Sulfenic acids in the gas phase: A photoelectron study

Article abstract of DOI:10.1021/ja9511331

Thermolysis of methyl methanethiosulfinate and methyl tert-butyl sulfoxide has been studied by photoelectron (PE) spectroscopy. The electronic structure of methanesulfenic acid (1) generated from both compounds has been determined, and the thermal stability of 1 was checked. 1 appears rather stable in the gas phase, giving rise to thioformaldehyde and water at high temperature. Thermolysis of vinyl tert-butyl sulfoxide gives rise to ethanethial S-oxide (6). At the thermolysis onset, sulfine 6 is observed in a mixture with a compound identified as ethenesulfenic acid (4). These results imply either an easy isomerization of 4 to 6, in agreement with previous theoretical evaluations, or an alternative thermolysis pathway of the starting sulfoxide, directly leading to sulfine 6. The obtained PE spectra complement previous microwave and/or mass spectrometry data and provide a further insight in the electronic structure and thermal stability of sulfenic acids 1 and 4. The experimental ionization potentials are compared throughout this study with ab-initio calculated vertical ionization potentials either within Koopmans' approximation or by difference between the ionic and ground state energies.

Full text of DOI:10.1021/ja9511331

J. Am. Chem. Soc. 1996, 118, 1131-1138
1131
Sulfenic Acids in the Gas Phase: A Photoelectron Study
S. Lacombe,* M. Loudet, E. Banchereau, M. Simon, and G. Pfister-Guillouzo
Contribution from the UniVersite´ de Pau & des Pays de l’Adour, Laboratoire de Physico-Chimie
Mole´culaire, URA CNRS 474, AVenue de l’UniVersite´, 64000 PAU, France
ReceiVed April 7, 1995X
Abstract: Thermolysis of methyl methanethiosulfinate and methyl tert-butyl sulfoxide has been studied by
photoelectron (PE) spectroscopy. The electronic structure of methanesulfenic acid (1) generated from both compounds
has been determined, and the thermal stability of 1 was checked. 1 appears rather stable in the gas phase, giving rise
to thioformaldehyde and water at high temperature. Thermolysis of vinyl tert-butyl sulfoxide gives rise to ethanethial
S-oxide (6). At the thermolysis onset, sulfine 6 is observed in a mixture with a compound identified as ethenesulfenic
acid (4). These results imply either an easy isomerization of 4 to 6, in agreement with previous theoretical evaluations,
or an alternative thermolysis pathway of the starting sulfoxide, directly leading to sulfine 6. The obtained PE spectra
complement previous microwave and/or mass spectrometry data and provide a further insight in the electronic structure
and thermal stability of sulfenic acids 1 and 4. The experimental ionization potentials are compared throughout this
study with ab-initio calculated vertical ionization potentials either within Koopmans’ approximation or by difference
between the ionic and ground state energies.
Sulfur in sulfenic acids, R-S-OH, has an intermediate
oxidation number (0) and thus stands between thiols (-II) and
sulfinic acids, RSO2H (II). For this reason, sulfenic acids and
sulfenates are involved in the oxidation chain of reduced sulfur
compounds such as thiols, sulfides, and disulfides (sulfur
oxidation numbers -II and -I, respectively) and are of current
interest in organic, bioorganic,1 and atmospheric chemistry.2
Furthermore, sulfenic acids participate in the reversible thermal
rearrangment of sulfoxides bearing a â-hydrogen atom.3 On
the other hand, R,â-unsaturated sulfenic acids readily isomerize
to sulfines, which are the active compounds in the chemistry
of allium.4
In order to further investigate the structure of methanesulfenic
acid (1), we undertook a photoelectron (PE) study of flash-
vacuum thermolysis (FVT)12 of two known precursors of
CH3-S-OH (1), namely, methyl methanethiosulfinate (2) and
methyl tert-butyl sulfoxide (3).
We also attempted to characterize by this way the even more
reactive ethenesulfenic acid (4, CH2dCH-S-OH) starting from
vinyl tert-butyl sulfoxide (5), although 4 is known to isomerize
into (Z)-ethanethial S-oxide (6):13
In spite of their importance, the great instability of simple
alkyl or alkenyl sulfenic acids has hampered their direct
characterization. The elusive methanesulfenic acid was studied
in the gas phase for the first time by microwave spectroscopy5
and later by mass spectrometry.6,7 The characterization of
ethenesulfenic acid is only based on mass spectrometry stud-
ies,7,8 while heavier alkyl or aromatic sulfenic acids have been
identified by matrix-IR spectroscopy in the condensed phase.9,10
On the whole, sulfenic acids are easily trapped by alkynes to
give R,â-unsaturated sulfoxides; this indirect method allows a
reliable characterization of these compounds.11
Method
In the following, the results of these PE experiments are
reported. The assignment of the spectra of sulfenic acids 1 and
4 or of their isomer(s), based on experimental arguments, is
further supported by ab-initio calculations. Much of this
theoretical work was aimed at estimations of ionization poten-
tials (IPs) as extensive studies of the energies and geometries
of these compounds have already been carried out.8,14,15 Vertical
X Abstract published in AdVance ACS Abstracts, January 15, 1996.
(1) (a) Davis, F. A.; Billmers, R. L. J. Am. Chem. Soc. 1981, 103, 7016-
7018 and references cited. (b) Van der Broek, L. A. G. M.; Delbressine, L.
P. C.; Ottenheijm, H. C. J. In The Chemistry of sulphenic acids and
deriVatiVes; Patai, S., Ed.; J. Wiley and Sons: Chichester, 1990; pp 701-
722.
(2) Yin, F.; Grosjean, D.; Seinfeld, J. J. Atmos. Chem. 1990, 11, 309-
364, 365-399. Barnes, J.; Becker, K.; Mihalopoulos, N. J. Atmos. Chem.
1994, 18, 267-289.
(11) Bell, R.; Cottam, P.; Davies, J.; Jones, D. J. Chem. Soc., Perkin
Trans. I 1981, 2106-2115. Davis, F. A.; Yocklovich, S.; Baker, S.
