Gas-phase fragmentation of the protonated benzyl ester of proline: Intramolecular electrophilic substitution versus hydride transfer

Article abstract of DOI:10.1002/jms.3162

In this study, the gas phase chemistry of the protonated benzyl esters of proline has been investigated by electrospray ionization mass spectrometry and theoretical calculation. Upon collisional activation, the protonated molecules undergo fragmentation reactions via three primary channels: (1) direct decomposition to the benzyl cation (m/z 91), (2) formation of an ion-neutral complex of [benzyl cation + proline]⁺, followed by a hydride transfer to generate the protonated 4,5-dihydro-3H-pyrrole-2-carboxylic acid (m/z 114), and (3) electrophilic attack at the amino by the transferring benzyl cation, and the subsequent migration of the activated amino proton leading to the simultaneous loss of (H₂O + CO). Interestingly, no hydrogen/deuterium exchange for the fragment ion m/z 114 occurs in the d-labeling experiments, indicating that the transferring hydride in path-b comes from the methenyl hydrogen rather than the amino hydrogen. For para-substituted benzyl esters, the presence of electron-donating substituents significantly promotes the direct decomposition (path-a), whereas the presence of electron-withdrawing ones distinctively inhibits that channel. For the competing channels of path-b and path-c, the presence of electron-donating substituents favors path-b rather than path-c, whereas the presence of electron-withdrawing ones favors path-c rather than path-b. Copyright 2013 John Wiley & Sons, Ltd. Copyright

Full text of DOI:10.1002/jms.3162

Research article
Received: 31 October 2012
Revised: 11 December 2012
Accepted: 19 December 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.3162
Gas-phase fragmentation of the protonated
benzyl ester of proline: intramolecular
electrophilic substitution versus
hydride transfer
Fei Li,a Xiaoping Zhang,b Huarong Zhanga and Kezhi Jianga*
In this study, the gas phase chemistry of the protonated benzyl esters of proline has been investigated by electrospray
ionization mass spectrometry and theoretical calculation. Upon collisional activation, the protonated molecules undergo
fragmentation reactions via three primary channels: (1) direct decomposition to the benzyl cation (m/z 91), (2) formation of
an ion-neutral complex of [benzyl cation + proline]+, followed by a hydride transfer to generate the protonated 4,5-dihydro-
3H-pyrrole-2-carboxylic acid (m/z 114), and (3) electrophilic attack at the amino by the transferring benzyl cation, and the
subsequent migration of the activated amino proton leading to the simultaneous loss of (H2O + CO). Interestingly, no hydrogen/
deuterium exchange for the fragment ion m/z 114 occurs in the D-labeling experiments, indicating that the transferring hydride
in path-b comes from the methenyl hydrogen rather than the amino hydrogen. For para-substituted benzyl esters, the presence
of electron-donating substituents significantly promotes the direct decomposition (path-a), whereas the presence of
electron-withdrawing ones distinctively inhibits that channel. For the competing channels of path-b and path-c, the presence
of electron-donating substituents favors path-b rather than path-c, whereas the presence of electron-withdrawing ones favors
path-c rather than path-b. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: benzyl cation transfer; hydride transfer; activation of the amino hydrogen; the substituent effect; INC
activation of benzylated cations favors to the formation of an ion/
neutral complex (INC) intermediate consisting of the benzyl cation
Introduction
Electrospray ionization mass spectrometry (ESI-MS), including
tandem mass spectrometry (MS/MS) in particular, represents an in-
dispensable physicochemical method for the structural elucidation
of various compounds.[1–4] A clear understanding of the fragmenta-
tion patterns of the protonated molecules (MH+) is quite desirable
for effective structural elucidation in MS. Prior to any dissociation,
MH+ is often observed to undergo isomerization to form more
reactive species, in which the ionizing proton migrates from the
original protonation site to the thermodynamically less favorable
sites. Such proton migration processes have been used in the past
decades to explain the dissociation pattern observed in the tandem
mass spectra of organic molecules and peptides in tandem mass
spectra.[5–11] However, the structural information based on the
interpretation of the fragmentation pattern of [M + H]+ is often
obscured because of the occurrence of widespread rearrangement
reactions. Among these reactions, the benzyl cation transfer is well
documented and has attracted great interest since the early days of
organic mass spectrometry.[12–19]
and a neutral counterpater, which can further undergo intracomplex
reactions to yield some interesting fragment ions.[14–19,26–28] INCs
are reactive intermediates that exist extensively in gas-phase ionic
reactions, and their existence and importance in unimolecular
fragmentation reactions during the MS process have been well
documented in several past decades.[29–34] Under conventional
