Phenylsulfinyl Radical: Gas-Phase Generation, Photoisomerization, and Oxidation

Article abstract of DOI:10.1021/jacs.8b05055

Arylsulfinyl radicals are key intermediates in sulfoxide chemistry. The parent molecule, phenylsulfinyl radical PhSOa€￠, has been generated for the first time in the gas phase through high-vacuum flash pyrolysis of PhS(O)R (R = CF₃ and Cl) at a

Full text of DOI:10.1021/jacs.8b05055

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Article
Phenylsulfinyl Radical: Gas-phase Generation,
Photoisomerization, and Oxidation
Jian Xu, Zhuang Wu, Huabin Wan, Guohai Deng, Bo Lu, Andre K. Eckhardt,
Peter R. Schreiner, Tarek Trabelsi, Joseph S. Francisco, and Xiaoqing Zeng
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05055 • Publication Date (Web): 10 Jul 2018
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Phenylsulfinyl Radical: Gas-phase Generation, Photoisomeri-
zation, and Oxidation
Jian Xu,1 Zhuang Wu,1 Huabin Wan,1 Guohai Deng,1 Bo Lu,1 André K. Eckhardt,2 Peter R. Schreiner,2
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Tarek Trabelsi,3 Joseph S. Francisco,4,* and Xiaoqing Zeng1,*
1College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123 (China)
2Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen (Germany)
3University of Nebraska – Lincoln, Lincoln, Nebraska 68526 (USA)
4Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, (USA).
KEYWORDS: Reactive sulfur species, photochemistry, ring-opening, oxidation, matrix-isolation spectroscopy
ABSTRACT: Arylsulfinyl radicals are key intermediates in sulfoxide chemistry. As the parent molecule, phenylsulfinyl radical
PhSO• has been generated for the first time in the gas phase through high-vacuum flash pyrolysis (HVFP) of PhS(O)R (R = CF3
and Cl) at about 1000 K. Upon UV light irradiation (365 nm), PhSO• isomerizes to a novel oxathiyl radical PhOS• in cryogenic
matrices (2.8 K). Prolonged irradiation causes further isomerization of PhOS• to 2-hydroxyphenylthiyl radical, the formation of
which has been also observed in the 193 nm laser photolysis of matrix-isolated 2-hydroxybenzenethiol. Concomitantly, ring-
opening occurs during the UV photolysis of PhOS• and 2-hydroxybenzenethiol and forms an acyclic thioketoketene radical.
Phenylsulfinyl radical reacts partially with molecular oxygen in the gas phase and yields phenyl radical Ph• and OSOO. Upon irra-
diation (365 nm), the isomeric oxathiyl radical also combines O2 with immediate dissociation to phenoxy radical PhO• and SO2.
The identification of the intermediates with IR and UV/Vis spectroscopy is supported by quantum chemical computations at the
B3LYP/def2-TZVPP and UCCSD(T)/aug-cc-pV(D+d)Z levels of theory. The isomerization of PhSO• has been discussed based on
the computed potential energy profile and the comparison with the intensively explored photochemistry of phenylperoxy radical
PhOO•.
INTRODUCTION
cleavage of diphenyl sulfoxide was confirmed by nanosecond
transient absorption spectroscopy with observed transitions
(λmax) at 300 and 450 nm.35 Recently, a transient absorption at
460 nm was observed in the photolytic C–S bond cleavage of
the aryl benzyl sulfoxide radical cation.31-33 More recently,
PhSO• has been detected by EPR spectroscopy in functionaliz-
ing nanostructured SBA-15 silicas with diazene precursors.36
Sulfur-containing radicals like thiyl radicals R-S•,1-5 sulfinyl
radicals R-SO•,6-8 sulfonyl radicals R-SO2•,9-12 and sul-
fonyloxyl radicals R-SO3•13-15 are highly relevant to atmos-
pheric chemistry, biology, and organic synthesis. For instance,
simple sulfinyl radicals HSO•16 and CH3SO•17,18 form as fleet-
ing intermediates in the oxidation of volatile organic sulfur
compounds (VOSCs) in the atmosphere. More complex cyste-
ine sulfinyl radicals are involved in the oxidative damage of
proteins induced by reactive oxygen species (ROSs).19,20 Aryl-
sulfinyl radicals ArSO• have been proposed as the key inter-
mediates in a variety of chemical transformations, such as the
thermal disproportionation of aryl arenethiolsulfinates,21-24 the
photolytic decomposition of arenesulfinate esters, aryl thiosul-
finate,25-27 and vic-disulfoxides,28 the photochemical racemiza-
tion of aryl sulfinamides,29,30 the photosensitized oxidation
aryl arenesulfoxides,31-33 and the copper-catalyzed aerobic
oxidation of thiols.34
Scheme 1. Phenyl-substituted sulfur-centered radicals
PhSOx• (x = 0–2) and isomers.
