First experimental observation on different ionic states of the CH3SS radical: A HeI photoelectron spectrum

Article abstract of DOI:10.1063/1.1343081

The photoelectron (PE) spectrum of methyl disulfide radical beam produced by pyrolysis was developed. Different ionic states of methyl disulfide radical were calculated to study the photoelectron spectroscopy (PES) bands and density functional theory (DFT). The analysis of the five sharp peaks of dimethyl disulfide PE spectrum showed that the G2 calculations correctly predicted the ionization energies of different ionic states and also improved the DFT calculations.

Full text of DOI:10.1063/1.1343081

First experimental observation on different ionic states of the CH 3 SS radical: A HeI
photoelectron spectrum
Ge Maofa, Wang Jing, Sun Zheng, Zhu Xinjiang, and Wang Dianxun
Citation: The Journal of Chemical Physics 114, 3051 (2001); doi: 10.1063/1.1343081
View online: http://dx.doi.org/10.1063/1.1343081
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 114, NUMBER 7
15 FEBRUARY 2001
First experimental observation on different ionic states of the CH SS
3
radical: A HeI photoelectron spectrum
Ge Maofa, Wang Jing, Sun Zheng, Zhu Xinjiang, and Wang Dianxuna)
The Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080,
People’s Republic of China
͑
Received 12 July 2000; accepted 1 December 2000͒
A continuous flowing CH SS radical beam is produced in situ by pyrolysis of CH SSCH at
3
3
3
2
15(Ϯ0.5) °C. An obvious and complete photoelectron ͑PE͒ spectrum of the CH SS radical is
3
recorded in situ for the first time. Five sharp peaks at 8.63, 9.36, 9.94, 10.29, 10.72 eV and one
broader band at 11.82 eV are observed in the PE spectrum below 12 eV. The first peak with the
lowest ionization energy, 8.63 eV, is attributed to removal of an electron from the highest occupied
ϩ
1
molecular orbital ͑HOMO͒ 5aЉ of the CH SS radical, corresponding to the CH SS (X AЈ)
3
3
2
Ϫ1
�
CH SS(X AЉ) ionization. The vibrational spacing 600Ϯ60 cm on the first peak corresponds to
3
ϩ
the S–S stretch mode excited in the CH SS cation upon photoionization process. The sharp peaks
3
3
at 9.36 and 9.94 eV come from removal of the electron from the 16aЈ orbital, leading to AЉ and
1
AЉ ionic states of the CH SS radical, and the sharp peaks at 10.29 and 10.72 eV should be the result
3
of removal of the electron from the 15aЈ orbital of the CH SS radical. To assign the photoelectron
3
spectroscopy ͑PES͒ bands of the CH SS radical, both GAUSSIAN2 ͑G2͒ and improved density
3
functional theory ͑DFT͒ calculations on different ionic states of the CH SS radical are also
3
performed. The PES experimental ionization energies of the CH SS radical agree with those
3
calculated with G2 and DFT. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1343081͔
INTRODUCTION
It is well-known that dimethyl disulfide CH SSCH is an
In this paper, we present a PES study on different ionic
states of the CH SS radical.
3
3
3
important precursor in the atmospheric sulfur chemistry
EXPERIMENT
cycle that could contribute to formation of acid rain.1 Nu-
–6
merous experimental and theoretical studies on CH SSCH3
The experiment is performed on a double-chamber UPS
machine-II which was built specifically to detect transient
species as described elsewhere.15 A continuous flowing
CH3SS beam is produced in situ by pyrolysis of CH3SSCH3
at 215(Ϯ0.5) °C in a quartz tube using a double-heater inlet
system. That is to say, the new species is produced within
1–2 cm of the photoionization region of the spectrometer,
and then it passes through the photoionization point and is
quickly pumped out. The purity of CH3SSCH3 is 99% and it
was bought from the ACROS Company. The PE spectrum of
CH3SSCH3 in the low temperature is the same as in Ref. 16.
