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Formation and characterization of the sulfur-containing distonic radical anion,E.CH2-SE-CHCN, in the gas phase
Born, M.; Ingemann Jorgensen, S.; Nibbering, N.M.M.
Published in:
Journal of the American Society for Mass Spectrometry
DOI:
10.1016/1044-0305(94)00089-I
Link to publication
Citation for published version (APA):
Born, M., Ingemann Jorgensen, S., & Nibbering, N. M. M. (1995). Formation and characterization of the sulfurcontaining distonic radical anion,E.CH2-S-E-CHCN, in the gas phase. Journal of the American Society for Mass
Spectrometry, 6, 71. DOI: 10.1016/1044-0305(94)00089-I
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SHORT COMMUNICATION
Formation and Characterization of the SulfurContaining Distonic Radical Anion,
CH2--S--CH--CN,
in the Gas Phase
Monique Born, Steen Ingemann, and Nico M. M. Nibbering
Institute of Mass Spectrometry, Umverslty of Amsterdam, Amsterdam, The Netherlands
The reactions of the atomic oxygen radical anion O - with C H 3 - - S - - C H 2 - - C N m the gas
phase have been examined with Fourier transform ion cyclotron resonance in combination
with tandem mass spectrometric experiments performed with a double-focusmg quadrupole
hybrid mstrument. Deuterium labelmg has revealed that the O - ion reacts with
CH3--S--CH2--CN
by proton abstraction from the methylene group as well as by
competing 1,1- and 1,3-H~ abstractions to afford isomeric radical anions. High kinetic
energy (8 keV) collision-induced charge reversal experiments mdicate that the 1,1-H~-'abstraction leads to a C H B - - S - - C ' - - C N carbene ion, whereas the 1,3-H~ abstraction
yields a novel sulfur-containing distomc radical anion, wt'uch is formulated as
" C H 2 - - S - - C H - - C N . (J Am Soc Mass Spectrom 1995, 6, 71-75)
D
istomc radical cations have been studied exten)sively and are well recognized as stable species
and key intermediates in unimolecular as well
as bimolecular reactions in the gas phase [1-5]. The
interest m these i n s arises in part from the findmg
that some sunple distomc radical cations that contain a
halogen, mtrogen, or oxygen atom (e.g., "CH2OH ~)
are more stable than their conventional isomers (e.g.,
CHBOH +') [2, 6]. The oppomte situation applies to
simple sulfur-containing dlstomc radical cations [1,
7-13], which according to ab mitio calculations are less
stable than the conventional isomers [1, 7]. For example, the distonic radical cation C H 2 - - S H - - C H 3 is
estimated to be about 80 kJ tool -1 higher in energy
than the conventional C H 3 - - S + - - C H 3 ion [7] In
contrast to the extensive literature concerning dlstonic
radical cations, very little is known about distonic
radical anions. A few radical anions that may be classified as distonic have been generated. The reported
examples include the " C H 2 - - C ( O - ) ~ C H 2 ion generated by a 1,3-H~" abstraction in the reaction of O with acetone [14] and the "CH2CO 2 ion formed in the
reaction of the acetyl amon with molecular oxygen
[15]. The latter ion has been reported to arise also by
high kinetic energy collision-induced dissociation (CID)
Address repnnt requests to Professor Dr N M M Nlbbermg, Inshtute of Mass Spectrometry, Umverslty of Amsterdam, Nleuwe
Achtergracht 129, 1018 WS Amsterdam, The Netherlands
© 1995 American Society for Mass Spectrometry
1044-0305/95/$9 50
of carboxylate anions derived from cx,~-dlcarboxylic
esters together with the homologous 'CH2(CH2),CO 2
(17 = 1 and 2) species [16]. More recent examples include the C6H4CO 2 ion generated by addition of the
orthobenzyne radical anion to carbon dioxide [17], the
C6H3F- sons formed by competing 1,4- and 1,3-H~
abstractions m the reaction of O - w~th fluorobenzene
[18], and the radical anion of tetramethylene-ethane
generated by a 1,4-H +' abstraction from 2,3-dimethyl1,3-butadiene [19]. In addition, a theoretical study of
some srmple metal-containmg radical anions indicates
that the classical isomers generally represent the preferred form of these species [20]. In particular, the
isomers, H 2 C - - L i H - and H 2 C - - N a H - , are predicted to be much higher in energy than the CH3Liand C H 3 N a - species.
