Gas-phase and matrix-isolation photochemistry of methyl thioglycolate, CH3OC(O)CH2SH: Influence of the presence of molecular oxygen in the photochemical mechanisms

Article abstract of DOI:10.1016/j.jphotochem.2017.05.003

The photochemistry of methyl thioglycolate (MTG), CH₃OC(O)CH₂SH, in gas phase and in matrix isolation conditions was studied by means of FTIR spectroscopy, and the influence of the presence of molecular oxygen on the photochemical me

Full text of DOI:10.1016/j.jphotochem.2017.05.003

Accepted Manuscript
Title: Gas-phase and matrix-isolation photochemistry of
methyl thioglycolate, CH OC(O)CH SH: influence of the
3
2
presence of molecular oxygen in the photochemical
mechanisms
Authors: Yanina B. Bava, Luciana M. Tamone, Luciana C.
Juncal, Samantha Seng, Yeny A. Tob o´ n, Sophie Sobanska, A.
Lorena Picone, Rosana M. Romano
PII:
S1010-6030(16)31161-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.05.003
JPC 10630
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
6-12-2016
3-5-2017
3-5-2017
Please cite this article as: Yanina B.Bava, Luciana M.Tamone, Luciana C.Juncal,
Samantha Seng, Yeny A.Tob o´ n, Sophie Sobanska, A.Lorena Picone, Rosana
M.Romano, Gas-phase and matrix-isolation photochemistry of methyl thioglycolate,
CH3OC(O)CH2SH: influence of the presence of molecular oxygen in the
photochemical mechanisms, Journal of Photochemistry and Photobiology A:
Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.05.003
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Gas-phase and matrix-isolation photochemistry of
methyl thioglycolate, CH OC(O)CH SH: influence of the
3
2
presence of molecular oxygen in the photochemical
mechanisms
†
†
†
‡
Yanina B. Bava, Luciana M. Tamone, Luciana C. Juncal, Samantha Seng,
‡
‡
†
,†
Yeny A. Tobón, Sophie Sobanska, A. Lorena Picone, Rosana M. Romano*
†
CEQUINOR (UNLP, CCT-CONICET La Plata). Departamento de Química,
Facultad de Ciencias Exactas, Universidad Nacional de La Pla ta. Blvd. 120 N°
1
465, CC 962, La Plata (CP 1900), Argentina.
‡
Laboratoire de Spectrochimie Infrarouge et Raman, UMR CNRS 8516,
Université de Lille 1 Sciences et Technologies, Bât. C5, 59655 Villeneuve
d’Ascq Cedex, France.
Graphical Abstract
1

Highlights


The UV-visible photochemistry of CH
3
OC(O)CH
2
SH was studied by FTIR spectroscopy.
The photoproducts of the photolysis of CH
Ar are different.
3
OC(O)CH
2
SH in gas phase and isolated in solid

The photolysis of CH
3
OC(O)CH
2
SH in the presence of O
2
conducts to the formation of SO ,
2
either in gas phase or in matrix conditions.
Abstract
The photochemistry of methyl thioglycolate (MTG), CH
3
OC(O)CH SH, in gas
2
phase and in matrix isolation conditions was studied by means of FTIR
spectroscopy, and the influence of the presence of molecular oxygen on the
photochemical mechanisms was investigated. The UV-visible broad-band
photolysis of MTG in gas phase originates CH
matrix conditions the photoproducts are CH OH, H
inside the matrix cage. The photolysis of MTG in the presence of O
to the formation of SO , independent of the experimental conditions, being
CH OH, HC(O)OH, and CO the other photoproducts for the gas-phase
3
OC(O)CH
CS, and CO, interacting
conducts
3
and S
8
, while in
3
2
2
2
3
photolysis, and H
matrix conditions.
Introduction
2
CO, CH
4
, and CO
2
for the experiments performed in Ar
Thiols are widely used in free-radical photoinitiated polymerization
reactions. Several reviews on this subject were published, dedicated to
understand the role of the structure of the thiols and the presence of molecular
oxygen on the kinetic and mechanistic aspects of the polymerization process
[
1,2]. In particular, it was reported that the use of methyl thioglycolate (MTG),
CH OC(O)CH SH, results in greater photopolymerization reaction rates,
attributed to a weakening of the sulfur–hydrogen bond by hydrogen bonding of
3
2
2