The PE spectrum of the CH3SS radical is measured at a
3
have been also performed. The laser photofragmentation
time-of-flight mass spectroscopic technique investigated the
photodissociation process of CH SSCH . The kinetic energy
3
3
threshold measurement suggests that the CH3S2 radical
formed at the threshold of the above-mentioned process has
the CH SS structure, and gives the first ionization energy
3
7
8
8
.67Ϯ0.03 eV for the CH SS radical. In fact, Butler et al.
3
proposed first by threshold photoelectron photoion coinci-
dence ͑PEPICO͒ method that the CH S species has CH SS
3
2
3
structure.
Our laboratory has demonstrated the ability to generate a
continuous flowing beam of radicals via the pyrolysis of par-
ent species or the microwave discharge of suitable com-
ϩ
2
resolution of about 30 meV as indicated by the Ar ( P3/2)
photoelectron band. Experimental vertical ionization ener-
pounds, allowing us to perform HeI photoelectron spectros-
gies ͑I in eV͒ are calibrated by simultaneous addition of a
small amount of argon and methyl iodide to the sample.
v
copy ͑PES͒ studies on reactive open-shell species,9
–14
such
1
1
as the I ͑Ref. 9͒ and Br ͑Ref. 10͒ atoms, the NO ,
3
1
2,13
R N͑RϭCH ,C H ͒,
and CH O ͑Ref. 14͒ radicals. The
GAUSSIAN2 „G2… AND IMPROVED DFT
2
3
2
5
3
CH SS radical is produced from pyrolysis of the same pre-
CALCULATIONS
3
1
4
curssor as the CH S radical, but at lower temperature:
3
In order to assign the PES bands of the CH SS radical,
3
2
2
85͑Ϯ0.5͒ °C
G2 calculations have been performed on the AЉ ground state
•
•
CH SSCH ——→ CH S ϩCH S ,
1
3
3
3
3
of the CH SS radical in C symmetry, the ground AЈ, and
3
s
ϩ
2
15͑Ϯ0.5͒ °C
several ionic states of the CH SS cation. The G2 theoretical
procedure has been described in detail by Curtiss et al.
•
3
3
•
CH SSCH ——→ CH SS ϩCH .
17
3
3
3
Briefly, the G2 method uses MP2͑full͒/6-31G* to
get the structure, subsequent single point calculations
͓at the MP4/6-311G(d,p), MP4/6-311ϩG(d,p), MP4/6-
a͒Author to whom correspondence should be addressed; electronic mail:
wangdx@xxl.icas.ac.cn
0021-9606/2001/114(7)/3051/4/$18.00
3051
© 2001 American Institute of Physics
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1

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J. Chem. Phys., Vol. 114, No. 7, 15 February 2001
Maofa et al.
311G(2df,p), QCISD͑T͒/6-311G(d,p), and MP2/6-311
ϩG(3df,p) levels, respectively͔ are then carried out to ob-
tain total energies at the QCISD͑T͒/6-311ϩG(3df,p) level,
making certain assumptions about additivity. The additivity
approximations have been examined in a separate study and
have been found to be valid in most cases. Finally, a small
semiempirical correction, referred to as the higher level cor-
rection ͑HLC͒ is applied. This is based on the number of ␣
and ␤ electrons. The computed ionization energies ͑E in eV͒
are obtained from the difference of the total energy of the
2
resulting cation / AЉ ground state in C symmetry.
s
The density functional theory ͑DFT͒ calculations have
also been performed for getting the vertical ionization ener-
gies on different ionic states of the CH SS radical, because
3
the DFT calculation was applied successfully to the assign-
ment of the PES2,13,1b8ands for the NO3,11 R N͑R
2
1
14
ϭCH ,C H ,C H ͒,
CH S, and CH O ͑Ref. 14͒ radi-
3 3
3
2
5
3
7
cals. The DFT calculation is carried out with the Amsterdam
density functional ͑ADF͒ program package in which a den-
sity fitting procedure is used to obtain the Coulomb potential,
and an elaborate 3D numerical integration technique is em-
ployed to calculate the Hamilton matrix elements. The
Vosko–Wilk–Nusaair parametrization of uniform electron
gas data is used for the exchange-correlation energy and po-
tentials, which is usually called the local spin density ap-
proximation ͑LSDA͒. The gradient correction of exchange
energy due to Becke and that of correlation energy devel-
oped by Perdew have been performed. The molecular orbit-
als were expanded in an uncontracted triplet ␨-STO basis set
augmented by polarization basis functions.