As a member in the family of phenyl-substituted sulfur-
centered radicals PhSOx• (x = 0–2, Scheme 1), the parent aryl-
sulfinyl radical PhSO• has been frequently studied in solution.
The first-time detection of PhSO• in the photolysis and pyrol-
ysis of S-aryl arenethiosulfonates and diaryl sulfoxides was
accomplished using electron paramagnetic resonance (EPR)
spectroscopy.27 Later on, its presence in the photo-induced α-
Similar to other sulfinyl radicals (CH3SO•,37 CF3SO•,38 and
t-BuSO•6), computations of PhSO• at the B3LYP/6-31+G(d,p)
level suggest that the spin density is almost entirely localized
on the sulfur and oxygen atoms with equal contribution.35
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However, unlike the spontaneous oxidation of CH3SO• by
Page 2 of 9
Matrix-isolation IR Spectroscopy. Matrix IR spectra were
recorded on a FT-IR spectrometer (Bruker 70V) in a reflec-
tance mode by using a transfer optic. A KBr beam splitter and
liquid-nitrogen cooled MCT detector were used in the mid-IR
region (4000–400 cm–1). Typically, 200 scans at a resolution
of 0.5 cm–1 were co-added for each spectrum. The gaseous
PhS(O)CF3, PhS(O)Cl, and 2-hydroxybenzenethiol were
mixed by Ar (1 : 1000) through a U-trap containing ca. 20 mg
of corresponding samples at –28, 18, and –10 °C, respectively.
The mixtures were passed through an aluminum oxide furnace
(o.d. 2.0 mm, i.d. 1.0 mm), which was heated over a length of
ca. 25 mm by a tantalum wire (o.d. 0.4 mm, resistance 0.4 Ω),
deposited (2 mmol h–1) in a high vacuum (~106 pa) onto the
Rh-plated Cu block matrix support (2.8 K for PhS(O)CF3 and
PhS(O)Cl, 15 K for 2-hydroxybenzenethiol) using a closed-
cycle helium cryostat (Sumitomo Heavy Industries, SRDK-
408D2-F50H) inside the vacuum chamber. Temperatures at
the second stage of the cold head were controlled and moni-
tored using a LakeShore 335 digital cryogenic temperature
controller a Silicon Diode (DT-670). The voltage and current
used in the pyrolysis experiments are 5.1 V and 3.34 A, re-
spectively. Photolysis was performed using ArF excimer laser
(Gamlaser EX5/250, 3 Hz) and UV flashlight (365 nm, Boyu
T648, 20 W).
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molecular oxygen in a solid Ar matrix under cryogenic condi-
3
tions,13 the reactivity of PhSO• with O2 in solution at room
temperature is very low, and no oxidation product was identi-
fied.35 Furthermore, solution studies demonstrate that PhSO•
in the 2A" ground state is a relatively inefficient singlet oxygen
quencher due to the absence of a low-lying 1A' state.39
In comparison to the thermally persistent sulfinyl radicals
R-SO•, computational studies conclude that the isomeric ox-
athiyl radicals R-OS• have lower stability, which are the low-
est among the related radicals R-SO• > R-SS• > R-OO• > R-
OS•.40-42 Nevertheless, the exceptionally high activation barri-
ers associated with the interconversion between R-SO• and R-
OS• (e.g., H, CH3, CF3, > 40 kcal mol–1) allowed isolation of
an oxathiyl radical CF3OS• during UV light irradiation (266 or
254 nm) of the sulfinyl radical CF3SO• in solid noble gas ma-
trices.38 In sharp contrast, the oxathiyl radical PhOS• remains
barely known, although the structurally related phenylperoxy
radical PhOO•43-45 and other phenyl-substituted sulfur-centered
radicals like PhS•11 and PhSO2•46 have already been intensive-
ly studied in the last decades.