the thiol group [1]. As far as we know, there are no reports on photochemical
studies of MTG, which are of fundamental importance for a complete
understanding of the mentioned photoinitiated mechanism.
On the other hand, MTG constitutes the smaller exponent of the series of
CH
3
OC(O)(CH
2
n
) SH compounds, reported as intermediated in the organic
sulfur cycle in marine environments, produced by marine phytoplankton [ 3,4].
Sulfur-containing volatile organic compounds are highly reactive and play an
important role in atmospheric chemistry, particularly in the acid rain
production and the formation of secondary aerosols [5,6]. Although the
atmospheric reactivity of sulfur-rich species in gaseous phase is well known,
their photoreactivity is still sparsely investigated. In this context, the study of
the photolysis of MTG, and also its photochemical evolution in the presence of
molecular oxygen, becomes significant for a deep comprehension of the
atmospheric cycle of sulfur.
In this paper, and as part of a general project dealing with photochemical
studies of sulfur-containing compounds with atmospheric interest, we present
the gas-phase photolysis of MTG, in absence and in presence of molecular
oxygen, followed by FTIR spectroscopy. The gas-phase studies are compared
with the photochemistry in matrix-isolation conditions, which allow the
detection of reactive intermediates of mainly unimolecular mechanisms,
constituting a very useful technique for the complete elucidation of
photochemical mechanisms. The photoproducts and intermediate species were
detected by their IR spectra, and the photochemical mechanisms were proposed.
In a recent investigation, the IR spectra of gaseous and matrix-isolated MTG
were studied [7]. The gas phase IR spectrum was interpreted by the presence of
3

the most stable conformer only, which structure was determined by Fantoni et
al by means of microwave spectroscopy [8,9,10]. On the other hand, the matrix-
isolated spectra present evidences of the presence of a high energy conformer.
Additionally, the MTG dimer was also isolated by increasing the proportion of
MTG in the matrix. According with DFT calculations, (MTG)
2
presents two
hydrogen-bond interactions, each of them between the oxygen atom of the CO
group of one of the subunit with the hydrogen of the S H group of the other
one [7].
Experimental
A commercial sample of methyl thioglycolate (MTG, Aldrich 95 %) was
purified by repeated trap-to-trap distillation in vacuum. Its purity was checked
by means of gas phase FTIR spectrum. The molecular oxygen and Ar (both from
AGA) were passed through a trap cooled to 100 °C to retain possible traces
of impurities.
The gaseous samples were prepared by standard manometric methods. The
gas phase experiments were performed in a cross-shaped glass cell, equipped
with quartz and CsI windows, which allows the measurement of FTIR spectra
during the UV-Vis irradiation. The IR spectra of the gas phase mixtures were
recorded with a resolution of 0.5 cm-1 and 1 cm , with 64 and 4 scans,
respectively, on a Nexus Nicolet instrument equipped with either an MCTB or
-1
-
1
a DTGS detector (for the ranges 4000-400 or 600-180 cm , respectively).
The matrices were prepared by pulsed depositions [11,12] of the gaseous
mixtures onto a cold CsI window (~10 K) of a Displex closed -cycle refrigerator
(
SHI-APD Cryogenics, model DE-202). In our experimental conditions, the IR
4

-
1
spectra of the matrices were measured with a resolution of 0.125 and 0.5 cm ,
with 256 and 64 scans, respectively.
The gas phase samples and the matrices were exposed to broad -band UV-
visible radiation (200 ≤ λ ≤ 800 nm) from Spectra-Physics Hg-Xe lamp
operating at 800 W in order to study the photolysis processes. The output of
the lamp was limited by a water filter to absorb infrared radiation for
minimizing any heating effect. For the gas phase experiments the FTIR spectra
were collected before, during and after the irradiation. For the matrix
experiments the FTIR spectra were recorded before and after various times of
irradiation.
Results and Discussion
Photolysis of MTG in gas phase
A gas phase sample of MTG was exposed to broad-band UV-Vis radiation
(
200 ≤ λ ≤ 800 nm) and FTIR spectra were collected before, during (more than
00 spectra with only 4 scans each were taken in every experiment to closely
1
monitor the changes occurring during the photolysis) and after irradiation.
Figure 1 shows the IR spectra of a vapor sample of MTG before and during the
photolysis at different irradiation times. Several changes were observed on IR
spectra as a consequence of the photolysis. The intensity of all the absorptions
previously assigned to MTG decreases following the same kinetic behavior.
Besides of the decay of the IR bands corresponding to MTG, the appearance of
new absorptions can be observed in the spectra. To help in the interpretation of
the spectra and the assignment of the new signals to the different
photoproducts, the IR bands were grouped together, according to their kinetic
behavior. Although some of the IR absorptions of MTG and the photoproducts
5