Tetrahedron Lett. 1978, 97-100. Davis, F. A.; Jenkins, R. H.; Rizvi, S.
Q.; Yocklovich S. G. J. Org. Chem. 1981, 46, 3467-3474.
(12) (a) Bock, H.; Solouki, B. Angew Chem., Int. Ed. Engl. 1981, 20,
427-444. (b) Lacombe, S.; Gonbeau, D.; Cabioch, J. L.; Pellerin, B.; Denis,
J. M.; Pfister-Guillouzo, G. J. Am. Chem. Soc. 1988, 110, 6964-6967.
(13) Block, E.; Penn, R. E.; Revelle, L. K. J. Am. Chem. Soc. 1979,
101, 2200-2201.
(3) Braverman, S. In ref 1b, pp 311-359.
(4) Block, E. Angew Chem., Int. Ed. Engl. 1992, 31, 1135-1178.
(5) Penn, R. E.; Block, E.; Revelle, L. J. Am. Chem. Soc. 1978, 100,
3622-3623.
(6) Turecek, F.; Drinkwater, D. E.; Mc Lafferty, F. W. J. Am. Chem.
Soc. 1989, 111, 7696-7701.
(7) Turecek, F.; Brabec, L.; Vondrak, T.; Hanus, V.; Hajicek, J.; Havlas,
Z. Collect. Czech. Chem. Commun. 1988, 53, 2140-2158.
(8) Turecek, F.; Mc Lafferty, F.; Smith, B. J.; Radom, L. Int. J. Mass
Spectrom. Ion Processes, 1990, 101, 283-300.
(14) Turecek, F. J. Phys. Chem. 1994, 98, 3701-3706.
(15) Hoz, T.; Basch, H. In Supplement S. The chemistry of sulfur-
containing functional groups; Patai, S., Rappoport, Z., Eds.; J. Wiley and
Sons: New York, 1993; pp 1-100.
(9) Davis, F. A.; Billmers, R. L. J. Org. Chem. 1985, 50, 2593-2595.
(10) Barret, G. C. In ref 1b, pp 1-22.
0002-7863/96/1518-1131$12.00/0 © 1996 American Chemical Society

1132 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Lacombe et al.
IPs have been calculated either within Koopmans’ approxima-
tion or more precisely as differences between the ionic and
ground state energies, both obtained on the neutral ground state-
optimized geometry.
The thermolysis of all precursors was performed in an FVT
oven directly coupled to the photoelectron spectrometer; this
assembly provides an efficient detection of reactive compounds
from thermolysis.16 In all our experiments, the compounds of
interest are obtained in a mixture with FVP byproducts such as
thioformaldehyde or isobutene. In the latter case, digital
subtraction of a prerecorded spectrum of isobutene unmasks the
ionization pattern of the sulfenic acids 1 and 4 or of their
isomerization or decomposition products. Such a procedure has
already proven valuable for the PE caracterization of short-lived
species such as ClPS2,17 FPS2 and FPS,18 and N-cyanometha-
nimine,19 among others.
Before describing the PE spectra, it is worth mentioning that
on going from He I (21.21 eV) to He II (40.81 eV) excitation
energies, the bands arising from the ejection of an electron from
an orbital localized on sulfur decrease in intensity. Conversely
bands arising from the ejection of an electron from oxygen
orbitals smoothly increase. These relative intensity changes are
attributed to variations in the one- and/or two-center contribu-
tions to the photoionization cross sections of the corresponding
molecular orbitals.20 These intensity changes are particularly
useful for the assignment of the PE spectra of oxy sulfur
compounds.
Thermolysis of Methyl Methanethiosulfinate (2) and
Methyl tert-Butyl Sulfoxide (3)
The PE spectra of methyl methanethiosulfinate (2) and its
thermolysis products at 793 and 1073 K are displayed in Figure
1. The spectrum of the starting compound is characterized by
a set of three well-defined bands with decreasing intensity at
9.12, 9.51, and 10.38 eV. Three broader bands are then
observed with higher IPs, namely, 12.26, 12.98, and 14.72 eV
(Figure 1a). On going from He I to He II excitation energy,
the relative intensities of the three first bands are strongly
modified; the decrease of the intensities of the first two bands
is evident, while the intensity of the third one at 10.38 eV
remains unchanged. This observation clearly indicates that the
first two bands arise from the ionization of orbitals heavily
localized on sulfur, while the third band originates from an
orbital weakly localized on sulfur.
Compound 2 begins to decompose around 573 K and
disappears completely at 633 K, giving rise, up to 1073 K, to
a clean new spectrum with the well-known ionization pattern
of thioformaldehyde at 9.38 eV (narrow and high-intensity
band), 11.76 eV (broad signal with a vibrational structure), and
13.85 eV21 (Figure 1b). Besides these signals, five other bands
are observed at 9.01, 11.08, 12.90, 14.95, and 16.80 eV. The
He II thermolysis spectrum at 793 K indicates the obvious
decrease of the intensity of the two first bands at 9.01 and 9.38
eV and the slighter decrease of the intensity of the fourth one
Figure 1. Photoelectron spectra of (a) methyl methanethiosulfinate
(2), (b) its thermolysis products at 793 K, and (c) its thermolysis
products at 1073 K.
at 11.76 eV (the two latter assigned to thioformaldehyde
ionizations), while the intensity of the 11.08 eV band remains
roughly constant.
On further heating, this new spectrum does not drastically
change up to 1073 K. At this temperature the following changes
are observed (Figure 1c): decrease of the intensity of the 9.01
eV band resulting in a slight shift toward higher IP of this band
maximum (9.15 eV), decrease of the intensity of the 11.08 eV
band relative to those of thioformaldehyde, and appearance of
a new band at 10.44 eV and a narrow band at 12.60 eV
assignable to water.