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In continuation of our interest in the photochemistry of sul-
fur-containing radicals (e.g., CH3SO3•,14 CH3SO2•,12 and
CF3SO•38), we herein report the first gas-phase generation of
the phenylsulfinyl radical PhSO•, its photochemistry including
isomerization and ring-opening, and the oxidation with molec-
ular oxygen (Scheme 2), which is achieved by combining ma-
trix isolation spectroscopy (IR and UV/Vis) with quantum
chemical computations.
Matrix-isolation UV/Vis Spectroscopy. The cryostat used
was an APD Cryogenics HC-2 closed-cycle refrigerator sys-
tem with KBr windows for IR and BaF2 windows for UV/Vis
measurements. IR spectra were recorded with a Bruker IFS 55
FT-IR spectrometer (spectral range of 4000–350 cm–1 with a
resolution of 0.7 cm–1, 50 scans per measurement accumulat-
ed), UV/Vis spectra with a Jasco V670 spectrometer (spectral
range of 190–850 nm with a scanning speed of 1 nm s–1). For
the combination of HVFP with matrix isolation, a small,
home-built, water-cooled oven directly connected to the vacu-
um shroud of the cryostat was used. The pyrolysis zone con-
sisted of a completely empty quartz tube (inner diameter 8 mm,
length of heating zone 50 mm) resistively heated by a coax
heating wire. The temperature was controlled through a
Ni/CrNi thermocouple. A gas mixture PhS(O)CF3 and Ar
(1:1000) was prepared in a 2 L storage bulb and evaporated at
room temperature into the quartz pyrolysis tube. Immediately
after leaving the tube, at a distance of ca. 50 mm, all pyrolysis
products were condensed (typically 30 to 60 mbar in one hour)
on the surface of the 12.0 K matrix window. For irradiation, a
high-pressure mercury lamp (HBO 200, Osram) with a mono-
chromator (Bausch & Lomb) or a low-pressure mercury spiral
lamp (Gräntzel) with interference filter (254, 365, and 436 nm)
was used.
Scheme 2. Generation, photochemistry, and oxidation of
PhSO•.
Computational Details. Molecular structures and IR fre-
quencies of stationary points were computed using DFT
UB3LYP/def2-TZVPP48 and UCCSD(T)/aug-cc-pV(D+d)Z.49-
51
Local minima were confirmed by vibrational frequency
EXPERIMENTAL AND COMPUTATIONAL DETAILS
analysis, and transition states were further checked by intrinsic
reaction coordinate (IRC) computations.52,53 Time-dependent
(TD) DFT (B3LYP/def2-TZVPP) calculations54,55 were per-
formed for the prediction of vertical excitations. The natural
spin densities were computed at UB3LYP/def2-TZVPP level
with the Natural Bond Orbital (NBO) method.56 These compu-
tations were performed using the Gaussian 09 software pack-
age.57 The CCSD(T) calculations were performed using the
MOLPRO 2015 package.58
Sample Preparation. The commercially available
PhS(O)CF3 and 2-hydroxybenzenethiol were checked by 13C
NMR and gas-phase IR spectroscopy. Sulfinyl chloride
PhS(O)Cl was prepared according to the published protocol,47
and the purity was checked by using NMR spectroscopy (1H
NMR (400 MHz, CDCl3, tetramethylsilane (TMS)) δ = 7.55
(m, 5H) ppm, 13C NMR (100 MHz, CDCl3, TMS) δ = 123.35,
129.14, 133.38, 148.18 ppm) with a Bruker Avance ІІІ HD400
spectrometer at 25 °C.
