Experimental and Theoretical Studies on Gas-Phase Fragmentation Reactions of Protonated Methyl Benzoate: Concomitant Neutral Eliminations of Benzene, Carbon Dioxide, and Methanol

Article abstract of DOI:10.1007/s13361-018-1997-8

Protonated methyl benzoate, upon activation, fragments by three distinct pathways. The m/z 137 ion for the protonated species generated by helium-plasma ionization (HePI) was mass-selected and subjected to collisional activation. In one fragmentation path

Full text of DOI:10.1007/s13361-018-1997-8

B American Society for Mass Spectrometry, 2018
J. Am. Soc. Mass Spectrom. (2018)
DOI: 10.1007/s13361-018-1997-8
RESEARCH ARTICLE
Experimental and Theoretical Studies on Gas-Phase
Fragmentation Reactions of Protonated Methyl Benzoate:
Concomitant Neutral Eliminations of Benzene, Carbon
Dioxide, and Methanol
Hanxue Xia, Yong Zhang, Athula B. Attygalle
Center for Mass Spectrometry, Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ
07030, USA
Abstract. Protonated methyl benzoate, upon ac-
tivation, fragments by three distinct pathways.
The m/z 137 ion for the protonated species gen-
erated by helium-plasma ionization (HePI) was
mass-selected and subjected to collisional activa-
tion. In one fragmentation pathway, the protonat-
ed molecule generated a product ion of m/z 59 by
eliminating a molecule of benzene (Pathway I).
The m/z 59 ion (generally recognized as the
methoxycarbonyl cation) produced in this way,
then formed a methyl carbenium ion in situ by decarboxylation, which in turn evoked an electrophilic aromatic
addition reaction on the benzene ring by a termolecular process to generate the toluenium cation (Pathway II).
Moreover, protonated methyl benzoate undergoes also a methanol loss (Pathway III). However, it is not a simple
removal of a methanol molecule after a protonation on the methoxy group. The incipient proton migrates to the
ring and randomizes to a certain degree before a subsequent transfer of one of the ring protons to the alkoxy
group for the concomitant methanol elimination. The spectrum recorded from deuteronated methyl benzoate
showed two peaks at m/z 105 and 106 for the benzoyl cation at a ratio of 2:1, confirming the charge-imparting
proton is mobile. However, the proton transfer from the benzenium intermediate to the methoxy group for the
methanol loss occurs before achieving a complete state of scrambling.
Keywords: HepI, Helium-plasma ionization, Methyl benzoate, Fragmentation
Received: 17 March 2018/Revised: 7 May 2018/Accepted: 13 May 2018
philic species, which is the methyl carbenium ion, is generated
in situ and is added to the benzene ring to generate a toluenium
Introduction
ewis acid-catalyzed electrophilic aromatic substitution is
a classical reaction widely used in synthetic organic
chemistry [1]. The reaction occurs via an initial adduct called
the Wheland intermediate [2]. One of the simplest examples of
this reaction is the generation of toluene from benzene under
Friedel-Crafts conditions [3–5]. For this reaction, the electro-
ion [6, 7]. Analogous alkylations have been investigated in gas
phase for many decades [ 8–16]. Some alkylation reactions
have been conducted in gas phase with olefins and arenium
ions generated by chemical ionization [17]. The mechanism of
aromatic electrophilic substitution has been thoroughly scruti-
nized, and the role that arenium ions play as intermediates has
been well established [18]. Particularly for gas-phase reactions,
the intermediacy of complexes formed between arenium ions
and neutral complexes has been well accepted [10, 19, 20]. The
primary benefit of investigating ion–molecule interactions in
the gas phase with mass-selected ions under mass spectromet-
ric conditions is that such reactions can be thoroughly
L
Electronic supplementary material The online version of this article (https://
doi.org/10.1007/s13361-018-1997-8) contains supplementary material, which
is available to authorized users.