conditions, many of these reactions would appear unusual and
impossible to occur especially when the two components are
connected by a covalent bond. Well-documented examples of
these reactions include electrophilic aromatic substitution,[13–15]
hydride transfer,[26] electron transfer[27] and nucleophilic aromatic
substitution.[14]
Electrophilic aromatic substitution reactions, initiated by the
attack of the benzyl cation, have been reported in the collision-
induced dissociation (CID) mass spectrometry of protonated
dibenzylated molecules.[16–19,34] In such backbone rearrangements,
The benzyl cation is thermodynamically stable in the gas phase
as a consequence of resonance stabilization. Previously published
theoretical calculations have indicated that tropylium ion is the
lower energy isomer of the benzyl cation, but the energy barrier
of the isomerization of the benzylium ion to the tropylium ion is
particularly substantial.[20–23] Thus, in most ESI-MS analysis,
fragmentation of benzylated cations prefers to the formation of
benzyl cations rather than the tropylium ion.[14,24–26] Collision
*
Correspondence to: Ke-Zhi Jiang, Key Lab of Organosilicon Chemistry and
Material Technology, Hangzhou Normal University, 222# Wenyi Road, Hangzhou,
Zhejiang, (310012) China. E-mail: jiangkezhi@hznu.edu.cn
a Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou
Normal University, Hangzhou, 310012, China
b College of Chemical Engineering and Material Science, Zhejiang University of
Technology, Hangzhou, 310014, China
J. Mass Spectrom. 2013, 48, 423–429
Copyright © 2013 John Wiley & Sons, Ltd.

F. Li et al
the electrophilic attack of the benzyl cation on another benzyl ring
results in a benzyl–benzyl interaction, and the formed isomeric ion
then undergoes dissociation to give the fragment ion at m/z 181.
Furthermore, such electrophilic attack at the phenyl ring activates
the ring hydrogen to be an ionizing proton. Migration of the activated
proton can lead to the dissociation of the precursor ion.[15] The
amino nitrogen possesses a higher level of nucleophilicity
towards the benzyl cation than the phenyl ring carbon, suggesting
that it would be feasible for the benzyl cation to undergo the
electrophilic attack at the amino nitrogen. To the best of our
knowledge, however, there have not been any reports in literature
describing the occurrence of this type of electrophilic attack.
The motivation in the current work was to perform a detailed
investigation on the activation of the amino nitrogen by electro-
philic attack of the benzyl cation. Benzyl esters of amino acid can
provide both the donor and acceptor (the amino nitrogen) units
required for the benzyl cation transfer in ESI-MS/MS. At the same
time, benzylation is one of the most popular methods available
for the protection of carboxylic groups in the organic and
bioorganic synthesis. With this in mind, we selected benzyl esters
of proline as a model molecule to further explore the fragmentation
reaction originating from the migration of the benzyl cation.
energy was taken at 25%.1 Each mass spectrum was the average
of over 50 scans.
Accurate MS of the product ions were measured by a micrOTOF
QII quadrupole time-of-flight (Q-TOF) mass spectrometer (Bruker
Company, USA), equipped with an ESI ion source. The collision
energy of the CID for the selected ions was set at 8 eV with Argon
being used as the collision gas. The instrument was operated at a
resolution higher than 15 000 full width at half maximum using
the micrOTOF-Q Control program ver. 2.3. The data were analyzed
using the Data Analysis ver. 4 software package delivered by Bruker
Daltonics.
Theoretical calculations
The theoretical calculations were performed using the Gaussian
03 program.[37] The structures of the reactants, transition states,
intermediates and products were optimized using the density
functional theory (DFT) method at the B3LYP/6-31 + G(2d,p) level.
All reactants, intermediates and products were identified as the
true minima by the absence of imaginary frequencies. Transition
states, on the other hand, were identified by the presence of one
single imaginary vibration frequency and the normal vibrational
mode. The transition states were further confirmed using the
intrinsic reaction coordinates calculations. The energies discussed
here are the sum of the associated electronic and thermal free
Energies. The DFT-optimized structures were shown by Gauss
View (Version 3.09) software to give higher quality images of
these structures. Hard data on geometries of all the structures
considered are available in the Supplementary Material.
Experimental
Materials
O-Benzylated proline derivatives (compounds 1–7 in Scheme 1)
were synthesized according to the classical method, involving
the reaction of proline with the corresponding benzyl alcohol in
the presence of SOCl2.[35] The N-benzylated proline derivative
(compound 8) was obtained from the reaction of proline with
benzyl chloride at a temperature of 40ꢀC, according to a procedure
described in the literature.[36] All compounds were purified after
synthesis, and their structures were further confirmed by MS.