developed on photolysis partially overlap, making difficult the integration of
the area of the bands, plots of the IR intensities of the absorptions against the
irradiation time are presented in Figure 2.
An intermediate species, mainly characterized by two broad absorptions,
-
1
about 2570 and 1738 cm , first grows on photolysis reaching a maximum at
approximately 10 min of irradiation, and then completely disappears at the end
of the experiences. Any attempt to associate these signals with a known
compound, by comparison with literature values, failed. Considering that the
shifts, and also their relative intensities, of these bands with respect to the
-
1
corresponding absorptions of MTG in gas phase, of 27 cm for (SH) and
32 cm-1 for (C=O), coincide with the bands observed for the MTG dimer

isolated in Ar-matrix [7], the intermediate photoproduct was assigned to a
dimer of MTG. Moreover, several bands presented as shoulders of the MTG IR
absorptions, that accompanied the kinetic behavior of these two signals, can
also be identified as arising from the dimeric form of MTG. Table 1 presents
the comparison of the IR bands assigned to (MTG) formed by photolysis of
2
gas phase MTG with the ones corresponding to the IR spectrum of the liquid,
interpreted by the presence of a dimeric form, and the matrix -isolated MTG
dimer [7]. As can be observed in the table, there is a very good agreement of
the wavenumbers of the intermediate species formed by gas-phase photolysis
of MTG and its dimer. At this point, and trying to understand this unexpected
result, it is important to reiterate that the FTIR spectra were taken
simultaneously with the irradiation of the sample.
6

The main photoproduct, that continuously growth on photolysis starting
from approximately 5 min of irradiation and remains as a stable species after
photolysis, recognized by the IR absorptions centered at 1060, 1248, 1375,
-
1
1
771 and 2966 cm , among others, with their characteristic band-shapes, was
methyl acetate (CH OC(O)CH ) [13]. The S formation was also corroborated
3
3
8
by its typical yellowish deposit on the windows of the photochemical cell. The
mechanism probably initiates by the formation of free radicals through the
rupture of one chemical bond. As it is well known, the photochemistry of thiol
compounds, RSH, generally starts trough the breaking of the S H single bond,
forming RS radicals and hydrogen atoms [14]. In the present case, the
intermediacy of MTG dimer would favor the free radical mechanism, since the
SH bond is weakened in the dimeric structure by the intramolecular hydrogen
bond with the carbonylic oxygen. In coincidence with this explanation, it has
been previously reported that the photoinitiated thiol -ene polymerization using
glycolate esters results in greater reaction rates compared with the reactions
performer with other thiols. This difference was attributed to the debilitation
of the SH bond by hydrogen bonding interactions [15]. The formation of
methyl acetate can be interpreted as a desulfurization of MTG, probably
mediated by (MTG) , according to eq. 1 and 2:
2
CH
CH
3
3
OC(O)CH
OC(O)CH
2
2
SH + h → CH
S + H → CH
3
OC(O)CH
2
S + H
+ 1/8 S
(eq. 1)
(eq. 2)
3
OC(O)CH
3
8
This mechanism constitutes a very interesting result, since desulfurization
of thiols is a very important process, particularly for the petrochemical
industry. Several different methods were reported, including for example the
oxidation [16], or the reaction with coordination complexes [17]. A
7

