Conformational and spectroscopic study of xanthogen ethyl formates, ROC(S)SC(O)OCH2CH3. Isolation of CH3CH2OC(O)SH

Article abstract of DOI:10.1016/j.saa.2014.12.086

ROC(S)SC(O)OCH₂CH₃, with R = CH₃, (CH₃)₂CH and CH₃(CH₂)₂, were obtained through the reaction between potassium xanthate salts, ROC(S)SK, and ethyl chloroformate, ClC(O)OCH₂CH₃. The liquid compounds were identified and characterized by ¹H and ¹³C NMR and mass spectrometry. The conformations adopted by the molecules were studied by DFT methods. 6 conformers were theoretically predicted for R = CH₃ and (CH₃)₂CH, while the conformational flexibility of the n-propyl substituent increases the total number of feasible rotamers to 21. For the three molecules, the conformers can be associated in 3 groups, being the most stable the AS forms - the CS double bond anti (A) with respect to the CS single bond and the SC single bond syn (S) with respect to the CO double bond - followed by AA and SS conformers. The vibrational spectra were interpreted in terms of the predicted conformational equilibrium, presenting the ν(CO) spectral region signals corresponding to the three groups of conformers. A moderated pre-resonance Raman enhancement of the ν(CS) vibrational mode of CH₃(CH₂)₂OC(S)SC(O)OCH₂CH₃ was detected, when the excitation radiation approaches the energy of a n → π? electronic transition associated with the CS chromophore. UV-visible spectra in different solvents were measured and interpreted in terms of TD-DFT calculations. The unknown molecule CH₃CH₂OC(O)SH was isolated by the UV-visible photolysis of CH₃OC(S)SC(O)OCH₂CH₃ isolated in Ar matrix, and also obtained as a side-product of the reaction between potassium xanthate salts, ROC(S)SK, and ethyl chloroformate, ClC(O)OCH₂CH₃.

Full text of DOI:10.1016/j.saa.2014.12.086

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Conformational and spectroscopic study of xanthogen ethyl formates,
ROC(S)SC(O)OCH2CH3. Isolation of CH3CH2OC(O)SH
⇑
Luciana C. Juncal, Melina V. Cozzarín, Rosana M. Romano
CEQUINOR (CONICET-UNLP), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Argentina
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ROC(S)SC(O)OEt molecules present
three groups of different conformers
in equilibrium.
The syn–syn conformers are stabilized
in liquid phase by intermolecular
interactions.
The unknown CH3CH2OC(O)SH was
isolated by Ar-matrix photolysis of
CH3OC(S)SC(O)OEt.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2014
Received in revised form 7 December 2014
Accepted 17 December 2014
Available online 30 December 2014
ROC(S)SC(O)OCH2CH3, with R = CH3A, (CH3)2CHA and CH3(CH2)2A, were obtained through the reaction
between potassium xanthate salts, ROC(S)SK, and ethyl chloroformate, ClC(O)OCH2CH3. The liquid com-
pounds were identified and characterized by 1H and 13C NMR and mass spectrometry. The conformations
adopted by the molecules were studied by DFT methods. 6 conformers were theoretically predicted for
R = CH3A and (CH3)2CHA, while the conformational flexibility of the n-propyl substituent increases the
total number of feasible rotamers to 21. For the three molecules, the conformers can be associated in
3 groups, being the most stable the AS forms – the C@S double bond anti (A) with respect to the CAS sin-
gle bond and the SAC single bond syn (S) with respect to the C@O double bond – followed by AA and SS
conformers. The vibrational spectra were interpreted in terms of the predicted conformational equilib-
Keywords:
Conformations
Vibrational spectroscopy
Photochemistry
rium, presenting the
m
(C@O) spectral region signals corresponding to the three groups of conformers.
(C@S) vibrational mode of CH3(CH2)2-
A
moderated pre-resonance Raman enhancement of the
m
OC(S)SC(O)OCH2CH3 was detected, when the excitation radiation approaches the energy of a n ? p⁄ elec-
tronic transition associated with the C@S chromophore. UV–visible spectra in different solvents were
measured and interpreted in terms of TD-DFT calculations. The unknown molecule CH3CH2OC(O)SH
was isolated by the UV–visible photolysis of CH3OC(S)SC(O)OCH2CH3 isolated in Ar matrix, and also
obtained as a side-product of the reaction between potassium xanthate salts, ROC(S)SK, and ethyl chlo-
roformate, ClC(O)OCH2CH3.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
abilities as flotation collectors, particularly for metallic sulfides of
Cu(II) and Zn(II), have been known since 1927 [1,2]. The advantage
Xanthogen formates are compounds with the general formula
R1OC(S)SC(O)OR2, with R1 and R2 alkyl or aryl groups. Their
of the use of xanthogen formates instead of xanthate salts in min-
ing and metallurgical processes is related with the stability of the
former compounds in acidic media, while the salts usually need
pH above 10 to avoid decomposition [3]. The collector properties
of a series of different molecules having in common the isopropyl
⇑
Corresponding author.
E-mail address: romano@quimica.unlp.edu.ar (R.M. Romano).
http://dx.doi.org/10.1016/j.saa.2014.12.086
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
347
substituent, including not only a xanthogen formate and a xan-
thate salt but also a dixantogen, a dithiophosphate and a thiocarba-
mate, have been comparatively evaluated, concluding that the
xanthogen formate is an excellent collector for copper sulfide [4].
The role of the substituents R1 and R2 in the collector ability of
the xanthate salts and xanthogen molecules has been evaluated by
Ackerman et al. [3], finding that for the xanthates the main effect of
the alkylic group in the flotation process is the increment of the
insolubility and hydrophobicity of the adsorbed species. On the
other hand, they proposed a chelation mechanism for the interac-
tion between the xanthogens and the metallic sulfides, occurring
probably by both, the oxygen and the sulfur atoms of the carbony-
lic and thiocarbonylic groups, respectively. Although the chelate
complex is unknown, the structure and conformations of the mol-
ecules, as well as the substituents R1 and R2, are expected to play
an important role in the collector properties. A molecular confor-
mation that allows the formation of a ring involving five atoms
of the xanthogen, ꢁ ꢁ ꢁS@CASAC@Oꢁ ꢁ ꢁ, and the mineral surface,
should surely favor the chelation mechanisms.
