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Toluene and isotoluene radical cations alkoxymethyl substituents as a probe to study the formation fragmentation and isomerization

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JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 34, 408È420 (1999)
Toluene and Isotoluene Radical Cations :
Alkoxymethyl Substituents as a Probe to Study the
Formation, Fragmentation and Isomerization¤
Natascha Harting and Hans-Friedrich GruŽ tzmacher*
FakultaŽt fuŽr Chemie der UniversitaŽt Bielefeld, UniversitaŽtsstr. 25, D-33615 Bielefeld, Germany
The unimolecular reactions of toluene radical cations and tautomeric isotoluene radical cations (methylenecyclohexadiene radical cations) substituted by an ROCH group (R = CH , C H , n-C H , i-C H ) were studied by
2
3 2 5
3 7
3 7
mass-analyzed ion kinetic energy (MIKE) spectrometry and collisional activation mass spectrometry. The
alkoxymethyl-substituted toluene radical cations were obtained directly by 70 eV electron ionization, while the
substituted isotoluene radical cations were prepared by elimination of an aldehyde or ketone molecule from ionized
bis(alkoxymethyl)benzenes. It is shown that the fragmentation reactions induced by the alkoxymethyl side-chain in
the toluene radical cations and isotoluene radical cations can be used conveniently to distinguish unambiguously
between the tautomers. The substituted toluene radical cations eliminate abundantly the alkoxy group of the sidechain as aldehyde or ketone by a well known fragmentation mechanism. Typical of the substituted isotoluene
radical cations is an abundant elimination of alcohol from the side-chain of metastable ions, which with the exception of some para-substituted ions surpasses the abundance of the loss of aldehyde/ketone. Further, the substituted
toluene radical cations typically fragment by loss of the methyl group of the aromatic ring. The analysis of the
MIKE spectra reveals that Æ10% of the isotoluene racial cations are isomerized into the toluene isomers if these
ions are generated by energetically favorable processes from the bis(alkoxymethyl)benzenes. Otherwise, extensive
tautomerization between toluene and isotoluene-like ions takes place. Thus, (substituted) toluene and isotoluene
radical cations are stable species which can be detected by tandem mass spectrometry. However, the activation
barrier between the tautomeric ions cannot be very large, and energetically excited isotoluene radical cations are
converted into the more stable toluene isomer. Copyright ( 1999 John Wiley & Sons, Ltd.
KEYWORDS : isoarene radical cation ; ion structure ; isomerization ; fragmentation mechanism ; tandem mass spectrometry
INTRODUCTION
The discussion about isomers 2, 3 and 4 of toluene 1
(Scheme 1) and their radical cations has been an important topic not only in mass spectrometry, but also in
solution. Horning1 discussed the so-called “isoaromatizationÏ of 1 to isotoluenes on a theoretical basis
in the context of “alicyclicÈaromaticÏ isomerization.
There he stated isotoluene not to be preparable because
of its lack of stability and easy transformation to 1.
Nevertheless, around 20 years later Bailey and
Baylouny2 managed to prepare and to isolate isotoluene, and subsequently di†erent methods of preparing o-isotoluene (2)3 and p-isotoluene (3)4,5 (Scheme
1) were published. The stability of both isomers is
strongly dependent on solvent e†ects and the absence of
oxidants or acids/bases. Even without heating, 2 isom-
* Correspondence to : H.-F. GruŽtzmacher, FakultaŽt fuŽr Chemie der
UniversitaŽt Bielefeld, UniversitaŽtsstr. 25, D-33615 Bielefeld, Germany
¤ Dedicated to the memory of Professor Dr Wilhelm J. Richter
GruŽtzmacher=chemie.uni-bielefeld.de
Contract/grant sponsor : Fonds der Chemischen Industrie.
CCC 1076È5174/99/040408È13 $17.50
Copyright ( 1999 John Wiley & Sons, Ltd.
erizes immediately to 1 in the presence of catalytic
amounts of sulfuric acid.
Because of its reactivity, 2 has been brought to reaction with di†erent partners, such as styrene.5 Although
both isomers 2 and 3 have been calculated by Bartmess6
to be 23 kcal mol~1 (1 kcal \ 4.184 kJ) less stable than
Scheme 1.
Received 7 October 1998
Accepted 11 December 1998
TOLUENE AND ISOTOLUENE RADICAL CATIONS
409
1, the para-isomer 3 can be distinguished from the
ortho-isomer 2 by its lower “ability to undergo an ene
reaction.Ï7 Its transfer of the active hydrogen to the
reaction partner in order to form a benzyl radical is
apparently slow. The so far missing isomer 4, misotoluene, which was mentioned by Dreiding and coworkers,8 can be interpreted as either a biradical (4a) or
a homofulvene (4b) (Scheme 1). This isomer polymerizes
when heated without solvent and does not rearrange to
toluene.9
Regarding toluene and isotoluene radical cations in
solution and in the solid, Kelsall and Andrews10 produced [C H ]`~ radical cations from cycloheptatriene
7 8
6, 1 and 2 in a solid argon matrix and studied them by
photochemical experiments. It was proved that the
radical cation 2`~ of o-isotoluene is formed by
reversible rearrangement of 1`~ and 6`~ (Scheme 2), as
was known earlier from mass spectrometric studies.11
Additionally, it could be shown that the radical cation
of norbornadiene, 7`~, also exhibits the ability to form
2`~, but in this case the equilibrium is shifted to 2`~ and
6`~, respectively.
Concerning mass spectrometry, numerous experimental studies of [C H ]`~ rearrangements have been
7 8
accompanied by theoretical calculations on di†erent
levels with special regard to activation barriers and
intermediates of the interconversion of isomers.12
Bartmess6 produced the radical cations 2`~ and 3`~ in
an ion cyclotron resonance (ICR) cell directly from the
neutral species for the Ðrst time. Further, the fragmentation of the molecular ions of several alkylbenzenes into
[C H ]`~ ions has been studied by mass spectrom7 8
etry.13,14 The radical cations of isoarenes are formed
obviously from various types of precursors.14 One of
the Ðrst unimolecular reactions, in which 2`~ was discovered to be formed by speciÐc H-migration from a
side-chain and loss of a neutral molecule, was the analogue of the McLa†erty rearrangement of nbutylbenzene.15h20 The H-atom used for this formation
of 2`~ originates exclusively from the c-position of the
side-chain (Scheme 3, I).
In principle, the mechanism of this process is characterized by two factors : the H-transfer and the carbonÈ
carbon bond cleavage.16 The H-transfer is inÑuenced by
the CÈH bond strength at the c-position of the sidechain of the alkylarene radical cation and by the basicity of the accepting ortho-position of the arene ring. The
Scheme 2.