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computed vibrational displacement vectors for PhSO•, the S–
RESULTS AND DISCUSSION
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4
5
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8
O stretching vibration couples asymmetrically with the C–S
stretching mode and occurs at 1088.9 cm–1 (calcd. 1097 cm–1,
B3LYP/def2-TZVPP) as the strongest IR band, which exhibits
a discernible isotopic shift of 4.7 cm–1 (calcd. 5.7 cm–1) for the
naturally abundant 34S. The symmetrically coupled stretching
vibration appears at 1059.5 cm–1 (calcd. 1069 cm–1) with an
isotopic shift of 4.5 cm–1 (calcd. 5.1 cm–1). These two band
positions are close to those of the S–O stretching vibrations
found in CF3SO• (1072.6 cm–1, Ar-matrix)38 and CH3SO•
(1068.2 cm–1, Ar-matrix).63 The C–S stretching mode appears
at 704.9 cm–1 (calcd. 711 cm–1) as a very weak band, which is
between those in CF3SO• (751.9 cm–1) and CH3SO• (669.9
cm–1). The COS deformation in PhSO• locates at 470.4 cm–1
(calcd. 475 cm–1) and is higher in frequency that that in
CF3SO• (340.8 cm–1). In PhOS•, the IR fundamentals for the
C–O and O–S stretching modes occur at 1135.9 cm–1 (calcd.
1147 cm–1) and 895.3 cm–1 (calcd. 906 cm–1), and the latter is
associated with a weak band at 891.7 cm–1 for the naturally
abundant 34S.
Generation and photochemistry of PhSO• and PhOS•.
The generation of PhSO• was achieved first by high-vacuum
flash pyrolysis (HVFP) of the sulfoxide PhS(O)CF3 (1, Figure
1A) at about 1000 K. The IR spectrum of the pyrolysis prod-
ucts (Figure 1B) isolated in solid Ar-matrix at 2.8 K suggests
that nearly 80% of the sulfoxide decomposes and yields the
expected fragmentation species •CF3 (2) with characteristic IR
bands at 1250.5, 1086.4, and 703.3 cm–1.59 In addition, the
formation of a new species is evidenced by the presence of the
IR bands (3) mainly at 1472.2, 1444.7, 1088.9, 1059.5, 737.6,
and 681.4 cm–1. In order to aid the assignment, the IR spec-
trum for the remaining fragment PhSO• (Figure 1D) was com-
puted, showing good agreement with the experimental obser-
vations (Table 1).
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Figure 1. (A) IR spectrum of Ar-matrix isolated PhS(O)CF3 (1).
(B) IR spectrum of Ar-matrix isolated HVFP products of 1. (C) IR
difference spectrum reflecting the change of the Ar-matrix isolat-
ed HVFP products of 1 upon 10 min UV light irradiation (365
nm); (D) Simulated IR difference spectrum based on the comput-
ed IR spectra of PhSO• (3) and PhOS• (4) at the B3LYP/def2-
TZVPP level. The IR bands of CF3• (2) and H2O (*) are labeled.
Figure 2. (A) IR spectrum of Ar-matrix isolated PhS(O)Cl (5).
(B) IR spectrum of Ar-matrix isolated HVFP products of 5. (C)
IR difference spectrum reflecting the change of the Ar-matrix
isolated HVFP products of 5 upon 10 min UV light irradiation
(365 nm). The IR bands of H2O (*) are labeled.
Considering the computed (TD-B3LYP/def2-TZVPP) in-
tense vertical transitions at 398 (oscillator strength f = 0.0179)
and 377 nm (f = 0.0086) for PhSO• (Table S1), the matrix
containing the pyrolysis products of 1 was subjected to UV
light (365 nm) irradiation. The resulting IR difference spec-
trum (Figure 1C) demonstrates the sole depletion of PhSO•
and the formation of a new species (4) with strong IR bands at
1483.8, 1185.2, 1135.9, 895.3, and 752.3 cm–1. These band
positions nicely coincide with the computed IR frequencies for
the isomeric oxathiyl radical PhOS• (Table 1 and Figure 1D).
Similar RSO• → ROS• isomerization has been previously
observed only for CF3SO• under the 266 nm laser irradiation
conditions,38 whereas, the most frequently computationally
explored HSO• → HOS• isomerization remains experimental-
ly unobserved.60-62
Attempts to convert PhOS• back to PhSO• were made by
using visible (560–400 nm) and UV light irradiations (365 and
266 nm), however, no interconversion took place. Instead, the
UV light slowly depleted both radicals simultaneously (Figure
3A). By following the changes of the IR spectra obtained at
different photolysis time, at least three new species can be
distinguished among the photolysis products. In the corre-
sponding IR difference spectrum (Figure 3B), one prominent
band at 2133.5 cm–1 (labeled with 8), which is very close to
the characteristic asymmetric C=C=O stretching vibrations
observed in ketenes such as H2CCO (2141.8 cm–1, Ar-
matrix).64 Therefore, it is very likely that ring-opening occurs
in PhOS• during the photolysis, resembling the known photo-
induced ring-opening of the closely related phenylperoxy radi-
cal PhOO• to a ketoketene radical, for which the asymmetric
C=C=O stretching vibration was observed in the range of
2150–2050 cm–1 in solid Ar-matrix.43
Gas-phase generation of PhSO• was also observed when
sulfinyl chloride PhS(O)Cl (5, Figure 2A) was used as the
precursor under the same pyrolysis conditions with PhS(O)CF3.