photochemical desulfurization of some thiols using triethylphosphite and
triethylborane was also reported [18].
Alternative photochemical channels of gaseous MTG, although in much less
extent, produced carbon monoxide, methane and carbonyl sulfide, undoubtedly
identified by their characteristic gas-phase IR spectra (see Figure 1). The low
intensity of these absorptions, and also the partial overlapping with other bands
in the spectra, prevents a clear determination of their kinetic behavior.
However, from a qualitative inspection of the spectra we can conclude that
these photoproducts were most probably simultaneously formed in, at least, two
or three different photochemical channels.
Photolysis of MTG in gas phase in presence of O
The photochemical reaction in gas phase between MTG and molecular
oxygen was also studied. Mixtures of MTG:O in 1:2 proportion were prepared
2
2
using standard manometric methods. The fresh mixtures were exposed to broad-
band UV-Vis radiation. The spectra were collected before, during and after
irradiation, as explained previously. No chemical reaction between MTG and
molecular oxygen was observed in the time previous to the irradiation. Figure
3
shows typical IR spectra of a mixture of MTG:O in proportion 1:2 before
2
and during the photolysis for various irradiation times. The decay of the IR
bands corresponding to MTG can be observed together with the appearance of
new absorption in the spectra. After 42 minutes of irradiation ca. 90 % of MTG
was consumed. Two main photochannels are discerned, according to the kinetic
behavior of the IR new bands. The first one leads to the formation of sulfur
dioxide, carbon monoxide, methanol, and formic acid, detected in its dimeric
form. The photochemical mechanisms, starting by the formation of the thiyl
8

radical (eq. 1), can be tentatively described by eq. 3-7 (the detected
photoproducts are indicated in bold). Eq. 8 represents the whole photochemical
process.
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
OC(O)CH
OC(O)CH
OC(O)CH
OC(O)CH
OC(O)CH
2
2
2
2
2
SH + h → CH
3
OC(O)CH
2
S + H
SOO
+ SO
OO
OC(O) + HC(O)OH
OH + CO
+ h → SO
(eq. 1)
(eq. 3)
S + O
→ CH
OC(O)CH
2
3
2
SOO → CH
OC(O)CH

(eq. 4)
3
2
2

+ O
2
→ CH
3
OC(O)CH
2
(eq. 5)
OO → CH
(eq. 6)
3
OC(O) + H → CH
OC(O)CH SH + 2 O
(eq. 7)
3
3
2
2
2
+ HC(O)OH + CH
3
OH + CO (eq. 8)
The second mechanisms, with a slower kinetic evolution with respect to the
first one, conducts to the formation of methyl acetate, and can be attributed to
the photolysis of MTG without intervention of molecular oxygen, as previously
described by eq. 1-2. Small amount of OCS and CH
alternative mechanisms or by secondary channels.
Photolysis of MTG isolated in solid Ar
4
are also formed by
The matrix isolated MTG (proportion 1:1000, 1:750 and 1:200) was exposed
to broad-band UV-Vis radiation (200 ≤ λ ≤ 800 nm) and IR spectra of the matrix
were taken after different irradiation times (see Figure 4). Several ne w
absorptions, listed in Table 2, appeared in the FTIR spectra after photolysis.
To help in the interpretation of the experiments and the identification of the
photoproducts, the intensity of the bands were plotted as a function of the
irradiation time, and grouped together according with their kinetic behavior.
This strategy for the analysis of the results allowed the determination of three
9

different alternative photochemical channels. The plots of the intensities of
some of the absorptions against the irradiation times are presents in Figure 5.
The main mechanism, described by eq. 9, conducts to the extrusion of the
CO molecule, and the concomitant formation of methanol and
thioformaldehyde. A similar mechanism was previously observed after the
fragmentation of MTG upon electron impact, investigated by means of mass
analyzed ion kinetic energy (MIKE). The initial loss of methanol from MTG +
was followed by the loss of CO and the concomitant formation of SCH +
CH OC(O)CH SH + h → CH OH + CO + H CS (eq. 9)
The wavenumbers of the absorptions assigned to CO and CH
compared with the ones of FTIR Ar-matrix spectra of these molecules measured
by our research group. H CS was identified by its most intense absorption on
the basis of the monomeric [20] and complexed [21] reported matrix-isolated
spectra. The three species, CO, CH OH and H CS, were formed inside a matrix
[19].
2
3
2
3
2
3
OH were
2
3
2
cage, without possibility to escape from it. For this reason, and as a
consequence of intermolecular interactions, the IR spectra show visible signs
of molecular complexation, evidenced by the wavenumber shifts with respect
to the pure isolated compounds. The formation of methanol and
thioformaldehyde was previously observed after UV light irradiation of
dimethyl sulfoxide (DMSO) in low temperature solid argon matrix [ 22]. In the
first step of the photolysis DMSO isomerizes to sulfenic ester, CH
3
SOCH
3
,
which further dissociates to form both CH SHOCH and CH OHSCH
3
2
3
2
complexes. None IR band observed after photolysis of MTG could be identified
as belonging either to CH SOCH intermediate or to CH SHOCH complex.
The groups originally present in the MTG molecule, CH O– and –CH S–, that
3
3
3
2
3
2
1
0