The effect of the substituents consists mainly in an alteration of
the electronic density over the interacting O and S atoms. Recently,
a QSAR analysis of the selectivity of different xanthogen molecules
in the flotation of chalcopyrite (CuFeS2) was published [5]. Six the-
oretical descriptors were employed in this study, being the elec-
tronic density of the LUMO orbital one of the most important
descriptors that influences the flotation selectivity. In a previous
theoretical work, not only the energy and character of the LUMO
orbital but also the characteristic of the HOMO orbital are pro-
posed to explain the reactivity and consequent collector ability of
different compounds [6,7].
roformate, ClC(O)OCH2CH3. The reactants were mixed at 0 °C, and
the reaction mixtures were stirring during 5 h, allowing reaching
slowly ambient temperatures. Pure samples of ROC(S)SC(O)OCH2-
CH3 were isolated from reduced-pressure distillation after filtration
of the solids formed during the reaction as yellow liquids, and sub-
sequently purified by repeated trap-to-trap distillation in vacuum
conditions. A more detailed description of the synthesis conditions
is presented in the Supplementary material. The purity of the sam-
ples was controlled along the purification processes by GC–MS. As a
side-product of the reactions CH3CH2OC(O)SH was identified.
Gas chromatography–mass spectrometry
The GC–MS analysis was carried out on a Shimadzu QP-2010.
Details are given in Table S1 of the Supplementary material.
Figs. S1–S3 show the chromatograms obtained during the purifica-
tion processes from CCl4 solutions of approximately 200 ppm. In
the conditions specified in Table S1, the elution times were 6.9,
8.5 and 9.6 min for methyl, isopropyl and n-propyl compounds,
respectively, while the chromatographic peak assigned to CH3CH2-
OC(O)SH was observed at 5.5 min.
NMR spectroscopy
The 1H (200 MHz) and 13C (50 MHz) NMR spectra of the sam-
ples were measured at 298 K on a Varian Mercury Plus 200 spec-
trometer. Each of the compounds was dissolved in CDCl3 in a
5 mm NMR tube. Chemical shifts, d, are given in ppm relative to
TMS (d = 0 ppm).
Despite the interest in xanthogen compounds regarding their
collector properties, as described above, no structural, conforma-
tional and spectroscopic studies were found in the literature. In
this work the experimental and theoretical conformational, vibra-
tional and electronic studies of three xanthogen ethyl formates,
ROC(S)SC(O)OCH2CH3, with R = CH3A, (CH3)2CHA and CH3(CH2)2A,
are presented. The compounds were synthesized following a
reported procedure [8], and the identity and purity of the mole-
cules were checked by 1H and 13C NMR and GC–MS. CH3CH2-
OC(O)SH was detected as a side-product of the xanthogens
synthesis. To the best of our knowledge, there is no previous report
of this molecule in the literature. Only its salts were identified as a
decomposition product during the flotation process [9]. The
matrix-isolation photochemistry of the smallest molecule, CH3-
OC(S)SC(O)OCH2CH3, was also investigated. The main photochan-
nel conducts also to the formation of CH3CH2OC(O)SH.
FTIR spectroscopy
The FTIR spectra were recorded on a Nexus Nicolet instrument
equipped with either an MCTB or a DTGS detector (for the ranges
4000–400 cm-1 or 600–100 cmꢂ1, respectively) at room tempera-
ture and with a resolution of 4 cmꢂ1. The IR spectra of the neat liq-
uids were measured between KBr, CsI and polyethylene windows,
to cover the range between 4000 and 100 cmꢂ1
.
Raman spectroscopy
The FTRaman spectra were measured in a Bruker IFS 66 FTR-
aman spectrometer, using a resolution of 4 cmꢂ1, in the region
between 3500 and 100 cmꢂ1. The samples, placed in a sealed
Experimental
Materials
Reagents (ROH, with R = CH3A, (CH3)2CHA and CH3(CH2)2A,
CS2, KOH and ClC(O)OCH2CH3) were purchased reagent grade and
used without further purification. Solvents (CH3CH2OCH2CH3, CH3-
C(O)CH3, CCl4, CHCl3, CH3OH and CH3CN) were dried using molec-
ular sieves. Potassium xanthate salts, ROC(S)SK with R = CH3A,
(CH3)2CHA and CH3(CH2)2A, were prepared and purified according
to literature procedures from KOH, CS2, and the corresponding
alcohol, ROH [10].
Synthesis
Xanthogen ethyl formates, ROC(S)SC(O)OCH2CH3 with R = CH3A,
(CH3)2CHA and CH3(CH2)2A, were prepared by the reaction of the
corresponding potassium xanthate salts ROC(S)SK, and ethyl chlo-
Scheme 1. Schematic representation of the syn(S) and anti(A) orientation in terms
of the dihedral angles
molecules.
s1(S@CASAC) and s2 (CASAC@O) for ROC(S)SC(O)OEt

348
L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
Table 1
FTIR spectra were recorded on
a Nexus Nicolet instrument
Relative energies and populations of the most stable conformers of ROC(S)SC(O)OCH2-
CH3, with R = CH3 and (CH3)2CHA, calculated with the B3LYP/6-31 + G⁄
approximation.
described previously. Following deposition and IR analysis of the
resulting matrixes, the samples were exposed to broad-band UV–
visible radiation (200 6 k 6 800 nm) from a Spectra-Physics Hg–
Xe arc lamp operating at 1000 W. The output from the lamp was
limited by a water filter to absorb IR radiation and so minimize
any heating effects. The IR spectra of the matrixes with 0.5 and
0.125 cmꢂ1 resolution were then recorded at different times of
irradiation in order to monitor closely any change in the spectra.
Conformera
AS
AA
SS
AS-A
AS-G
AA-A
AA-G
SS-A
SS-G
CH3OC(S)SC(O)OCH2CH3
D
E (kcal/mol)
0.00
48.2
83.3
0.47
35.1
1.42
6.0
9.4
1.75
5.4
1.40
3.3
5.4
1.82
2.1
% (298 K)b
(CH3)2CHOC(S)SC(O)OCH2CH3
UV–visible spectroscopy
D
E (kcal/mol)
0.00
51.9
89.3
0.48
37.4
1.79
3.4
5.7
1.92
2.3
1.68
2.7
4.9
2.17
2.2
% (298 K)b
UV–visible spectra in the 200–800 nm range of solutions of the
samples in solvents of different polarity and at different concentra-
tions were recorded at room temperature on a Hewlett Packard
UV–VIS spectrometer using a 1 cm-quartz cell.
a
The first letter, A or S, refers to the anti or syn orientation of the C@S with
respect to the CAS, the second letter corresponds to the orientation of the SAC with
respect to the C@O, and the third one describes the two possible orientation of the
ethyl terminal group, anti (A) or gauche (G).
b
Calculated from
DG° according to the Boltzmann law, with 2 multiplicity for the
Theoretical calculations
gauche forms.
