Theoretical and experimental study of tropylium formation from substituted benzylpyridinium species

Article abstract of DOI:10.1002/jms.1461

Fragmentation pathways of unsubstituted and substituted benzylpyridinium compounds were investigated using mass-analysed kinetic energy (MIKE) technique in combination with high level of quantum chemical calculations in the gas phase. Fast atom bombardmen

Full text of DOI:10.1002/jms.1461

Research Article
Received: 29 January 2008
Accepted: 29 May 2008
Published online in Wiley Interscience: 10 July 2008
(
www.interscience.com) DOI 10.1002/jms.1461
Theoretical and experimental study of
tropylium formation from substituted
benzylpyridinium species
a∗
a
b
a
Emilie-Laure Zins, Claude Pepe, David Rondeau, Sophie Rochut,
c
d
Nicolas Galland and Jean-Claude Tabet
Fragmentation pathways of unsubstituted and substituted benzylpyridinium compounds were investigated using mass-
analysed kinetic energy (MIKE) technique in combination with high level of quantum chemical calculations in the gas phase.
Fast atom bombardment (FAB) source was used for ionisation of the studied compounds. The formation of both benzylium
and tropylium species were investigated. Hybrid Hartree-Fock/Density Functional Theory calculations have been performed to
assess the geometries and the energies of the transition states and intermediates. For each cases, different reaction pathways
were investigated, and particularly in the case of the formation of tropylium species, the formation of the seven-membered ring
before or after the loss of pyridine were studied. The effect of para-methyl and para-methoxy substituents on the activation
energy of the rearrangement process to form thermodynamically stable tropylium compounds has been studied. Theoretical
calculations showed competition between direct bond cleavage and rearrangement reactions to form benzylium and tropylium
compounds, respectively. Experimental results also suggested that the rearrangement process takes place to yield stable
tropylium under ”soft ionisation techniques”, such as FAB. Copyright ꢀc 2008 John Wiley & Sons, Ltd.
Keywords: benzylpyridinium fragmentation; tropylium formation; activation energy; FAB; MIKE; DFT calculations
Introduction
direct bond cleavage has been defined by the ꢀꢀHf, as follows:
+
+
R
In mass spectrometry, the ratio constants of fragment ions de-
pend on the internal energy (its average value and its distribution)
received by the molecular species in the ionisation source.[1] Con-
ventional methods such as fast atom bombardment (FAB) and
electrospray ionisation (ESI) are useful for the formation of even-
ꢀꢀH = [ꢀH Py + ꢀH R ] − ꢀH Py
[4–9]
f
f
f
f
However,tothebestofourknowledge,nodetailedinvestigation
on substituted benzylpyridinium fragmentation pathways are
available in the literature. In this work, we focused our attention
on the fragmentation of p-methyl- and p-methoxy-substituted
benzylpyridinium species.
The formation of tropylium species from various alkylbenzene
and halogen-substituted alkylbenzene has been extensively
studied in the gas phase using mass spectrometry (under
[
2,3]
electron molecular species from fragile compounds.
By using
EI source, the internal energy distribution and the mean internal
energy gained by the molecule is not much affected by changing
experimental conditions slightly (i.e. geometry of the instrument
[
1–3]
and source temperature).
On the contrary, in the ESI source
conditions the internal energy of the ions depends on different
parameterschosenbytheoperator-likeapplieddesolvatationvolt-
age,natureoftheusedsolvent,temperature,andalsounadjustable
conditions (geometry of the source and the analyser). Because of
these reasons, the acquired ESI mass spectra of the same com-
poundsusingdifferentinstrumentsmaydifferfromeachother.[
In the earlier studies, substituted benzylpyridinium chloride
electronic impact ionisation conditions) and also using theoretical
calculations.[
14–21]
Different transition states (TSs) have been
proposed for the isomerisation pathways of benzyl cation to
form stable tropylium cation, and these processes are highly
4–9]
∗
Correspondence to: Emilie-Laure Zins, Laboratoire de Dynamique Interactions
−
+
salts (Cl Py R) were used as molecular thermometers, which was
et R e´ activit e´ LADIR, Universit e´ Pierre et Marie Curie, Case Courrier 49, 4 place
Jussieu, CNRS, UMR 7075, Paris 75005, France. E-mail: zins@spmol.jussieu.fr
a method developed by De Pauw to fit internal energy distribution
[
4,6,10,11]
of ions formed under FAB and ESI source conditions.