Correspondence to: Athula Attygalle; e-mail: aattygal@stevens.edu

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
scrutinized without the impediments mediated by solvent and
counterion effects [21–23]. As part of our pursuit on intricate
details of gas-phase ion fragmentation mechanisms [24], we
investigated protonation of methyl benzoate under helium-
plasma ionization (HePI) conditions [25]. From the spectra
recorded from protonated methyl benzoate, we noted a peak
at m/z 93 for the toluenium ion that originates from an internal
electrophilic aromatic substitution reaction.
held at 1.16 × 10−4 mbar, and the laboratory-frame collision
energy was kept at about 15 eV (unless otherwise stated). Liquid
samples (about 1–5 μL) were mixed with silica gel (200 μm;
5 mg) and added to the bottom of a one-side-sealed glass tube
(1.7 o.d. × 1.3 i.d. × 11.0 mm), and the tube was attached to the
inner side of the cylindrical glass enclosure with the aid of a small
wad of Bmuseum putty.^ For gas-phase deuterium-exchange
experiments, a cotton swab soaked in D2O (20–30 μL) was
inserted into the source enclosure through a hole in the front
panel of the source. A stream of hot nitrogen (150 °C) via the
Bdesolvation gas^ line was deployed to facilitate the evaporation
of D2O from the cotton swab. Accurate mass product-ion spectra
were recorded on a Synapt G2 HDMS (Waters Corp., MA) mass
spectrometer equipped with a HePI source. Nitrogen was used as
the nebulizer, desolvation, and cone gases. Mass calibration (m/z
10–1200) was performed using a solution of sodium formate
(100 ppm). Mass spectra were acquired in the positive-ion mode
over a range of m/z 10–150. The capillary voltage was set at
3.00 kV; sampling cone, 11.0 V; extraction cone, 1.5 V;
desolvation-gas flow rate, 370 L/h; Vernier-probe adjuster posi-
tion, 5.92 mm; trap collision energy, 2 eV; transfer collision
energy, 11.8 eV. The source and desolvation-gas temperatures
were held at 80 and 100 °C, respectively.
Although chemical ionization of esters has been the subject
of extensive studies [26, 27], detailed investigations have been
carried out generally under negative-ion generating modes
[28]. To rationalize the gas-phase dissociation of protonated
esters, three major fragmentation pathways have been proposed
[26]. Although the loss of an alcohol molecule by the acyl
carbon-oxygen bond cleavage to generate an acylium cation is
one of the major fragmentation pathways for most esters, it
appears that for esters made from higher alcohols, the loss of an
alkene molecule to form a protonated acid takes precedence.
However, it is irrational to expect an alkene loss from methyl
esters. In the third mechanism, the protonated ester eliminates
the neutral acid and generates a carbocation. Such a mechanism
is also not expected from methyl esters because the formation
of a methyl carbenium ion is thermodynamically unfavorable.
Thus, the second and third mechanisms do not contribute to
fragmentation pathways of protonated methyl benzoate. In
other words, the protonated methyl benzoate molecule neither
fragments to generate protonated benzoic acid nor generates
methyl cation by a direct cleavage mechanism; instead, it loses
a molecule of methanol as envisaged by the first mechanism.
However, we noted that it is not a simple and straightforward
reaction. The results of our detailed study are presented here.
Chemical Synthesis
Synthesis of [2H3]Methyl Benzoate
Methanol-d4 (5 μL) was mixed with benzoic acid (10 mg),
dicyclohexylcarbodiimide (10 mg), and a catalytic amount of
4-dimethylaminopyridine in dichloromethane (1 mL) at room
temperature. The mixture was heated at 50 °C for 1 h and
flashed through a silica gel column with hexane/diethyl ether
(5:5). 70-eV EI-MS, m/z (%) 139(M+·, 41), 106(9), 105(100),
77(68), 51(42), 50(25).