All of the quantum chemical calculations were performed with
the Gaussian 03 program system [13], using the B3LYP method in
combination with a 6-31+G⁄ basis set. Geometry optimizations
were sought using standard gradient techniques by simultaneous
relaxation of all the geometrical parameters. The calculated vibra-
tional properties correspond in all cases to potential energy min-
ima for which no imaginary vibrational frequency was found. The
electronic spectra were simulated using the TD-DFT formalisms
over the previously optimized structures, with a maximum of
100 states and S = 1 [14,15]..
2 mm glass capillary, were excited with a 1064 nm Nd-YAG laser.
The resonance or pre-resonance Raman effect was investigated
using an Horiba-Jobin-Yvon T64000 Raman spectrometer, with a
confocal microscope and CCD detection, employing different exci-
tation wavelength from Ar and Kr multiline lasers. The wavenum-
bers were calibrated with the 459 cmꢂ1 band of CCl4. CH3CN was
used as internal standard for the pre-resonant Raman study.
Matrix isolation and photochemistry
Results and discussion
1 L vacuum flasks containing the vapor of each of the com-
pounds were filled with ca. 500 Torr of Ar. The gas mixtures were
deposited on a CsI window cooled to ca. 15 K by means of a Displex
closed-cycle refrigerator (SHI-APD Cryogenics, model DE-202)
using the pulse deposition technique [11,12]. The matrix-isolated
Mass spectra
The most intense peaks in the mass spectrum of CH3-
OC(S)SC(O)OCH2CH3 (Fig. S4) correspond to m/z = 29 (CH3CH+2,
100%), m/z = 75 (CH3OCS+, ꢃ90%), m/z = 108 (CH3OC(S)SH+, ꢃ45%)
and m/z = 15 (CH+3, ꢃ30%). On the other hand, the most intense
ionic fragment in the mass spectrum of both propyl derivatives
(Figs. S5 and S6) is the propyl group, C3H+7, at m/z = 43. This fact
clearly denotes the higher stability of the RAO single bond for
R = CH3 compared with R = C3H7, when the molecules are exposed
to electron impact ionization. A complete list of the peaks observed
in the mass spectra of the three compounds, their relative abun-
dances and their proposed assignment is presented as Supplemen-
tary material (Table S2). The parent ion, at m/z = 180 for R = CH3
and at m/z = 208 for R = C3H7, is present in each of the spectra,
allowing the identification of the samples.
CH3CH2OC(O)SH, obtained as a side-product of the reactions
performed for the synthesis of the xanthogens, was clearly identi-
fied by its mass spectrum (presented in Fig. S7 and Table S3). The
molecular ion was observed at m/z = 106, while the most intense
peak corresponds to the CH3CH2+ fragment (m/z = 29). Several sig-
nals could be also explained by rearrangement, being the
Table 2
Relative energies and populations of the most stable conformers of CH3(CH2)2-
OC(S)SC(O)OCH2CH3 calculated with the B3LYP/6-31+G⁄ approximation.
Conformera
D
E (kcal/mol)
% (298 K)b
A-AS-A
G -AS-A
A-AS-G
0.19
0.00
0.72
0.54
0.55
0.55
0.54
1.81
1.89
2.31
2.39
2.59
2.59
2.39
1.72
1.73
2.24
2.18
2.15
2.26
2.18
29.9
27.2
13.6
4.4
4.8
4.8
4.4
1.6
1.2
1.0
0.4
0.2
0.2
0.4
1.0
2.5
0.8
0.4
0.4
0.4
0.4
G+-AS-G+
G+-AS-Gꢂ
Gꢂ-AS-G+
Gꢂ-AS-Gꢂ
A-AA-A
G -AA-A
A-AA-G
G+-AA-G+
G+-AA-Gꢂ
Gꢂ-AA-G+
Gꢂ-AA-Gꢂ
A-SS-A
G -SS-A
A-SS-G
G+-SS-G+
G+-SS-Gꢂ
Gꢂ-SS-G+
Gꢂ-SS-Gꢂ
a
The first letter, A or G, refers to the orientation of the n-propyl group, the second
letter, A or S, correspond to anti or syn orientation of the C@S with respect to the
CAS, the third letter refers to the orientation of the SAC with respect to the C@O,
and the last one describes the two possible orientation of the ethyl terminal group,
syn-anti
syn-gauche
Fig. 1. Molecular models of the most stable conformers of CH3CH2OC(O)SH
calculated with the B3LYP/6-311++G⁄⁄ approximation (syn denotes the orientation
of the CAS with respect to the C@O, and anti or gauche the conformation of the ethyl
group).
anti (A) or gauche (G).
b
Calculated from
DG° according to the Boltzmann law, with 2 multiplicity for the
gauche forms.

L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
349
the syn-anti form corresponds to a high energy conformer, and will
not be considered. The dihedral angles around each CAO bond, s3
and s4 in Scheme 1, prefer the syn conformation. These CAO bonds
have a double bond character, due to an electron delocalization,
which explain the planarity of the RAOAC(S)ASAC(O)AOAEt moi-
ety. Additionally to the three conformers previously described, the
ethyl terminal group can adopt either anti or gauche conformation,
giving a total number of six different structures. Figs. S14 and S15
of the Supplementary material depict the molecular models of the
six conformers of CH3OC(S)SC(O)OCH2CH3 and (CH3)2-
CHOC(S)SC(O)OCH2CH3, respectively, calculated with the B3LYP/
6-31+G⁄ approximation and Table 1 gives the theoretical relative
energies and abundances.
The vibrational spectra (IR and Raman) were simulated for each
of the conformers, to help in the interpretation of the experimental
spectra. From the comparative analysis of the theoretical wave-
numbers and relative IR and Raman intensities of the different con-
formers we can conclude that some of the vibrational modes are
sensitive to the conformation adopted by the molecules, but not
to the orientation of the ethyl group. For this reason, a relative
abundance of each form –SA, AA and SS–, with independence of
the conformation of the ethyl group, is also presented in Table 1.
The flexibility of the n-propyl group adds more conformational
possibilities to the CH3(CH2)2OC(S)SC(O)OCH2CH3 molecule with
respect to the ones previously described. In addition to the anti
or gauche conformation of the terminal ethyl group, the isopropyl
moiety can also adopt either anti or gauche forms. For each of the
three main conformers –SA, AA and SS– seven forms are expected,
according with the orientation of both the terminal n-propyl and
ethyl groups: A–A, A–G, G–A, G+–G+, G+–Gꢂ, Gꢂ–G+, and Gꢂ–Gꢂ.