Results and discussion
The gas-phase benzyl cation transfer reactions have been explored
by investigating of the fragmentation pattern of the protonated
substituted-benzyl esters of proline (Scheme 1). A typical CID
spectrum for the [M+ H]+ ion of the proline benzyl ester (1) is given
in Fig. 1(a), and the CID MS results are summarized in Table 1. The
most abundant ion at m/z 91 (P-a), observed in the fragmentation
of [1+ H]+, is attributed to the benzyl cation, originating from
the direct decomposition of the precursor ion. The fragment ion
at m/z 114 (P-b) is assigned as the protonated molecule of 4,5-
dihydro-3H-pyrrole-2-carboxylic acid, derived from the loss of
toluene from the precursor ion. This process involves the
formation of an INC of [benzyl cation + proline]+, followed by a
hydride transfer process.[26] Furthermore, the occurrence of
hydride abstraction reaction demonstrates that the C7H+7 ion
in the complex must have the benzylium structure because
Mass spectrometry
The ESI-MS/MS experiments were performed on an LCQ advan-
tage mass spectrometer (ThermoFisher Company, USA),
equipped with an ESI ion source in the positive ionization mode,
with data acquisition using the Xcalibur software (Version 1.4).
Typical parameters used for the operation of the ESI-MS were
as follows.
The compounds were dissolved in a mixed solvent of methanol
and water (V: V = 1 : 1) at a concentration of about 10 ppm. The
diluted solutions were introduced into the source chamber at a
flow rate of 5 mL minꢁ1. The sheath gas (N2, 99.99%) was set at 25
arbitrary units. The spray voltage was set at ꢁ4.5 kV, and the heated
ion transfer tube (250 ꢀC) was set at +55 V for ion introduction. The
ion trap pressure of approximately 2 ꢂ 10ꢁ5 Torr was maintained
with a Turbo pump, and pure helium (99.999%) was used for the
trapping and collision activation of the selected ions. The CID MS
spectra of the protonated molecules were obtained by isolation
and activation of the precursor ions, and the normalized collision
the tropylium ion is known to be inert towards hydride
[26,38]
abstraction.
The fragment ion at m/z 160 (P-c) is assigned to
the N-benzylated 3,4-dihydro-2H-pyrrolium cation, C4H7N–CH2C6H+5,
formed by the simultaneous loss of (H2O + CO) (46 Da).[39] The
elemental compositions of these three product ions have been
confirmed by determining their accurate masses using high
resolution mass spectrometer (Table S1 and Fig. S1 in the
Supporting Information).
1The relative collisional energy in this ion trap instrument (LCQ) varies from 0%
to 100%, where 0–100% relative collisional energy correspond to 0–5 V peak
to peak of the resonance excitation RF voltage, e.g. 25% relative collisional
energy means 1.25 V (5 V ꢂ 25%).
Scheme 1. Structures of O- and N-benzylated proline derivatives.
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J. Mass Spectrom. 2013, 48, 423–429

Reactions of benzyl cation
Figure 1. Collision-induced dissociation mass spectra of (a) the [M + H]+ ion at m/z 206, (b) the [M + D]+ ion at m/z 207, and (c) the [M - H + 2D]+ ion at
m/z 208 of compound 1.
Table 1. The collision-induced dissociation mass spectrometry data of the protonated proline acid substituted-benzyl esters at the normalized
collision energy of 25%
[M + H]+
m/z (%)
P-a
P-b
P-c
RC6H4CH+2
m/z (%)
C5H8NO+2
m/z (%)
RC11H13N+
m/z (%)
Compound
R
1
2
3
4
5
–H
206 (25.5)
236 (1.5)
91 (100)
121 (100)
105 (100)
109 (100)
125 (100)
127 (100)
169 (100)
171 (100)
136 (2.4)
114 (37.2)
114 (0.2)
114 (6.0)
114 (19.9)
114 (12.0)
114 (11.8)
114 (7.6)
114 (7.0)
114 (1.7)
160 (39.7)
190 (0.03)
174 (0.6)
178 (10.3)
194 (9.3)
196 (9.7)
238 (11.9)
240 (12.1)
205 (38.1)
–OCH3
–CH3
–F
220 (14.9)
224 (11.5)
240 (26.1)
242 (17.3)
284 (14.7)
286 (18.0)
251 (100)
–
–
–
–
35Cl
37Cl
79Br
81Br
6
7
–NO2
Compounds 2–8 display similar fragmentation behaviors in the
CID MS spectra (Fig. S2 in the Supporting Information), and the
MS data are summarized in Table 1. Both P-a and P-c showed a
mass shift corresponding to the mass of the corresponding
substituent R- in the mass spectra. However, no mass shift was
observed for P-b. Generally, three major dissociation reactions
were observed for all the compounds studied, as described in
Scheme 2. The dissociation reactions were further confirmed by
investigating the breakdown curves of the typical compound 1
at the different collision energies (Fig. S3 in the Supporting
Information). Path-a describes the direct decomposition of the
protonated molecule 1-b to form the benzyl cation (P-a). In the
reaction channel of path-b, decomposition of 1-b leads to an
INC of [benzyl cation + proline]+, which undergoes intracomplex
hydride transfer to give the fragment ion of P-b (m/z 114). Activation
of the protonated molecule also results in the isomerization of 1-b
to give 1-c via the migration of the benzyl cation to the amino
nitrogen. The subsequent dissociation of 1-c gives rise to the
product ion P-c with the concomitant loss of (H2O + CO) in path-c.