The spectra of methyl tert-butyl sulfoxide (3) and its
thermolysis products are depicted in Figure 2. The sulfoxide 3
is characterized by two low-IP bands at 8.64 and 9.65 eV
followed by broader signals centered at 12.12 and 15.31 eV
(Figure 2a). On heating, this spectrum begins to change at 673
K and is completely different at 723 K (Figure 2b). A broad
high intensity band is now observed at 9.18 eV followed by a
set of broad signals centered at 11.10, 11.90, 12.92, and 14.94
eV. By digital subtraction of isobutene spectrum, the difference
spectrum of Figure 2e is obtained; the position and intensity of
the bands at 9.01, 11.10, 12.92, 14.94, and 16.80 eV closely
match those of the new compound obtained by thermolysis of
2 between 633 and 1073 K (Figure 1b). However, the bands
in the difference spectrum from 3 are less clear-cut than in the
thermolysis spectrum of 2, as broad shoulders on the low-IP
(16) Bourdon, F.; Ripoll, J. L.; Valle´e, Y.; Lacombe, S.; Pfister-Guillouzo,
G. J. Org. Chem. 1990, 55, 2596-2600. Bourdon, F.; Ripoll, J. L.; Valle´e,
Y.; Lacombe, S.; Pfister-Guillouzo, G. New J. Chem. 1991, 15, 533-537.
(17) Meisel, M.; Bock, H.; Solouki, B.; Kremer, M. Angew Chem., Int.
Ed. Engl. 1989, 28, 1373-1376.
(18) Bock, H.; Kremer, M.; Solouki, B.; Binnewies, M.; Meisel, M. J.
Chem. Soc., Chem. Comun. 1992, 9-11.
(19) Bock, H.; Dammel, R. Angew Chem., Int. Ed. Engl. 1987, 26, 504-
526.
(20) Schweig, A.; Thiel, W. J. Electron Spectrosc. Rel. Phenom. 1974,
3, 27-38. Schweig, A.; Thiel, W. Mol. Phys. 1974, 27, 265-268.
(21) Solouki, B.; Rosmus, P.; Bock, H. J. Am. Chem. Soc. 1976, 98,
6054-6055.

Sulfenic Acids in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1133
Figure 2. Photoelectron spectra of (a) methyl tert-butyl sulfoxide (3) and (b-c) and (d) its thermolysis products at 723, 1073, and 1233 K and
(e-g) the difference spectra of the thermolysis products at 723, 1073, and 1233 K after digital subtraction of isobutene spectrum (shaded: isobutene
spectrum).
side of the first two bands may be noticed (8.31 and 10.55 eV)
and broad signals remain around 11.90 and 14.00 eV.
eV), some remaining compound observed at lower temperature
(9.01 and 11.10 eV), unidentified bands at 10.07 and 10.44 eV,
and a sharp band at 9.38 eV which is not inconsistent with
thioformaldehyde. However, the other bands of this product
(11.76 and 13.85 eV) are not clearly distinguished due to the
presence of numerous thermolysis byproducts.
Assignment of the Spectra and Discussion. The starting
compounds are the substituted sulfoxides 2 and 3. For methyl
tert-butyl sulfoxide (3), the interpretation is straightforward and
follows from the known spectra of different sulfoxides.7,22
For methyl methanethiosulfinate (2) (Figure 1a), from the
data on sulfoxides7,22 and the He II experiment, the second (9.51
eV) and third (10.38 eV) bands are assigned to the ejection of
an electron from the orbitals at the sulfoxide group (interacting
On further heating, no changes are observed up to 853 K,
where a weak intensity band begins to increase at 10.44 eV. At
1073 K, the intensity of this 10.44 eV band still increases (Figure
2c) and is clearly observed besides the previously described
bands (9.01 and 11.10 eV) after digital subtraction of isobutene
spectrum (Figure 2f). Bands due to water ionization begin to
appear at 12.60 eV. At 1233 K the thermolysis spectrum is
totally different (Figure 2d,g): The first band is slightly shifted
toward higher IP (9.37 eV), and its shape is modified. The
second band at 11.10 eV has strongly decreased; the previously
described band at 10.44 eV is broadened with a shoulder at
10.07 eV, and the water band at 12.60 eV is clearly distin-
guished. After digital subtraction of the isobutene spectrum,
the spectrum exhibits some features assignable to water (12.60
(22) Bock, H.; Solouki, B. Angew Chem., Int. Ed. Engl. 1972, 11, 436-
438. Bock, H.; Solouki, B. Chem. Ber. 1974, 107, 2299-2318.

1134 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Lacombe et al.
Table 1. Calculated and Experimental Vertical IPs (eV) of CH3SOH (1)
Koopmans
∆E ) E(1•+) - E(1)
Gaussian 92
Gaussian 92
CIPSI
IP
attribution
AM1a
MP2/6-311G** b
MP2/6-311G** b
8.86
MP2/6-311G** b
expt
1
2
3
4
ns - πCH
nS - nO
8.74
11.31
13.03
14.42
9.63
12.08
14.38
15.68
8.92
10.65
9.01
11.08
12.92
14.95
3
σ
σ
SO, σCH
CS, σCH
a Geometry optimization at the AM1 level (d in Å, angles in deg): C-S, 1.745; S-O, 1.707; O-H1, 0.961; C-H4, 1.114; C-H5, 1.115; C-H6,
1.113; CSO, 100.70; SOH1, 106.06; H4CS, 111.51; H5CS, 106.81; H6CS, 112.25; H1OSC, 101.83; H4CSO, 42.91; H5CSO, 161.87; H6CSO, -78.93.
b Geometry optimization at the MP2/6-31G* level (first and second values in parentheses refer respectively to MP2(FULL)/6-31+G(d)14 and
experimental data5 : C-S, 1.797 (1.795, 1.806); S-O, 1.695 (1.699, 1.658); O-H1, 0.976 (0.977, 0.957); C-H4, 1.092 (1.095, -); C-H5, 1.091
(1.091, -); C-H6, 1.094 (1.095, -); CSO, 99.56 (99.3, 100.1); SOH1, 106.24 (107.8, 107.7); H4CS, 111.97 (110.4, -); H5CS, 110.35 (112, -);
H6CS, 106.77 (106.5, -) ; H1OSC, 92.38 (95.5, 93.9); H4CSO, 66.40 (56, -).
with the sulfenyl orbital for the former) and the first one (9.12
eV) to the ejection of an electron from a nearly pure lone pair
orbital at the sulfenyl sulfur.
of methanethiol,24 thus implying significant Franck-Condon
effects upon ionization. Actually, the first ionic state has been
calculated with a planar CSOH framework,14 and according to
this theoretical study, the difference between the adiabatic and
vertical ionization energies amounts to 0.46 eV. From our
measurements this difference appears similar, although the
determination of the adiabatic IP is difficult because of the low-
energy tail of the bands. As already stated by Turecek,14 the
previous mass spectrometry measurement of the first IP (9.07
eV)7, inferred from the ionizing efficiency curve of CH3SOH•+,
“did not locate the true threshold corresponding to adiabatic
ionization”; indeed, this value is much closer to our value of
the vertical IP (9.01 eV).