The IR spectrum of the pyrolysis products (Figure 2B) sug-
gests that the only byproduct is atomic chlorine (IR inactive).
The fully reproducible PhSO• → PhOS• photoisomerization
(Figure 2C) enables the unambiguous identification of the very
weak IR bands associated with both radicals. According to the
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Table 1. Observed and computed vibrational frequencies (1700–400 cm−1) of PhSO• isomers.a
1
2
3
4
5
6
7
8
9
PhSO• (3)
Observed
Ar-matrixb
PhOS• (4)
ortho-HOPhS• (6)
Computed
B3LYPc
Observed
Computed
Observed
Computed
CCSD(T)d Ar-matrixb B3LYPc
1600.2 (6) 1634 (4)
1590.6 (12) 1625 (3)
CCSD(T)d Ar-matrixb
B3LYPc
CCSD(T)d
1616 (<1) 1599.8
1610 (1)
1508 (5)
1477 (10) 1431.7
1354 (2)
1325 (1)
1201 (<1) 1178.1
1184 (<1) 1162.2
1104 (3)
1623.4
1611.3
1586.3 (40)
1546.7 (15)
1621 (55)
1583 (37)
1633
1595
1573.1 (2)
1472.2 (9)
1444.7 (15)
1326.2 (2)
1300.1 (3)
1595.0
1462.6
1483.8 (59) 1516 (30) 1476.0
1460.8 (17) 1493 (4) 1448.2
1450.8 (100) 1484 (139) 1486
1477 (46)
1386 (45)
1341 (43)
1270 (59)
1213 (83)
1185 (3)
1142 (11)
1085 (8)
1038 (6)
997 (<1)
978 (2)
1473
1389
1338
1279
1205
1162
1125
1062
1127
938
931
842
833
755
694
661
626
547
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
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42
43
44
45
46
47
48
49
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53
54
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59
60
1333.4
1296.7
1351 (<1) 1337.0
1333 (<1) 1298.6
1365.2 (23)
1335.4 (17)
1312.9 (19)
1234.5 (13)
1213.6 (13)
1158.5 (52)
1113.3 (10)
1066.7 (6)
1304.1 (1)
1185.2 (38) 1207 (12) 1184.7
1182 (<1) 1153.7
1135.9 (52) 1147 (10) 1142.0
1071.0 (10) 1101 (8)
1023.8 (7) 1044 (5)
1136.9
1088.9 (100) 1097 (64) 1082.8
1059.5 (19)
1020.6 (3)
1078.9
1024.3
1069 (13) 1070.5
1044 (4) 1021.8
1023 (<1) 975.8
1014 (1) 937.1
1000 (<1) 914.0
1021 (<1) 987.2
1010 (<1) 921.9
994 (<1)
940 (6)
993.4 (2)
907.8 (2)
903.4
876.5
861.0
811.1
737.0
730.9
600.4
583.0
540.4
470.6
878 (2)
945 (2)
883.1
832.4
720.3
690.7
604.5
560.2
460.7
430.9
895.3 (100) 906 (29)
849 (<1)
752.3 (80)
839.1 (21)
750.1 (13)
728.5 (4)
699.2 (4)
620.8 (38)
851 (5)
774 (47)
752 (8)
707 (7)
678 (75)
560 (6)
865 (<1)
767 (47)
711 (1)
707 (32)
628 (<1)
475 (23)
466 (6)
737.6 (56)
704.9 (3)
681.4 (27)
780 (56)
760 (6)
703 (26)
627 (<1)
555 (3)
505 (7)
682.1 (24)
470.4 (28)
524 (5)
512 (4)
503
495
488.3 (8)
a Full list of the computed IR fundamentals are given in the Supporting Information. b Band positions in Ar-matrix at 2.8 K, relative
integrated intensities (%) in parentheses. Harmonic frequencies (unscaled) at the def2-TZVPP basis set, IR intensities (km mol−1)
c
in parentheses. d Harmonic frequencies (unscaled) at the UCCSD(T)/aug-cc-pV(D+d)Z level.