This method combined with Mass Kinetics software
a Laboratoire de DynamiqueInteractionset R e´ activit e´ LADIR, Universit e´ Pierre et
Marie Curie, Case Courrier 49, 4 place Jussieu, CNRS, UMR 7075, Paris 75005,
France
[
1,12,13]
has
been used to perform calibration of internal energy distribution
deposited on desolvated ions, which were produced by ESI source
conditions.[ For these studies, substituted benzylpyridinium
compounds have been used, because it was considered that
b Universit e´ d’Angers, Chimie, Ing e´ nierie Mol e´ culaire et Mat e´ riaux d’ Angers
CIMMA, UFR Sciences-B aˆ t. K,2, Bd Lavoisier, Angers 49100, France
6]
(
1) their fragmentation occurred through a common direct bond
c Laboratoire de Spectrochimie et Mod e´ lisation, UPRES-EA 1149, 1, quai de
Tourville-BP 1026 44035 NANTES, CEDEX 01, CNRS, UMR 2465, France
cleavagebetweenNandCtoyieldstablesubstitutedbenzylcation,
and (2) this direct bond cleavage was not accompanied by any
reverse activation energy process.
d Universit e´ Pierre et Marie Curie, Structure et Fonction de Mol e´ cules Bioactives,
Boîte courrier 182, 4 place Jussieu, Paris 75005, CNRS, UMR 7613, France
[
4–9,11]
The energy barrier for the
J. Mass. Spectrom. 2009, 44, 12–17
Copyright ꢀc 2008 John Wiley & Sons, Ltd.

Tropylium formation from benzylpyridinium species
According to the starting geometry, different intermediate ions
and TS structures can be obtained.[ Pre-established theoretical
process is not available to find a suitable TS structure. However,
differentcalculationscanbeusedtoobtainsuitableTSs.[ Among
these methods, the QST3 calculations are applicable for studying
the reaction pathways of big molecules.[
28]
29]
30]
For the study presented here, we used partial optimisations
followed by QST3 calculations, i.e. the structures obtained under
constrained optimisations were used as starting guesses for a full
optimisation of TSs.
This study was completed by Intrinsic Reaction Coordinate
IRC) and frequency calculations, for the most favourable reaction
pathways.
Figure 1. Fragmentation of benzylpyridiniums by: (A) direct cleavage (DC)
of the N–C bond and (B) rearrangement (RR) into tropylium formation.
(
[
15]
endothermic (288.0 kJ/mol; Ignatyev and Sundius, 329 kJ/mol,
Fridgen et al.[ ). Owing to the high energetic barrier, these
isomerisation pathways have never been considered under ESI
Experimental study
16]
Chemicals
[
16]
source conditions. Fridgen et al.
reported the effect of the
nature and position of substituents, i.e. ortho, meta and para
on activation energy barrier. Disselkamp et al. also observed the
p-Methoxy benzylpyridinium and p-methyl benzylpyridinium salts
were prepared by treating the corresponding benzyl halides with
excess anhydrous pyridine at 60 C for 3 h. After the reaction, the
◦
[
22]
same phenomenon for substituted anilines.
To the best of
excess pyridine was evaporated to give a solid. The compounds
were analysed by mass spectrometry and used without further
our knowledge, there are no reports on the mass spectral and
theoretical studies towards the fragmentation of substituted
benzylpyridinium compounds available in the literature. This
prompted us to investigate the dissociation mechanisms of
substituted benzylpyridinium compounds.
[6]
purification. Reagents for synthesis, and 3-nitrobenzylalcohol
(NBA)werepurchasedfromSigma-Aldrich(SaintQuentinFallavier,
France).
Before determining the initial internal energy distribution
deposited on substituted benzylpyridinium species under ESI
source conditions, the main objective of our study was to estimate
the activation energies of fragmentation reactions of these ions.