The other point of concern is the thermal stability of CH3-
SOH, as we observe neither isomerization nor decomposition
under 853 K. At higher temperature, the appearance of a slowly
growing new band at 10.44 eV from both precursors is noticed
(Figures 1c and 2c,f). However owing to the presence of
numerous products in our spectra, no conclusion may be drawn
as to the structure of this new compound.
Our PE experiments indicate that thioformaldehyde is ob-
tained by FVT of 2 and isobutene by FVT of 3. From the shape
and relative intensities of the bands of the other FVT product
in both cases, the formation of the same compound from these
two precursors is obvious, thiosulfinate 2 being cleaved at lower
temperature than sulfoxide 3. The accompanying FVT product
is thus most probably CH3-S-OH. The question then arises
on which isomer(s) we are dealing with.
On the basis of microwave spectrometric data,5 it has been
concluded that methanesulfenic acid exists in the hydroxyl form
1 in the gas phase rather than in its isomeric sulfoxide form
H-SO-CH3. From this experiment, and from high level of
theory geometry optimizations (MP2(FULL)/6-31+G(d)),14
methanesulfenic acid (1) is found with a gauche conformation
of the hydroxyl relative to the CSO plane (experimental HOSC
dihedral angle 93.9°). Ab-initio calculations at the MP2/6-31G*
level indicate the sulfenic form 1 to be more stable than the
sulfoxide isomer by 98.74 kJ mol-1 15
However matrix-IR
.
At still higher temperatures, our observations confirm the
previous observation of thermal decomposition of CH3SOH into
CH2dS and H2O.5 These decomposition products appear at
lower temperature from 2 (1073 K, Figure 1c) than from 3 (1233
K, Figure 2d,g).
spectroscopy studies on higher homologs of sulfenic acids have
shown the existence of both isomers in the condensed phase.9,10
On qualitative grounds, it is evident that we are not dealing
with the sulfoxide isomer between 573 and 853 K: Actually,
In order to further confirm our assignment, vertical IP
evaluations at different levels of calculations have been per-
formed for CH3SOH. Examination of our ab-initio results
(Table 1) is a good illustration of the inadequacy of Koopmans’
approximation for this molecule containing a third-row atom.
The first two orbital energies are clearly too deep after the
Gaussian 92 SCF calculation. This gives rise to overestimated
IPs by 0.62 eV for the former and 1.00 eV for the latter. This
inaccuracy may be accounted for by the neglect of polarization
and correlation effets.
A good estimate of the first vertical and adiabatic IPs has
been obtained14 from the energetic difference between the first
ionic and ground state energies at the G2 (MP2) level: IP(v)
) 9.17 eV and IP(a) ) 8.71 eV, while smaller values were
calculated at the QCISD(T)/6-311+G(3df,2p) level: IP(v) )
9.04 eV and IP(a) ) 8.58 eV. Our own estimation of the first
vertical IP at a lower level (MP2/6-311G**) amounts to 8.86
eV. However owing to the lack of symmetry of the molecule,
it is not possible to converge on the second ionic state with the
Gaussian 92 set of programs. We thus attempted to estimate
this energy within the CIPSI formalism which includes, through
variational and perturbational configuration interactions treat-
ment, both polarization and correlation effects. However in this
the separation between the first two IPs of sulfoxide species
7,22
RSOCH3 or RSOCHdCH2
is typically in the range 1-1.3
eV, whereas in the new spectrum (Figures 1b and 2b,e) the
separation is 2.07 eV. Moreover, from substituent effects, it
may be deduced that the first band of H-SO-CH3 will appear
at higher IP than the corresponding one of dimethyl sulfoxide
(9.10 eV). The obtained value of 9.01 eV definitely rules out
the occurrence of this isomer between 573 and 853 K.
The PE spectrum of methanesulfenic acid is probably close
to the one of the structurally related cyclic sulfenate, 1,2-
oxathiolane, for which the first two bands are observed at 8.51
eV (mainly nS) and 10.89 eV (mainly nO).23 The important gap
between the first two bands (2.38 eV) results from the CSOC
dihedral angle which is estimated between 30° to 50°. Ac-
cording to the shape and position of the first (9.01 eV) and
second (11.08 eV) bands (IP2 - IP1 ) 2.07 eV) in the new
compound spectrum (Figures 1b and 2b,e), and to the result of
the He II experiment at 793 K (decrease of the intensity of the
9.01 eV band, constancy of the intensity of the 11.08 eV band),
this new spectrum may thus be assigned to methanesulfenic acid
(1).
The shape of both bands appears rather broad and poorly
resolved. The shape of the former more closely matches the
one of dimethyl disulfide rather than the thin and narrow one
(24) Handbook of HeI photoelectron spectra of fundamental organic
molecules; Kimura, K., Karsumata, S., Achiba, Y., Yamazaki, T., Iwata,
S., Eds.; Japan Scientific Societies Press: Tokyo, Japan, 1981.
(23) Jorgensen, F.; Carlsen, L. Chem. Ber. 1983, 116, 2374-2377.