Experimental
Materials
Methyl [2,3,4,5,6 -2H5]Benzoate
All chemicals including methyl benzoate, [2,3,4,5,6-
2H5]benzoic acid, [carboxyl-13C]benzoic acid, methanol-d4,
D2O (99.9 atom % D), and H218O (97 atom % 18O) were
purchased from Sigma-Aldrich Chemical Co. (St Louis, MO)
and used without further purification. Structures of all synthetic
products were confirmed by mass spectrometry. EI mass spec-
tra (70 eV) were recorded by GC–MS.
To a solution of [2,3,4,5,6-2H5]benzoic acid (1.2 mg) in Et2O
(1 mL), methanol (5 μL) and p-toluene sulfonic acid (1 mg)
were added, and the mixture was stirred overnight at 60 °C.
After adding H2O (1 mL), the reaction mixture was extracted
three times with diethyl ether (5 mL × 3). The desired product
was isolated as described above. 70-eV EI-MS, m/z (%)
141(M+·, 38), 110(100), 82(66), 54(34).
Mass Spectrometry
Methyl [Carboxyl-13C]benzoate
A Micromass Quattro Ultima triple-quadrupole tandem mass
spectrometer (Manchester, UK) was used to obtain all collision-
induced dissociation mass spectra. Gaseous ions from samples
were generated by HePI procedures [25]. A stream (~30 mL
min−1) of high-purity helium (99.999%) was passed through the
metal capillary held typically at about 3.6 kV. The source tem-
perature was held at 100 °C. The hot (100 °C) desolvation gas
(N2), at a flow of 90 L/h, was used to heat the samples. Typically,
the cone voltage for the ion transfer was set at 10 V. For CID
experiments, the pressure of the argon in the collision cell was
This compound was synthesized starting from [carboxyl-
13C]benzoic acid, by a procedure similar to that described for
methyl [2,3,4,5,6 -2H5]benzoate. 70-eV EI-MS, m/z (%)
137(M+·, 32), 106(100), 77(77), 51(37), 50(21).
[18O2]Benzoic Acid
To a solution of benzoic acid (1 mg) in 18O-water (95%,
50 μL), concentrated H2SO4 (1 μL) was added, and the mixture
was stirred overnight at 60 °C. The mixture was extracted three

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
times with Et2O (0.5 mL × 3). The ether layers were separated,
combined, and evaporated to obtain the desired product.
the methoxy group. Moreover, the high Gibbs free energies
obtained for the four ring-protonated isomers indicate that
direct ring protonation is disfavored, which agrees with the
experimental observation that protonated methyl benzoate
when exposed to D2O vapor undergoes essentially only one
H/D exchange (Fig. 1a, b). It is well known that the benzenium
ion rapidly interacts with water [21, 33, 37–39]. In other words,
if the charge-imparting proton migrates rapidly to the ring to
form a benzenium ion, more than one H/D exchanges are
expected when methyl benzoate is exposed to D2O in a HePI
source. For example, such a tendency is witnessed with pro-
tonated methyl phenylacetate, which undergoes at least 3–4
H/D exchanges under similar ion-source conditions (Fig. 1a,
d). Proton shuttling mechanisms mediated by water within the
timeframe of acquiring mass spectra have been well investigat-
ed [40, 41]. The suggestions we made on charge-imparting
proton migrations are supported by our computational results
which indicated that the ring protonation in methyl
phenylacetate is not as prohibitive as that observed for methyl
benzoate. For methyl benzoate, a protonation at any ring site of
the molecule increases its relative energy to a higher level than
that predicted for the alkoxy protonation. Thus, it is not sur-
prising that protonated methyl phenylacetate undergoes more
than one H/D exchanges because a protonation on any ring
carbon of methyl phenylacetate is favored over a protonation
on its alkoxyl oxygen atom (Table 1). Moreover, the Gibbs free
energy barrier that the proton must surmount for a transfer from
the carbonyl-oxygen site of methyl phenylacetate to the ring
ortho position via the six-membered transition state (TS 2a-
2c2) is relatively low (5.76 kcal/mol) compared to 37.18 kcal/
mol required to reach the TS 1a-1c2 transition state of proton-
ated methyl benzoate (Fig. S2).