In this report, we present a theoretical and mass spectral study
concerning the reaction pathways for the fragmentation of two
substituted benzylpyridinium ions. We measured the kinetic
energy release (KER) from the fragment ions using mass-analysed
Kinetic energy release determination by MIKE
The measurements of KER of the substituted benzyl pyridinium
dissociation were obtained using a JMS-700 (JEOL Ltd, Akishima,
Tokyo, Japan) double focusing reversed geometry mass spectrom-
eter. FAB was used as ionisation method in positive ion mode with
NBA as matrix. A 2-µl matrix solution containing the benzylpyrid-
ium salt was directly deposited onto the stainless-steel FAB target
maintained on direct introduction probe. The sample was intro-
duced into the ion source of the mass spectrometer and irradiated
[
23–26]
ion kinetic energy (MIKE) technique
under FAB ionisation
source conditions. Theoretically, estimated activation energies
associatedtodifferentreactionpathwayswerecomparedtoobtain
the most suitable energy barrier for the fragmentation of these
species.Forthisstudy,weinvestigatedtheformationofbenzylions
and tropylium species first by the direct bond cleavage between
N and C from substituted benzylpyridinium compounds, and then
through the rearrangement process of benzyl ions (Fig. 1).
with a xenon gun at 5 keV energy and an emission current of
−5
10.6 mA. The source pressure was kept at 2 × 10 Torr at room
source temperature. A 10-kV acceleration voltage was applied and
nominal resolution was 2000 (10% valley definition). Under such
conditions, a Gaussian-shaped peak was observed for the main
beam. Metastable ion spectra were recorded by the MIKE tech-
nique, selecting a given precursor ion using the magnetic sector,
observing fragmentation in the second field free region (FFR2)
and determining the fragmentation product ions by scanning the
electrostatic analyser (ion kinetic measurements). To obtain an
acceptable signal-to-noise ratio, metastable peak profiles were
determined averaging a large number of electric sector voltage
scans (in 5- to 10-min overall acquisition time) in a narrow energy
window. All studies were performed under metastable ion condi-
tions, i.e. without introduction of collision gas in the first free field
region (the used mass spectrometer was not combined with a
collision cell in the FFR2). Analysis of MIKE spectra were performed
using the JEOL scan data analysis software. The average KERs were
determined from the MIKE spectra, using the Beynon and Cooks
method.[
Methods
Theoretical study
Optimisations
The ab initio and density functional theory (DFT) calculations have
[
27]
been performed using the Gaussian 03 software. All structures
were initially optimised using the abinitio HF/STO-3G method and
∗
then using the correlation level B3LYP/6-31G . Finally, the most
∗
favourable reaction pathways were studied using MP2/6-31+G
level of theory. All the values reported at this level of theory take
the correction of the zero point energy, and the entropic factor
into account (i.e. these values correspond to the free enthalpies of
formation).
23]
Results and Discussion
Transition-state structures
Owing to the large number of possible reaction pathways, finding
the most favourable reaction pathway is not a trivial problem.
Theoretical calculations (DFT) of the studied compounds 1 (un-
substituted benzylpyridinium), 2 (p-methoxy benzylpyridinium)
J. Mass. Spectrom. 2009, 44, 12–17
Copyright ꢀc 2008 John Wiley & Sons, Ltd.
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E.-L. Zins et al.
Figure 2. Structures of the most stable reagent ions and product ions
studied.
Figure 3. Cleavage of the N–C bond: (A) two-step reaction corresponding
to an increase of the N–C bond until breaking before relaxation
and (B) single-step reaction in where an increase of the N–C bond
and 3 (p-methyl benzylpyridinium) show similarity in their
geometry, i.e. both the aromatic rings are oriented orthogonally
◦
◦
and simultaneously relaxation take place. α1 = 111.5 , α2 = 124.0 ,
◦
β = 111.5 , d1 = 1.52 Å, d2 = 2.18 Å.
(Fig. 2). In the same way, the DFT-optimised ion structures of
substituted and non-substituted benzylium are close to each
other and independent of the nature of the substituents. The CH2
group of benzylium lies on the same plane as the ring, and the
charge is delocalised all over the structure (Fig. 2).