A total of 21 conformers, with relative stabilities below 3 kcal/
mol with respect to the most stable form, were found according
to the B3LYP/6-31+G⁄ approximation. The relative energies and
percentage of each form calculated at ambient temperature are
presented in Table 2, and the molecular models are depicted in
Fig. S16 of the Supplementary material. All of the structures corre-
spond to true minima, with no imaginary frequencies in the calcu-
lated vibrational spectra.
Two conformers of CH3CH2OC(O)SH are predicted to coexist at
ambient temperature, differing in the orientation of the ethyl
group, and with the SH single bond syn with respect to the C@O
double bond. Fig. 1 shows the molecular models of the two con-
formers calculated with the B3LYP/6-311++G⁄⁄ approximation.
The most stable form presents the ethyl group in anti position,
while the second conformer – with gauche orientation – is pre-
dicted 0.48 and 0.25 kcal/mol higher in energy, according with
the B3LYP/6-311++G⁄⁄ and MP2/6-311++G⁄⁄ approximations,
respectively. In accordance with the Boltzmann law, and taking
into consideration the 2 multiplicity of the gauche form, almost
equal amount of each conformer is predicted at ambient tempera-
ture(syn-anti:syn-gauche 59:41 and 58:42 for B3LYP and MP2 cal-
culations, respectively).
Fig. 2. IR (black trace between 3500 and 400 cmꢂ1 and red trace between 600 and
100 cmꢂ1) and Raman (lower blue trace, between 3500 and 100 cmꢂ1) spectra of
liquid CH3OC(S)SC(O)OCH2CH3. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
HOC(O)SH+ (m/z = 78) the most intense peak due to a McLafferty
rearrangement.
NMR spectroscopy
The analysis of both 1H and 13C NMR spectra is important not
only as purity criteria of the compounds but also as a structural
characterization of the molecules. Table S4 lists 1H NMR chemical
shits, their multiplicities, and the 3J values of the different signals
for the three compounds. The comparison presented in this table
serves to reinforce the proposed assignment and to verify the dif-
ferent listed 3J values. Table S5 lists the corresponding 13C NMR
values for the three compounds. The 1H and 13C NMR spectra are
shown in Figs. S8–S13. Again, the comparison of the values corrob-
orates the assignment, the structure of the molecules and their
purity.
Theoretical calculations
The structures and conformations of the molecules were theo-
retically investigated using DFT (B3LYP) and MP2 methods, mainly
to help in the interpretation of the vibrational spectra in terms of
different possible conformers in equilibrium. Considering the var-
ious dihedral angles of the compounds, several conformations are
expected. Only the conformers laying 3 kcal/mol or less above
the most stable structure were taking into account for the discus-
sion, considering that higher-energy forms will not appreciable
contribute to the population.
Starting from the AC(S)ASAC(O)A central moiety, two different
conformations around each of the CAS single bond can be antici-
pated, as shown in Scheme 1. According with the calculations,
the preferred structure is the one named AS in Scheme 1, this is
the C@S double bond anti (A) with respect to the CAS single bond
and the SAC single bond in syn (S) position with respect to the C@O
double bond. This result is in agreement with the reported X-ray
structure of (CH3)2CHOC(S)SC(O)OCH3 [8]. The second and third
conformers are structures named AA and SS, respectively, while
Vibrational analysis
Fig. 2 shows the IR and Raman spectra of a liquid sample of CH3-
OC(S)SC(O)OCH2CH3 and Table 3 lists the wavenumbers observed
in these spectra and also in the IR of an Ar-isolated matrix. The
assignment of the bands has been performed with the aid of the
predictions of theoretical calculations and also by the comparison
with related molecules [14,16–20]. Although the calculations were
performed for the isolated molecule, and therefore ignoring the
intermolecular interactions present in the liquid phase, the agree-
ment between the experimental and theoretical spectra is very
good.

350
L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
Table 3
Experimental (IR and Raman) and theoretical (B3LYP/6-311++G⁄⁄) vibrational wavenumbers (cmꢂ1) of CH3OC(S)SC(O)OCH2CH3.
Experimental
FTIR liquid
B3LYP/6-311++G⁄⁄b
Assignmentc
FTRaman liquid
3031
Ar-matrixa
3034
2983
3167
3106
3103
3097
3080
3059
3040
m
m
m
m
m
m
m
m
m
m
as(CH3)methyl
as(CH2)ethyl
as(CH2)methyl
as(CH2)ethyl
(AS)
(AS)
(AS)
2968
2946
2899
2872
2846
2763
1781
1750
2945
2904
2872
2845
2762
1782
1751
(AS)
s(CH2)ethyl
s(CH3)ethyl
(AS)
(AS)
s(CH3)methyl
(AS)
(C@O)
+ m(OACH2)(AS)
(AS)
1816
1805
(C@O)(SS)
(C@O)(AS)
8
1763:1
>
>
<
1761:7
1760:4
1758:0
>
>
:
ꢀ
1719
1717
1776
m
(C@O)(AA)
1743:4
1741:1
1472
1444
1474
1445
1481.0
1488
1480
d(CH2)ethyl
d(CH3)ethyl
(AS)
(AS)
8
1451:3
>
>
>
>
<
1449:6
1448:8
1446:1
1445:0
1264:6
1263:3
>
>
>
>
:
ꢀ
1258
1138
1258
1130
1285
m
(CAO)[O–C(S)]
(AS)
1143.8
1154
1139
m
m
(CAO)[O–C(O)]
(CAO)[O–C(O)]
(AS-A)
(AS-G)
ꢀ
1130:8
1128:2
8
<
1044
1044
1062
m
(C@S)(AS)
1065:5
1062:6
1054:9
:
1010
935
1008
935
1026
941
m
m
(OACH2)(AS)
(OACH3)(AS)
8
<
962:0
960:0
952:6
:
779
557
537
499
479
464
399
335
289
249
779
573
540
508
477
464
393
327
287
250
846
658
566
507
470
453
391
338
282
258
q(CH3)(AS)
doop(C@O)(AS)
doop(C@S)(AS)
m
m
m
sCASAC(AS)
asCASAC(AS-G)
asCASAC(AS-A)
d(OACAC)ethyl
d(SACAO)(AS)
(AS)
s
s
(CACAOAC)(AS-G)
(CACAOAC)(AS-A)
a
b
c
Different matrix sites are indicated between the key symbol.
The wavenumber of the most stable form of each group is listed.
AS, AA, SS, AS-A and AS-G denote the conformers defined in Table 1 and Fig. S14.
