Investigations of the fragmentation pathways of benzylpyridinium ions under ESI/MS conditions

Article abstract of DOI:10.1002/jms.1672

Benzylpyridinium ions are often used as 'thermometer ions' in order to evaluate the internal energy distribution of the ions formed in sources of mass spectrometers. However, the detailed fragmentation pathways of these parent ions were not well establish

Full text of DOI:10.1002/jms.1672

Research Article
Received: 28 July 2009
Accepted: 27 August 2009
Published online in Wiley Interscience: 12 October 2009
(www.interscience.com) DOI 10.1002/jms.1672
Investigations of the fragmentation pathways
of benzylpyridinium ions under ESI/MS
conditions
a∗
b
c
d
´
Emilie-Laure Zins, David Rondeau, Philippe Karoyan, Celine Fosse,
Sophie Rochute and Claude Pepee
Benzylpyridinium ions are often used as ‘thermometer ions’ in order to evaluate the internal energy distribution of the ions
formed in sources of mass spectrometers. However, the detailed fragmentation pathways of these parent ions were not well
established. In particular, fragmentation involving a rearrangement (RR) process may be influencing the simulated distribution
curves. In a previous study, we suggested that such RR actually occurred under electrospray ionization/mass spectrometry
(ESI/MS) and fast atom bombardment/mass spectrometry (FAB/MS) experiments. Here, we present a systematic study of
different substituted benzylpyridinium ions. Theoretical calculations showed that RR fragmentation leading to substituted
tropylium ions could occur under ‘soft ionization’ conditions, such as ESI or FAB. Experimental results obtained under gas-phase
reactivity conditions showed that some substituted benzylpiridinium compounds actually undergo RR fragmentations under
ESI/MS conditions. Mass-analyzed kinetic experiments were also carried out to gain information on the reaction pathways that
actually occur, and these experimental results are in agreement with the reaction pathways theoretically proposed. Copyright
c
ꢀ 2009 John Wiley & Sons, Ltd.
Keywords: benzylpyridinium ions; ion/molecule reactions; mass spectrometry; tropylium ions; DFT calculations
Introduction
existence of a reaction pathway for the rearrangement to tropy-
lium. However, thesefirstexperimentalresultsconcernedonlyFAB
ionization sources. Another preliminary study dealing with para-
H, para-CH3, and para-t-butyl benzylpyridinium ions suggested
that the formation of tropylium ions could occur from some sub-
stituted benzylpyridinium ions even under ESI/MS conditions.[13]
However, the precise influence of the nature and the position of
the substituent on the activation energies of the rearrangement
(RR) processes versus DCs remained unclear. Owing to the fre-
quently chosen benzylpyridinium ion for the calibration studies,
the knowledge of those compounds which undergo RR prior to or
in the course of fragmentation is thus of prime importance.