The formation of the substituted and non-substituted benzyl
cations from the molecular ions are due to the cleavage of bond
between C and N, and this can be possible by the following two
pathways:
1.
Stepwise process: the length of the N–C bond increases until
breaking and then relaxation process occurs consecutively
(
(
related to the change of two dihedral and two direct angles
Fig. 3(A))).
2.
Single-stepprocess:thestabilisedbenzylcationcanbeformed
by the increase of the N–C bond length until breaking and
the relaxation (related to the change of angles) process takes
place simultaneously (Fig. 3(B)).
Figure 4. Cleavage of the N–C bond, of the non-substituted benzylpyri-
∗
dinium, at the B3LYP/6-31G level of theory. Influence of the reaction
In the stepwise process, Fig. 4(A) shows the evolution of the
pathway on the potential energy curve: (A) two-step reaction: fragmen-
tation prior to relaxation and (B) one-step reaction: full relaxation during
fragmentation.
potential energy along the reaction pathway. In the first step,
the potential energy of the system increases gradually until
stabilisation, and in the second step the resulted benzyl cation
relaxesbyobtaining themoststableconformation, andtheenergy
of the system decreases. This relaxation is not associated with any
collision or radiative effect, but it is due to the variation of ion
structures. In this case, the calculated internal energy is very high,
i.e. 326 kJ/mol for compound 1.
In the single-step process, the potential energy curve is funda-
mentally different compared to stepwise process (Fig. 4(B)). The
DFT-calculated activation energy for the single-step mechanism is
28 kJ/mol for compound 1.
Similar results were obtained in the case of compounds 2 and 3:
respectively, which are formed by the neutral loss of pyridine
moiety from the corresponding molecular ions. The determined
average KER values from these peaks are close to 40 and 150 meV
for both the studied compounds 2 and 3, respectively. The
difference between these values suggests that the fragmentation
pathway of compounds 2 and 3 depends on the substitution.
The obtained KER value for compound 3 (150 meV) cannot
be explained only by a direct bond cleavage mechanism. For
this reason, we assume that a rearrangement process also occurs
during the fragmentation of this ion, to form stable tropylium.
To confirm this hypothesis, the reaction pathway concerning
this rearrangement process is theoretically investigated for the
compounds 2 and 3.
2
for each case, the stepwise process is energetically unfavourable.
That is the reason why we have studied the single-step process
∗
at the MP2/6-31+G level of theory. At this level of calculation,
the free enthalpies of formation of benzyl ions are 202, 209 and
1
96 kJ/mol, respectively, for compounds 1, 2 and 3.
Formation of substituted tropyliums
Kinetic energy release measurements
The recorded MIKE spectra of the studied compounds 2 and 3
show Gaussian-shaped metastable peaks at m/z 121 and 105,
As similar as in the case of direct cleavage reactions, it is necessary
to know the structure of substituted tropylium species before
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Copyright ꢀc 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 12–17

Tropylium formation from benzylpyridinium species
E ( kJ/mol)
TS1RR
TS2RR
+
+
≠
Int1RR
+
≠
Py
+
Int2RR
+
I
2
11
+
198
143
145
1
23
Reaction coordinate
Figure 7. Potential energy evolution during the tropylium formation from
∗
the p-H benzylpyridinium, at the MP2/6-31+G level of theory.
Figure 5. Formation of the tropylium ion after cleavage of the N–C bond:
(
A) TS1 as opening-ring structure and (B) TS2 as bicyclic structure.
the benzylpyridinium species in the FAB source is close to
2
00 kJ/mol[ and this energy is not sufficient to form above
31]
discussed TSs.
In the other case, the rearrangement was considered to occur
before the N–C bond cleavage to produce the stable tropylium.
This reaction pathway is more complex than the previously
discussed mechanism, as it proceeds through many TSs and
reactiveintermediates.However,thetheoreticalstudiessuggested
that the reaction had proceeded mainly from four steps, as shown
in Fig. 7.
During the first step of this reaction, the relative orientation of
the two cycles was changed by C–N bond flipping, and the first
[
32−35]
from
TS structure resulted in an 1–2 hydrogen migration
benzylic position to its adjacent carbon (TS1 , Fig. 7).