The reported examples of distomc radical anions
suggest that the reactions of O-" with suitable organic
substrates in the gas phase may provide a general and
relatively facile method for the formation of these
species. As part of our ongoing research mto the formation and reactivity of various types of radical anions in the gas phase, we decided to explore whether
distoruc radical anions can be formed from simple
molecules that contam a group capable of stabilizmg a
negative charge and a second functionality capable of
stabilizing an adjacent radical center. Here we report
our result for CH3SCH2CN, which was selected as a
relatively simple and readily available model compound that fulfills these requtrements.
Received July 11, 1994
Revised August 12, 1994
72
BORN ET AL.
Experimental
The results were obtained in part with the use of a
Fourier transform ion cyclotron resonance (FT-ICR)
[21-23] mass spectrometer designed and constructed
at the University of A m s t e r d a m [24]. The experimental
operating procedure of this instrument has been outlined elsewhere [25, 26]. The O - ions were generated
from N 2 0 by dissociahve capture of electrons with an
energy of 1.0-1.5 eV. The partial pressures of N 2 0 and
thiomethylacetonitrile were both = 3 x 10 -5 Pa as
measured with an uncalibrated ionization gauge placed
in a side arm of the main p u m p i n g line. Unwanted
runs were ejected from the FT-ICR cell by applying
appropriate radiofrequency (rf) pulses to the excitation
plates of the cell [27].
The CID and charge reversal (CR) spectra [28-30]
were recorded with a Fisons ZAB-2HFqQ (Fisons Instruments, Manchester, UK) reverse geometry doublefocusing quadrupole hybrid mass spectrometer [31,
32]. The CID experiments were carried out with He as
the collision gas, whereas 0 2 was used as the collision
gas in the CR experiments. In both instances the gas
was admitted to the collision cell located in the second
field-free region unt:l the intensity of the main b e a m of
the mass-selected negative ions was reduced by 50%.
The methylthioacetonitrile sample was introduced into
the combined electron impact (EI)-chemical ionization
(CI) source with a direct insertion probe. The pressure
of nitrous oxide was about 10 -2 Pa as measured with
an uncahbrated ionization gauge placed in a side arm
of the entrance to the diffusion p u m p situated beneath
the ion-source housing.
Table 1. Tune dependence of the normahzed abundances (%)
of the ions formed m the reaction of the O- ion with
CH3SCD2CNin the gas phase as determined by Fourier
transform ion cyclotron resonancea
Time
(ms) CH3S--CD--CN CH3--S--C--CN CH2S--CD--CN
100
200
400
600
800
1000
55
58
70
80
81
85
32
28
17
9
8
4
All samples were commercially available with the exception of the CH3SCD2CN (>__95% d 2) compound,
which was prepared from the unlabeled c o m p o u n d by
h y d r o g e n - d e u t e r i u m exchange in D 2 0 at room temperature. After removal of the water layer no further
purification was needed. The identity of the compound, the label content, and the position of the deuterium atoms were confirmed by EI mass spectrometry.
and
Discussion
The O-" radical anion is k n o w n to react with a variety
of organic compounds by abstraction of a hydrogen
atom and a proton in an overall process, which is
described commonly as a formal H~" abstraction [33,
34]. In the reaction of O-" with methylthioacetonitrile
in the FT-ICR cell, competing proton transfer and H~"
abstraction occur. Deuterium labeling reveals that proton abstraction involves only the methylene position
(eq 1), whereas two channels are open in the latter
process; that is, competing 1,1- and 1,3-H~-" abstractions take place (eqs 2 and 3).
12
14
13
11
11
11
a See text and eqs 4 - 6
O- + H3CS--CD2--CN
----* H 3 C S - - C D - - C N
+ DO"
(1)
m/z 87
H3CS--C--CN
+ D20
(2)
m/z 85
H2C'S--CD--CN +
HDO
(3)
m/z 86
In Table 1 the relatwe abundances of the different
product ions are given as a hmctmn of reaction time.
The radical a m o n formed by D~ abstraction from the
labeled c o m p o u n d reacts relatwely readily with the
parent c o m p o u n d to form the C H 3 - - S - - C D - - C N
anion either by abstraction of a deuteron or a deuterium atom (eq 4).