Scheme 3.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 408È420 (1999)
410
N. HARTING AND H.-F. GRU TZMACHER
ease of carbonÈcarbon bond cleavage in the second step
depends distinctly on the stability of the neutral lost.
These e†ects are in agreement with a concerted mechanism and also with a two-step process. With increasing
length of the alkyl chain, concurring H-migrations
directly to the benzylic position or the ipso-position at
the ring also become possible. In the radical cation of
n-butylbenzene, the speciÐc H-migration to the benzylic
position which yields 1`~ directly, but demands a fourcenter concerted rearrangement (Scheme 3, II), was
rejected by deuteration experiments.18 To the contrary,
it has been proved17 that 4@,4@-dimethylpentylbenzene
does indeed exhibit an H-migration involving an
H-atom at a d position. In contrast to path III, pathway
IV (Scheme 3, III, IV) is favored since the aromatic ring,
which carries charge and radical, does participate in the
migration. Apart from various ionized alkylbenzenes,
the elimination of phenylalkenes for the radical cations
of a,u-diphenylalkanes with varying chain length were
investigated.14,21 The H-DÈexchange observed between
c-, ortho- and benzylic positions for speciÐcally deuterated derivatives during this process established Ðrmly a
two-step mechanism for this type of fragmentation.
As mentioned above, the stability of the released
neutral fragment has an important e†ect on the possible
formation of isoarene radical cations by rearrangement
of ionized alkylarenes and related aromatic compounds.
For instance, deuterated benzyl ethyl ether22,23 was
examined by the mass-analyzed ion kinetic energy
(MIKE) technique in order to reveal whether the loss of
acetaldehyde by the rearrangement process is concerted
or stepwise and includes a distonic intermediate24
(Scheme 4).
Selecting p-bis(ethoxymethyl)benzene as a model,
Derrick and co-workers25 demonstrated by a detailed
investigation of isotope e†ects on the elimination of
acetaldehyde and by the lack of any H-DÈexchange that
this mechanism is likely to be concerted. Other
authors23,24 contradicted and demonstrated a stepwise
loss of acetaldehyde from the molecular ion of benzyl
ethyl ether. These authors prepared the distonic intermediate (Scheme 4) using 3-methyl-2,3-dioxyspiro[5.5]
undeca-8,10-diene as a precursor and demonstrated its
further fragmentation into 2`~ and 1`~. They suggested
also a new mechanism for the formation of the isotoluene radical cation 2`~ including an ionÈneutral
complex (Scheme 5), which explains the release of only a
small amount of kinetic energy during this reaction.
Furthermore, 1`~ is also formed in low-energy processes, which may correspond to an isomerization “catalyzedÏ by the acetaldehyde in the complex. Thus, the
rate-determining step of the reaction was found to be
the initiating proton transfer leading to the distonic ion.
In addition to interest in the details of the mechanism
of formation of [C H ]`~, numerous studies have also
7 8
addressed the question
of whether the [C H ]`~
7 tech8
isomers 2`~È4`~ can be trapped by appropriate
niques in their potential energy wells. In a review
Kuck14 stated that “the initial formation of ionized isotoluenesÏ from n-alkylbenzenes “is well acceptedÏ but
“subsequent rearrangement appears likely.Ï McLa†erty
and co-workers26 applied collisional activation (CA)È
MIKE-techniques in order to distinguish ionized
toluene 1`~, ionized cycloheptatriene 6`~ and the
[C H ]`~ ions generated from n-butylbenzene and 27 8
phenylethanol
radical cations,19,20,27,28 respectively.
They discovered only slight di†erences between the CA
spectra of these ions and stated that the isotoluene
radical ion, if generated, has isomerized prior to
analysis. Supporting these results, Nibbering and de
Boer28 found complete scrambling in the [C H ]`~
7 8
ions generated from isotopomeric 2-phenylethanols,
which indicated that the di†erent [C H ]`~ isomers
7 8spectrometer
generated as fragment ions in a mass
cannot be isolated without isomerization. Similar
results have been reported also by other authors.11
However, Bursey et al.18 have demonstrated in ICR
studies that the structure of the [C H ]`~ ions gener8
ated from n-butylbenzene is not the7 toluene
structure.
Under these conditions, “the toluene radical cation
exhibits the ability to abstract NO from neutral alkyl
nitrates,Ï whereas the [C H ]`~ ion2 generated from n7 8
butylbenzene does not. Further,
Dunbar and Klein19
observed that 1`~ and suggested ions 2`~ generated
from n-butylbenzene and 2-phenylethanol radical
cations do not give identical photodissociation spectra
Scheme 4.
Scheme 5.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 408È420 (1999)
TOLUENE AND ISOTOLUENE RADICAL CATIONS
in ICR. Some evidence for stable 2`~, even under MIKE
conditions, was presented by Burgers et al.20 They used
CA and charge stripping techniques to investigate
[C H ]`~ ions generated from isotopomers of the
7 8 cation of n-butylbenzene and 2-phenylethanol,
radical
respectively. However, eventually they also came to the
conclusion that [C H ]`~ ions generated from these
7 8
precursors “indeed represent a mixture of at least two
structures, i.e. 2`~ and an additional, so far unknown
structure.Ï
These results lead to the conclusion that 2`~ is initially formed in the McLa†erty rearrangement whereas
other isomeric [C H ]`~ ions are formed by a sub7 8
sequent isomerization. In general, two possibilities for
the isomerization of 1`~ to 2`~ are suggested. The Ðrst
is a facile rearrangement of [C H ]`~ ions forced by
7 8
excess energy which makes it impossible
to trap 2`~
even as metastable ions. The second is the shuttle
mechanism, suggested by Turecek et al.24 for the special
case of ethyl benzyl ether (Scheme 5), which, however,
may operate also in other cases.
Regarding this situation, it is still of interest to distinguish unambiguously by MIKE spectrometry
between the isomers of [C H ]`~ ions generated by
fragmentation reactions and7 to8 obtain more information about their isomerization processes. This is conveniently achieved by studying ionized bis(alkoxymethyl)benzenes and alkoxymethylmethoxymethylbenzenes as model substances because of the special e†ects
of alkoxy substituents on rearrangement processes
typical of ionized alkylbenzenes.29 One crucial e†ect of
an alkoxymethyl substituent is that the typical tolueneÈ
cycloheptatriene rearrangement of ionized alkylbenzenes does not occur because benzylic radicals and
benzylic ions formed during rearrangement processes
are stabilized. For the fragmentations studied here the
loss of a stable aldehyde or ketone molecule from the
Ðrst alkoxymethyl side-chain by H-migration to the
benzene ring ensures uncomplicated formation of an
isotoluene radical cation substituted by the second alkoxymethyl group. It will be demonstrated that the
further fragmentation initiated by this group as opposed
to the reaction of this side-chain in the isomeric methylbenzyl alkyl ethers can be used as an “indicator reactionÏ
to distinguish isomers with a toluene and an isotoluene
structure. In addition, it will be shown that the indicator reactions caused by these side-chains can be also
used to distinguish meta- and para-isomers of the substituted isotoluene radical cations by MIKE spectrometry.