The tentative assignment is also supported by the agreement
with the computed strongest IR fundamental at 2198 cm–1 (IR
intensity: 1880 km mol–1) for the thioketoketene radical 8 in
the most favorable conformation (Scheme 2, Table S3). In the
ring-opening of PhOO• (Scheme S1),43 a seven-membered
ring intermediate, 2-oxepinoxy radical, bearing characteristic
C=O stretching vibration at 1726.9 cm–1 is involved. By anal-
ogy, 2-thiepinoxy radical should be also formed via the ring-
expansion of 4. However, no distinguishable IR band at
around 1700 cm–1 in the IR spectrum (Figure 3A) for the char-
acteristic C=O stretching vibration (calcd. 1711 cm–1, Table
S3) could be observed, which might be due to the photolability
of 2-thiepinoxy radical towards the light (365 nm) applied in
photolysis of 4.
stretching vibrations in thione isomers produced via recombi-
nation of 6 and hydrogen atoms in Ar-matrix cages.
Among the photolysis products of PhOS• (4), another new
species exhibits IR bands (6) at 1586.3, 1546.7, 1450.8,
1365.2, 1335.4, 1312.9, 1234.5, 1213.6, 1158.5, 750.1, and
620.8 cm–1. These band positions coincide with the computed
IR fundamentals for 2-hydroxyphenylthiyl radical (6, Table 1),
an isomer of PhOS• (4). Given the known photo-induced hy-
drogen-elimination in phenol PhOH (→ PhO• + H•, 248 nm)65
and PhSH (→ PhS• + H•, 285–290 nm) in solid Ar-matrices,66
the photochemistry of 2-hydroxybenzenethiol (7) in solid Ar-
matrix with an ArF excimer laser (193 nm) was also studied.
The corresponding IR difference spectrum (Figure 3B) shows
the formation of both 2-hydroxyphenylthiyl radical (6) and the
acyclic thioketoketene radical (8). Notably, a broad and weak
band centered at 1667.8 cm–1 keeps increasing its intensity
during photolysis, it can be tentatively assigned to the C=C
Figure 3. (A) IR difference spectrum reflecting the change of the
Ar-matrix isolated PhOS (4)• upon UV light irradiation (365 nm,
30 min). (B) IR difference spectrum reflecting the change of the
Ar-matrix isolated 2-hydroxybenzenethiol (7) upon ArF laser
irradiation (193 nm, 50min). IR bands of 2-hydroxyphenylthiyl
radical (6), the thioketoketene radical (8), and unknown species (#)
are labeled.
The photochemistry of PhSO• (3) in solid Ar-matrix was al-
so followed by UV/Vis spectroscopy (Figure 4). Consistent
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with the TD-B3LYP/def2-TZVPP computed vertical transi- could be identified in the IR spectrum (Figure 5A) probably
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tions at 521 (f = 0.0006), 398 (f = 0.0179), and 377 nm (f =
0.0086) for PhSO• (Table S1), the HVFP products of
Ph(O)CF3 (1) shows very weak absorption starting at around
475 nm and terminating at 410 nm, which is also in agreement
with the previously observed transient absorptions at around
450 nm for PhSO• in solution.31-33,35 In line with the computed
intense absorptions at around 300 nm (311 nm, f = 0.0554; 295
nm, f = 0.0432), a much stronger absorption centered at 300
nm with superimposed vibrational fine structures vanishes
upon the 365 nm irradiation, hence, it should also belong to
the same carrier PhSO•. Apparently, the n → π* transition in
PhSO• (λmax = 465 nm) is significantly blue-shifted in compar-
ison with those in CH3SO• (530 nm)63 and CF3SO• (550 nm).38
due to low IR intensities.
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Figure 5. (A) IR spectrum of Ar-matrix isolated HVFP products
of a mixture of PhS(O)Cl (5), O2, and Ar (1:50:1000). (B) IR
difference spectrum reflecting the change of the Ar-matrix isolat-
ed HVFP products of a mixture of PhS(O)Cl (5) and O2 (1:50)
upon 130 min UV light irradiation (365 nm). IR bands of PhSO•
(3), PhOS• (4), SO2 (9), OSOO (10), and PhO• (11) are labeled.