Inthisreport,wepresentasystematicstudyofthefragmentation
for a series of substituted benzylpyridinium ions, i.e. the parent
compound (p-H) and the derivatives o-CH3, m-CH3, p-CH3, p-F,
An important aspect in monitoring the properties of gaseous
ions and understanding their reactivities is the internal energy
content imposed to the species emerging from the ion source. The
classical method of electron ionization, for example, is well known
to generate ‘hot’ ions including excited electronic states several
electron volts above the ground-state species.[1,2] In the case of
electrospray ionization/mass spectrometry (ESI/MS), the internal
energy of the parent ions may depend on the instruments.[3–7]
In order to compare the mass spectra obtained using different
mass spectrometers, some methods were proposed to calibrate
the instruments with another.[3–9]
In these earlier studies, substituted benzylpyridinium chloride
salts (R-C6H4CH2-NC5H5+ X−) were used as molecular thermome-
ters. To this end, De Pauw fitted the internal energy distribution
of ions formed under fast atom bombardment (FAB) and electro-
sprayionization(ESI)sourceconditions[3–6] usingtheMassKinetics
software[7–9] to determine the distribution of internal energies de-
posited in desolvated ions produced by ESI source conditions.[3]
For these studies, substituted benzylpyridinium compounds were
used because it was considered that (1) their fragmentation oc-
curred through a common direct bond cleavage (DC) between
N and C to yield stable substituted benzyl cation and (2) this DC
was not accompanied by a significant reverse activation energy
process.[3,4,6,10,11]
∗
´
Correspondence to: Emilie-Laure Zins, Universite Pierre et Mzarie Curie,
Laboratoire de Dynamique, Interactions et re´activite´, 4 place Jussieu, Paris,
75005, France. E-mail: zins@spmol.jussieu.fr
´
Universite Pierre et Mzarie Curie, Laboratoire de Dynamique, Interactions et
a
´
´
reactivite, 4 place Jussieu, Paris, 75005, France
´
Universited’Angers, CIMMACNRS, 2boulevardLavoisier, Angers, 49045, France
b
c
ˆ
`
UPMC, Chemistry, case 45, Batiment F, piece 735, 4 place Jussieu, Paris, 75005,
France
However, based on mass-analyzed kinetic energy release ex-
periments of para-H-, para-CH3-, and para-OCH3-substituted ben-
zylpyridinium ions,[12] we previously proposed that tropylium ions
can also be formed during the fragmentation of benzylpyridinium
ionsgeneratedbyFAB.Complementarycalculationssupportedthe
´
d
e
Universite Pierre et Marie Curie, Ecole Nationale SuperieureDe Chimie De Paris,
boˆıte courrier 1130, 75231 Paris Cedex 05, France
´
Universite Pierre et Marie Curie – Laboratoire de Dynamique, Interactions et
´
´
Reactivite, case courrier 49, 4 place Jussieu, 75005 Paris, France
c
J. Mass. Spectrom. 2009, 44, 1668–1675
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

Fragmentation pathways of benzylpyridinium ions
p-Cl, p-CN, p-CF3, m-OCH3 and p-NO2.[3,4,6,11] In addition to these
compounds, traditionally used as ‘thermometer ions’, studies of
multiply methylated derivatives are likely to provide additional
insightsintothefragmentationpathways;p-(t-butyl),3,5-dimethyl,
2,5-dimethyl, 2,4-dimethyl and pentametyl benzylpyridiniums
were also included in both the experimental and theoretical
studies in this present work.
to 0%. For the introduction of toluene as a neutral reaction partner,
the helium inlet of the ion-trap mass spectrometer was extended
by a reservoir containing liquid toluene over which the helium
gas was swept. The relative toluene content in the helium can be
varied by cooling of the reservoir. In addition to toluene, C6H5CH3,
C6H5CD3 and C6D5CD3 were also employed in order to confirm the
results obtained for the nondeuterated compound and to study
the benzylpyridinium o-CH3, m-CH3 and p-CH3 precursor ions (see
below).
The kinetic energy releases (KERs) associated with the disso-
ciation of metastable benzylpyridinium ions were determined
with a reverse geometry sector-field mass spectrometer as de-
scribed elsewhere.[10] Ions were generated using FAB (Xe, 8 keV),
the parent ions were mass-selected using a magnetic sector and
the product ions evolving from unimolecular dissociations of the
metastable parent ions in the field-free region following the mag-
net were monitored using an electrostatic analyzer (MIKE spectra).
The average KERs and the resulting kinetic energy release dis-
tribution (KERD) curves were determined from the MIKE spectra
obtained using Eqn (1), which employs sigmoid derivatives to
reproduce the experimental data of Gaussian-shaped peaks.