O
R
S
O
ν
C
S
C
O
Et
ν
ν
Mineral Surface
Scheme 2. Schematic representation of the potential chelating ability of the SS
conformer of ROC(S)SC(O)OEt molecules during a flotation process.
and 1719 cmꢂ1. The same pattern is present in the Raman
spectrum. According to the theoretical vibrational shifts, and also
considering the predicted relative abundances of the different
conformers, these bands were assigned to the SS, SA, and AA
conformers, respectively (see Table 3). On the other hand, the con-
formation adopted by the terminal ethyl group has no influence in
the carbonylic wavenumber. The IR spectrum of the molecule iso-
lated in solid Ar, that freeze at approximately 15 K the conforma-
tional composition of the gas phase at ambient temperatures,
shows only two absorptions, with their corresponding matrix-site
Wavenumber (cm-1)
Fig. 3. IR spectra of liquid CH3OC(S)SC(O)OCH2CH3 between 1830 and 1650 cmꢂ1
.
As previously reported for similar molecules [14,15,18], the car-
bonylic stretching mode can be considered as a conformational
sensor. In this case, three bands are clearly observed in the IR spec-
trum of liquid CH3OC(S)SC(O)OCH2CH3 (see Fig. 3), at 1782, 1751

L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
351
Table 4
Experimental (IR and Raman) and theoretical (B3LYP/6-31+G⁄) vibrational wave-
numbers (cmꢂ1) of (CH3)2CHOC(S)SC(O)OCH2CH3.
Experimental
FTIR liquid
B3LYP/6-31 + G⁄
Assignmenta
FTRaman liquid
3319
3267
2983
2
2
m
m
m
m
m
m
m
(C@O)(AS)
(C@O)(AA)
2984
2969
2939
2926
2900
2873
2767
2733
1783
1750
1718
1451
3141
3139
3129
3125
3080
3059
3058
3056
1814
1803
1775
1519
as(CH2)ethyl
(AS)
as(CH3)isopropyl
as(CH2)ethyl
as(CH3)isopropyl
(AS)
(AS)
2938
(AS)
2905
2873
s(CH2)ethyl
(AS)
ms (CH3)ethyl
(AS)
m
m
m
m
m
s(CH3)isopropyl
s(CH3)isopropyl
(C@O)(SS)
(C@O)(AS)
(C@O)(AA)
(AS)
(AS)
1781
1748
1716
ꢀ
d(CH3)ethyl
1466
1445
(AS)
(AS)
1386
1374
1351
1333
1273
1244
1172
1134
1087
1018
899
1387
1516
1509
1395
1376
1310
1306
1173
1158
1124
1025
1019
912
872
843
806
783
667
567
501
475
d(CH2)ethyl
d(CH3)isopropyl
d(HACAO)(AS)
(AS)
1350
1332
1271
d(HACACH3)(AS)
m
m
m
m
m
m
m
m
m
(CAO)[O–C(S)]
(CAO)[O–C(S)]
(CAO)[O–C(O)]
(CAO)[O–C(O)]
(CAO)[O–isopropyl]
(C@S)(AS)
(OACH2)(AS)
s(CACAC)(AS)
(CAS)[C(O)–S]
(AS)
(AA)
1182
1144
1086
1017
899
(AS-A)
(AS-G)
Fig. 4. IR (black trace between 3500 and 400 cmꢂ1 and red trace between 600 and
100 cmꢂ1) and Raman (lower blue trace, between 3500 and 100 cmꢂ1) spectra of
liquid (CH3)2CHOC(S)SC(O)OCH2CH3. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
(AS)
874
840
800
777
697
666
562
500
464
404
378
358
324
306
238
212
850
799
778
697
667
562
505
458
403
378
354
322
300
241
210
(AS)
q
q
(CH3)(AS)
(CH3)(AS)
m(CAS)[C(S)–S]
(AS)
doop(C@O)(AS)
doop(C@S)(AS)
d(OACAS)(AS)
d(OACAC)ethyl
(AS)
d(OACAC)isopropyl
d(CACAC)isopropyl
d(OACAC)ethyl
d(OACAS)(AS)
d(SACAO)(AS)
461
457
400
378
324
294
234
(AS)
(AS)
(AS)
s
s
(CACAOAC)(AS)
(CAOAC@S)(AS)
a
AS, AA, SS, AS-A and AS-G denote the conformers defined in Table 1 and Fig. S15.
act as a chelating agent in the flotation process, as schematically
shown in Scheme 2.
Some vibrational modes are also sensitive to the conformation
adopted for the terminal ethyl group. For example, two bands are
observed for m(CAO)[O–C(O)], masCASAC, and s(CACAOAC), attribut-
able to the SA-A and SA-G conformers, as listed in Table 3.
The position of the vibration modes assigned to the stretching
of the four CAO single bonds of the molecule follows the following
trend:
Fig. 5. IR (black trace between 3400 and 400 cmꢂ1 and red trace between 600 and
100 cmꢂ1) and Raman (lower blue trace, between 3400 and 100 cmꢂ1) spectra of
liquid CH3(CH2)2OC(S)SC(O)OCH2CH3. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
m
ðC ꢂ OÞ
>
m
ðC ꢂ OÞ
>
m
ðC ꢂ OÞ
>
m
ðC ꢂ OÞ
.
½OꢂCðSÞꢄ
½OꢂCðOÞꢄ
½OꢂCH2
ꢄ
½OꢂCH3ꢄ
This trend denotes the double bond character of the CO bonds
attached to the thiocarbonylic group, in major extent, and also to
the carbonylic group, due to electronic delocalization.
splitting. These bands, at 1763 and 1743 cmꢂ1, were assigned to
the SA and AA conformers, while no signs of the third SS form were
observed. The relative high intensity of the carbonylic absorption
of the SS conformer in the liquid IR spectra compared with its pre-
dicted relative abundance, together with their absence in the IR
matrix isolated spectrum, suggests the stabilization of this form
in the liquid phase by, presumably, intermolecular interactions.
According with the discussion presented in the introduction of this
paper, the SS form would be the most appropriated conformer to
Figs. 4 and 5 present the IR and Raman spectra of a liquid sam-
ple of ROC(S)SC(O)OCH2CH3, with R = isopropylA and n-propylA,
respectively, while the experimental wavenumbers, together with
the predicted ones and a tentative assignment, are listed in Tables
4 and 5. As presented above for the methyl derivative, and based
on the purity of the compounds determined either by GC–MS
and NMR analysis, the vibrational spectra can be interpreted only
if a mixture of different conformers in equilibrium is considered

352
L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
Table 5
Experimental (IR and Raman) and theoretical (B3LYP/6-31 + G⁄) vibrational wave-
numbers (cmꢂ1) of CH3(CH2)2OC(S)SC(O)OCH2CH3.