EXPERIMENTAL
411
USA) and the software Complement (Version 1.5). The
samples were introduced by a septum inlet system at
ambient temperature and ionized by 70 eV electrons.
The Autospec mass spectrometer uses an acceleration
voltage of 8 kV and the JMS instrument a voltage of 10
kV.
For the MIKE and CA spectra at the Autospec, the
ions were selected in the 3rd Ðeld-free region (FFR), and
the spectra were obtained by scanning the second electrostatic analyzer (ESA) to record the spectrum. Argon
was added to the collision cell of this region as CA gas.
The MIKE and CA spectra at the JMS instrument were
obtained under similar conditions by using only three
sectors. The precursor ions were focused into the 3rd
FFR after the 1st magnetic sector, and the spectrum
was recorded by scanning the 2nd ESA. For a better
“front-end resolution,Ï some MIKE spectra were
obtained by focusing the precursor ions into the 4th
FFR after the 2nd ESA and recording the spectrum by
scanning the 2nd magnet. CA spectra were measured by
introducing air into the collision cell of the 3rd FFR or
the 4th FFR. The relative abundances of the ions in the
MIKE and CA spectra were obtained from the peak
areas and normalized to the total fragment ion abundance. The relative abundances are reproducible to
^5% (relative), but at least to ^1% (absolute). All
tables, including EI mass spectra, contain abundances
relative to the total ion abundances.
The kinetic energy release distribution was obtained
from the peak shape of the relevant process in the
MIKE spectrum by using the META program of Szilagyi and Vekey.30
Compounds
For puriÐcation, all compounds were distilled using a
BuŽchi GKR apparatus and controlled by gas chromatography (Hewlett-Packard 5890 Series II gas chromatograph ; 30 m HP-5 column, wide bore, carrier gas
nitrogen). The structure of all compounds was conÐrmed by 1H NMR spectrometry (Bruker AC 250P, 250
MHz) and EI mass spectrometry (VG Autospec).
Because of the lack of intense peaks of the molecular
ions and large [M [ H]` signals in the EI mass
spectra, the isotopic purity was estimated from the
reagents applied for deuteration to be at least 95%.
The syntheses of both the symmetrical bis(alkoxymethyl)arenes25,31 and the monoethers of the xylenes
have been described elsewhere.32 Alkoxymethyl
methoxymethyl ethers were prepared by reaction of
chloromethylmethoxymethylbenzenes with the corresponding alcoholates of potassium in the dried alcohol
or by reaction of hydroxymethylmethoxymethylbenzenes with the appropriate alkyl bromide by treatment
with a suspension of NaH in dry DMF.
Mass spectrometry
MIKE spectra were measured with a double-focusing
mass spectrometer of EBE geometry (VG Autospec,
Fisons, Manchester, UK), operated by Opus V3.1X.
Additional experiments were performed with a highresolution four-sector Ðeld instrument with EBEB
geometry (JMS-HX/HX110A, Jeol, Peabody, MA,
Copyright ( 1999 John Wiley & Sons, Ltd.
RESULTS AND DISCUSSION
The reactions of the investigated radical ions depend
strongly on the length of the alkyl chain of the ether
group. Hence it is convenient to discuss the results in
J. Mass Spectrom. 34, 408È420 (1999)
412
N. HARTING AND H.-F. GRU TZMACHER
two sections. In the Ðrst section, the fragmentation reactions of the molecular ions 8m`~ and 8pp`~, m/z 136, of
methoxymethyltoluenes and of the isomeric ions 9m`~
and 9p`~, m/z 136, of an isotoluene structure
(methoxymethylmethylenecyclohexadiene radical ions,
Scheme 6) will be discussed. Here, m and p refer to the
meta- and para-position of the side-chain relative to the
methyl group and methylene group at the ring, respectively. In the second section the MIKE and CA spectra
of the molecular ions 14`~, 16`~ and 18`~, m/z
136 ] 14n (n \ 1, 2), of the alkoxymethyltoluenes with
larger O-alkyl groups and their isomeric radical cations
15`~, 17`~ and 19`~ (alkoxymethylmethylenecyclohexadiene radical cations) will be treated. Again, m and
p indicate the meta- and para-isomers with respect to
the position of the alkoxymethyl side-chain to the
methyl group or methylene group.
Scheme 7.
Formation and fragmentation of methoxymethyltoluene
radical cations and isomeric isotoluene ions (m/z 136)
The methoxymethyl-substituted toluene ions 8`~ were
prepared by 70 eV EI of the methylbenzyl methyl ethers
8 assuming that the molecular ions surviving fragmentation retain the toluene structure. The corresponding isotoluene radical cations 9`~ (Scheme 6) were prepared by
fragmentations of suitable precursors.
C H CH OCH radical ions, m/z 136, of initially
7 7 structure
2
3 are generated from the molecular
unknown
ions of the alkoxymethylmethoxymethylbenzenes 11È13
(Scheme 7) by loss of the appropriate aldehyde or
ketone. The relative abundances of these ions in the EI
mass spectra and in the MIKE spectra of the molecular
ions of the precursors are presented in Table 1.
The loss of formaldehyde from the molecular ions of
the bis(methoxymethyl)benzenes 10m`~ and 10p`~ is
obviously not favored since the abundance of the ions
at m/z 136 is low in the EI spectra and also not very
high in the MIKE spectra (Table 1 ; see also Fig. 1). In
contrast, the losses of acetaldehyde, propionaldehyde
and acetone from the precursors 11`~, 12`~ and 13`~
respectively, lead to intense signals at m/z 136 in both
the EI and the MIKE spectra. Further, the signals for
the ion at m/z 136 in the MIKE spectra of 10m`~ and
10p`~ (Fig. 1) are broad and round-topped, indicating a
large kinetic energy release (KER) and a substantial
reverse activation barrier for the loss of formaldehyde
and the presence of excess energy in these ions m/z 136.