The vertical ionization energies ͑Ev in eV͒ computed
using the DFT program are obtained from the total energy
difference of the resulting cation/neutral species in this
study. The sum method proposed by Ziegler–Rauk–
Baerends is used to calculate the total energies of singlet
states described by multideterminatal wave functions. The
geometries used in the calculation for the neutral radical are
taken from optimizational results at the UMP2/6-31G* level
of theory.
FIG. 1. HeI photoelectron spectrum of CH SS radical which is produced by
3
pyrolysis of CH SSCH at 215͑Ϯ0.5͒ °C.
3
3
photoionization studies of this species, i.e., 8.67Ϯ0.03 eV
͑Ref. 7͒ and 8.62Ϯ0.05 eV,19 respectively. This assignment
is also supported by an analysis of fine vibrational structure
on the first PES peak of this species. The vibrational spacing
Ϫ1
600Ϯ60 cm on the first PES peak of the species is close to
Ϫ1
the reported vibrational frequency 610Ϯ160 cm
of the
neutral CH3SS species,20 because the vibrational spacing on
the PES band caused by electron ionization of a nonbonding
orbital should be close to vibrational frequency with the neu-
tral species. The position of the first PES peak compares well
RESULTS AND DISCUSSION
Figure 1 gives the PE spectrum of the new species py-
rolyzed CH SSCH at 215(Ϯ0.5) °C. An expanded PE spec-
3
3
trum in which a fine vibrational structure on the first peak
could be clearly seen in the low ionization energy region
͑Ͻ12.50 eV͒ is given in Fig. 2.
From Figs. 1 and 2, it is clearly seen that the first peak at
8.63 is a sharp peak with some structures in the higher ion-
ization energy side. This phenomenon shows that the highest
occupied molecular orbital ͑HOMO͒ of the species pyro-
lyzed CH SSCH should be slightly bonding or slightly an-
3
3
tibonding orbitals, because removal of the electron from
these orbitals always leads to sharp peak with some structure
1
6
in the higher ionization energy side.
The peak at 8.63 eV is assigned to be the first PES band
of the CH SS radical, because the measured ionization en-
3
ergy (8.63Ϯ0.02 eV) is in excellent agreement with adia-
FIG. 2. The expanded PE spectrum of CH SS radical in the low ionization
3
batic ionization energy values obtained from two previous
energy region ͑Ͻ13.00 eV͒.
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J. Chem. Phys., Vol. 114, No. 7, 15 February 2001
Ionic states of CH SS
3
TABLE I. The experimental vertical ionization energies ͑Iv in eV͒, calculated ionization energies ͑E in eV͒,
according to C symmetry by using the G2 and DFT methods, and relative intensities of PES signals observed
s
from different ionic states of the CH SS radical.
3
PES
experimental
Iv(eV)
G2
energies
E͑eV͒
DFT
energies
E͑eV͒
Relative intensity
Cationic
states
This workb
Stat. ratio
a
1
AЈc
8
9
9
0.29
0.72
1.82
.63
.36
.94
8.524
9.175
10.340
10.558
11.101
12.613
8.702
9.321
10.125
10.534
11.226
12.708
3
1
3
1
3
d
d
e
e
AЉ
AЉ
AЉ
AЉ
3.05
1.00
2.97
1.00
3
1
3
1
1
1
1
AЈf
a
Ϯ0.02 eV.
Corrected for analyzer sensitivity Ϯ0.02.
Ionic state comes from electron ionization of the HOMO 5aЉ for CH SS radical.
Ionic state comes from electron ionization of the SHOMO 16aЈ for CH SS radical.
b
c
3
d
3
e
f
Ionic state comes from electron ionization of the 15aЈ orbital for CH SS radical.
3
Ionic state comes from electron ionization of the 4aЉ orbital for CH SS radical.