The mass-selected m/z 137 ion, upon collisional activation,
fragments by several different pathways. The base peak of the
spectrum was observed at m/z 59 (Fig. 2a). For the formation of
the m/z 59 ion, a molecule of benzene must be eliminated from
the protonated precursor molecule. Accurate mass measure-
ments confirmed the elementary composition of m/z 59 ion to
be C2H3O2 (Fig. S3). Spectra recorded from protonated methyl
[carboxyl-13C]benzoate (Fig. 2b), methyl [carbonyl-18O]ben-
zoate (Fig. 2c), [2H3]methyl benzoate (Fig. 2e), and methyl
[2,3,4,5,6-2H5]benzoate (Fig. 2f) showed peaks at m/z 60, 61,
62, and 59, respectively, which indicated that the carbonyl-
carbon and all the three hydrogen atoms on the methyl group
are retained when the m/z 59 product ion is formed. Further-
more, gas-phase H/D exchange experiments showed that the
incipient proton that bestows the charge to the molecule is
specifically lost with the elimination of the benzene molecule
(Fig. 2d). Based on this supportive data, the elemental compo-
sition of the m/z 59 ion was further confirmed to be C2H3O2.
The m/z 59 ion is a frequently encountered fragment in the
mass spectra of carboxylic compounds [42]. Many familiar,
and some unfamiliar, isomeric structures are possible for this
m/z 59 ion [43–46]. The structures of isomeric [C2H3O2]+ ions
have been investigated extensively using computational
methods [44, 47]. Blanchette et al. considered 11 isomeric
Methyl [Carbonyl-18O]Benzoate
This compound was synthesized in a manner similar to that for
methyl [2,3,4,5,6 -2H5]benzoate, starting from [18O2]benzoic
acid. 70-eV EI-MS, m/z (%) 138(M+·, 16), 137(4), 136(27),
108(4), 107(50), 106(7), 105(78), 77(100), 51(59), 50(33).
Computational Methods
All calculations were done using Gaussian 09 [29]. Geometries
of all species involved in several possible pathways were fully
optimized by using the widely used B3LYP [30, 31] method
with a large 6–311++G(2d,2p) basis set. Frequency calcula-
tions were done at the same level to verify the nature of each
stationary state on the potential energy surface, i.e., reactants/
products are associated with all positive vibrational frequen-
cies, and transition state (TS) is associated with only one
imaginary frequency, for which the normal vibrational mode
corresponds to the expected bond formation/breaking move-
ments in a specific reaction pathway. Gibbs free energies were
calculated at room temperature and ambient pressure.
Results and Discussion
Helium-plasma ionization (HePI) provides a potent way of
forming protonated molecules even from gaseous substances
[25]. For example, methyl benzoate vapor when subjected to
HePI generates an ion of m/z 137 for the protonated species
(Fig. 1a). The determination of the precise protonation sites of
gas-phase aromatic molecules has attracted considerable atten-
tion of researchers for many years [32–36]. Thus, the first
question we wanted to address in this study was the initial
protonation site of methyl benzoate.
In addition to the four different ring sites, there are two other
positions in methyl benzoate that could undergo protonation
(Table 1). The relative Gibbs free energy values calculated for
the optimized structures for the putative positive ions showed
that the carbonyl-oxygen is the thermodynamically most favor-
able protonation site (Table 1).