Sulfenic Acids in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1135
Figure 3. Photoelectron spectra of (a) vinyl tert-butyl sulfoxide (5) and (b-f) its thermolysis products at 653, 673, 853, 1073, and 1173 K and
(g-k) the difference spectra of the thermolysis products after digital subtraction of isobutene spectrum (shaded: isobutene spectrum).
case, although the first calculated IP remains of the same order
of magnitude as the previous one (8.92 eV), the second one is
too low relative to Koopmans’ approximation and amounts to
10.65 eV for an experimental value of 11.08 eV. Such an
exaggerated correction has already been encountered within this
formalism for the second A′ ionic state of phosphaethene
localized on the phosphorus lone pair.12b
The results of AM1 calculations have been included in this
table, as the calculated IPs, even within Koopmans’ approxima-
tion, appears as reliable as the more expensive and time-
consuming results of more elaborate ab-initio methods; actually
the first IP is calculated at 8.74 eV (experimental 9.01 eV) and
the second at 11.31 eV (experimental 11.08 eV) for a gauche
AM1-optimized geometry (dihedral CSOH angle 101.8° close
to the ab-initio result (95.5°)14).
evidenced for CH3SOH, these calculations nevertheless confirm
the consistency of our analysis as the experimentally observed
large gap (2.07 eV) between the first two ionic states is
calculated regardless of the algorithm used (AM1, IP2 - IP1 )
2.57 eV; Koopmans-MP2/6-311G**, IP2 - IP1 ) 2.45 eV;
CIPSI-MP2/6-311G**, IP2 - IP1 ) 1.73 eV). From these
studies, the experimental first band at 9.01 eV thus arises from
the ejection of an electron from a pure sulfenyl lone pair orbital
(perpendicular to the heavy atoms planes), and the second one
at 11.08 eV arises from the ejection of an electron from the
antibonding orbital between the sulfur and oxygen lone pairs
(in the CSO plane).
Thermolysis of Vinyl tert-Butyl Sulfoxide (5)
The PE spectra of 5 and its thermolysis products are given
in Figure 3. The spectrum of the unsaturated sulfoxide is
characterized by three low-intensity bands on the low-IP side
On the whole, while a rather approximative evaluation of the
IPs (and especially the second one) by different methods is

1136 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Lacombe et al.
at 8.70, 9.85, and 10.55 eV followed by broad signals centered
around 11.67, 12.90, and 15.20 eV (Figure 3a). (Some
remaining water, identified by a sharp band at 12.60 eV, is
observed in the spectrum of the starting product, even after
extensive trap-to-trap distillation.) This compound begins to
react at 573 K, and its spectrum is totally different at 653 K
(Figure 3b). The first band, slightly shifted toward higher IP
(8.78 eV), appears before a broader two-components signal: The
former component of this band, centered around 9.32 eV, clearly
displays a vibrational fine structure (υ ) 1370 cm-1) and may
be assigned to isobutene,24 while the latter and broader part of
the band is centered around 9.71 eV. Then, another thin band
is observed at 10.35 eV followed by broad signals around 11.50,
13.00, and 14.90 eV. This spectrum is very sensitive to the
pyrolysis temperature and is totally modified in the 653-853
K temperature range (Figure 3b-d). A careful examination of
these spectra and their difference spectra with isobutene (Figure
3g-i) allows the following conclusions to be made. (1) A first
set of two bands at 8.78 and 11.47 eV (this latter rather broad
and probably containing two ionizations) very quickly disappears
between 653 and 673 K and is no longer observed at 853 K.
These bands are assigned to compound A. (2) Another set of
four bands at 9.71, 10.35, 13.05, and 14.92 eV is already present
at 653 K (Figure 3b,g). Their intensity smoothly increases with
increasing temperature up to 853 K relative to the intensity of
the band of isobutene (9.32 eV) and of the previously described
bands of compound A (8.78 and 11.47 eV) (Figure 3b-d,g-i).
This set of three bands is attributed to compound B.
An He II experiment, performed at 873 K (i.e., at a
temperature where only compound B is present) indicates, after
subtraction of isobutene, no intensity change for the first two
bands of the spectrum (9.71 and 10.35 eV). This result implies
that these two bands do not arise from the ejection of an electron
from orbitals strongly localized on sulfur but rather from orbitals
where sulfur and oxygen both participate.
At higher temperature (1073 K), another change is still
noticed (Figure 3e,j): The 9.71 eV band decreases together with
the sharp component at 10.35 eV, and a modification of the
shape of the 9.32 and 10.35 eV bands is clearly observed. A
new weak band is detected at 8.90 eV as a shoulder on the
low-IP side of the 9.32 eV band, while the intensity of the 12.60
eV band, assignable to water, noticeably increases. The weak
intensity narrow band at 8.90 eV may tentatively be assigned
to minute amounts of thioketene,25 although the other bands of
this compound (11.32, 12.14, and 14.55 eV) are not observed
in our complex pyrolysis mixture.
Figure 4. Correlation diagram between methanesulfenic acid (1) and
ethylene and between methanethiol and ethylene (experimental IPs in
eV) (nS refers to a lone pair sulfur orbital perpendicular to the heavy
atoms plane).
with isobutene, and (3) over 1073 K where decomposition
products begin to appear which are the only thermolysis products
at 1173 K (isobutene, acetaldehyde, carbon monoxide, water,
and possibly H2S and thioketene).
Assignment of the Spectra. The spectrum of the starting
vinyl sulfoxide 5 is easily analyzed according to the previous
study on this class of compounds.7,22 The presence of isobutene
in each thermolysis spectrum makes it clear that the expected
thermolysis reaction is going on (vide supra).
The spectrum of compound B alone at 853 K (Figure 3d,i)
is more easily analyzed as it shows a close resemblance to the
spectrum of methanethial S-oxide (CH2dSdO, 7)26,27 in its
overall feature (band shapes and relative intensities). The only
noticeable difference concerns the shift of all bands to lower
IPs relative to 7: 9.71, 10.35, 13.05, and 14.92 eV vs 10.46,
10.70, 13.59, and 15.43 eV. Accordingly compound B is the
methylated derivative of 7, i.e., ethanethial S-oxide (6). This
analysis is confirmed by the He II experiment. The constancy
of the first two band intensities is in agreement with the
description of the two highest orbitals from which the corre-
sponding electrons are ejected: πnb for the former (located on
carbon, oxygen, and slighter on sulfur) and nS - nO for the
latter (located on sulfur and oxygen).26
A careful comparison of the starting product spectrum (Figure
3a) with the first two thermolysis spectra (Figure 3b,c) indicates
that between 653 and 673 K another product (A) is present in
minor amounts besides isobutene and ethanethial S-oxide (6).