3
fine structure. The vibrational spacing 600Ϯ60 cmϪ1 on the
with results of both G2 and DFT calculations, and can be
1
assigned as the AЈ ionic state of the CH SS radical ͑see
3
first peak at 8.63 eV is close to the reported vibrational fre-
Ϫ1
Table I͒. Table I gives the PES experimental vertical ioniza-
tion energies ͑IV in eV͒ and corresponding G2 and DFT
computed ionization energies ͑E in eV͒ for different ionic
quency 610Ϯ160 cm of the S–S stretch mode for the neu-
2
0
tral CH SS radical.
3
An analysis for orbital character of the CH SS radical
3
states of the CH SS radical. According to the Frank–Condon
3
finds that both 16aЈ and 15aЈ orbitals of the CH SS radical
3
principle, the energies of the ionic states are given using the
are sulfur lone-pair orbitals with contributions from the two
sulfur atoms. This means that the removal of the electrons of
vertical ionization energy (I ), chosen as the maxim of the
V
PES bands, and the computed ionization energy ͑E͒ should
be obtained from the difference of total energy of the result-
both 16aЈ and 15aЈ orbitals for the CH SS radical should
3
lead to four sharp peaks on the PE spectrum, because each of
both 16aЈ and 15aЈ orbitals will lead to both triplet and
singlet states of the ion. The intensity of the PES band, cor-
ing cation/neutral radical in CS symmetry for the CH SS
3
radical. That is to say, the previous photoionization experi-
mental studies and both G2 and DFT calculations support
that the first PES peak at 8.63Ϯ0.02 eV for the species py-
rolyzed CH SSCH is the first PES peak of the CH SS radi-
3
responding to the triplet state AЉ should be three times that
1
of the singlet state AЉ, because the intensity of the PES
3
3
3
band is the representation of relative ionic statistical weights.
A careful analysis for the intensities of the four sharp
peaks at 9.36, 9.94, 10.29, and 10.72 eV finds that the inten-
sity of the peak at 9.36 eV is about 3.05 times that of the
peak at 9.94 eV, and the intensity of the peak at 10.29 eV is
cal, i.e., it represents the generation of the CH SS radical.
3
From Figs. 1 and 2, it is also seen that the four peaks at
9.36, 9.943, 10.29, and 10.72 eV which together appear on
the PE spectrum accompanied with the first peak at 8.63 eV
are very sharp. These phenomena show that ionizations cor-
responding with the four sharp peaks come also from re-
moval of the electron from nonbonding orbitals of the spe-
cies pyrolyzed CH SSCH , because the vertical ionization
2
.97 times that of the peak at 10.72 eV. That is to say, the
intensity ratio of the four sharp peaks for species pyrolyzed
CH SSCH are also in good agreement with relative ionic
3
3
3
3
statistical weight led by ionization of both 16aЈ and 15aЈ
energy ͑IV in eV͒ is equivalent to the adiabatic ionization
orbitals of the CH SS radical. The PES band centered near
3
energy ͑I in eV͒ on the PES bands.
A
1
1.82 eV could be attributed to removal of the electron from
It was also recognized that the assignment of the PES
bands for a new species can be also supported by theoretical
the 4aЉ orbital or maybe comes from removal of the elec-
trons from more orbitals in the deep valence shell of the
analysis on the new species proposed. According to C sym-
s
CH SS radical, because the band is very broad.
metry, the molecular orbitals of valence shell of the CH SS
3
3
The above assignment for the PES bands of the CH SS
radical would be in the following order of increasing energy:
3
radical is supported by the results of both G2 and DFT cal-
culations. As indicated in Table I, experimental and theoret-
ical results agree reasonably well.
2
2
2
2
2
1
ϳ13aЈ 14aЈ 4aЉ 15aЈ 16aЈ 5aЉ .
The removal of an electron from the HOMO will leave the
1
ion in the singlet state AЈ, whereas removal of an electron
In a word, from the previous other experimental
studies,7
,8,19,21
and from the analysis on the intensities of the
in turn from each of the other valence shell orbitals can leave
both triplet and singlet states of the ion.