Of particular importance with regard to the structures of
the resulting fragment ions are earlier experiments of Dunbar,
+
Ausloos and McLafferty, who showed that some C7H7 ions
undergo consecutive reactions with neutral toluene.[14–20] More
precisely, these ion–molecule reactions proved that the tropylium
ions are unreactive toward toluene, whereas the benzylium
+ [14–32]
Accordingly, we employ this
ions mediate a formal CH2
.
method to determine whether tropylium, benzylium or both
isomers are formed during the fragmentation of benzylpyridinium
compounds.
Experimental and Theoretical Methods
The details of the theoretical methodology have been described
elsewhere.[12] Briefly, ab initio and density functional theory (DFT)
calculations were performed using the Gaussian 03 software.[33]
All optimizations and frequency calculations were carried out
at the B3P86/6-31+G∗ level, unless otherwise specified. First,
the different structures considered for this study were built up
using the results previously obtained from the intrinsic reaction
coordinate (IRC) calculations in the case of the p-CH3 and
p-OCH3 benzylium ions.[12] Full optimizations were carried out
to know the structures of the initial states, the final states
and the intermediates. Transition structures were searched via
partial optimizations followed by QST3 calculations. The energies
reported below include zero point energy (ZPE).


a




d
Ei − Ef
Ei − Ef
b
ae−
1 + e−
b
I =
=
(1)
ꢀ
ꢁ
2
dE
Ei − Ef
b
b
1 + e−
This model includes only three adjustable parameters, a, b, and
Ef . In our case, it was possible to reproduce all the experimental
data without any constraint on the values of these adjustable
parameters because none of the experimental peaks was a
composite one. The KERDs were accordingly derived using Eqn (2).
The samples of substituted benzylpyridinium salts were pre-
pared by treating the corresponding benzyl halides with excess
anhydrous pyridine at 60 ◦C for 3 h. After the reaction, the
excess of pyridine was evaporated to give a solid. The com-
pounds were analyzed by mass spectrometry and used without
further purification.[4] Reagents for synthesis, toluene and 3-nitro-
benzylalcohol (NBA), were purchased from Sigma-Aldrich (Saint
Quentin Fallavier, France).


ꢀ
ꢁ
2
Ei − Ef
2
e−
Ei − Ef
b












a(Ei − Ef )
2γ b2
e−
b
P(E) =
−
ꢀ
ꢁ
ꢀ
ꢁ
2
3
Ei − Ef
Ei − Ef
1 + e−
1 + e−
b
b
(2)
For most mass-spectrometric experiments, we used a modified
Finnigan LCQ Deca XP Plus (ThermoFinnigan, San Jose, CA, USA)
ion-trap mass spectrometer equipped with an ESI source and
operated in positive ion mode. Nitrogen was used as the nebulizer
gas. The operating conditions were set as follows: spray voltage,
5.0 kV; capillary voltage, 40 V; heated capillary temperature, 75 ◦C;
tube lens offset, 55 V and sheath gas flow rate, 10 arbitrary units.
The samples were introduced into the ESI source via a needle at
a flow rate of 5 µl/min. Mass spectra were recorded from m/z 80
to 2000. Collision-induced dissociation (CID) of the mass-selected
precursor ions was achieved by rf-excitation of the trapped ions
using the helium buffer gas as the collision partner. The collision
energy was optimized for each experiment and is expressed in
terms of the manufacturer’s normalized collision energy (%). The
range from 0 to 100% corresponds to a resonance excitation a.c.
signal of 0–2.5 V (zero-to-peak) at the secular frequency of the
interest ion.[34] This energy depends on the m/z value of the parent
ion, but this value is not expressed directly in voltage units.