In contrast, the losses of acetaldehyde, propionaldehyde
and acetone from 11`~È13` are associated with much
smaller KERs (Fig. 1). Thus, only C H CH OCH
7
2
3
radical ions, m/z 136, generated from the7ethoxymethyland propoxymethylmethoxymethylbenzenes 11`~È13`
are considered to give isotoluene radical cations 9`~ of
low internal energy.
To establish the structure of these C H CH OCH
7 7
2 their3
radical ions, m/z 136, formed by fragmentation,
MIKE spectra were compared with the MIKE spectra
of the substituted toluene radical cations 8m`~ and
8p`~. The relative abundances of the fragment ions in
the MIKE spectra are listed in Tables 2 and 3.
All processes observed in the MIKE spectra, i.e. the
loss of a methyl radical and the loss of CO, formaldehyde and methanol, are common to the ions 8`~ and
the fragment ions C H CH OCH , m/z 136, but their
7
2
3
relative abundances 7di†er
considerably.
It is obvious
from Tables 2 and 3 that each of the series of meta- and
para-substituted derivatives are grouped into three
classes of C H CH OCH radical ions, m/z 136, which
7
2
3 in the MIKE spectra. The
give distinct7 peak
patterns
Table 1. Relative signal intensity of the ion at m/z 136 (% total
ion current) in the EI and MIKE spectra of the molecular ions of alkoxymethylmethoxymethyl benzenes
10–13, and kinetic energy release (KER, meV) during
their formation
Signal intensity (%)
Scheme 6.
Copyright ( 1999 John Wiley & Sons, Ltd.
Molecular ion
EI
MIKE
T * (meV)a
10m½~
10p½~
11m½~
11p½~
12m½~
12p½~
13m½~
13p½~
1.1
1.6
4.5
3.0
5.3
4.9
4.8
4.3
17.9
9.8
90.0
58.6
62.8
63.3
b
66.3
384
414
24
43
22
41
b
43
a T * is the most frequent amount of KER.
b KER not determined because of low signal intensity.
J. Mass Spectrom. 34, 408È420 (1999)
TOLUENE AND ISOTOLUENE RADICAL CATIONS
413
Table 2. Relative signal intensities (% total ion current) in the
MIKE
spectra
of
the
meta-substituted
[ C H CH OCH ] ‘~ radical cations, m/z 136,
7
7
2
3
derived from 10m–13m.
Ion at m /z 136 generated from
Process
m /z
8m½~
10m½~
11m½~
12m½~
13m½~
ÉCH
3
ÉCO
ÉCH O
2
ÉCH O~
3
ÉCH OH
3
Othera
Othera
Othera
Othera
121
108
106
105
104
92
91
80
79
24.8
5.1
16.5
—
52.8
—
0.3
0.1
—
10.4
5.1
20.0
—
56.5
1.0
0.9
0.5
0.5
2.9
3.8
6.3
—
87.0
—
—
—
—
3.1
2.4
8.8
—
85.2
0.5
—
—
—
3.7
0.8
10.8
1.6
83.1
—
—
—
—
a These ions may have more than one precursor and cannot be
assigned to a specific process.
C H CH OCH , m/z 136, derived from the ionized
7 7
211`~È13`~
3
bisethers
which give almost identical MIKE
spectra characterized by only one large signal due to
loss of CH OH and a small signal for the loss of ~CH .
3
Finally, the3 fragment ions C H CH OCH , m/z 136,
7
7
2
3
originating from 10`~ by loss of formaldehyde correspond to the third class of ions at m/z 136 with a peak
pattern between those of the other two types of ions.
This corroborates the assumption that energetically
excited ions at m/z 136 arise from 10`~, and that these
ions at m/z 136 subsequently isomerize to a mixture of
structures.
Although the MIKE spectra of the ions
C H CH OCH , m/z 136, arising from the molecular
7 7 of 211È13 3 by the loss of acetaldehyde, propiions
onaldehyde and acetone, respectively, give similar
MIKE spectra which are di†erent to those of the molecular ions of methylbenzyl methyl ether 8, it is not
certain that these ions correspond only to ions 9`~ of an
isotoluene structure. To arrive at this conclusion, a
more detailed discussion of the fragmentation reactions
observed in the MIKE spectra is needed. Apart from
the loss of CO, which has only minor signiÐcance, the
Table 3. Relative signal intensities (% total ion current) in
the MIKE spectra of the para-substituted
[ C H CH OCH ] ‘~ radical cations, m/z 136,
7 7
2
3
derived from 10p–13p.
Ions at m /z 136 generated from
Figure 1. Partial MIKE spectra of the molecular ions of (a)
bis(methoxymethyl)benzene 10p½~, (b) ethoxymethylmethoxymethylbenzene 11p½~, (c) n -propoxymethylmethoxymethylbenzene 12p½~ and (d) isopropoxymethylmethoxymethylbenzene
13p½~.
Ðrst class consists of the methoxymethyl-substituted
toluene radical cations 8m`~ and 8p`~, which exhibit
abundant losses of ~CH , CH O and CH OH in their
3 class
2 contains fragment
3
MIKE spectra. The second
ions
Copyright ( 1999 John Wiley & Sons, Ltd.
Process
m /z
8p½~
10p½~
11p½~
12p½~
13p½~
ÉCH
3
ÉCO
ÉCH O
2
ÉCH O~
3
ÉCH OH
3
Othera
Othera
Othera
Othera
Othera
Othera
Othera
121
108
106
105
104
94
92
91
80
79
59
45
27.6
4.8
15.9
—
50.7
—
—
0.8
0.3
—
—
—
10.9
12.5
27.1
5.3
34.1
1.2
4.8
1.8
1.4
0.9
—
—
2.2
10.3
9.3
—
76.4
—
1.7
—
—
—
—
—
9.7
7.7
13.8
—
66.4
—
2.3
—
—
—
—
—
3.2
5.7
12.9
—
71.3
—
1.5
2.8
1.1
—
0.8
0.9
a These ions may have more than one precursor and cannot be
assigned to a specific process.
J. Mass Spectrom. 34, 408È420 (1999)
414
N. HARTING AND H.-F. GRU TZMACHER
abundant processes during the decomposition of ions
C H CH OCH , m/z 136, are initiated by a hydrogen
7 7
2
3
or proton transfer. The main dissociation of the
C H CH OCH radical ions is the loss of methanol,
7 7 requires
2
which
a3 mobile hydrogen on the ring, which is
eventually transferred to the ether group (Scheme 8).