Migration of the benzyl cation in path-c has been supported by
the comparison of the fragmentation behaviors of the protonated
O-benzylated proline derivative (1) and the protonated N-
benzylated proline derivative (8). [8 + H]+ readily undergoes the
synchronous loss of (H2O + CO)[39] and yields the base peak ion
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F. Li et al
Scheme 2. Proposed reaction channels for fragmentation of [1 + H]+. INC, ion/neutral complex.
Figure 2. Collision-induced dissociation mass spectra 3 (m/z 206 ! m/z 160 !) spectra of the protonated 1 (a) and 8 (b).
at m/z 160, corresponding to the N-benzylated 3,4-dihydro-2H-
pyrrolium cation [Fig. S2(g) in the Supporting Information]. The
subsequent dissociation of the species almost generates the
only fragment ion of benzyl cation (m/z 91) in the MS3 spectrum
[Fig. 2(a)]. The CID MS spectrum of P-c, originating from the
decomposition of [1+ H], is very similar to that of the N-benzylated
3,4-dihydro-2H-pyrrolium cation (Fig. 2). The single fragment ions
of these species possess much similar levels of intensity (54.8% vs
53.3%). In addition, protonation on the carboxylic oxygen in 1-b
weakens the O8–C9 ester bond rendering it more susceptible
to fragmentation to form a benzyl cation. The amino N atom
possesses a high level of nucleophilicity towards the benzyl
cation, indicating a facile process to yield P-c via the benzyl
cation transfer. This result therefore effectively confirmed the
structure of P-c to be the N-benzylated 3,4-dihydro-2H-pyrrolium
cation and that the formation of P-c resulted from the migration
of the benzyl cation during the fragmentation of 1-b.
Electrophilic attack by the benzyl cation at the amino nitrogen
activates the amino hydrogen to be an ionizing proton in the
structure of 1-c, which has previously been reported to be the
key step in the dissociative benzyl cation transfer process.[15]
Migration of the activated amino proton in path-c has been
supported by the deuterium labeling experiments (Fig. 1). An
abundant deuterated molecule [1 + D]+ at m/z 207 was generated
by spraying a freshly prepared methanol-d4 solution of 1 in the
positive mode. Whereas a di-deuterated ion [1 ꢁ H + 2D]+ at m/z
208 was obtained from the same solution after several minutes
for the hydrogen/deuterium (H/D) exchange of the amino proton.
Decomposition of the [1+ H]+, [1+ D]+ and [1 ꢁ H + 2D]+ species
resulted in the formation of the fragment ion P-c with the same
mass (m/z 160) via the loss of (H2O + CO), (HDO + CO) and
(D2O + CO), respectively. On the basis of the aforementioned
D-labeling CID experiments, the two protons in the neutral
H2O fragment were confirmed to come from the external
proton and the amino hydrogen, indicating the occurrence of
an amino hydrogen transfer in path-c.
The D-labeling CID experiments allowed for the rationalization of
the mechanism in the reaction channel of path-b. As shown in Fig. 1,
the m/z value of P-b is observed to shift to 115 in the CID spectrum
of [1+ D]+, and to 116 in the CID spectrum of [1ꢁ H + 2D]+.
Interestingly, no H/D exchange occurs for the formation of the
fragment ion at m/z 114, indicating that the product ion P-b
contains both the external proton and the amino hydrogen. The
aforementioned results confirm that the hydride transferring to
the benzyl cation in path-b comes from the methylene hydrogen
of the five-membered proline ring rather than the amino proton.