12
10
8
Experimental
FTIR liquid
B3LYP/6-31 + G⁄
Assignmenta
FTRaman liquid
2982
2979
2971
3144
3127
3125
3120
3106
3081
3058
3043
1816
1804
m
m
m
m
m
m
m
m
m
m
as(CH2)ethyl
(AS)
as(CH2)npropyl
as(CH3)npropyl
as(CH2)ethyl
(AS)
2969
2937
(AS)
2939
2926
2906
2881
2854
(AS)
as(CH2)npropyl
s(CH2)ethyl
(AS)
(AS)
2880
2855
s(CH3)ethyl
(AS)
s(CH3)nPropyl
(C@O)(SS)
(AS)
6
1782
ꢀ
1749
(C@O)(AS)
1755
1745
ꢀ
ꢀ
460
470
480
490
500
510
1777
m
(C@O)(AA)
1723
1719
1713
1470
1711
1473
1464
1456
1444
1437
1419
1389
1377
1364
1350
1309
Laser Wavelength (nm)
1535
1524
1520
1514
1508
1500
1444
1438
1407
1395
1327
1304
1300
1296
1276
1265
1200
1174
1150
1140
1052
1039
1036
955
d(CH2)ethyl
(AS)
d(CH2)npropyl
d(CH2)npropyl
d(CH3)ethyl
Fig. 6. Raman excitation profile of the 1030 cmꢂ1 band assigned to
m(C@S) of
(G-AS)
1451
1436
CH3(CH2)2OC(S)SC(O)OCH2CH3 with respect to the 380 cmꢂ1 band of CH3CN.
(A-AS)
(AS)
d(CH3)npropyl
d(CH3)npropyl
d(CH2)ethyl
(A-AS)
(G-AS)
(see Figs. S15 and S16). Again, the carbonylic stretching spectral
regions present the more clear evidence of the presence of the
three groups of conformers in liquid phase: AS, AA, and SS. The
matrix IR spectrum of (CH3)2CHOC(S)SC(O)OCH2CH3 shows only
signals of the AS conformers (see Table 4). From the theoretical
results presented in Table 1, the relative stabilities of the second
group of conformers (AA) are smaller for the isopropyl derivative
than for the methyl one. On the other hand, the stabilization of
conformers AA and SS in liquid phase compared with gas phase
becomes evident.
1389
1379
1365
1352
1308.2
1294
1273
1268
1234
1223.8
(AS)
d(CH3)npropyl
d(CH2)ethyl
(AS)
(AS)
d(CH2)npropyl
d(CH2)npropyl
d(CH2)ethyl
(AS)
(AS)
(AS)
1279
1267
m
m
(CAO)[O–C(S)]
(CAO)[O–C(S)]
(AS)
(AA)
d(CH2)npropyl
(AS)
1233
1171
1133
1105
m
m
m
m
(CAO)[O–C(S)]
(CAO)[O–C(O)]
(CAO)[O–C(O)]
s(CACAC)(AS)
(SS)
(AA)
(AS)
1132
1110
1094
The vibrational (IR and Raman) spectra of CH3(CH2)2-
OC(S)SC(O)OCH2CH3 are composed mainly by signals arising from
the AS conformer, with some of the bands, as for example in the
q(CH2)ethyl
(AS)
1048
1029
1011
937
923
896
m
m
m
m
m
as(CACAC)(AS)
(C@S)(AS)
1030
1010
935
922
892
868
849
793
777
761
694
672
662
m
(C@O) spectral region, originated by the AA and SS forms. In some
(CAC)ethyl
(AS)
cases, bands corresponding to the A-AS, G-AS, AS-A and AS-G con-
formers can be distinguished. A complete list of the wavenumbers
and a tentative assignment is presented in Table 5.
Table 6 shows a comparison of the IR vibrational bands assigned
to the C@O, C@S, OAC(S), and OAC(O) stretching modes in the
liquid IR spectra of the three compounds. As can be observed in
the table, the values are very similar. The main difference is noticed
(OAC)npropyl
(OAC)npropyl
(A-AS)
940
909
872.7
843
818
781
769
706
682
(G-AS)
(AS)
q(CH2)npropyl
867
841
m
m
(OAC)ethyl
(CAS)[C(O)–S]
(AS)
(AS)
q
q
q
(CH2)ethyl
(CH2)npropyl
(CH2)npropyl
(AS)
776
694
(G-AS)
(A-AS)
(A-AS)
(G-AS)
m
m
(CAS)[C(S)–S]
(CAS)[C(S)–S]
for the
wavenumbers, denoting a stronger bond than in the other two
compounds. The (CAO)[O–C(S)] follows the opposite trend.
m(C@S) mode of the methyl derivative, observed at higher
ꢀ
666
doop(C@O)(AS)
669
m
665
558
507
567
504
463
455
411
386
372
325
309
250
doop(C@S)(AS)
d(OACAS)(AS)
d(OAC@S)(AS)
d(CACAC)(AS)
d(CACAO)ethyl
d(CACAO)ethyl
d(OACAC)npropyl
d(OACAS)(AS)
d(SACAO)(AS)
521
469
Pre-resonance Raman spectroscopy
471
458
419
396
379
331.5
307
The resonance or pre-resonance Raman effect was investigated
for CH3(CH2)2OC(S)SC(O)OCH2CH3. Raman spectra of 1:1 mixtures
of the compound with CH3CN, used as internal standard, were
taken using eight different excitation lines of an Ar laser between
528.7 and 457.9 nm. The intensity of the band assigned to
415
(AS-A)
(AS-G)
(AS)
308.7
249
s
(CACAOAC)(AS)
m
(C@S), at 1030 cmꢂ1, presents a moderated enhancement as the
a
AS, AA, SS, A-AS, G-AS, AS-A and AS-G denote the conformers defined in Table 2
excitation energy increases. Fig. 6 depicts the pre-resonance exci-
tation profile of the 1030 cmꢂ1 band with respect to the
380 cmꢂ1 band of CH3CN. This pre-resonant behavior is associated
with an electronic absorption observed at 376 nm, and assigned to
the n ? p⁄ of the C@S chromophore.
and Fig. S16.
Table 6
Comparison of the IR wavenumbers of the most stable conformers of
ROC(S)SC(O)OCH2CH3, with R = CH3A, (CH3)2CHA, and CH3(CH2)2A.
Electronic spectroscopy
R
m
(C@O)
m
(C@S)
m
(CAO)[O–C(S)]
m(CAO)[O–C(O)]
The UV–visible spectra of the three compounds were measured
in solutions of several concentrations in solvents with different
polarities (carbon tetrachloride, ethylic ether, chloroform, acetone,
methanol, and acetonitrile). A complete list of the absorption
CH3A
(CH3)2CHA
CH3(CH2)2A
1751
1748
1745
1044
1018
1029
1258
1273
1279
1138
1134
1133

L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
353
Table 7
Comparison of the experimental absorption maxima (k, nm) and extinction coefficients (e
, L molꢂ1 cmꢂ1) of the UV–visible spectra of ROC(S)SC(O)OCH2CH3, with R = CH3A,
(CH3)2CHA, CH3(CH2)2A with the energy and oscillator strength of the electronic transitions calculated with the TD-B3LYP/6-31+G(d) approximation.