clearly favor the formation of CH
CH SHOCH products in the photolysis of MTG.
second photochemical mechanism produces carbon diox ide and
ethanethiol, according to eq. 10.
CH OC(O)CH SH + h → CO CH
3
OH andH
2
CS, explain the lack of the
3
2
A
3
2
2
3
CH
2
SH
(eq. 10)
The products were again identified by the comparison with the IR spectra of
the pure isolated compounds in Ar matrices (Table 2). The interaction between
the two molecules is evidenced not only by the shift of the absorptions, but
also by the splitting of the bending mode of CO
2
, as a consequence of the loss
complexation [23]. Small
of degeneration of these fundamentals, typical of CO
2
amounts of OCS are also produced in a third mechanism, that presumable leads
also to the formation of dimethyl ether (eq. 11). Although the bands of
CH
3
OCH could not be discarded in the spectra of the irradiated matrices, due
3
to the expected overlapping with the absorptions of other photoproducts
produced in greater amount, their presence was not confirmed.
CH
3
OC(O)CH
2
SH + h → OCS + CH
3
OCH
3
(?)
(eq. 11)
-
1
Three other IR absorptions, at 2801, 1129 and 1064 cm , present a clear
intermediate behavior, as can be observed in Figure 5. The assignment of these
bands to one or more species is not trivial, due on one hand for the possible
hiding of some of the absorptions by other photoproducts and on the other for
the expected shift of the wavenumbers on complexation with the other
molecules hosted in the same matrix cage. One plausible explanation is the
formation of CH
3
OCH SH complexed with CO, as an intermediate product of
2
eq. 9. The synthesis of methoxymethanethiol was performed from the reaction
of thioacetic acid S-methoxymethyl ester with NaOH [24]. Although some IR
1
1

(
ATR) absorptions of a benzene solution of the compound were reported, no IR
spectrum of the pure sample was found in the literature. A close inspection of
the irradiated spectra in the region of the CO molecule seems to support this
hypothesis, since initially an absorption centered at 2130 cm -1 is formed, while
bands at 2133, 2136 and 2138 cm-1 are developed as the irradiation times
increases. These experimental findings indicate that CO is interacting with
different photoproducts, first with the intermediate and at longer times with
methanol and thioformaldehyde.
Photolysis of MTG isolated in solid Ar in presence of O
Irradiation experiments of MTG with O in solid Ar matrices were conducted
with MTG:O :Ar proportions of 1:1:200, 1:2:200 and 1:20:200. According to
2
.
2
2
the interpretation of the new IR absorptions appearing after photolysis
compiled in Table 3, the main photochemical reaction mechanism of a 1:1:200
MTG:O
CH OC(O)CH
The only effect observed in the irradiated spectra as the proportion of O
the matrix increases was the growth of the production of CO , and the
2
:Ar matrix can be represented by eq. 12:
3
2
SH + O + h → CO + SO + H
2
2
2
CO + CH
4
(eq. 12)
2
in
2
concomitant decrease of the signal assigned to CO (see Figure 6). The IR
absorptions are shifted with respect to the values of the isolated compounds, as
expected from intermolecular interactions. In this particular case, the final
production of four different species in the same matrix cage, SO
2
, H
2
CO, CH
4
and CO (or CO ), makes the interpretation of the interactions with a theoretical
2
model very difficult. Additionally, the formation of molecular complexes
between the photoproducts and oxygen molecules present in the matrix cannot
be ruled out. However, the comparison with related molecular complexes can
1
2