PES peaks for the species pyrolyzed CH3SSCH3 and both G2
and DFT calculations on different ionic states of the CH3SS
radical, it is concluded that the PE spectrum presented for the
The HOMO (5aЉ) of the CH SS radical is an essentially
3
lone-pair orbital on the end sulfur atom with some S–S an-
tibonding character. This means that removal of an electron
species pyrolyzed CH SSCH3 is the PE spectrum of
3
from the HOMO (5aЉ) of the CH SS radical, leading to the
ion in singlet state AЈ, should give a sharp PES peak with
the CH SS radical, i.e., the pyrolysis of CH SSCH at
3
3
3
3
1
215͑Ϯ0.5͒ °C generates the CH SS radical.
3
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3054
J. Chem. Phys., Vol. 114, No. 7, 15 February 2001
Maofa et al.
An interesting phenomenon is also attended that the first
and Xinjiang Zhu would like to thank the Chinese Academy
of Sciences for receipt of Scholarship during the period of
this work.
2
2,23
PES band at 9.84 eV for the methyl CH radical
is not
3
seen on the PE spectrum of the species pyrolyzed
CH SSCH . A reasonable explanation may be that the CH
3
3
3
radical generated by pyrolysis of CH SSCH3 transforms
3
fastly into the ethane,
1
R. P. Wayne, Chemistry of Atmospheres ͑Clarendon, Oxford, 1991͒.
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2
CH*ϩCH*→CH CH ,
3
3
3
3
3
91, 14417 ͑1986͒.
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4
5
2
4
16
J. G. Calvert and J. N. Pitts, Photochemistry ͑Wiley, New York, 1996͒.
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alkylthio radical. The first PES band with the highest in-
tensity for the ethane generated buries below the PES band
6
7
centered near 11.82 eV of the CH SS radical.
3
CONCLUSIONS
8
9
0
1
J. J. Butler, T. Baer, Jr., and S. A. Evans, J. Am. Chem. Soc. 105, 3451
͑
1983͒.
A continuous flowing CH SS radical beam is produced
in situ by pyrolysis of CH SSCH at 215͑Ϯ0.5͒ °C. An ob-
vious and complete PE spectrum of the CH SS radical is in
situ recorded. Five sharp peaks at 8.63, 9.36, 9.94, 10.29, and
3
D. X. Wang, Y. Li, S. Li, and H. Q. Zhao, Chem. Phys. Lett. 222, 167
͑1994͒.
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3
3
1
1
3
9
7, 59 ͑1998͒.
D. X. Wang, P. Jiang, X. M. Qian, and G. Y. Hong, J. Chem. Phys. 106,
003 ͑1997͒.
1
5
1
0.72 eV, led by removal of the electrons from the HOMO
aЉ, second highest occupied molecular orbital ͑SHOMO͒
6aЈ and 15aЈ orbitals of the CH SS radical, respectively, in
3
12
C. H. Qiao, G. Y. Hong, and D. X. Wang, J. Phys. Chem. A 103, 1972
͑1999͒.
C. H. Qiao, M. F. Ge, and D. X. Wang, Chem. Phys. Lett. 305, 359
3
13
the low ionization energy region ͑Ͻ12.00 eV͒, appear clearly
on the PE spectrum of the CH SS radical. The broad band at
͑
1999͒.
3
14
X. J. Zhu, M. F. Ge, J. Wang, Z. Sun, and D. X. Wang, Angew. Chem. Int.
Ed. Engl. 39, 1940 ͑2000͒.
11.82 eV is observed and assigned. Both G2 and DFT cal-
15
culations on different ionic states of the CH SS radical are
also performed for assigning the PES bands of the CH SS
radical. The PE spectrum of the CH SS radical provides an-
other example in which the G2 calculation can predict cor-
rectly the ionization energies of different ionic states for the
species studied. And the improved DFT calculation can also
give good results for both the lowest and higher ionic states
of the radicals that are not too large.
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ACKNOWLEDGMENTS
͑
1991͒.
This project was supported by the National Natural Sci-
ence Foundation of China ͑Contract No. 29973051͒. Dr. Ge
Mao-Fa gratefully acknowledges the support of K. C. Wong
Education Foundation, Hong Kong. Zheng Sun, Jing Wang,
2
2
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23
24
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1