Transition
n ? p⁄ (C@S)
p
?
p⁄ (C@S)
p
?
p⁄ (C@S)
p ?
p⁄ (C@O)
CH3OC(S)SC(O)OCH2CH3
kexp [mn]a
(
ktheor [mn]
(Oscillator strength)
361
298
273
–
216
b
eexp [L molꢂ1 cmꢂ1])a
(100)
362
(20ꢁ103)
260
(10ꢁ103)
150
188
(11ꢁ10ꢂ4
)
(20ꢁ10ꢂ2
)
)
)
(89ꢁ10ꢂ3
)
(13ꢁ10ꢂ2
)
(CH3)2CHOC(S)SC(O)OCH2CH3
kexp [mn]a
376
(54)
302
276
224
(
eexp [L molꢂ1 cmꢂ1])a
(15ꢁ103)
262
(16ꢁ103)
193
(12ꢁ103)
179
ktheor [mn]
(Oscillator strength)
366
(14ꢁ10ꢂ4
)
(19ꢁ10ꢂ2
(10ꢁ10ꢂ2
)
(10ꢁ10ꢂ2
)
CH3(CH2)2C(S)SC(O)OCH2CH3
kexp [mn]c
376
(43)
354
272
220
(
eexp [L molꢂ1 cmꢂ1])c
(88ꢁ102)
259
(53ꢁ102)
209
ktheor [mn]
(Oscillator strength)
(1ꢁ10ꢂ4
)
(19ꢁ10ꢂ2
(9ꢁ10ꢂ2
)
a
b
c
Taken from the spectrum measured in methanol solutions.
The experimental could not be measured due to overlap of the bands.
Taken from the spectrum measured in acetonitrile solutions.
e
Table 8
Energy (eV) and approximate character of selected molecular orbitals calculated using
the NBO analysis at the B3LYP/6-31+G⁄ level of approximation for the most stable SS
conformers of ROC(S)SC(O)OCH2CH3, with R = CH3A, CH3CH2A, (CH3)2CHA, and
CH3(CH2)2A.
maxima and the calculated absorption coefficients are listed in
Table S6, and some of the spectra are presented in Fig. S17.
To help in the interpretation and assignment of the absorption
maxima to the electronic transitions of the molecules, TD-DFT cal-
culations were performed. The calculated electronic spectra are
almost invariable for the different conformers of each compound.
For this reason, only the data corresponding to the most stable
form of each molecule are presented. Table 7 compares the exper-
imental maxima with the calculated ones using the TD-B3LYP/6-
31+G(d) approximation. The strongest absorptions corresponds to
MO
Character
CH3A
CH3CH2A
(CH3)2CHA
CH3(CH2)2A
LUMO+1 p⁄(C@O)
LUMO
ꢂ0.58639
ꢂ2.30992
ꢂ5.85738
ꢂ7.68267
ꢂ0.62149
ꢂ2.33580
ꢂ5.91806
ꢂ7.71695
ꢂ0.65197
ꢂ2.38828
ꢂ5.96703
ꢂ7.75641
ꢂ0.61414
ꢂ2.31917
ꢂ5.90527
ꢂ7.69981
p⁄(C@S)
npS
HOMO
HOMO-2
npO
p
?
p⁄ transitions localized at the C@O and C@S groups. A weak
absorption, appearing at 361 nm for the compound with
R = CH3A and at 376 nm for each of the propyl derivatives, is
assigned to an n ? p⁄ electronic transition localized at the C@S
group, according with the TD-DFT calculations, and also in agree-
ment with the assignment for related molecules [16]. The Raman
enhancement of the C@S stretching vibrational mode in pre-reso-
nance with this electronic transition reinforces the assignment.
Considering that the energy and character of the HOMO and
LUMO orbitals of xanthogen compounds are proposed to be very
important for the evaluation of the reactivity, selectivity, and col-
lector ability of the molecules [5], these orbitals were predicted
using the NBO analysis. Only the SS conformers were evaluated,
since the flotation process occur most probably through these
forms, the only ones that present the possibility to form a six mem-
ber ring (see Scheme 2). Table 8 lists the energy of four frontier
orbitals calculated with the B3LYP/6-31+G⁄ approximation,
selected as the most important ones to evaluate the interaction
of the molecules with the mineral surface, of the three compounds
studied in this work. Fig. 7 depicts the schematic representation of
these molecular orbitals calculated for CH3(CH2)2OC(S)SC(O)OCH2-
CH3. For comparison purposes, the ethyl derivative was also
included in the table. The interaction between the xantogens and
the mineral surface occurs mainly through lone electron pairs
located at the sulfur and oxygen atoms of the C@S and C@O groups.
The HOMO and HOMO-2 orbitals, assigned to npS and npO, respec-
tively, are then the most relevant ones to evaluate their reactivity.
The higher the energy of the HOMO and/or HOMO-2 orbitals, the
greater the ability and selectivity of the xanthogen molecule to
form a covalent bond. On the other hand, the LUMO and LUMO+1
orbitals, with p⁄(C@S) and p⁄(C@O) character, are also important
for the interaction with d-orbitals of the metal. According
with the data listed in Table 8, the collector selectivity will be
LUMO+1
LUMO
OMO
OMO-
Fig. 7. Schematic representation of selected molecular orbitals of CH3(CH2)2-
OC(S)SC(O)OCH2CH3 calculated using the NBO analysis at the B3LYP/6-31+G⁄ level
of approximation.

354
L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
Table 9
Table 10
Band position (in cmꢂ1
)
and assignment of the IR absorptions appearing after
Experimental (liquid and Ar-matrix IR) and theoretical (B3LYP/6-311++G⁄⁄) vibra-
photolysis of CH3OC(S)SC(O)OCH3 isolated in solid Ar.
tional wavenumbers (cmꢂ1) of CH3CH2OC(O)SH.