Owing to the known thermolysis pathway of 5,13 and to the
fast disappearance of compound A in favor of sulfine 6 between
653 and 853 K, the tentative identification of compound A as
ethenesulfenic acid (4) has been checked. A qualitative
estimation of the first bands of ethenesulfenic (4) may be drawn
from the correlation diagram of Figure 4 starting from the
previously described PE spectrum of CH3SOH. For 4 a planar
CCSO framework with, as for parent CH3SOH, the hydroxyl
hydrogen orthogonal to the heavy atom plane (dihedral HOSC
This change is more evident at 1173 K (Figure 3f,k). At
this temperature, the 10.35 and 9.71 eV bands disappear and
the signal is completely modified to give rise to two thin bands
at 10.21 eV (acetaldehyde)24 and 10.44 eV (possibly H2S).24
Water (12.60 eV) and carbon monoxide (13.98 eV)24 are also
observed in these spectra besides isobutene (9.32 eV). At this
temperature, the band at 8.90 eV is less intense than at 1073 K.
Although the difference spectra at these temperatures (Figure
3j,k) are much less intense than at lower temperature (Figure
3g-i) and exhibit a rather poor signal-to-noise ratio, they
nevertheless allow the identification of the previously described
bands and have thus been included.
From this analysis, three temperature ranges are thus clearly
defined for the thermolysis of 5: (1) between 653 and 853 K
where two products, A (8.78 and 11.47 eV) and B (9.71, 10.35,
13.05, and 14.92 eV), are observed besides isobutene, (2)
between 853 and 1073 K where only compound B is obtained
angle 88°) is inferred from the results of Turecek and al.8
A
parallel correlation diagram is drawn for ethenethiol, the PE
spectrum of which is analyzed on the basis of a planar CCSH
arrangment.28
The comparison between the spectrum of ethenethiol and the
spectrum tentatively assigned to ethenesulfenic acid (4) further
supports our analysis. (1) The important gap between the first
(26) Block, E.; Bock, H.; Mohmand, S.; Rosmus, P.; Solouki, B. Angew
Chem., Int. Ed. Engl. 1976, 15, 383-384.
(27) Valle´e, Y.; Ripoll, J. L.; Lafon, C.; Pfister-Guillouzo, G. Can. J.
Chem. 1987, 65, 290-291.
(25) Bock, H.; Solouki, B.; Bert, G.; Rosmus, P. J. Am. Chem. Soc. 1977,
99, 1663-1664.
(28) Chin, W. S.; Mok, C. Y.; Huang, H. H. J. Electon Spectrosc. Rel.
Phenom. 1994, 67, 173-179.

Sulfenic Acids in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1137
Table 2. Total Energies in Hartree (1 Hartree ) 2625.5 kJ mol-1
)
Table 3. Calculated Vertical IPs (eV) for 4, 6, and 7a
of the Isomers of Ethenesulfenic Acid
IP
isomer
HF/6-31G* a
MP2/6/-311G** b
1
2
3
CH2dCH-S-OH (4)
CH3-CHdSdO (6)
a Geometry optimization at the HF/6-31G* level. b Geometry opti-
mization at the MP2/6-31G* level.
-550.371362
-550.356500
-551.056097
-551.057924
CH2dCH-S-OH (4)
attribution
Koopmans
∆E
nS - π
nS - nO
π + nS
8.87
8.40
8.78
12.52
12.24
expt
11.47
CH3-CHdSdO (6)
and second band (around 2.7 eV) is of the same order of
magnitude as for ethenethiol (2.73 eV) and results from a strong
interaction between the sulfur lone pair perpendicular to the
heavy atoms plane (nS ) and the π orbital in both cases. (2)
The position of the first band (8.78 eV) slightly higher than the
corresponding one of CH3SOH (9.01 eV) is expected from the
relative positions of the first bands of CH3SH (9.46 eV) and
CH2dCHSH (8.92 eV). Our value of the first vertical IP for 4
(8.78 eV) is furthermore in excellent agreement with the
previous mass spectrometric determination (8.70 eV).7 (3) The
second broad band at 11.47 eV of 4 probably contains two
ionizations, one arising from the ejection of an electron from
the π + nS orbital and the other from the nS - nO orbital in
the molecular plane.
attribution
Koopmans
∆E
πnb - πCH3 (A′′)
nS - nO (A′)
σ, πb
9.69
9.59
9.71
11.32
10.23
10.35
14.34, 14.84
expt
13.05
CH2dSdO (7)
attribution
Koopmans
∆E
π
nb (A′′)
nS - nO (A′)
11.65
σ, πb
14.72, 15.38
10.18
10.08
10.46
10.58
10.70
expt26,27
13.59
a Gaussian 92 MP2/6-311G** results on MP2/6-31G*-optimized
geometries. See ref29 for optimized geometries.
(10.46 and 10.70 eV26,27), although slightly underestimated.
Accordingly, the ∆E calculation for the first two ionic states of
sulfine 6 (9.59 and 10.23 eV) agrees satisfactorily well with
the experimental spectrum (9.71 and 10.35 eV) and further
supports our qualitative assignment: 9.71 eV (πnb), 10.35 eV
(nS - nO).
In order to further confirm this assignment, the previous
calculations of Turecek and al.8 on ethenesulfenic acid (4) and
its isomer ethanethial S-oxide (6) have been completed by IP
evaluations for both compounds (MP2/6-311G** level). In the
case of sulfine 6, a parallel calculation on parent methanethial
One conclusion clearly arises from those results: The first
vertical IP of sulfine 6 (9.59 eV) is calculated much larger than
that of ethenesulfenic acid (4) (8.40 eV). For this latter
compound , as for CH3SOH, the gap between the first and
second IPs is expected to be rather large (around 2 eV from
Table 3) if the overestimation of the energy of the second ionic
state by about 1 eV within Koopmans’ approximation is taken
into account as for CH3SOH (Table 1). These theoretical results
are thus fully consistent with the previous qualitative correlation
diagram built for ethenesulfenic acid (4) (Figure 4) and further
support our assignment. It is worth noticing that our calculated
values of the first vertical IP are either identical with those of
Turecek and al. at the MP4/6-31G* level8 for sulfine 6 (9.59
eV) or very close for ethenesulfenic acid (4) (8.40 vs 8.35 eV8).