The molecular ions 8m`~ and 8p`~ can mobilize this
hydrogen by H transfer from the methyl group on to
the aromatic ring. It should be noted that this is also
the Ðrst reaction step in the interconversion of toluene/
cycloheptatriene radical cations and is known to require
a substantial activation energy.12 In contrast, the ions
9`~ already have such a mobile hydrogen because of the
isotoluene structure, and this H atom just needs to be
transferred to the ether group. The loss of alcohol will
be discussed in more detail in the next section, but it is
obvious that loss of methanol should be a much more
abundant process for metastable isotoluene radical ions
9`~, as observed experimentally. Nevertheless, the elimination of CH OH cannot be used to distinguish
between toluene3 and isotoluene structures of the ions
C H CH OCH , m/z 136, since this is an abundant
7 7 for
2 both3isomers.
process
This is also true for the second abundant fragmentation of the ions C H CH OCH , m/z 136, by loss of an
7 7 In2 the 3case of the toluene-like
aldehyde or a ketone.
ions 8`~ this is the well known fragmentation of ionized
benzyl ethers23h25 which was presented in the Introduction (Scheme 4) and in which the initial reaction step
is the transfer of a H-atom to the aromatic ring.
However, it is known that H-atoms are also accepted by
ionized CÈC double bonds during the fragmentation of
the molecular ions of alkenes and in particular of polyenes.33 Consequently, elimination of an aldehyde or
ketone is expected to occur also from the isotoluene
isomers 9`~, although with a reduced abundance
because of the energetically less favored product ion
(Scheme 9).
Finally, the methyl radical eliminated from ions
C H CH OCH originates from the ring and not from
7 alkyl
7
2chain,3as demonstrated by deuteration of the
the
Scheme 8.
Copyright ( 1999 John Wiley & Sons, Ltd.
Scheme 9.
O-methyl group. Thus, only ions 8`~ with a toluene
structure are expected to lose easily a methyl radical by
initial H-transfer from the benzylic position of the
methoxymethyl side-chain on to the ring by a 1,2-H
shift. A “ring walkÏ mechanism34 brings the H-atom
eventually into the ipso-position to the methyl group
where it can assist the methyl cleavage (Scheme 10). In
the case of the ions 9`~ of an isotoluene structure, very
likely the methyl group has to be created Ðrst by an
H-transfer from the ring to the methylene group, corresponding to an isomerization of 9`~ into 8`~, as an
essential step of the methyl loss (Scheme 10).
Assuming that the loss of a methyl radical occurs
only from toluene-like ions 8`~, the intensity of the
signal for loss of methyl in the MIKE spectra can be
used to estimate the fraction of ions 8`~. This will give
an upper limit of the amount of isomerization. By this
Scheme 10.
J. Mass Spectrom. 34, 408È420 (1999)
TOLUENE AND ISOTOLUENE RADICAL CATIONS
criterion, the ions at m/z 136 generated from 10m`~ and
10p`~ contain about 42% ions 8`~, demonstrating a signiÐcant amount of isomerization of these fragment ions
because of the excess energy present. With one exception, at the maximum a fraction of 15% of ions 8`~ is
calculated for the other fragment ions C H CH OCH ,
7 7
2
3
m/z 136. The exception concerns the ions at m/z 136
generated from 1-n-propoxymethyl-4-methoxymethylbenzene (12p), which exhibit an unusually abundant loss
of methyl in the MIKE spectrum. This would correspond to an appreciable isomerization of 35% especially
of these ions C H CH OCH , m/z 136. However, in
7 7
2
3
agreement with the literature,17,18 this abundant loss of
a methyl radical from these ions at m/z 136 can be the
result of a concurring H-migration from the d-carbon
into the benzylic position (Scheme 11), which opens a
direct route to the toluene ion 8`~. Indeed, of the compounds studied, 12m`~ and 12p`~ are the only precursor ions which allow this H-transfer from a d-position.
Obviously, this H-transfer can compete with the favored
McLa†erty rearrangement only in the para-isomer
12p`~, probably because of a stabilizing e†ect of the
para-substituent.
The isomers 8`~ and 9`~ can be distinguished not
only by their MIKE spectra but also their CA spectra
(Tables 4 and 5). This is an uncommon result since the
unsubstituted isotoluene 2`~ and toluene 1`~ radical
cations are known to be indistinguishable by their CA
spectra. However, although CA spectra are believed to
be more suited for mixture analysis, the CA spectra of
Scheme 11.
Table 4. Relative signal intensities (% total ion current) in the
CA
spectra
of
the
meta-substituted
[ C H CH OCH ] ‘~ radical cations, m/z 136,
7 7
2
3
derived from 10m–13m.
Ion at m /z 136 generated from
Process
m /z
8m½~
10m½~
11m½~
12m½~
13m½~
ÉH~
ÉCH
3
ÉCH
5
ÉCO
ÉCH O
2
ÉCH O~
3
ÉCH OH
3
Othera
Othera
Othera
Othera
135
121
119
108
106
105
104
91
77
65
63
12.4
14.1
1.8
1.4
5.4
13.9
29.4
7.7
4.9
2.4
1.6
7.4
2.5
6.8
2.1
7.2
9.6
30.0
6.6
5.4
3.7
4.0
3.6
1.6
6.0
2.1
3.2
4.7
63.4
5.5
3.6
1.3
1.2
3.9
2.0
1.1
1.6
4.6
6.4
56.9
5.9
4.0
2.0
3.8
2.0
1.1
1.2
5.3
6.7
56.3
5.8
4.4
1.8
a These ions may have more than one precursor and cannot be
assigned to a specific process.
Copyright ( 1999 John Wiley & Sons, Ltd.
415
Table 5. Relative signal intensities (% total ion current) in
the
CA
spectra
of
the
para-substituted
[ C H CH OCH ] ‘~ radical cations, m/z 136,
7
7
2
3
derived from 10p–13p.
Process
m /z
8p‘~
10p‘~
ÉH~
ÉCH~
3
ÉCH
5
ÉCO
ÉCH O
2
ÉCH O~
3
ÉCH OH
3
Othera
Othera
Othera
Othera
Othera
CH OCH½
3
2
Othera
135
121
119
108
106
105
104
91
77
65
63
51
45
39
10.7
17.4
2.1
1.9
5.2
14.9
25.5
7.7
4.8
2.1
1.7
2.4
1.7
1.9
6.3
6.4
4.5
2.9
8.3
9.7
17.5
8.7
5.9
4.7
5.1
6.7
7.5
5.7
ions m /z 136 generated from
11p‘~
12p‘~
13p‘~
4.0
3.4
2.0
7.0
5.0
6.6
28.5
10.4
5.5
2.8
2.6
3.6
13.5
3.0
6.1
6.9
1.4
3.7
6.2
10.4
30.4
10.3
4.5
2.3
1.6
2.7
11.4
2.3
4.5
2.4
1.4
2.1
5.7
8.8
32.7
11.7
3.6
1.9
1.4
2.2
19.4
1.9
a These ions cannot be assigned directly to a specific process.
the ions derived from 10`~È13`~ give no additional
information about an interconversion of toluene ions
8`~ and isotoluene ions 9`~ since most of the CA fragmentations are structurally not speciÐc in this respect.