Ion/molecule reactions in the trap were performed using the
CID routine of the software while setting the activation amplitude
Results and Discussion
Theoretical activation energies
As a starting point for the search of the relevant transition
structures, we supposed that the general reaction pathway
leading to the formation of tropylium ions is not affected by the
substituents. Consequently, thereactionpathwaysobtainedinthe
case of p-H, p-CH3 and p-OCH3 compounds[12] were used to search
forthemostendothermictransitionstructuresfortheRRprocesses
of all other benzylpyridinium ions. A previous study[12] showed
that the RR process should involve two intermediates and two
transition states. As an example, Fig. 1 shows the reaction pathway
proposed in the case of the fragmentation of the unsubstituted
benzylpyridinium compound. In a first step, a triciclic structure is
formed(TS1RR,Int1RR).Then,aring-openingresultsintheformation
of a seven-member ring (TS2RR, Int2RR). A DC occurs from Int2RR to
form a tropylium ion. On the basis of these calculations, the most
endothermal transition state is the tricyclic sctructure TS1RR. But
c
J. Mass. Spectrom. 2009, 44, 1668–1675
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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E.-L. Zins et al.
promote RR processes.[5] The characterization of the daughter
ions experimentally formed is likely to shed some light on the
fragmentation pathways of the parent ions. Therefore, gas-phase
reactivitywascarriedoutincombinationwithESI/MSexperiments.
Energy (kJ/mol.)
TS1
RR
RR
TS2
+
+ ≠
+ ≠
Int2
RR
Int1
+ Py
+
RR
+
Gas-phase reactivity studies
+
211
198
143
145
The reactivity difference between tropylium ions and benzylium
ions allows to distinguish the isomeric ions by means of simple
ion/molecule reactions.[11–17] Accordingly, we were tempted
to probe if the same concept can also be applied to the
substituted benzylium and tropylium ions evolving upon CID
of the corresponding benzylpyridinium precursors. In fa+ct, the
123
Reaction coordinate
Figure 1. Potential energy evolution during tropylium formation from the
unsubstituted benzylpyridinium at the MP2/6-31+G∗ level of theory.[12]
+
formal CH2 transfer to neutral toluene to afford C8H9 (m/z
105) is not systematically favored thermochemically (see below).
However, a preliminary study of the unsubstituted- and para-t-
butyl-substituted benzylpyridinium ions showed the possibility of
using such experiments to probe the structure of the fragment
ions.[13] Hence, we may use the subsequent reactivity of the
primary fragments formed upon loss of neutral pyridine from
the benzylpyridinium ions to assess the ratio of DC to afford the
corresponding benzylium ions versus the RR to the respective
tropylium isomers (Fig. 2).
thefinalstateislessstablethanthistransitionstate.Thus,unlikethe
general case, this RR process occurs without any reverse energy.
The activation energies associated with the RR and direct cleavage
processes were determined for all the substituents considered
(Table 1). These results show that irrespective of the nature
and the position of the substituent the formation of tropylium
ions is systematically more favorable than that of direct bond
processes. But do RR processes actually take place under ESI/MS
conditions? This question is important because ESI sources are
often considered as a soft ionization method that does not
It should be noted that these gas-phase reactions cannot be
carried out from the o-CH3, m-CH3 and p-CH3benzylpyridinium
Table 1. Influence of the substitution on the enthalpies of formation of the benzylium and tropylium ions from substituted benzylpyridinium ions
and some relevant data of the associated transition structures (TS) as obtained using B3P86/6-31+G∗ calculations
Sum of electronic and thermal energy (Hartree)
Enthalpy of formation (kJ/mol)
Substituent
Parent ions
Benzyl ion
Tropylium ion
TS
νimag (cm−1
)
Benzyl ion
Tropylium ion
TS
p-H
−520.43781
−559.87359
−559.87547
−559.87592
−271.39878
−310.84091
−310.8399
−310.84738
−429.14141
−370.78493
−731.31065
−363.83149
−608.99075
−386.1823
−476.26462
−350.28087
−350.28168
−350.28869
−468.59207
−271.4127
−520.4
−559.8
−559.8
−559.9
−678.2
−619.8
−980.3
−76
−81
−78
−76
226
209
217
198
189
172
176
176
154
o-CH3
m-CH3
p-CH3
p-terbu
p-F
−310.85516
−310.85516
−310.85596
−429.14881
−370.79437
−731.316206
−363.84054
−609.00296
−386.20614
−476.27828
152.6
155.1
10.58
−619.82003
−980.34488
−612.87841
−858.03853
−635.21715
−725.31791
−599.31289
−599.31101
−79
−72
215
213
246
249
215
263
207
200
191
198
223
217
152
227
155.8
155.1
p-Cl
p-CN
p-CF3
−858
−80
−78
151.2
147.7
m-OCH3
p-NO2
3,5 DiMe
2,5 DiMe
2,4 DiMe
PentaMe
−635.2
−599.3
−599.2
−599.2
−86
−155
−147
149.9
171.1
−717.60617
−468.59735
160
146
2E + 06
In this table, νimag is the imaginary frequency associated to the transition state.