In summary, while the formation of ethanethial S-oxide (6)
between 653 and 853 K is firmly estabished during the FVT of
5, the identification of ethenesulfenic acid (4) in a mixture with
6 at 653 K only rests on the following arguments: (1) difference
spectrum consistent with the qualitative correlation diagram built
for 4, (2) good agreement between mass spectrometry and PE
data, (3) fast disappearance of 4 in favor of sulfine 6 observed
between 653 and 673 K as already described,13,8 and (4)
consistency of the experimental and calculated IPs.
S-oxide (7) (the spectrum of which has long been known26,27
)
has been carried out to assess the accuracy of our method. First,
our energetic results and optimized geometries29 are in agree-
ment with Turecek’s previous study:8 Compounds 4 and 6 are
found almost isoenergetic, the sulfine 6 being favored by 4.8
kJ mol-1 at the highest level of theory used (Table 2). It is
worth noticing that the previously calculated activation energy
between 4 and 6 amounts to 137 kJ mol-1 (MP4(SDTQ)/6-
31G(d,p) level8) and is indicative of an easy isomerization
pathway linking 4 to 6.
As expected from both experimental and theoretical previous
statements,4,30 (Z)-6 is calculated more stable than (E)-6 by 9.86
kJ mol-1, and once again our results are in good agreement
with Turecek’s study.8 For both optimized structures 4 and 6,
the short hydroxyl hydrogen methylenic carbon (3.17 Å) or
conversely oxygen-methyl hydrogen distance (2.57 Å) is worth
noticing.29
In Table 3, different vertical IP evaluations for the three
studied compounds have been summarized. The more precise
∆E calculations between the ionic and ground states energies
could only be performed on the first IP in the case of compound
4; actually, due to the lack of symmetry of these molecule, no
separation between ionic states can be achieved and no
convergence on the second ionic state is obtained with the
Gaussian 92 set of programs. On the contrary, sulfines 6 and
7 have a Cs symmetry, and the first two ionic states (A′′ and A′
symmetry) are thus calculated.
In any case, ethenesulfenic acid (4) is hardly at all observed
by FVT of 5, contrary to sulfine 6. Two assumptions may
account for this result: (1) either fast isomerization of sulfenic
acid 4 into sulfine 6 (this assumption is supported by the
relatively low calculated activation barrier between 4 and its
more stable isomer 6 (137 kJ mol-1)8 and by previous chemical
trapping experiments of 413) or (2) a competitive thermolysis
pathway leading directly, through a six-center transition state,
to sulfine 6.
The calculated vertical IPs for model compound 7 (10.08 and
10.58 eV) are in good agreement with experimental results
(29) MP2/6-311G**-optimized geometries for (values in parentheses from
ref 8): 4, C1C2 1.339 (1.317), C2S 1.753 (1.758), SO 1.687 (1.648), OH4
0.977 (0.950), C1H 1.084 (1.073), H1C1C2 120.2 (120.2), H2C1C2 121.7
(122.2), H3C2C1 121.9 (121.6), C1C2S 126.7 (127.4), C2SO 101.3 (102.0),
SOH4 106.3 (108.8), OSC2C1 0.0 (3.6), H1C1C2S 180, H2C1C2S 0, H4OSC2
87.9 (88.1), distance C1H4 3.17; (Z)-6, C1C2 1.491 (1.498), C2S 1.634
(1.593), SO 1.499 (1.466), C2H4 1.089 (1.077), C1H 1.094 (1.086), H3C1C2
109.5 (110.5), H1C1C2 110.9 (110.2), H2C1C2 110.9, C1C2H4 122.15 (120.1),
C1C2S 124.6 (126.5), C2SO 114.3 (114.4), H3C1C2S 0 (0), H1C1C2S -120,
H2C1C2S 120 (120.7), distance H3O 2.57; 7, CS 1.628, SO 1.495, CH 1.085,
OSC 115.35, SCH1 116.1, SCH2 122.7, H1CSO 180, H2CSO 0.
(30) Block, E.; Penn, R. E.; Bazzi, A.; Cremer, D. Tetrahedron Lett.
1981, 22, 29-32.
Sulfine 6 is stable between 653 and 1073 K. Above this
temperature, new decomposition products are observed indicat-

1138 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Lacombe et al.
monitored by a microcomputer supplemented with a digital analog
converter. The spectra are calibrated on the known ionizations of xenon
(12.13 and 13.43 eV) and argon (15.76 and 15.93 eV). The IPs are
accurate within 0.02 eV. The short path thermolysis system has been
described elsewhere.33 Products are introduced at a partial pressure
around 10-2 mbar. In these conditions, it is estimated that the detection
of decomposition products with lifetimes in the 10-1-10-3 s range may
be performed.19,34
Methyl methanethiosulfinate (2) was prepared by careful oxidation
of dimethyl disulfide with m-chloroperbenzoic acid in CHCl3 at 273
K.36 Methyl tert-butyl sulfoxide (3) was prepared by oxidation of
methyl tert-butyl sulfide by hydrogen peroxide in glacial acetic acid.37
Vinyl tert-butyl sulfoxide (5) was prepared in two steps from dimethyl
sulfite. First, from a Grignard reaction between tert-butylmagnesium
chloride and dimethyl sulfite, methyl tert-butyl sulfinate was prepared.38
A second Grignard reaction was then performed between vinylmag-
nesium chloride and methyl tert-butyl sulfinate.39 All the synthesized
compounds were purified by extensive trap-to-trap distillation on a
vacuum line and identified by comparison with reported data.
Preliminary calculations were performed in the AM1 formalism with
the AMPAC set of programs40 on fully optimized geometries. Ab-
initio calculations were carried out with the Gaussian 92 program.41
Full MP2 geometry optimizations were performed at the MP2/6-31G*
level. Energies of ground and ionic states were then calculated at the
MP2 level on the MP2/6-31G* ground state geometries, using the
6-311G** basis set.