Using again the assumption that loss of methyl occurs
only from toluene-like ions 8`~, slightly larger amounts
of isomerization (\20% with the exception of 12p`~) of
the isotoluene-like ions derived from 11`~È13`~ are
obtained than calculated from the MIKE spectra.
Anyway, the clear di†erences observed between the
three classes of ions C H CH OCH , m/z 136, also in
7 7 by CA
2 the3 fragmentations of
the CA spectra prove that
the isomers 8`~ and 9`~ proceed faster than isomerization. Therefore, we conclude that the ions
C H CH OCH , m/z 136, from 10`~È13`~ are formed
2
3 rearrangement predominantly as isoby7 a7McLa†erty
toluene ions 9`~, and that only a small proportion of
these ions have sufficient excess energy to rearrange into
the more stable ion 8`~, with the exception of ions at
m/z 136 generated by loss of formaldehyde from 10`~.
These results corroborate the view that 8`~ and 9`~ are
two stable species which do not interconvert easily but
exist separately with a substantial energy barrier in
between.
Fragmentations of alkoxymethyltoluene ions and
alkoxymethyl isotoluene ions (alkyl = ethyl, n-propyl and
isopropyl), m/z (136 + 14n)
Alkoxymethyl-substituted toluene radical cations 14`~,
16`~ and 18`~ with ethyl-, n-propyl and isopropyl ether
groups, respectively, were obtained by 70 eV EI of the
corresponding methylbenzyl alkyl ethers (Scheme 12).
The corresponding isomeric isotoluene radical ions
15`~, 17`~ and 19`~ (Scheme 12) were produced by loss
of acetaldehyde, propionaldehyde and acetone, respectively, from the molecular ions of the symmetrically
substituted bis(alkoxymethyl)benzenes 20È22. In both
series of ions the isomers with the alkoxymethyl substituent in the meta- (suffix m) and para-positions (suffix p)
J. Mass Spectrom. 34, 408È420 (1999)
416
N. HARTING AND H.-F. GRU TZMACHER
Scheme 12.
were studied. Compared with the bis(methoxymethyl)benzenes 10 and the alkoxymethylmethoxymethylbenzenes 11È13, the metastable molecular ions of the
bis(alkoxymethyl)benzenes 20È22 prefer decomposition
to the ions C H CH OR`~ by elimination of an alde7 molecule.
7
2 It can be seen from the partial
hyde or ketone
MIKE spectra of 10p`~ in Fig. 1 and 20p`~È22p`~ in
Fig. 2 that the loss of the aldehyde or ketone competes
more and more e†ectively with the elimination of
alcohol with an increasing size of the O-alkyl group.
This agrees with the known e†ects of the CÈH bond
energy and the stability of the neutral lost in that type
of a McLa†erty rearrangement (see discussion in the
Introduction and Scheme 3).
The relative intensities of the signals in MIKE spectra
of the alkoxymethyl-substituted toluene radical ions
14`~, 16`~ and 18`~ and their isotoluene isomers 15`~,
17`~ and 19`~ are presented in Tables 6 and 7 for metaand para-substituted derivatives, respectively. The corresponding CA mass spectra are shown in Tables 8 and
9.
Figure 2. Partial MIKE spectra of the molecular ions of (a)
bis(ethoxymethyl)benzene
20p½~ ; (b) bis(n -propoxymethyl)
benzene 21p½~ and (c) bis(isopropoxymethyl)benzene 22p½~.
In close analogy to the methoxymethyl-substituted
toluene and isotoluene radical ions 8`~ and 9`~, the corresponding higher homologues give di†erent MIKE
spectra for isomeric arene and isoarene radical cations.
Table 6. Relative signal intensities (% total ion current) in the MIKE spectra of the
meta-substituted [ C H CH OCH ] ‘~ radical cations, m/z 136, derived
7 7
2
3
from 14m–22m.
Process
ions m /z 150 from
14m‘~
20m‘~
ÉCH~
3
ÉH O
2
ÉC H
2 4
ÉC H~
2 5
ÉCH CHO
3
ÉC H O~
2 5
ÉC H OH
2 5
Copyright ( 1999 John Wiley & Sons, Ltd.
6.8
—
3.8
0.8
82.3
—
6.9
1.7
0.5
0.9
0.3
5.1
0.2
89.6
Process
16m‘~
21m‘~
ÉCH~
3
ÉH O
2
ÉC H
3 6
É~C H
3 7
ÉC H CHO
2 5
ÉC H O~
3 7
ÉC H OH
3 7
7.4
—
6.0
1.1
77.6
—
8.0
—
—
—
—
11.0
3.1
85.9
ions m /z 164 from
18m‘~
22m‘~
7.0
—
16.5
0.2
69.2
—
—
1.0
0.4
15.3
4.1
79.4
J. Mass Spectrom. 34, 408È420 (1999)
TOLUENE AND ISOTOLUENE RADICAL CATIONS
417
Table 7. Relative signal intensities (% total ion current) in the MIKE spectra of
the para-substituted [ C H CH OCH ] ‘~ radical cations, m/z 136, derived from
7 7
2
3
14p–22p.
Process
ÉCH~
3
ÉH O
2
ÉC H
2 4
ÉC H~
2 5
ÉCH CHO
3
ÉC H O~
2 5
ÉC H OH
2 5
ions m /z 150 from
14p‘~
10p‘~
14
—
2.2
0.7
70.6
—
12.5
1.5
2.2
5.1
1.7
66.1
3.8
18.0
Process
16p‘~
ions m /z 164 from
21p‘~
18p‘~
22p‘~
ÉCH~
3
ÉH O
2
ÉC H
3 6
ÉC H
3 7
ÉC H CHO
2 5
ÉC H O~
3 7
ÉC H OH
3 7
2.7
—
3.8
0.8
81.5
—
11.1
4
13.0
9.5
2.9
58.8
3.6
8.2
As expected from the results discussed in the previous
section, the MIKE spectra of the alkoxymethyltoluene
radical ions are dominated by the signal due to the
elimination of an aldehyde or ketone while the MIKE
spectra of the fragment ions C H CH OR`~ are char7 7 elimination
2
acterized by a more abundant
of the
alcohol. Further, the signiÐcant loss of ~CH is again
more abundant in the MIKE spectra3 of the
alkoxymethyl-substituted toluene ions.
