calcula-
tion.26,28 The ns-TR3 spectrum during the pulse radiolysis of
MTA showed intense ns-TR3 peaks at 1120 and 1603 cmꢂ1 as
well as weak peaks at 503, 644, 719, 804, 988, 1427, 1457, and
1498 cmꢂ1 (Fig. 5a). The ns-TR3 signals decayed without any
spectral change within 5 ms (Fig. S13†). Such behavior is in
agreement with the ns-TA of MTA. These peaks are consistent
with those of MTAc+ calculated by the TD-DFT as shown in
Fig. 5b. In the case of MTAc+, the upshi of C–S stretch
group is coplanar to benzene ring (Fig. 4b, f ¼ 0.0ꢁ). The C1–S
˚
˚
bond length decreases from 1.798 A to 1.720 A upon oxidation.
Furthermore, the C2(5)–C3(6) bonds in benzene ring decrease
˚
˚
from 1.392 (1.391) A to 1.372 (1.376) A upon oxidation, while
˚
C
1(3)–C2(4) bonds increase from 1.398 (1.399) A to 1.428 (1.419)
˚
A. These dihedral angle and bond length changes suggest
a semi-quinoidal structure including S atom and benzene ring
for MTPMc+ as shown in Scheme 1. In the above section, the
upshi and downshi of C–S and C]C vibrational modes,
respectively, were conrmed. These Raman peak shis are
consistent with the formational of semi-quinoidal structure
upon oxidation.
Fig. S6† shows the ns-TA spectra of 4-(methylthio)phenyl-
acetic acid (MTPA) observed at 500 ns aer an electron pulse
during the pulse radiolysis of MTPA in aqueous solution. The
ns-TA peak at 570 nm attributable to MTPA radical cation
(MTPAc+) was clearly observed when the sample included 1 mM
MTPA. On the other hand, the absorption peaks around 540 and
800 nm were observed by increasing the MTPA concentration to
7.5 mM, indicating the formation of s- and p-(MTPA)2c+.
Fig. S7a† shows the ns-TR3 spectrum of MTPA observed at
500 ns aer an electron pulse during the pulse radiolysis of
MTPA (0.5 mM). The peaks of ns-TR3 spectrum are consistent
with those of MTPAc+ calculated by the TD-DFT method
(Fig. S7†). The downshied C]C stretching vibration of
benzene ring was observed around 1580 cmꢂ1 with very weak
intensity as expected from the TD-DFT calculation. The
upshi of C–S stretch vibration of MTPAc+ was also observed.
vibration (1119 cmꢂ1) was observed upon oxidation. The CPh
–
O mode around 1300 cmꢂ1 became obscure, while intensity of
C]C of benzene ring is enhanced. The geometry of MTAc+
optimized by the DFT calculation is compared with that of
neutral MTA (Fig. 6). In the neutral state, the SCH3 of one of
the two conformers is perpendicular to the benzene ring, while
in the oxidized state SCH3 is coplanar to the benzene ring in
the both conformers. Among conformers, the conformers with
Fig. 5 (a) ns-TR3 spectrum observed at 500 ns after an 8 ns electron
pulse during the pulse radiolysis of MTA (1 mM) in N2O-saturated
aqueous solution containing NaBr (100 mM). Probe: 532 nm. (b and c)
Scheme 1 Dimerization of thioanisole radical cations with semi-qui- Raman peaks of MTAcc+ and MTAdc+, respectively (see Table S5 in the
noidal and quinoidal structures.
ESI†), calculated by TDDFT at UBP86/TZVP level.
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Table
2
Mulliken atomic spin density for ArSCH3c+ by the DFT
calculation at the UB3LYP/6-311+G(d,p) levela
Atom
MTPMc+
MTPAc+
MTTc+
MTBc+
MTAc+
C1
C2
C3
C4
C5
C6
0.177
0.020
0.168
0.132
0.170
0.136
0.135
0.075
0.214
0.011
ꢂ0.093
ꢂ0.033
ꢂ0.035
ꢂ0.104
ꢂ0.006
0.302
0.289
0.311
0.309
0.178
ꢂ0.038
0.156
ꢂ0.097
0.042
ꢂ0.090
0.0407
ꢂ0.049
0.145
0.042
0.050
Fig. 6 Optimized structures of (a) MTA and (b) MTAc+ by the DFT
calculation at the UB3LYP/6-311+G(d,p) level. Numbers are calculated
a
Mulliken atomic spin density of the most stable conformers
(MTPMdc+, MTPAfc+, MTTcc+, MTBcc+, and MTAdc+, see Tables S1–S5
bond lengths (A˚). Yellow and red colors show S and O atoms, in the ESI) are indicated.
respectively.
The positive charge on the benzene ring increases in the
order of MTAc+ (0.506) < MTBc+ (0.525) < MTPMc+ (0.549) <
MTPAc+ (0.563) < MTTc+ (0.621), indicating that the positive
charge of MTPMc+, MTPAc+, MTTc+ and MTBc+ with semi-
quinoidal structure localized on ArS is higher than that of
MTAc+ with the quinoidal structure. Therefore, ArSCH3c+ with
semi-quinoidal structure dimerizes to s- and p-(ArSCH3)2c+ by
the face-to-face interaction between ArS and ArS chromophores
as shown in Scheme 1. In other words, the positive charge
localization on S atom and benzene ring is important for the
formation of s- and p-(ArSCH3)2c+.
the lower energy are compared (MTAb and MTAd, see
Table S5†).