Figure 2. Protocol for the distinction of RC7H6+ isomers by their reaction with toluene.
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Fragmentation pathways of benzylpyridinium ions
Relative
intensity
1
Parent ion
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
m/z 91.07
m/z 99.93
m/z 105.07
m/z 116.53
m/z 131.53
m/z 145.13
m/z 148.73
m/z 159.07
m/z 170.07
m/z 182.67
m/z 188.73
m/z 196.73
m/z 202.80
m/z 105
Fragment ion
0
50
100
150
200
250
300
350
Activation time (ms)
Figure 3. Effects of the activation time on the mass spectra obtained for the unsubstituted benzylpyridinium ion under gas-phase reaction conditions.
Normalized collision energy: 20%.
compounds. Indeed, for these cases, the daughter ions have the
A
1.0
0.8
0.6
0.4
0.2
0.0
same m/z ratio as that of the species resulting from the gas-phase
reactivity (m/z = 105).
In the analysis of the results, we may distinguish three+scenarios:
(1) Complete conversion of the daughter ion via CH2 transfer
indicates that the parent ions exclusively underwent DC of the
C–N bond to yield the corresponding benzylium ions. (2) Partial
reactivity of the daughter ions suggests that two populations of
ions are formed with the reactive fraction due to the benzylium
structure formed via DC and the nonreactive fraction due to the
tropylium structure and thus rearrangment (RR). (3) Absence of
reactivity indicates either that all ions follow the RR route to afford
tropylium ions or CH2+ transfer.
m/z 91
m/z 105
Daughter ion (m/z 125)
Parent ion (m/z 217)
Accordingly, MS/MS experiments were carried out in which the
mass-selected benzylpyridinium precursor ions were subjected
to CID in the presence of neutral toluene as a reagent, which
means that fragmentation and the gas-phase reactions take
place during the same MSn step. As a variable, we employed
the activation time (tact), i.e. the length of the rf-excitation pulse
applied to the mass-selected benzylpyridinium precursor. For
all benzylpyridinium ions studied here, suitable fragmentation
periods for the parent ions were between 10 ms ≤ tact ≤ 500 ms.
Figure 3 shows the evolution of the ion abundances with tact in
the case of the unsubstituted benzylpyridinium compound, using
a normalized collision energy of 20%. In the first part of this graph,
for 25 ms ≤ tact ≤ 75 ms, the parent ion (C6H5 –CH2 –NC5H5+, m/z
170) disappears gradually and a proportional increase is observed
in the daughte+r ion (C7H7+, m/z 91), but a part of this ion already
reacts to C8H9 (m/z 105), indicating the presence of benzylium
ion. In the second part of the graph (tact > 75 ms), the intensity
0
10
20
30
40
50
NCE (%)
B
1.0
0.8
0.6
0.4
0.2
0.0
m/z 105
Daughter ion (m/z 119)
Parent ion (m/z 211)
0
10
20
30
40
50
+
of the parent ion continues decreasing and C7H7 also gradually
NCE (%)
disappears. Theintensityoftheion m/z 105increasescontinuously
and becomes the major product ion at extended activation times.