RR
From this TS, a tricyclic intermediate (bicyclo [4.1.0] hepta (2.4)
diene pyridinium) was formed. Many structures were considered
for this intermediate, and our theoretical results suggested that
RR
the most stable structure for this intermediate is Int1 (Fig. 7).
RR
Inthefollowingstep,thebicyclicintermediateInt2 wasformed
RR
from Int1 by changing its bond angles and bond lengths,
Figure 6. Activation energy for the fragmentation of the non-substituted
benzylpyridiniumattheB3LYP/6-31G leveloftheory:(I) benzylpyridinium,
resulting in the opening of the three-membered ring. The TS
∗
RR
associated to this step is TS
2
(Fig. 7). Similar to the case of
(
A) simple cleavage of the N–C bond without energy release, (F) tropylium
RR
intermediate Int1 , many conformations were considered for the
ion and pyridine. Formation of tropylium ion after the lost of pyridine,
through a (TS2) bicyclic transition state or a (TS1) non-cyclic transition
state.
RR
structure of Int2 (i.e. chair, boat and planar). Theoretical studies
showed that the boat conformation is more stable than the other.
In the final step C–N, bond breakage took place and resulted
in the formation of stable planar tropylium ion, and the loss of
pyridine. This step corresponds to a direct bond cleavage, and
occurs without any reverse energy.
studying the reaction mechanism. The optimised structure of the
un-substituted tropylium compound is presented in Fig. 2.
First, we performed the DFT calculations for the formation
of the un-substituted tropylium. These theoretical calculations
showed two endothermic reaction pathways for the formation of
stable tropylium ion: (1) the cleavage of the N–C bond followed
by rearrangement and (2) the rearrangement followed by bond
cleavage between N and C.
Using these results, we propose a potential energy surface
(PES) surface for the formation of the un-substituted tropylium
RR
compound (Fig. 7). IRC calculations performed using TS1 and
RR
TS2 structures showed that the proposed structures are indeed
suitable for TSs. Frequency calculations were also performed, for
all mentioned stationary points on the PES, to check the imaginary
frequencies (their number and their associated movement of
vibration). Table 1 summarises the results obtained at the MP2/6-
In the case of the rearrangement process, after the bond
cleavage two different types of intermediate steps were studied,
which are (1) ring opening step (TS1) and (2) bicyclic (bicyclo
∗
31+G (the free enthalpies of formation for each stationary point,
[0, 1, 4] hepta-(2, 4)-diene) step (TS2) (Fig. 5). Figure 6 shows
the number of imaginary frequencies, and the compatibility with
either a local minimum or a local maximum on the PES are
mentioned).
that the formation of ring opening TS1 is a very endothermic
∗
process (662 kJ/mol at the B3LYP/6-31G level of theory) to
produce tropylium. The TS2 structure is thermodynamically more
stable (451 kJ/mol at the B3LYP/6-31G level of theory) than
the TS1 one, for the formation of tropylium ion. Nevertheless,
these both processes are not possible under the FAB conditions.
In fact, the internal energy deposited on the N–C bond of
Finally, we determined the required energies for the formation
of substituted tropyliums from p-methyl and p-methoxy benzyl
pyridinium compounds. These theoretical studies showed that
the reaction pathways are independent of the substituent: the
most endothermal step is the migration of the H atom (Table 2).
∗
J. Mass. Spectrom. 2009, 44, 12–17
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E.-L. Zins et al.
◦
◦
DC
process (i.e. E – release energy >E , Table 3). This result
RR
Table 1. Theoretical results for the formation of tropylium from the
un-substituted benzylpyridinium, at the MP2/6-31+G level of theory
∗
was in agreement with the weak value of KER experimentally
obtained.
Number of
frozen
parameters frequencies
Number of
imaginary
Structure in
accordance
with a
•
In the case of compound 3, the activation energies associated
with direct cleavage process and with rearrangement process
are close, and the tropylium ion is more stable than the benzyl
ion(Table 3).Thatisthereasonwhyinthiscase,wecansuppose
that both direct cleavage process and rearrangement process
can occur. This result was in agreement with the stronger value
of KER experimentally obtained.