help in the interpretation of the IR matrix spectra. SO
2
molecule can form
molecular complexes acting either as electron density donor or acceptor, giving
place to different molecular geometries (see for example reference [ 25] and
references cited therein). In the photolysis of MTG in an Ar matrix doped with
-
1
O
2
, the asSO fundamental mode is 15 cm shifted with respect to the same
2
mode in the SO
2
isolated in the stable (S) Ar matrix site [26,27], while the
deformation vibrational mode is +5 cm-1 shifted. According to previous studies
[
25] this trend in the direction of the wavenumber shift might be indicative of
a complexation through the lone pair of one of the oxygen atom of the SO
molecule, acting as the electron donor. A complex between sulfur dioxide and
formaldehyde in O matrices was previously reported [28]. Although the bands
of the SO moiety were not informed, the authors observed a red shift in the
carbonyl absorption of H CO, coincident with our findings.
2
2
2
2
Other photochemical channels give rise to small amounts of CO and OCS.
The wavenumbers of the bands indicate that the other species hosted in the
same matrix cage were different than in the photolysis of an Ar matrix of MTG
without molecular oxygen. The absorptions of CO and OCS has been shown to
be very sensitive to the complexation, the direction of the shift with respect to
free CO depending on the complex geometry (see reference [ 29] and [30],
respectively, and reference cited therein).
Conclusions
The photochemical reaction mechanisms of MTG in gas-phase and in matrix
conditions, alone and in presence of molecular oxygen, were determined by the
FTIR detection of the photoproducts. The formation of numerous intermediate
species and/or possible molecular complexes transforms the photochemical
1
3

process description difficult and several photochannels can be considered.
Scheme 1 summarizes the main final products identified for each of these
experiences.
The photolysis of MTG in gas phase conducts to its desulfurization, mediated
by the dimeric form, and the concomitant formation of methyl acetate
(
CH
3
OC(O)CH
3
) and sulfur (S ). On the other hand, the photolysis in matrix
8
conditions, restricted mainly to unimolecular mechanisms and dominated by
low temperatures, that allow the isolation of species that are not stable at
normal conditions, originates methanol, thioformaldehyde, and carbon
monoxide, interacting inside the matrix cage. If molecular oxygen is present
during the photolysis, sulfur dioxide is formed in both conditions. While in gas
phase the other products are methanol, formic acid and carbon monoxide, in
matrix conditions formaldehyde, methane, and carbon dioxide are the
photoproducts formed together with SO . It is worthnoting that these
2
photoproducts are of high relevance for chemical processes occurring in the
atmosphere.
Acknowledgements
This work was supported by funds from the international cooperation program
ECOS-MinCyT (A13E05), Agencia Nacional Científica y Tecnológica ANPCyT
(
PICT11-0647 and PICT14-3266), Facultad de Ciencias Exactas of the
Universidad Nacional de La Plata (UNLP-11/X684), Consejo Nacional de
Investigaciones Científicas y Técnicas CONICET (PIP-0352), ) and the
"
LABEXCaPPA PIA Program (contract "ANR-11-LABX-0005-01).
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1
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[
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⋯N
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1
7

Figure 1. FTIR spectra of MTG in gas phase before and after 0.5, 2, 5, 10, 20
and 42 min of UV-visible broad-band irradiation in the 1900-3300 (top) and
6
00-1900 (bottom) cm-1 regions. The absorptions assigned to the different
species are indicated: A, MTG; B: dimer of MTG; C: CH
CO and F, OCS.
3
OC(O)CH
3
; D, CH ; E,
4
1
8

Figure 2. Plots of the selected IR intensities against the irradiation times
assigned to MTG (left), (MTG)
2
(middle), and CH
3
OC(O)CH (right) in the
3
photolysis of MTG in gas-phase.
1
9

Figure 3. FTIR spectra of a gaseous mixture of MTG:O
2
in a proportion 1:2
before and after 0.5, 1, 2, 5, 10, 20 and 42 min of UV -visible broad-band
-
1
irradiation in the 600-3500 cm region. The absorptions assigned to the
different species are indicated: A, MTG; D, CH
HC(O)OH and I, SO
4
2
; E, CO; F, OCS; G, CH
.
3
OH; H,
2
0

Figure 4. FTIR spectra of an Ar matrix initially containing MTG in a 1:1000
proportion before and after 0.5, 2, 5, 15, 40, 80 and 140 min of UV -visible
broad-band irradiation in the 600-3300 cm-1 region. The absorptions assigned
to the different species are indicated: A, MTG; G, CH
CO and K, CS
3
OH; E, CO; F, OCS; J,
2
2
.
2
1

Figure 5. Plots of the IR intensities against the irradiation times assigned to
CO (top), CH OH (middle) and to an intermediate product (bottom) in the
photolysis of an Ar matrix initially containing MTG in a 1:1000 proportion.
3
2
2