Using the criterion of “~CH loss only from
3
alkoxymethyl-substituted toluene radical
cationsÏ again
to estimate the toluene-like ions present in the fragment
ion mixture [C H CH OR]`~ generated from
7 7
220È22, fractions of 10È25%
bis(alkoxymethyl)benzenes
of ions were calculated which have undergone isomerization after formation as isotoluene radical cations.
However, a closer inspection of the MIKE spectra in
Tables 6 and 7 reveals that this procedure considerably
overestimates the amount of isomerization. For
example, from the loss of ~CH in the MIKE spectra of
15m`~ and [C H CH OR]`~ 3derived from 20m (Table
7 7 that2 25% of the fragment ions have
6), it is calculated
isomerized into 15m`~ (or are formed directly as this
isomer). However, the presence of 25% of ions 15m`~ in
the mixture of fragment ions would require a relative
intensity of more than 20% for the signal due to loss of
acetaldehyde in the MIKE spectrum of these ions
whereas only 5.1% is observed. This latter value would
correspond to a fraction of 6% of isomerized ions only,
but clearly acetaldehyde may be also eliminated from
the ethoxymethyl-substituted isotoluene radical cation.
Hence the amount of isomerized ions should be even
lower than 6%. Making similar “cross-checksÏ also for
the MIKE spectra of the other fragment ions
[C H CH OR]`~ reveals that for all these ions the
7 7 of2 ions isomerized by rearomatization is small
amount
and very likely always distinctly below 10%.
It is striking that the meta- and para-isomers of the
alkoxy-substituted isotoluene radical ions 15`~, 17`~
and 19`~ yield very di†erent MIKE and CA spectra
with respect to the abundance of the aldehyde/ketone
loss and of the elimination of alcohol. This e†ect is
much more pronounced than in the MIKE and CA
spectra of the methoxymethyl-substituted isotoluene
ions 9`~ (see Tables 2È5) and of the meta- and paraisomers of alkoxymethyl-substituted toluene radical
ions 14`~, 16`~ and 18`~. The loss of alcohol and the
formation of the radical cation at m/z 104 should dominate in the MIKE and CA spectra of isotoluene radical
ions because of the extra H-atom on the ring (see
Copyright ( 1999 John Wiley & Sons, Ltd.
6.6
—
5.7
0.2
84.3
—
3.0
2.2
0.3
10.6
3.6
73.2
—
8.2
Scheme 8), while elimination of the alkoxy group as an
aldehyde or ketone molecule should be a minor process.
However, this is observed only for meta-isomers, while
the para-isomers exhibit more abundant losses of
aldehyde/ketone. This is unexpected since the product
ion, m/z 104, of the elimination of alcohol from the
meta-isomers 15m`~, 17m`~ and 19m`~ must have a
meta-quinoid structure which is energetically much less
favored than the para-quinoid product ion arising from
the corresponding para-isomers 15p`~, 17p`~ and 19p`~
(Scheme 13).
One explanation could be a di†erent internal energy
of the fragment ions [C H CH OR]`~ arising from
7
2
meta- and para-substituted7 precursors,
but in this case
the di†erence should disappear in the CA spectra.
Scheme 13.
J. Mass Spectrom. 34, 408È420 (1999)
418
N. HARTING AND H.-F. GRU TZMACHER
Table 8. Relative signal intensities (% total ion current) in the
CA spectra of the meta-substituted [ C H CH OR ] ‘~ radical
7 7
2from 14m, 18m
cations (R = C H , n-C H , i-C H ) derived
3 7
3 7
and 20m–22m. 2 5
m /z
ions m /z 150 from
14‘~
20m‘~
m /z
16m‘~
ions m /z 164 from
21m‘~
18m‘~
22m‘~
149
135
122
121
119
106
105
104
93
91
77
65
63
59
51
39
31
—
—
4.2
3.4
3.0
2.4
2.8
38.6
12.4
14.4
3.5
6.0
3.9
1.4
1.0
0.7
1.1
0.8
0.4
—
—
163
149
146
122
121
119
106
105
104
93
91
77
73
65
63
51
43
41
39
2.8
2.9
—
5.3
3.0
1.4
41.8
13.1
13.6
2.6
4.4
2.9
0.4
0.8
0.6
0.7
1.9
1.1
0.8
—
—
—
1.5
—
—
5.3
14.0
63.5
—
3.5
2.1
1.3
1.1
1.1
1.1
3.1
1.3
0.9
—
—
—
1.4
0.6
3.5
8.0
65.3
—
4.8
4.2
1.9
1.4
3.8
1.6
1.6
1.9
—
—
—
2.0
—
8.9
2.9
1.4
39.2
10.7
16.6
2.8
4.7
3.0
—
1.2
0.8
1.1
2.8
0.8
1.1
—
—
—
1.2
1.6
1.1
6.9
16.0
59.3
—
4.5
2.5
—
0.9
0.7
0.9
3.0
0.7
0.7
Shown in Tables 8 and 9. However, the di†erence
between the meta- and para-isomers of the alkoxysubstituted isotoluene radical ions persists in the CA
spectra, so that this explanation can be disgarded. We
suggest that the reason for this di†erence is a substituent e†ect on the reacting conÐguration of the elimination of alcohol from isotoluene radical ions, and that
the crucial step is the H-transfer from the methylenecyclohexadiene ring on to the ether-O atom of the
side-chain. Owing to the steric constraints of the
Table 9. Relative signal intensities (% total ion current) in the
CA spectra of the para-substituted [ C H CH OR ] ‘~
7 7
2
radical cations (R = C H , n-C H , i-C H ) derived
2 5
3 7
3 7
from 14p, 18p and 20p–22p.
m /z
ions m /z 150 from
14p‘~
20p‘~
m /z
16p‘~
ions m /z 164 from
21p‘~
18‘~
22p‘~
149
135
122
121
119
106
105
104
93
91
77
65
63
59
51
39
31
—
—
—
2.0
7.3
2.0
2.9
1.5
29.5
17.4
17.8
3.0
5.7
4.4
1.6
1.1
—
1.7
1.4
0.8
—
—
—
163
149
146
122
121
119
106
105
104
93
91
77
73
65
63
59
51
43
41
39
—
2.5
3.5
4.9
3.9
2.4
15.5
8.9
11.2
3.2
6.3
3.1
9.3
3.3
3.2
—
3.6
7.9
3.7
3.7
—
0.5
2.5
4.4
2.9
0.9
18.5
10.8
14.4
1.9
6.7
3.8
11.5
2
2
—
2.8
9.2
2.7
2.6
3.7
1.7
3.8
1.9
1.2
27.9
11.0
22.8
—
7.0
4.1
1.4
1.1
7.5
1.8
1.8
1.4
—
—
—
Copyright ( 1999 John Wiley & Sons, Ltd.