Upon oxidation, the C2(5)–C3(6) bond length in benzene ring
˚
˚
decreases from 1.394 (1.390) A to 1.371 (1.368) A, while the C1(6)
–
˚
C
2(1) bond length increases from 1.397 (1.401) A to 1.425 (1.421)
˚
˚
˚
A. The C1–S bond length decreases from 1.787 A to 1.723 A upon
oxidation, indicating the increase in the double bond character.
˚
Furthermore, the C4–O bond length decreases from 1.367 A to
+
˚
1.317 A. These results suggest the MTAc has a quinoidal
structure different from other suldes as shown in Scheme 1.
The charges of ArSCH3c+ were calculated using the Merz-
Singh–Kollman as shown in Table 1.29 The charge on S atom
increases in the order of MTAc+ (0.115) < MTPMc+ (0.143) <
MTTc+ (0.144) < MTPAc+ (0.163) < MTBc+ (0.165), suggesting that
higher positive charge on S atom is responsible to the formation
of s-(ArSCH3)2c+ as shown in Scheme 1.
On the other hand, MTAc+ has a strong electron-donating
–OCH3 group and the positive charge is mostly delocalized on
MTAc+. The quinoidal structure of MTAc+ with the conjugation
between S and O atoms non-bonding electrons and p-electrons
of benzene ring induces double-bond character of CPh–S and
C
Ph–O bonds. The obscure C–O stretching vibration and the
The Mulliken atomic spin densities for ArSCH3c+ were also
shown in Table 2. The spin density on C4 of benzene ring
increases in the order of MTAc+ (0.178) < MTPAc+ (0.289) <
MTPMc+ (0.302) < MTBc+ (0.309) < MTTc+ (0.311), indicating the
semi-quinoidal structures for MTPMc+, MTPAc+, MTTc+ and
largest upshi of C–S stretching vibration were observed for
MTAc+. The formation of s- and p-MTA2c+ are unfavorable due
to the electronic repulsion of the positive charge delocalized on
two MTA molecules. The formation of s- and p-(ArSCH3)2c+ for
ArSCH3c+ with the quinoidal structure is inhibited (Scheme 1).
MTBc+ while the quinoidal one for MTAc+. The C1–S bond length
+
˚
˚
of ArSCH3c (1.72 A) is shorter than that of ArSCH3 (1.79 A) and
the C–S stretching vibration of ArSCH3c+ around 1100 cmꢂ1
showed the upshi compared with of ArSCH3. The results
indicated the double-bond character of C1–S of ArSCH3c+ with
the semi-quinoidal or quinoidal structures.
Conclusions
The structures of ArSCH3c+ in aqueous solution were studied by
ns-TR3 spectroscopy during pulse radiolysis and DFT calcula-
tions. Positive charge of ArSCH3c+ delocalizes on S atom and
benzene ring with increasing the double bond character of CPh
–
S bond. Semi-quinoidal structure with conjugation between S
atom non-bonding electron and p-electrons of benzene ring is
found to be important for formation of s- and p-(ArSCH3)2c+.
On the other hand, quinoidal structure of MTAc+ is not suitable
for formation of s- and p-(ArSCH3)2c+. To clarify the relation-
ship between structure and dimerization of ArSCH3c+ further
studies for vibrational structure of s- and p-(ArSCH3)2c+ are in
progress.
Table 1 The Merz-Singh–Kollman charge for ArSCH3c+ by the DFT
calculation at the UB3LYP/6-311+G(d,p) levela
Atom
MTPMc+
MTPAc+
MTTc+
MTBc+
MTAc+
C1
C2
C3
C4
C5
C6
S
0.068
0.132
0.105
0.121
0.123
0.105
0.144
0.069
0.050
0.208
0.037
0.017
0.165
0.067
0.068
ꢂ0.130
ꢂ0.069
ꢂ0.086
ꢂ0.079
0.402
0.344
0.455
0.627
ꢂ0.039
0.117
0.143
ꢂ0.001
0.063
0.163
ꢂ0.046
0.070
0.144
ꢂ0.179
0.179
0.115
Acknowledgements
We thank the members of the Research Laboratory for
Quantum Beam Science of ISIR, Osaka University for assistance
in the pulse radiolysis experiments. This work has been partly
a
Merz-Singh–Kollman charge of the most stable conformers (MTPMdc+,
MTPAfc+, MTTcc+, MTBcc+, and MTAdc+, see Tables S1–S5 in the ESI) are
indicated.
109338 | RSC Adv., 2016, 6, 109334–109339
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supported by a Grant-in-Aid for Scientic Research (Projects 18 I. Janik and G. N. R. Tripathi, J. Chem. Phys., 2013, 138,
25220806, 25288035, and 25390125) from the Ministry of 044506.
Education, Culture, Sports, Science and Technology (MEXT) of 19 M. J. T. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
Japanese Government.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
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