In some cases where the second ionic state could not be calculated
within this formalism due to the lack of symmetry of the molecule,
both the ground and the first two ionic states were calculated with the
CIPSI formalism42 (6-311G** basis set) on the previously established
MP2/6-31G* geometries of the ground state. In the CIPSI algorithm,
the effects of electronic correlation are estimated by configuration
interaction using a variation-perturbation method. A variational zeroth-
order wave function is built up from an iterative selection of the most
important determinants according to a threshold on the coefficients.
The perturbative step is a multireference second-order Mo¨ller-Plesset
treatment and includes all single and double excitations from the main
determinants.
ing either dehydration through intermediate 4 (1073 K) or
complete decomposition of 6 (1173 K) as already reported.13
Attempts have been made to better characterize 4 and 6.
However, tandem FVT experiments aimed at the study of the
isomerization 4 f 6 could not be performed. Actually, with
our device, such a tandem experiment can only be done in ovens
external to the photoelectron spectrometer, thus noticeably
increasing the length between the oven end and the ionization
head. In such conditions, sulfenic acids do not survive over
the increased residence time and their PE spectrum cannot be
obtained. Furthermore, an alternative route toward sulfine 6
was checked starting from ethanesulfinyl chloride, CH3CH2-
SOCl.26,31 However FVT of this latter compound gave rise to
a complex mixture of products, while HCl elimination32 over a
solid base did not lead to the seeked sulfine 6.
Conclusion
Thermolysis of thiosulfinate 2 or tert-butyl sulfoxide 3
produces the corresponding sulfenic acid 1 together with
thioformaldehyde and isobutene, respectively. Methanesulfenic
acid (1) appears rather stable in the gas phase, as only a very
slight change of its spectrum is noticed above 853 K. Above
1073 K, methanesulfenic acid (1) decomposes into thioform-
aldehyde (CH2dS) and H2O, as already observed from a
microwave experiment.5
On the other hand, thermolysis of vinyl tert-butyl sulfoxide
(5) obviously leads, besides isobutene, to ethanethial S-oxide
(6). However, another thermolysis product is present at the
lowest FVT temperatures. A body of evidence indicative of
the formation of transient ethenesulfenic acid (4) has been
presented. These results imply either a poor stability of acid 4
quickly isomerizing to 6 or an alternative decomposition
pathway of the starting sulfoxide 5 directly leading to sulfine
6.
To our knowledge, this work constitutes the first experimental
characterization of sulfenic acids 1 and 4 through their photo-
electronic spectra. The assignment of these spectra, based on
experimental grounds, has been completed by comparison with
ab-initio calculated IPs (Koopmans and ∆E). Both acids are
characterized by a large gap between the first two ionic states:
2.07 eV for 1 and around 2.7 eV for 4. (In this latter case, the
precise determination of the second IP is difficult due to poorly
resolved overlapping bands at 11.47 eV.) The first band
originates from the ejection of an electron from a lone pair
orbital heavily localized on sulfur, in interaction with the
ethylenic moiety in the case of 4. The second band arises from
the ejection of an electron from a nS - nO orbital in the heavy
atoms plane. This band is observed together with the one
originating from the ionization of the π + nS orbital in the
case of 4. Our results fairly confirm the previous mass
spectrometric determination of the first IPs of 1 and 4 by
Turecek7. Moreover the spectrum of ethanethial S-oxide (6) is
fully consistent with that of parent methanethial S-oxide 7.26,27
Acknowledgment. We thank ELF-ATOCHEM Co. for
financial support (grant for E.B.).
JA9511331
(33) Vallee, Y.; Ripoll, J. L.; Lacombe, S.; Pfister-Guillouzo, G. J. Chem.
Res. Synop. 1990, 21, 40-41; J. Chem. Res. Miniprint 401.
(34) If it is assumed that the products vaporized at ca. 10-2 mbar generate
a molecular flow, the maximum velocity of the molecules is calculated
around 102 m/s. For a more realistic velocity between 1 and 102 m/s, the
residence time of the species varies between 10-1 and 10-3 s for 10 cm
length between the top of the oven and the ionization head.19,35
(35) Pyrolytic methods in Organic Chemistry; Applications of flow and
flash-Vacuumpyrolytic techniques; Brown, R. F. C., Ed.; Academic Press
Inc.: New York, 1980; Vol. 41, pp 21-43.
(36) Small, L. D.; Bailey, J. H.; Cavallito, C. J. J. Am. Chem. Soc. 1947,
69, 1710-1713.
(37) Madesclaire, M. Tetrahedron 1986, 42, 5459-5495.
(38) Mikolajckyk, M.; Drabowicz, J. Synthesis 1974, 124-126.
(39) Rebiere, F.; Samuel, O.; Ricard, L.; Kagan, H. J. Org. Chem. 1991,
56, 5991-5999.
(40) Dewar, M. J. S.; Stewart, J. J. P. QCPE Bull. 1986, 6, 24.
(41) Frisch, M. J.; Trucks, G. W.; Head Gordon, M.; Gill, P. M.; Wong,
M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. a.;
Repogle, E. S.; Gomperts, J. L.; Andres, J. L.; Raghavachari, K.; Binkley,
J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Stewart, J. J.;
Popl, J. A. GAUSSIAN 92, Revision A; Gaussian Inc.: Pittsburg, PA, 1992.
(42) Huron, B.; Malrieu, J. P.; Rancurel, P. J. Chem. Phys. 1973, 58,
5745-5759. Pellissier, M. The`se, Universite´ Paul Sabatier, Toulouse,
France, 1980.
Experimental Section and Calculation Methods
Photoelectron spectra were recorded on an Helectros 0078 photo-
electron spectrometer equipped with a 127° cylindrical analyzer and
(31) Block, E.; Penn, R. E.; Olsen, R. J.; Sherwin, P. F. J. Am. Chem.
Soc. 1976, 98, 1264-1265.
(32) Zwanenburg, B.; Recl. J. R. Neth. Chem. Soc. 1982, 101, 1-27.