ꢀG
(kJ/mol)
Structure
Initial state
TS1RR
0
1
0
1
0
0
0
1
0
1
0
0
Minimum
Transition state
Minimum
0
202
152
154
129
160
RR
Int1
RR
TS2
Transition state
Minimum
RR
Int2
So, our theoretical and experimental results showed that the
Final state
Minimum
p-methoxybenzylpyridiniumcompoundundergoesadirectcleav-
age to form a benzyl ion, whereas the p-methyl benzylpyridinium
compound undergoes a rearrangement to form a tropylium ion.
ꢀ
G = G of the structure-G of the initial state.
The names of the structures are those used in Fig. 7.
Conclusion
Table 2. Comparison of the relative free enthalpies for the formation
of tropylium from the benzylpyridinium p-CH3, and p-OCH3, at the
MP2/6-31+G level of theory
Formation of benzyl and tropylium ions from p-methyl and p-
methoxy benzyl pyridinium compounds has been extensively
studiedbyexperimentalmethodsandtheoreticalcalculations.The
kineticenergyreleaseexperimentsfortheformationofsubstituted
∗
ꢀ
G (kJ/mol) from
ꢀG (kJ/mol) from
benzylpyridinium
p-CH3
benzylpyridinium
Structure
p-OCH3
+
C7H7 ion from substituted benzyl pyridinium compounds
+
showed different values and probably the substituted C7H7
Initial state
0
209
158
155
147
136
0
196
150
155
129
150
TS1RR
ion formed from different reaction pathways. We assumed two
reaction pathways, which are (1) direct bond cleavage between C
and N and (2) rearrangement process. Direct bond cleavage yields
to the formation of benzyl cations, whereas tropylium species
are formed by rearrangement process. The energy required for
these two processes were calculated by high-level theoretical
calculation. The formation of tropylium could be the result of
two reaction pathways that are (1) the cleavage of the N–C bond
followed by rearrangement and (2) the rearrangement followed
by bond cleavage between N and C. Theoretical calculations
confirmed that the cleavage of the N–C bond followed by
rearrangement is not possible under FAB conditions. On the basis
of our theoretical results, we propose another reaction pathway,
in which the rearrangement takes place prior to cleavage of
the N–C bond. In this case, the PES is mainly characterised by
two intermediate steps. IRC calculation showed that the most
endothermal step of this reaction pathway is the migration of a
hydrogen atom prior to the formation of a tricyclic structure. The
activation energy associated to this process suggested that this
reaction can occur under FAB conditions. This is in accordance
with the KER value obtained for the fragmentation of p-methyl
benzylpyridinium compound.
RR
Int1
RR
TS2
RR
Int2
Final state
ꢀ
G = G of the structure-G of the initial state.
The names of the structures are those used in Fig. 7.
◦
Table 3. Comparison of the activation energies (E ) for the frag-
mentation of p-H, p-methyl and p-methoxy benzylpyridinium, at the
∗
MP2/6-31+G level of theory
Formation
Formation of the
tropylium ions
of the
benzyl ions
Release
energy
(kJ/mol)
Thermodynamically
most favourable
reaction pathway
◦
◦
E
E
RR
DC
−
R
(kJ/mol)
(kJ/mol)
p-OCH3
p-CH3
209
196
73
46
118
182
DC
RR
RR, rearrangement reaction; DC, direct cleavage.
These reaction pathways will be further studied under ESI
conditions, and the substituted tropylium formation will be
investigated using a gas reacting selectively with one of the
product ion isomers.
The activation energies for the formation of substituted tropylium
ions through rearrangement process are 209 and 196 kJ/mol for
compounds 2 and 3, respectively (Table 3). Compared with the
activation energy for the formation of substituted benzyl ions, it
appears that the direct cleavage process is less endothermic than
the rearrangement process (Table 3). These theoretical results
suggested that both compounds 2 and 3 may undergo direct
bond cleavage, to form substituted benzyl ions. Moreover, Table 3
shows that:
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•
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Copyright ꢀc 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 12–17

Tropylium formation from benzylpyridinium species
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