The computational results predicted that the carbonyl-
oxygen to be the most favorable site to accept the incipient
proton. It also indicated that the protonation on carbonyl-
oxygen lengthens the C=O double bond to 1.293 Å with a
concomitant shortening of the Ccarboxyl―Oalkoxy single bond to
1.268 Å (Supporting Information, Fig. S1a). Moreover, once
protonated on the carbonyl-oxygen, the carboxylate group is
positioned closer to the phenyl ring to maximize intramolecular
interactions. The calculation results also showed that the pro-
tonation at the alkoxy oxygen site leads to an immediate
dissociation of the bond between this oxygen atom and the
carbonyl-carbon since the bond length is greatly elongated to
2.043 Å (Fig. S1b). Thus, the molecule should immediately
lose methanol to form the m/z 105 species, upon protonation on

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
Figure 1. A unit-mass resolution MS1 spectra recorded from samples of methyl benzoate (a) and methyl phenylacetate (c) placed in
the HePI source, at a cone voltage setting of 10 V. After 10 s of data acquisition, a cotton swab soaked in D2O was introduced to the
enclosed HePI source and spectra were re-recorded (b, d)
structures and concluded that the methoxycarbonyl cation to be
the thermodynamically most stable species [44].
generated by the initial fragmentation can remain associated
as a transient termolecular complex (TS 1c1-4). The methyl
cation in the complex, then acts as an electrophilic reagent
and attacks the benzene ring to generate the m/z 93 toluenium
cation (Scheme 1). Such addition products are not usually
observed in low-pressure collision cells. However,
Giacomello and Pepi [11] had demonstrated that when ben-
zene is exposed to the methoxycarbonyl cation generated
from the dissociation of methyl chloroformate in an ICR cell
at relatively high pressure (0.2–0.3 Torr), an electrophilic
reaction takes place to generate an m/z 93 ion. Herein, we
have shown that a similar reaction takes place intra-
molecularly in which the methoxycarbonyl cation acts as a
methylcarbenium ion donor in a manner that resembles the
well-known Friedel–Crafts substitution mechanism [3, 5].
Gibbs free energy calculations indicated that the energy
demand for the generation of the m/z 59 ion (39.4 kcal mol−1)
is only slightly higher than that needed for the m/z 93 ion
formation (TS 1c1-4, 39.07 kcal mol−1) (Fig. 3). At a first
glance, this result appeared to be incongruent with the
experimental result that the intensity of m/z 93 peak is
much weaker than that at m/z 59. However, because the
m/z 93 ion can further fragment to form m/z 91 and 77
ions [49, 50], it is evident that the intensity of m/z 93, 91,
and 77 peaks taken together is similar to that of the m/z 59
ion. In fact, this result is in agreement with the observation
that the m/z 93 ion is formed at lower collision energies
than those needed for the m/z 59 ion generation because
the formation energy for m/z 93 ion is somewhat lower
than that of the m/z 59 ion (Fig. S4).
The most interesting peak in the product–ion spectrum of
protonated methyl benzoate is that observed at m/z 93. To
generate an m/z 93 ion, a molecule of carbon dioxide should
be eliminated from the precursor m/z 137 ion. The loss of CO2
was confirmed by the spectra acquired from isotope-labeled
methyl benzoates. The spectra recorded from protonated meth-
yl [carbonyl-18O]benzoate (Fig. 2c) or methyl [carboxyl-
13C]benzoate (Fig. 2b) showed a peak at m/z 93 corresponding
to a loss of CO18O or 13CO2, respectively. This result indicated
that the atoms required for the loss of a 44-Da neutral molecule
from protonated methyl benzoate are the carbonyl-carbon atom
and the two oxygen atoms.