H3CS--C--CN
Materials
Results
J Am Soc Mass Spectrom 1995, 6, 71-75
+ H3CS--CD2--CN
--~
m/z 85
H3CS--CD--CN
+ H3CS--C--CN
(4)
m/z 87
The radical anion generated by H D +" abstraction reacts
very s l o w l y w i t h the parent c o m p o u n d to afford the
CHsmS--CD--CN
ion (eq 5) together with very
minor amounts of an ion with m/z 88 (eq 6). The
former ion can be generated either by deuteron abstraction or hydrogen atom transfer, whereas the m/z
88 ion m a y arise by deuterium atom abstraction from
the parent compound.
H2CS-CD-CN
+ H3CS-CD2-CN
m/z 86
H3CS-CD-CN
+ H2CS-CD2CN
(5)
/z 87
H2DCS-CD-CN
+ H3CS-CD-CN
(6)
m/z 88
These FT-ICR experiments indicate that three different
ions arise in the reaction of the O - ion with methyl-
J Am Soc Mass Spectrom 1995, 6, 71-75
thioacetonitrile. That is, proton abstraction leads to a
carbanion, whereas two isomeric species are generated
by the competing 1,1- and 1,3-H~ abstractions. To
structurally characterize these ions, high kinetic energy
CID experiments were performed with a reverse geometry double-focusing instrument (see Experimental).
The ion formed by deuteron abstraction eliminates
only a methyl radical in the high kinetic energy colhsion experiments, in agreement with the proposed
structure (see eq 1). The ion formed by D~-' abstraction
expels also a methyl radical upon collision, as expected
for the assumed structure (see eq 2). In contrast, no
negatively charged product ions are formed in these
experiments with the ion generated in the HD ÷" abstraction, thus precluding a structural characterization
of flus ion by CID. However, the collision-reduced CR
spectra of the three ions proved to be distinct as can be
seen in Figure 1.
The CR spectrum of the carbamon (Figure la) displays a peak that corresponds to C2DNS + ions generated by the expulsion of a CH 3 radical (eq 7) together
with peaks for the formation of CH3 S+ and C2DN ÷'
ions (eqs 7-9).
CR(O2)
>
C2DNS+'+ CH 3
(7)
>
CH3S++ C2DN
(8)
>
C2 DN+
(9)
H3CS--CD--CN
+
CH3S
100%-
50 % -
CR(O2)
)
C2NS++ CH 3
(10)
CH3S++ C2N
(11)
C2 N + + CH3S
(12)
Significantly, the CR spectrum of the radical anion
(Figure lc) generated by HD +' abstrachon contains a
peak that corresponds to an ion formed by the expulsion of a CH 2 fragment as well as signals for CH2 S+"
and C2DN +" ions (eqs 13-15).
CR(O~)
H2CS--CD--CN
"
'
C2DNS+'+ CH 2
(13)
)
CH2S+'+ C2DN
(14)
>
C2 DN+
(15)
+
CH2S
The CR spectra support the structural assignments of
the carbanion and the carbene radical anion. For the
ion generated by HD + abstraction, the expulsion of
CH 2 and the absence of a peak corresponding to
) 2 DNS*
C2DN"
2ooo
4000
6000
P
ioo~
Energy
8000
in ev
b
C2NS +
/
50 % -
",
2000
In the CR spectrum of the supposed carbene radical
amon, peaks that correspond to CH3 S+ and C2 N+
ions are noticed (Figure lb) together with a peak of an
ion formed by methyl radical loss (eqs 10-12).
H3CS--C--CN
73
SULFUR-CONTAINING DISTONIC RADICAL ANION
II
//
4000
6000
•
8000
Energy ,n eV
loo~
CH2S"
/
50 % C2ON+
~ I ~ I I ~ . i CDS÷
2 H 2
2000
4000
8000
m,
I
800o
Energy in eV
Figure 1. Colhslon-mduced charge-reversal spectra of the ions
formed m the reachons of the O- ion with methylttuo2,2-d2-acetomtnle in a chemical lomzahon source (a)
C H 3 - - S - - C D - - C N , (b) C H 3 - - S - - C ' - - C N , and (c)
CH2--SwCD--CN; see also text. The peaks marked with an
asterisk correspond to the charge-reversed and mass-selected
parent ion. A few spikes are present as very narrow signals in
spectra (a) and (c)
CH3 S+ (Figure lc) strongly suggest that a distonic
sulfur-containing radical anion is generated.