At larger reaction times, some additional ions are observed (e.g. an
ion at m/z 197 corresponding to an adduct of C8H9+ and toluene),
which are due to consecutive ion/molecule reactions and are not
further pursued.
Figure 4. Ion abundances in energy-dependent CID experiments of
(a) mass-selected [ClC7H6•C7H8]+ generated by CID of mass-selected
[ClC6H4CH2NC5H5]+ in the presence of toluene and (b) mass-selected
[(CH3)2C7H5•C7H8]+ generated by CID of mass-selected [(CH3)2C6H3CH2-
NC5H5]+ in the presence of toluene.
c
J. Mass. Spectrom. 2009, 44, 1668–1675
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E.-L. Zins et al.
Likewise, the ion/molecule reactions of all other benzylpyri-
dinium compounds were studied under these experimental
conditions. In many cases, the formation of adducts between
the daughter ions and one molecule of toluene was observed,
•
•
The daughter ions obtained from 2,4-dimethyl, 2,5-dimethyl,
3,5-dimethylandp-t-butylbenzylpyridiniumionspartiallyreact
with the toluene to give the ion m/z 105.
In the case of the pentamethyl- and p-OCH3 derivatives, and
benzylpyridinium compounds, the daughter ions do not react
with toluene to form an ion at m/z 105.
+
which formally correspond to the initial intermediates of CH2
transfer. Different MS3 experiments were carried out in order to
gain additional insight in this aspect. As an example, Fig. 4 shows
the CID breakdown curves of the adduct ions [ClC7H6•C7H8]+
and [(CH3)2C7H5•C7H8]+ of the primary fragments derived from
p-Cl-benzylpyridinium and 3,5-dimethyl-benzylpyridinium, re-
spectively, as a function of the normalized collision energy. These
examples show that upon CID the adduct ions can either decom-
pose into the components, e.g. ClC7H6+ (m/z 125) and C7H7+ (m/z
91), respectively, oraffordtheionatm/z 105, whichischaracteristic
for the CH2+ transfer to toluene.
According to Fig. 2, we assigned the first group to ions that only
correspond to the benzylium structure (i.e. exclusive DC pathway),
the second group points to a mixture of the DC and RR route,
while the third group of unreactive ions can be explained by
either exclusive RR or nonreactive benzylium ions. Considering
the donor character of the substituents involved in the last group,
nonreactive benzylium ions appear as the most likely explanation.
In a complementary manner, MS3 experiments were performed
in order to separate the fragmentation step and the gas-phase
reactivity process. Note that we did not observe strong changes in
the mass spectra obtained with the fragmentation conditions of
the parent ions. Furthermore, the experimental results obtained
under these conditions are very similar to those obtained in the
case of MS/MS experiments.
Similarresultswereobtainedinthecaseoftheuseofdeuteriated
gas reactants such as C6H5CD3 and C6D5CD3. Indeed, unreactive,
partially or totally reactive daughter ions, leading to the formation
of a peak at m/z 108, were observed (Table 2). In addition,
these experiments showed that the daughter ions obtained from
m-CH3 and p-CH3 benzylpyridinium compounds reacted partially
with toluene to give the ion m/z 108. Furthermore, the fragment
ion formed using o-CH3 benzylpyridinium precursor ion reacted
totally with toluene to give the ion m/z 108. These results are
summarized in Table 2.
In summary, the gas-phase reactivity study showed that some
substituted benzylpyridinium reacted with toluene, others were
unreactive and some showed partial reactivity, suggesting that
both tropylium and benzylium daughter ions coexist. Figure 5
illustrates the influence of the substitution of the parent ions
on the MS/MS spectra obtained in the case of a collision
energy of 40% and an activation time of 10 ms. The results
permit to categorize the benzylpyridinium ions into three
families.