Upon irradiation with UV light (365 nm, Figure 5B), deple-
tion of PhSO• (3) occurs and yields PhOS• (4), SO2 (9), and
PhO• (11, 1553.1, 1491.3, 787.2, and 633.3 cm–1).65 The ab-
sence of the IR bands for the O2-trapping product of PhSO• (3)
or derivatives suggests that no photooxidation happens. In-
stead, PhSO• (3) prefers isomerization to PhOS• (4), and the
latter combines O2 in the same matrix cages to furnish
PhOSOO• with immediate photofragmentation to PhO• (11)
and open-chain SOO, and the last species isomerizes to SO2
under the irradiation conditions (Figures S2-S3).69
Figure 4. (A) UV/Vis spectra of the HVFP products of Ph(O)CF3
in solid Ar-matrices at 12 K with 30 min deposition. (B) UV/Vis
spectra of the HVFP products of Ph(O)CF3 in solid Ar-matrices at
12 K with 90 min deposition. (C) Ten-fold expansion of the spec-
tra (B).
After the UV light irradiation (365 nm) of PhSO• (3), two
weak but distinguishable absorptions at 502 and 433 nm ap-
pear in the UV/Vis spectrum (Figure 4), which reasonably
account for the computed transitions at 522 (f = 0.0775) and
450 nm (f = 0.0131) for the expected isomer PhOS• (4). Fur-
thermore, the theoretically predicted absorptions at 693 nm (f
= 0.0131) for the photolysis products 2-hydroxyphenylthiyl
radical (6) and thioketoketene radical (8) at 570 nm (f =
0.0127) were also observed at 632 and 576 nm, respectively.
Quantum chemical computations. By referring to the in-
tensively explored mechanism for the decomposition of
PhOO•,42 the pathways for the isomerization of PhSO• were
computationally investigated (Figure 6).
In line with the general higher thermal stability of the sulfi-
nyl (RSO•) than the oxathiyl form (ROS•, R = H,48 CH3,40 F,70
and CF338), PhSO• (3) is lower in energy than PhOS• (4) by
12.8 kcal mol–1 at the CCSD(T)/cc-pVTZ//B3LYP/def2-
TZVPP level of theory. Importantly, the activation barrier
(TS1) associated with the conversion from 3 to 4 is formidable
(69.1 kcal mol–1). Considering the large C–S bond dissociation
energy (88.3 kcal mol–1), 3 should be thermally persistent in
the gas phase, which is consistent with the absence of any
isomerization or fragmentation species of 3 at the HVFP con-
dition (1000 K). Whereas, the 365 nm UV light (78.3 kcal
mol–1) enables the isomerization even under the matrix-
isolation condition (2.8 K).
Oxidation of PhSO• and PhOS•. The facile gas-phase
generation of PhSO• radicals allows the study of its oxidation
with molecular oxygen by pyrolysis a 1:50:1000 mixture of
Ph(O)Cl (5), O2, and Ar. The IR spectrum of the pyrolysis
products isolated at 2.8 K is shown in Figure 5a. Unlike the
efficient trapping reaction of CH3SO• with O2 (→
CH3S(O)OO•) in the gas phase,13 most PhSO• (3) radicals
survive under the pyrolysis conditions. Interestingly, traces of
SO2 (9, 1354.5 cm–1) and OSOO (10, 1231.1, 1045.1, and
597.1 cm–1)67 form, the presence of the latter implies the likely
formation of the unstable complex PhS(O)OO• followed by
fragmentation into Ph•68 and OSOO. This is consistent with
the observation of OS18O18O as the sole 18O-labeled species
among the HVFP products of a 1:50:1000 mixture of Ph(O)Cl
(5), 18O2, and Ar (Figure S1). However, none of the IR bands
for phenyl radical Ph•68 or its O2-trapping complex PhOO•41
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Figure 6. Computed potential energy profile for the isomerization of PhSO• (3) at the CCSD(T)/cc-pVTZ//B3LYP/def2-TZVPP and
B3LYP/def2-TZVPP (in parentheses) levels.