Figure 6. FTIR spectra of an Ar matrix initially containing MTG and O
2
in a
1
:20:200 MTG:O :Ar proportion before and after 0.5, 2, 4, 16, 32 and 64 min
2
-
1
of UV-visible broad-band irradiation in the 50-3500 cm region. The
absorptions assigned to the different species are indicated: A, MTG; I, SO
2
and J, CO .
2
2
3

Scheme 1. Schematic outline of the MTG photochemistry in gas phase and
isolated in solid Ar matrix, alone and in presence of molecular oxygen.
2
4

-
1
Table 1. Comparison of the vibrational wavenumbers (in cm ) of the gas phase
IR absorptions assigned to methyl thioglycolate dimer, (MTG) , produced by
photolysis of MTG, with (MTG) in liquid and isolated in solid Ar
Gas phase
Liquid phase a
2
2
Ar-matrix a
2959
Tentative assignment b
2
2
954
570
2954
2570
 (CH )
s
2


(SH) o.-p.
(SH) i.-p.
2566
1747


(C=O) o.-p.
(C=O) i.-p.
1
738
1740
1
1
435
414
1437
1414
-
-
δ (CH ); δ (CH )
3
2
i.-p.
i.-p.
δ (CH ); δ (CH )
3
2


as (CCO) i.-p.
as (CCO) o.-p.
1
1
279
197
1281
1192
1286
1198
δ (CH
2
2
)
)
i.p.
δ (CH
o.-p.
δ (CCO) i.p.
δ (CCO) o.-p.
1
033
1
004
1004
986
706
585
564
419
1012
992
709
598
564
419
δ (CSH); ν (CCO)
νs (CCO); δ (CSH)
ν (CS)
9
88
05
84
65
18
7
5
5
4
δ (CC=O); ν (CCS)
s
δ (CC=O); ν (CCS)
s
τ (CCSH)
a
b
Taken from reference [7].
as, antisymmetric stretching;  , symmetric stretching; δ, deformation; ,

s
torsion; i.-p., in phase; o.-p., out of phase.
2
5

Table 2. Wavenumbers and assignments of the IR absorptions appearing
after broadband UV-Vis photolysis of an Ar matrix containing MTG
-1
Vibrational mode a
Wavenumber (cm )
Molecule
3
3
500
530
}
 (OH)
CH
3
3
OH
OH
2
832
801
 (CH)
 (CH)
CH
2
CH
3
OCH SH
2
2
2
2
346
343ꢀ
340
as (CO)
CO
2
2
2
2
2
138
136
133
130
ꢁ
ꢄ
 (CO)
CO
2
2
2
2
2
2
058
055
053
051
049
045

(C=O)
OCS
ꢃ
ꢂ
1
373
252
129
064
053
 (COH)
 (CH2)
CH
3
OH
SH
1
1
1
1
CH CH
3
2
 (O CH
 (CH
 (CO)
 (CH
 (CO
ν (CS)
3
)
CH
3
3
OCH
2
SH?
2
)
CH
OCH
2
SH?
CH
3
OH
CS
CO
CH
9
9
97
92
}
}
2
)
H
2
6
6
61
53
2
)
2
5
89
CH
3
2
SH
a

s
, stretching;as, antisymmetric stretching; δ, deformation; , wagging.
2
6

Table 3. Wavenumbers and assignment of the IR absorptions appearing after
broadband UV-Vis photolysis of an Ar matrix containing MTG and O
2
-1
Vibrational mode a
s + as
Wavenumber (cm )
Molecule
3
3
703
598
CO
2
2
2 + as
CO
3
3
3
066
038ꢀ
018
as (C–H)
CH
4

as (CH
2
)
2
2
880
}
2
H CO
837

s
(CH )
2
2
2
2
2
343
}

as (CO
2
2
)
)
CO
2
342
278
13CO
}
as (CO
2
277
2
2
2
2
157
142
138
130
ꢁ
 (C=O)
CO
2
047
714
500
340
265
152
 (C=O)
 (C=O)
OCS
1
1
1
1
1
H
2
2
CO
CO
 (CH
2
)
H
 (SO )
as
2
SO
CO
SO
CO
CO
SO
2
 (H–C–O)
(SO
H
2

s
2
)
2
1
1
096
085
}
oop (CO)
H
2
6
6
62
56
}
 (CO
2
)
2
5
25
as, antisymmetric stretching;
out-of-plane deformation.
 (SO2)
2
a

s
, symmetric stretching; δ, deformation; δoop
,
2
7