To eliminate a carbon dioxide molecule from the precur-
sor ion, the methyl group must be transferred to the phenyl
ring by a specific rearrangement. To rationalize the forma-
tion of both the methoxycarbonyl (m/z 59) and the C7H9+ m/z
93 cations, we propose the fragmentation mechanism illus-
trated in Scheme 1. Upon activation of the precursor ion, the
charge-imparting proton attached to the carbonyl-oxygen
(1a) is initially transferred to the ortho-ring position via a
five-membered transition state (TS 1a-1c2) (Fig. 3). The
proton is then relocated to the ipso-ring carbon, and the
intermediate formed in this way (1c1) can then undergo
fragmentation by two channels (Scheme 1). In one pathway,
the bond between ipso-ring carbon and the carbonyl-carbon
elongates and breaks leading to the formation of the m/z 59
ion by losing a benzene molecule. Alternatively, the
methoxycarbonyl cation (m/z 59) and the benzene molecule

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
Table 1. Optimized structures and relative Gibbs free energies (in kcal/mol) of methyl benzoate and methyl phenylacetate protonated at different sites, computed for
298.15 K and 1 atm by the density functional theory method B3LYP using a 6-311++G(2d,2p) basis set
Optimized structures and computed relative Gibbs free energies (in kcal/mol)
Protonation site
Carbonyl oxygen
PhCO2Me
PhCH2CO2Me
0.00
0.00
2a
1a
Alkoxy oxygen
17.40
11.02
2b
1b
Ph-1
27.78
12.66
(ipso carbon)
2c1
1c1
Ph-2
23.41
4.67
(ortho carbon)
1c2
2c2
Ph-3
23.66
8.86
(meta carbon)
1c3
2c3
Ph-4
24.38
6.34
(para carbon)
2c4
1c4

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
Figure 2. Unit-mass resolution product-ion spectra of protonated (a) methyl benzoate, (b) methyl [carboxyl-13C]benzoate, (c)
methyl [carbonyl-18O]benzoate, (d) deuteronated methyl benzoate (generated by exposing methyl benzoate to D2O vapor in the
source; m/z 138), and those of (e) protonated [2H3]methyl benzoate and (f) protonated methyl [2,3,4,5,6-2H5]benzoate at a laboratory-
frame collision energy settings of 15 eV, recorded on a Quattro Ultima mass spectrometer
The CID spectrum of protonated methyl benzoate also
shows a prominent peak at m/z 105 (Fig. 2a), which represents
a methanol loss. The fragmentation pathway leading to this
apparently simple elimination is however not a straightfor-
ward reaction. If the charge-imparting proton is simply re-
moved together with the methoxy group without the inter-
vention of a ring intermediate, then a CH3OD molecule is
expected to be removed directly from deuteron-charged
methyl benzoate. However, the spectrum recorded from the
mass-selected monodeuterio isotopologue of protonated
methyl benzoate (m/z 138) showed two peaks of 2:1 relative
intensity at m/z 105 and 106 (Fig. 2d). Moreover, the spec-
trum recorded from m/z 142 ion of protonated methyl
[2,3,4,5,6-2H5]benzoate showed two peaks at m/z 109 and
Scheme 1. Proposed fragmentation pathway for the formation of Toluenium ions (m/z 93) and m/z 59 methoxycarbonyl cation

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
Figure 3. Relative Gibbs free energies [in kcal/mol computed for 298.15 K and 1 atm by the density functional theory method
B3LYP using a 6-311+ +G(2d,2p) basis set] and molecular structures of energy-optimized product ions and transition states
associated with the generation of m/z 59 and 93 ions from protonated methyl benzoate
110 (Fig. 2f). These results confirmed that the methanol loss
is not strictly confined to the transactions that take place at the
peripheral methoxycarbonyl functionality. The charge-
imparting proton ambulates to the ring and randomizes before
a subsequent transfer of oneof the ring protons to the methoxy
group for the concomitant methanol elimination. Previously,
we observed an analogous methanol loss from protonated
methyl dihydrocinnamate [47]. However, the benzenium
ion intermediate generated from protonated methyl
dihydrocinnamate underwent near statistical randomization
before the subsequent transfer of one of the ring protons to the
methoxy group. Not only the overall intermolecular H/D
exchange process of protonated methyl benzoate, but also
the intramolecular scrambling of the protons in the
benzenium ion intermediates is sluggish compared to those
processes of methyl dihydrocinnamate. If there were a com-
plete scrambling after the charge-imparting proton is trans-
ferred, then the m/z 105:106 peak ratio in Fig. 2d should have
been 1:5. The randomizationprocessisknown to proceed by a
Bring walk^ mechanism in protonated methyl benzoate,
which is a slow process compared to that of protonated ben-
zene, which is more-or-less instantaneous [33–38]. Accord-
ing to Kuck [21], the hydrogen ring walk in substituted
benzenium ions is markedly affected by the nature of the ring
substituents. Apparently, the methoxycarbonyl group does
not accelerate the randomization process. Various types of
scrambling mechanisms in gaseous ions have been reviewed
by Kuck [48].