Similar to the isomerization of the phenylperoxy radical to an
epoxide radical, the photochemically generated oxathiyl radi-
cal PhOS• (4) may further isomerize to a thiirane radical (13)
by overcoming a barrier (TS2) of 44.4 kcal mol–1. The absence
of this elusive species in the photolysis of 4 indicates further
reactions, including either ring-expansion (TS4) to a seven-
membered ring intermediate (14) or formation of an elusive 2-
thiolphenoxy radical (15) via hydrogen-shift (TS3). The latter
is slightly more favorable; however, the initially generated
intermediate 15 is unstable due to facile rearrangement to the
lower-energy 2-hydroxyphenylthiyl radical (6) by surmounting
a small barrier (TS6) of 4.8 kcal mol–1, which reasonably ex-
plains the experimental observation of 6 as the main photolysis
product of 4. Despite that a seven-membered ring intermediate
(2-oxepinoxy radical) has been identified during the photolysis
(> 400 nm) of PhOO•,41 the analogous sulfur intermediate 14
was not observed probably due to its sensitivity to the applied
365 nm UV light (calcd. 363 nm, f = 0.0022). Instead, its ring-
opening product 8 was tentatively identified according to the
characteristic IR band at 2133.5 cm–1 (Figure 3), although 8 is
significantly higher in energy than the ring species 14. Unlike
the formation of CO and CO2 elimination products in the pho-
tochemistry of PhOO•,43 no IR bands for fragmentation species
CO, CS, or OCS were observed in the photolysis of PhSO•.
Figure 7. Computed spin densities of PhSOx• (x = 0–2) at the
B3LYP/def2-TZVPP level.
CONCLUSIONS
The parent molecule of arylsulfinyl radicals PhSO• (3) has
been generated in the gas phase through high-vacuum flash
pyrolysis (HVFP) of precursors PhS(O)X (X = CF3 and Cl). In
addition to the IR and UV/Vis spectroscopic characterization,
the photochemistry and oxidation reaction with molecular ox-
ygen have been studied in solid Ar matrices. Three novel iso-
mers, phenoxathiyl radical PhOS• (4), 2-hydroxyphenylthiyl
radical (6), and an acyclic thioketoketene radical 8 were identi-
fied during the UV light (365 nm) photolysis of PhSO• (3). In
the gas phase, PhSO• partially combines molecular oxygen to
PhS(O)OO as followed by dissociation to Ph• and OSOO.
Whereas, the oxidation of PhOS• with O2 in cryogenic matrix
yields PhO• and SO2 under the UV light irradiation. To ac-
count for the observed isomerization reactions of PhSO• (3),
the corresponding pathways were computationally explored.
The computations suggest that a thiirane radical (13) plays the
key role in the formation of both 2-hydroxyphenylthiyl radical
(6) and the acyclic thioketoketene radical 8. Comparing to the
previous photolytic generation and complex secondary reac-
tions of PhSO• (3) in solution, its facile gas-phase generation
will aid further studies on its structure and reactivity. Addi-
tionally, the disclosed photoisomerization and oxidation is of
importance in understanding its implications in the atmospher-
ic oxidation of volatile organic sulfur compounds.
Natural population analyses (NPA) of the spin densities in
the phenyl-substituted sulfur-centered radicals PhSOx• (x = 0–
2) were computed at the B3LYP/def2-TZVPP level (Figure 7).
In accord with the recent computations for PhS• and PhSO2•,11
the densities for the unpaired electron spin in all these species
are largely localized on the heteroatoms. Similar to CF3SO•,38
the spin density in PhSO• is almost equally distributed on the S
(0.506) and O (0.379) atoms. In contrast the heavier S atom
(0.769) carries significantly more spin than the O atom (0.165)
in PhOS•.
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ASSOCIATED CONTENT
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Supporting Information
IR spectra for the 18O-labeling experiments, the computed vertical
transitions, vibrational frequencies, energies and atomic coordi-
nates for PhSO• (3) and isomers (4, 6, 8, 13, and 14). This materi-
al is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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*xqzeng@suda.edu.cn
*francisc@purdue.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
XQZ acknowledges the National Natural Science Foundation of
China (21673147 and 21422304) and the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions
(PAPD). A.K.E thanks the Fonds der Chemischen Industrie for a
scholarship.
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