Ar-
Molecule
Vibrational
mode
Wavenumber reported
previously
Experimental
FTIR liquid
B3LYP/6-311++G⁄⁄
Assignment
matrix
ꢀ
a
FTIR Ar-matrix
ꢀ
CO2
CO
m
m
as(O@C@O)
2346:3
2344:7 a
2339:1
2138.2b
2984
1790
3120.5
1779.8
m
m
as(CH3)
(C@O)
8
2340:2
8
<
1800:5
>
>
<
(CO)
2141:0
2138:2
2136:6
2057:5 OCS
2056:1
2054:9
2052:7
2049:7
1797:8
1795:6
1793:0
>
:
8
>
:
m
(C@O)
(C@O)
2049.3c
>
>
1386
1387.5
1351.2
1485.7
1426.6
1327.3
1196.9
d(CH3)
d(CH3)
d(CH2)
d(CH2)
>
>
>
>
>
>
<
1216.5
8
>
>
>
>
>
>
1123:4
>
>
>
>
>
>
<
1122:3
1121:3
1120:5
> 2046:9
>
:
2044:3
8
>
>
CH3CH2OC(O)SH
m
This work
1800:5
>
>
>
<
>
> 1119:5
>
:
1797:8
1795:6
1793:0
1718.0
1533:8
1532:3
1531:0
1527:6
1118:4
1110:0
1107:7
1106:2
1103:5
>
8
>
:
1103
1180.3
m
m
(CAO)[O–C@O]
>
>
<
CH2O
CS2
m
m
(C@O)
as(S@C@S)
1742.0d
1528.6e
8
>
>
:
>
>
>
>
ꢀ
ꢀ
ꢀ
>
>
<
1022
840
1035.1
951.6
677.7
(CAO)[O–CH2]
1025:7
1024:0
>
>
>
>
doop(SAH)
doop(C@O)
843:6
841:9
> 1524:2
>
:
1521:1
1387.5
CH3CH2OC(O)SH d(CH3)
SO2 as(SO2)
This work
666
667:7
666:9
ꢀ
ꢀ
m
1355:2
1354:0
1351.2
1355:6 f
1355:1
This work
1339:6 f
1335:1
a
Different matrix sites are indicated between the key symbol.
CH3CH2OC(O)SH d(CH3)
SO2
ꢀ
1334.0
mas(
34SO2)
1239.6
1216.5
CH2O
d(HACAO)
1245.1d
This work
1168.0d
were identified by their IR spectra, taken at different irradiation
times. Absorptions at 1800.5/1797.8/1795.5/1793.0 and 1110.0/
1107.7/1106.2/1103.5 cmꢂ1 were clearly formed and grown on
photolysis. From the analysis of all of the new IR signals observed
in the IR matrix spectra against the irradiation time, seven other
absorptions were found to follow the same kinetic pattern (see
Table 9). The formation of CH3CH2OC(O)SH was proposed to
explain the experimental findings in the IR spectra. As mention
in the introduction, as far as we know, no previous report of this
molecule was found in the literature. The IR wavenumbers were
compared with the ones observed in the liquid IR spectrum of a
sample obtained in this work as a side-product of the xanthogen
synthesis, and also with theoretical predictions. The experimental
and theoretical wavenumbers, together with the proposed assign-
ment were listed in Table 10. Formaldehyde was also identified as
a photoproduct, by comparison of the 1718.0, 1239.6 and 1183.0/
1179.0/1169.4 cmꢂ1 absorptions with Ar-matrix literature values
(see Table 9) [21].
Based on the identified photoproducts, the proposed photo-
chemical mechanism of CH3OC(S)SC(O)OCH2CH3 isolated in Ar-
matrix was depicted in Scheme 3. Similar mechanisms were previ-
ously reported for other molecules possessing a methyl group and
also a divalent sulfur atom, as for example CH3C(O)SX, with
X = HA, CH3A or CH3C(O)A [18]. No evidence of the CS molecule,
which IR absorption is expected in the 1280 cmꢂ1 spectral region,
is observed in the spectra taken after photolysis. However, the lack
of this band can be attributed to the low intensity of the vibrational
mode of this molecule in the IR spectrum [22].
CH3CH2OC(O)SH d(CH2)
CH2O (CAO)
8
m
1183:0
1179:0
1169:4
1123:4
1122:3
1121:3
1120:5
<
:
8
CH3CH2OC(O)SH d(CH2)
This work
>
>
>
>
>
>
<
>
>
>
>
> 1119:5
>
:
1118:5
1110:0
1107:7
1106:2
1103:5
8
CH3CH2OC(O)SH
CH3CH2OC(O)SH
m
m
(CAO)[O–C(O)]
This work
>
>
<
>
>
:
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(CAO)[O–CH2]
This work
This work
This work
1025:7
1024:0
CH3CH2OC(O)SH d(SAH)
CH3CH2OC(O)SH doop(SAH)
CH3CH2OC(O)SH doop(C@O)
843:6
841:9
843:6
841:9
This work
667:7
666:9
8
CO2
d(O@C@O)
663:6
661:9
663:8
<
663:5 a
:
662:0
a
b
c
d
e
f
Ref. [23].
Ref. [24].
Refs. [26,27].
Ref. [21].
Ref. [25].
Ref. [28].
Additionally, CO2 [23], CO [24], CS2 [25], OCS [26,27] and SO2
[28] were identified through their characteristic Ar-matrix IR
bands, presumably arising from secondary photochemical
channels.
CH3– > CH3(CH2)2– > CH3CH2– > (CH3)2CH–. The selectivity of some
of this compounds were experimentally determined, concluding
that CH3CH2– > (CH3)2CH– for ROC(S)SC(O)OCH2CH3, which is in
accordance with our predictions.
Conclusions
Matrix photochemistry
The conformational properties of three xanthogen ethyl for-
mates of general formula ROC(S)SC(O)OCH2CH3, with R = CH3A,
(CH3)2CHA and CH3(CH2)2A, were studied using theoretical
An Ar-matrix of CH3OC(S)SC(O)OCH2CH3 was irradiated with
broad-band light (200 6 k 6 800 nm) and the photolysis products

L.C. Juncal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 346–355
355
Scheme 3. Proposed photolysis main mechanism for CH3OC(S)SC(O)OCH2CH3 isolated in solid Ar.
methods and spectroscopic techniques. For all of the compounds
References
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The vibrational spectra (IR, Raman, and matrix-IR) of the mole-
cules are consistent with the presence of the three groups of con-
formers, AS, AA, and SS, and can be considered as an experimental
proof of the conformational equilibrium. Only some vibrational
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series, CH3OC(S)SC(O)OCH2CH3, was studied in matrix isolation
conditions. The previously unknown molecule, CH3CH2OC(O)SH,
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Acknowledgements
The authors thank the Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET) (PIP 4695), the Agencia Nacional
de Promoción Científica y Tecnológica (PICT 0647) and the Facultad
de Ciencias Exactas, Universidad Nacional de La Plata (UNLP – 11/
X684), for financial support.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.12.086.