The mechanism we propose to rationalize the methanol loss
is illustrated in Scheme 2. Prior to the elimination of a molecule
of methanol, the incipient proton on the carbonyl group is
transferred to the ring via a five-membered transition state
(TS 1a-1c2) (Fig. S5). However, before the protons attain a
completely randomized state in the substituted benzenium ion
1c2, the methoxycarbonyl group pivots and forms another
intermediate (5). A proton is then transferred from the
benzenium intermediate 5 to the methoxy oxygen (5 → 1b;
Scheme 2). Because a protonation of the methoxy oxygen is
dissociative, it leads to an immediate cleavage of the bond
between this oxygen and the carbonyl-carbon. The proposed
mechanism is supported by computational data which indicated
an elongation of the bond between carbonyl-carbon and
methoxy oxygen upon protonation of the methoxy group
2.043 Å (Fig. S1b).
From the spectra recorded from the isotope-labeled com-
pounds, it is evident that the incoming proton is preferentially
eliminated during the methanol loss process. The spectra re-
corded for protonated methyl [2,3,4,5,6-2H5]benzoate (Fig. 2f)
and methyl benzoate exposed in D2O vapor (Fig. 2d) showed
that loss of CH3OH and CH3OD, respectively, clearly prevail.

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
words, this observation shows that scrambling between the
oxygen atoms of methoxy and carbonyl groups does not occur
in protonated methyl benzoate.
Our proposed fragmentation mechanism was also support-
ed by Gaussian calculation results. Computations suggest that
the energy demand for the generation of the m/z 105 ion is
higher than that needed for the m/z 59 ion formation by
6.08 kcal mol−1 (TS 1c2-5, 45.52 kcal mol−1) (Fig. S5). This
result agrees with the experimental data that the intensity of
m/z 59 is about two to three times higher than that of the
m/z 105 ion (Fig. 2a and S4).
The spectrum of protonated methyl benzoate shows also a
peak at m/z 77 for the phenyl cation, which could originate
from two different pathways. Either the benzoyl cation formed
by the methanol loss could eliminate a carbon monoxide mol-
ecule, or the toluenium cation formed by the CO2 loss could
lose a molecule of methane to form the m/z 77 ion.
Scheme 2. Proposed fragmentation pathways for the forma-
tion of benzoyl cation by methanol loss from protonated methyl
benzoate
If the benzoyl cation is the sole precursor for the phenyl
cation, then the relative intensity ratio of m/z 105 and 106 peaks
from the product-ion spectrum of the m/z 138 ion generated
from methyl benzoate species exposed to D2O vapor (Fig. 2d)
should be reflected on those obtained for the m/z 77 and 78
peaks. However, the relative intensity ratio of m/z 105 and 106
peaks is about 2:1 and the relative intensity ratio of m/z 77 and
78 peaks is 1:2. It is well known that a loss of methane from
toluenium ion also lead to the formation of the phenyl cation
[9, 49]. Our spectrum for protonated [2H3]methyl benzoate
Incorporation of a hydrogen or a deuterium atom from the
phenyl ring provides only a minor contribution to the methanol
loss in both cases. Thus, the rate of proton back transfer from
the ortho-carbon to the methoxy group (5) must be faster than
the randomization rate of ring hydrogens in the benzenium ion
intermediate 1c2. In addition, the spectrum acquired from
protonated methyl [carbonyl-18O]benzoate (Fig. 2c) showed
only one peak at m/z 107 for the specific loss of [16O]methanol
originating exclusively from the methoxy group. In other
Figure 4. A unit-resolution MS1 spectrum recorded from a sample of methyl [2,3,4,5,6-2H5]benzoate placed in the HePI source at a
cone voltage setting of 98 V (a), and collision-induced (15 eV) product-ion spectra (MS2) of m/z 110 (b), m/z 109 (c), and m/z 98 (d)
ions generated from protonated methyl [2,3,4,5,6-2H5]benzoate by in-source fragmentation at a cone-voltage setting of 11, 17, and
11 eV, respectively

H. Xia et al.: Fragmentation of Protonated Methyl Benzoate
(Fig. 2e) corroborates the generalization that the methyl group
and one hydrogen atom in the aromatic ring are eliminated for
the methane loss. Because the hydrogens in methyl group also
can undergo scrambling with the ring hydrogens, a series of
peaks for phenyl cations with different numbers of deuterium
atoms were noted from the spectra of protonated [2H3]methyl
benzoate (Fig. 2e) and methyl [2,3,4,5,6-2H5]benzoate (Fig.