0.8
2.3
—
5.8
3.3
1.5
45.2
17.5
9.9
2.1
3.6
2.3
—
0.8
0.5
—
0.7
2.1
0.8
0.8
—
0.2
1.9
7.1
3.9
—
36.5
7
11.4
—
6.5
2.7
8.5
1.4
1.3
2.1
1.5
4.8
1.4
1.8
Scheme 14.
unsaturated ring, this transfer can occur only as a 1,3-H
shift from the ipso-position or as a 1,4-H shift from an
position ortho to the alkoxymethyl side-chain (Scheme
13). It is known35 that 1,3-H shifts require a large activation energy whereas 1,4- and 1,5-H shifts are much
more favorable. As can be seen from the reacting conÐguration of the favored 1,4-H shift within the metaand para-isomers of the alkoxymethyl-substituted isotoluene radical ions, only in the case of the metaisomers can an ortho- and para-quinoid structure be
adopted, while the reacting conÐguration of the paraisomers corresponds always to an unfavorable metaquinoid arrangement. Thus, the elimination of alcohol
from the meta- and para-isomers of the substituted isotoluene is obviously not governed by the stability of the
product ions but by the stability of a reaction intermediate necessary for a facile H-transfer.
The abundant elimination of an aldehyde/ketone
molecule from the para-isomers of the alkoxymethylisotoluene radical cations is unusual, since this is a typical
reaction of the radical cations of alkoxymethylsubstituted arenes. However, a fast isomerization of the
ions 15p`~, 17p`~ and 19p`~ into their toluene counterparts can be excluded by the weak loss of a ~CH in the
MIKE spectra (Table 7). Further, the 3 paraalkoxymethylisotoluene radical ions are still distinguished from the isomeric toluene radical ions by a
more abundant elimination of alcohol and a less abundant loss of aldehyde/ketone in the MIKE and CA
J. Mass Spectrom. 34, 408È420 (1999)
TOLUENE AND ISOTOLUENE RADICAL CATIONS
spectra. The loss of an aldehyde or ketone molecule
from the alkoxymethyl side-chain requires in every case
the transfer of an H-atom from the O-alkyl group to the
rest of the unsaturated radical ion. In principle, an
ionized polyene can act as the acceptor for the H
atom,32 but then the acceptor ability for this Hrearrangement must be very di†erent for the meta- and
para-isomers of the substituted isotoluene radical ions.
Indeed, assuming a 1,5-H shift with a six-membered
transition state as the most favored one as the initial
reaction step, the resulting distonic ion retains
maximum resonance stabilization only in the case of the
para-isomer. For this isomer, the H-atom from the Oalkoxy group migrates to the end of the polyene chain,
whereas in the case of the meta-isomer one of the inner
C atoms of the polyene chain is the acceptor of the
migrating H-atom (Scheme 14). Hence the elimination
of an aldehyde or ketone from m-alkoxymethylisotoluene radical cations is disfavored, either by generation of an unfavorable distonic intermediate or, avoiding the formation of this intermediate by avoiding the
1,5-H shift, by a larger activation energy for the H shift.
CONCLUSION
As expected from published results, the molecular ions
of the asymmetrically substituted and symmetrically
substituted bis(alkoxymethyl)benzenes 10-13 and 20È22
produce alkoxymethyl-substituted isotoluene radical
cations (alkoxymethylmethylencyclohexadiene radical
cations) by elimination of an aldehyde or ketone molecule. Our results conÐrm the earlier observations that
the strength of the CÈH bond broken during the initial
H transfer of this fragmentation and the stability of the
neutral lost have a strong inÑuence on the formation of
these ions. Therefore, in the series of bis(alkoxymethyl)benzenes studied here, the loss of formaldehyde from a
methoxymethyl side-chain is not abundant. This loss of
formaldehyde is associated with a large KER, which
results in a round-topped or Ñat-topped signal in the
MIKE spectrum. Thus, the alkoxymethyl-substituted
isotoluene radical cations arising from formaldehyde
loss obviously contain an appreciable amount of excess
energy, which has consequences for their further reactions.
419
As hoped for in the design of these experiments, the
alkoxymethyl substituent of toluene radical cations and
of the corresponding isotoluene radical cations is a
useful probe for the structure of these ions. Metastable
and collisionally activated alkoxymethyltoluene radicals
prefer the elimination of this side-chain as an aldehyde
or ketone molecule whereas the isotoluene isomers are
characterized by an abundant loss of alcohol. Additionally, the MIKE and CA spectra of the alkoxymethyl
toluene radical cations exhibit a distinct peak for the
loss of the methyl group from the ring. Using these differences in the MIKE spectra, it can be estimated that
less than 10% of the alkoxymethyl-substituted isotoluene radical cations formed by fragmentation have
isomerized into the more stable toluene structure before
further decomposition. This result shows unambiguously that (substituted) isotoluene radical cations are sufficiently stable to be studied by tandem mass
spectrometry. However, special care must be taken to
avoid deposition of excess energy in the isotoluene
radical cations during their formation, because the activation barrier for the isomerization cannot be very
large. This follows from the experiment in which the
methoxymethylisotoluene radical cations were generated by loss of a formaldehyde molecule. The MIKE
and CA spectrum of the resulting fragment ions show
clearly that in this case a large proportion of the initially formed methoxymethylisotoluene radical cations
has isomerized into the more stable toluene structure.
Finally, an alkoxymethyl side-chain with the O-alkyl
group corresponding to ethyl or a higher homologue
permits the di†erentiation between the meta- and paraisomers of the substituted isotoluene radical cations by
the relative abundances of the elimination of alcohol
and aldehyde or ketone, respectively. This is additional
proof that alkoxymethyl-substituted isotoluene radical
cations undergo fragmentation before any signiÐcant
isomerization.
Acknowledgements
We thank Professor S. Hammerum, University of Copenhagen,
Denmark, for the opportunity to use the Jeol mass spectrometer in
Copenhagen and Solveig K. Hansen, Katrine M. Petersen and Allan
Petersen, University of Copenhagen, for their support during these
measurements. The Ðnancial assistance of this work by the Fonds der
Chemischen Industrie is gratefully acknowledged.
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toluene, probl, stud, formation, substituents, fragmentation, isomerization, isotoluene, radical, cation, alkoxymethyl
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