Kinetic energy release studies
For all the benzylpyridinium compounds studied, Eqns (1) and
(2) were used to estimate the kinetic energy releases associated
with the fragmentation of the metastable ions. For instance, Fig. 6
shows the experimental and the fitted peak, the residue obtained
and the KERDs calculated in the case of the para-methoxy parent
ion. The KERs that were determined using this method showed no
significant influence of the substituents. As an example, Table 3
shows the values obtained from different parent ions.
Furthermore, the weak values obtained suggested that no
kinetic energy release was associated with these fragmentation
processes. Thus, two hypotheses are compatible with these
experimental results:
•
The benzylpyridinium ions with p-H, p-Cl, p-I, m-OCH3 and
p-NO2 yield daughter ions that react totally with toluene to
give the ion m/z 105.
•
the metastable ions underwent only DC fragmentations or
Table 2. Experimental results of the gas-phase reactivity
Gas-phase reactivity: formation of an ion at
R
m/z 105 with C6H5CH3
m/z 108 with C6H5CD3
m/z 108 with C6D5CD3
Conclusion
p-H
Total reaction
Total reaction
Partial reaction
Total reaction
Partial reaction
Partial reaction
Partial reaction
Total reaction
Total reaction
Total reaction
Partial reaction
Total reaction
Partial reaction
Partial reaction
No reaction
Total reaction
Partial reaction
Total reaction
Partial reaction
Partial reaction
Partial reaction
Total reaction
Total reaction
Total reaction
Partial reaction
Total reaction
Partial reaction
Partial reaction
No reaction
DC
RR + DC
DC
∗
o-CH3
∗
∗
m-CH3
p-CH3
RR + DC
RR + DC
RR + DC
DC
p-terbutyl
p-F
Partial reaction
Partial reaction
Total reaction
Total reaction
Total reaction
Partial reaction
Total reaction
Partial reaction
Partial reaction
No reaction
p-Cl
p-CN
DC
p-CF3
DC
m-OCH3
p-NO2
RR + DC
DC
3,5 Dimethyl
2,5 Dimethyl
2,4 Dimethyl
Pentamethyl
RR + DC
RR + DC
No reaction
No reaction
No reaction
DC, direct bond cleavage; RR, rearrangement process. Star indicates that these reactions cannot be achieved due to the fact that daughter ions have
a m/z ration of 105.
c
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J. Mass. Spectrom. 2009, 44, 1668–1675

Fragmentation pathways of benzylpyridinium ions
para-H
ortho-methyl
meta-methyl
para-methyl
para-terbutyl
para-fluoro
para-chloro
{Daughter ions + Toluene}
Daughter ions
para-cyano
m/z 105
para-triflouromethyl
meta-methoxy
para-nitro
3.5 dimethyl
2.5 dimethyl
2.4 dimethyl
pentamethyl
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Relative intensity
Figure 5. Influence of the substituent of the benzylpyridinium ions on the gas-phase reactivity between daughter ions and toluene. Collision energy of
40% was deposited on the parent ions selected in the ion-trap, and the activation time was 10 ms.
A
1.2
1.0
0.8
0.6
0.4
0.2
0.0
B
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
0.596 0.598 0.600 0.602 0.604 0.606 0.608 0.610 0.612 0.614
E/E0
0.596 0.598 0.600 0.602 0.604 0.606 0.608 0.610 0.612 0.614
E/E0
C
0.00005
0.000045
0.00004
0.000035
0.00003
0.000025
0.00002
0.000015
0.00001
0.000005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Kinetic energy (eV)
Figure 6. MIKE study of the para-methoxy benzylpyridinium ion fragmentation. A: experimental and theoretical peaks, B: residue in the fitted peak and
C: kinetic energy release distribution calculated.
c
J. Mass. Spectrom. 2009, 44, 1668–1675
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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E.-L. Zins et al.
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Adjustable
parameters
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•
•
•
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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