2f). Moreover, the product-ion spectrum of pentadeuterio-
toluenuim ion (m/z 98, Fig. 4d), which was generated in the
ion source from protonated methyl [2,3,4,5,6-2H5]benzoate by
in-source fragmentation, showed peaks at m/z 79, 80, and 81
for CHD3, CH2D2, and CH3D losses, respectively. This result
confirmed that the ring hydrogens can undergo scrambling
with the methyl group hydrogens before the methane loss
occurs. Analogously, the peaks at m/z 78 and 79 in the spec-
trum of protonated [2H3]methyl benzoate (Fig. 2e) correspond
specifically to this methane loss mechanism.
Furthermore, the spectrum of protonated methyl benzoate
also shows an intense peak at m/z 91, which can be attributed to
the well-known dihydrogen loss from the toluenium ion (Fig.
2a) [9, 49, 50]. As reported by Kuck et al. [49], the methyl
hydrogen atoms do not scramble completely with the ring
hydrogens before H2 or CH4 is eliminated. Our experimental
results from protonated [2H3]methyl benzoate (Fig. 2e) and
methyl [2,3,4,5,6-2H5]benzoate agree well with the Kuck’s
mechanism.
Experimental data acquired from protonated methyl
[carbonyl-18O]benzoate (Fig. 2c) and methyl [carboxyl-
13C]benzoate (Fig. 2b) confirmed the origins of the carbon
and oxygen atoms for the carbon monoxide loss because each
spectrum showed only one peak representing the C18O or 13CO
loss, respectively. With this CO-loss mechanism, it is obvious
that among all four phenyl cation products (m/z 79, 80, 81, and
82, Fig. 2f) of protonated methyl [2,3,4,5,6-2H5]benzoate, only
the m/z 81 and 82 ions originate from a 28-Da neutral loss from
the precursor m/z 109 and 110 ions. This hypothesis was
supported by the product-ion spectra of m/z 110, 109 ions
(Fig. 4b, c), which were generated in-source from protonated
methyl [2,3,4,5,6-2H5]benzoate. Each respective spectrum
showed only one peak either at m/z 82 or 81. Analogously, in
the three-peak cluster for phenyl cations in the spectrum of
protonated [2H3]methyl benzoate (Fig. 2e), only that at m/z 77
represents the CO loss from the benzoyl cation.
carbonyl-oxygen ambulates to the phenyl ring and then relo-
cates to the methoxy group to be eliminated as methanol.
Acknowledgements
This research was supported by funds provided by Stevens
Institute of Technology (Hoboken, NJ). We are grateful to Isra
Hassan for literature searches, and Prof. Dr. Dietmar Kuck,
Bielefeld University, Germany, for his comments that helped
us to improve the quality of the manuscript. We thank Bristol-
Myers Squibb (New Brunswick, NJ) for the donation of the
Quattro Ultima mass spectrometer.
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