Based on the present results it is not possible to
ascertain the location of the charge and radical site
74
BORN ET AL
J Am Soc Mass Spectrom 1995, 6, 71-75
w i t h i n the distonic ion. In the equations, w e h a v e
located the n e g a t i v e c h a r g e in a f o r m a l sense on the
c a r b o n a t o m in b e t w e e n the sulfur a t o m a n d the C N
function a n d the r a d i c a l p o s i t i o n on the c a r b o n a t o m
b o n d e d o n l y to the sulfur atom. This is b a s e d largely
on the o b s e r v a t i o n that C H 3 S C H 3 is a b o u t 85 kJ mo1-1
less acidic [35] than acetonitrile [36] in the gas phase,
w h i c h indicates that the c y a n o g r o u p stabilizes a n e g a tive c h a r g e on an a d j a c e n t c a r b o n a t o m better t h a n a
- - S C H 3 g r o u p . The d i s t o m c ion m a y , of course, also
be f o r m u l a t e d as - C H 2 - - S - - C H C N .
O n l y few hom o l y t i c C - - H b o n d d i s s o c i a t i o n e n e r g i e s (BDE) of
srrnple s u l f u r - c o n t a i n i n g c o m p o u n d s h a v e b e e n rep o r t e d . It m a y be noted, h o w e v e r , that the h o m o l y t i c
C - - H BDE of C H 3 S H ( = 387 kJ mo1-1) [37, 38] is
close to the v a l u e for CHBCN ( = 390 kJ mo1-1) [37,
39]. This s u g g e s t s that the s t a b i l i z a t i o n of the radical
site will n o t differ d r a s t i c a l l y if located o n the central
c a r b o n a t o m or o n the c a r b o n a t o m b o n d e d o n l y to the
sulfur atom. In other w o r d s , the stabilization of the
radical site m a y n o t be the crucial factor in d e t e r m i n ing the r a d i c a l / c h a r g e d i s t r i b u t i o n in the distonic ion.
The f o r m a t i o n of the distonic radical a n i o n can
i n v o l v e initial h y d r o g e n a t o m a b s t r a c t i o n f o l l o w e d b y
p r o t o n a b s t r a c t i o n as p r o p o s e d also for the f o r m a t i o n
of the o r t h o b e n z y n e radical a n i o n in the reaction of
O - w i t h b e n z e n e [17, 40]. If the h y d r o g e n a t o m abstraction occurs f r o m the m e t h y l g r o u p , p r o t o n abstraction f r o m the meth_ylene function can follow a n d
lead to the " C H 2 S - - C H - - C N
species (Scheme I).
H y d r o g e n a t o m a b s t r a c t i o n m a y also occur f r o m the
m e t h y l e n e g r o u p a n d be f o l l o w e d either b y p r o t o n
a b s t r a c t i o n f r o m this site w l t h f o r m a t i o n of the c a r b e n e
radical a n i o n or from the m e t h y l g r o u p . In the latter
instance, p r o t o n a b s t r a c t i o n g e n e r a t e s the distonic r a d ical a n i o n w i t h the c h a r g e o n the c a r b o n a t o m b o n d e d
o n l y to the s u l f u r a t o m (see S c h e m e 1). N e v e r t h e l e s s ,
electronic r e o r g a n i z a t i o n m a y occur a n d lead to the
a n t i c i p a t e d formal location of the n e g a t i v e c h a r g e in
b e t w e e n the functional g r o u p s . F u r t h e r w o r k o n the
stability a n d reactivity of the s u l f u r - c o n t a i n i n g distonic
radical a n i o n in c o m p a r i s o n to isomeric ions is in
progress.
O"
+ CH3S-CH2-CN
S
"-.
[ HO- + CH2S-CH2-CN ]*
1_,o
CH2S-CH-CN
Scheme I.
[ HO- + CHaS-CH-CN ]*
/.o
"CH2S-CH-CN
CHaS-'C-CN
Possible pathways that lead to formahon of the
distonic radical anion (see also text)
Acknowledgments
The authors thank the Netherlands Organlzahon for Scientific
Research (SON/NWO) for fmancml support The authors acknowledge H E K. Matwaba for his assistance durmg the experiments with the double-focusmg reverse geometry hybrid mstrument
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