Substituent effects are very useful in probing reaction mecha-
nisms.[9,11,26] A series of compounds bearing different substitutes
at the para position of the phenyl ring have been investigated
to further understand the reaction mechanisms. All of these
compounds exhibit similar fragmentation patterns, whereas the
relative intensities of the three competing product ions varied
as the substituent changes (Table 1). The abundance of the direct
fragment ions is significantly affected by the different substituents.
On the whole, the presence of the electron-donating groups on the
phenyl ring significantly promotes the direct decomposition to give
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Reactions of benzyl cation
the benzyl cations, whereas the presence of the electron-
withdrawing groups effectively inhibits such reaction channel. For
example, dissociation of [2+ H]+ almost generates p-methoxyl
benzyl cation (m/z 121), whereas dissociation of [7+ H]+ mainly
yields the N-nitrobenzylated 3,4-dihydro-2H-pyrrolium cation,
C4H7N–CH2C6H4NO+2 (m/z 205).
For the occurrence of path-b and path-c, the presence of the
electron-donating groups on the phenyl ring favors the hydride
transfer to form ion P-b, whereas the presence of the electron-
withdrawing groups favors the electrophilic attack of the benzyl
cation to give ion P-c, as indicated by the relationship between
To begin interpretation of the fragmentation of [1 + H]+, we
should first tackle the protonation site of the molecule. There
are two possible protonation sites for 1, including (i) the amino
N-atom (1-a) and (ii) the carbonyl O-atom (1-b). The calculated
free energy of 1-a is 106.0kJ/mol lower than that of 1-b, indicating
that the N-atom is the preferred site for protonation in ESI-MS anal-
ysis. The stability of 1-a is attributed to an intramolecular N–H. . .O
hydrogen bond (1.873 Å), as shown in Fig. S4 in the Supporting
Information. Several attempts have been made to optimize the
O-protonated 1 structure with orientation of the ionizing proton
towards the amino group, and all of the final structures result in
1-a. Thus, the rotation of the ester group around the C2–C6
bond of 1-b, with the energy barrier of 131.8kJ/mol relative to
1-a, of 1-b can be regarded as the mechanistic process of the
interconversion between the two isomers.
Protonation at the carboxylic O7 in 1-b weakens the C8–O9
bond, as indicated by the lengthened bond length (1.501 Å in
1-a vs 1.573 Å in 1-b, as shown in Fig. S4 in the Supporting
Information). Activation of 1-b results in fragmentation in the three
reaction channels, as shown in Scheme 2. P-a, resulting from the
direct decomposition of 1-b, is located 138.2kJ/mol above 1-a.
Compound 1-b surmounts a small energy barrier, which cannot
be calculated computationally, to generate an INC of [benzyl
cation+ proline]+, which lies 98.4 kJ/mol above 1-a. The subse-
quent intracomplex hydride transfer is the key step in path-b, with
an accumulated energy barrier (via TS-I) of 151.3 kJ/mol.
the intensity ratios of these two competitive product ions and
+[40]
the Hammett substituent constants sp
as shown in Fig. 3.
The logarithmic values of the abundance ratios of these two ions
are in line with sp+. The competition between the two reactions is
in agreement with the sensitivity of the corresponding energy
barrier affected by the different substituents,[9,26] which will be
further discussed in the succeeding sections.
To gain further insights into the mechanism of the present
benzyl cation migration reaction, DFT calculations for the
fragmentation of [1 + H]+ have been preformed at the B3LYP/6-
311 + G(2d,p) level of theory. A schematic potential energy
diagram for this reaction is given in Fig. 4, and full details of the
structures of the species involved are provided in the Supporting
Information.
The isomerization of 1-b to 1-c, through migration of the benzyl
cation via TS-b, only surmounts a small energy barrier of 123.5 kJ/mol.
It is noteworthy that TS-b is confirmed to be the right transition
state of the isomerization process by an intrinsic reaction
coordinates analysis, indicating that the benzyl cation transfer in
path-c occurs directly rather than via an INC. The resulting 1-c lies
152.4 kJ/mol below 1-b in free energy, indicating a much higher
benzylation nucleophilicity of the amino nitrogen than that of
the carboxylic oxygen. The transfer of the activated amino proton,
due to the electrophilic attack of the benzyl cation, to the carbox-
ylic hydroxyl O8 results in the fragment ion P-c by the simulta-
neous loss of (H2O + CO), via a five-membered ring transition state
TS-c with an accumulated energy barrier of 159.1 kJ/mol. Thereby,
the migration of the benzyl cation is a facile process as well as that
of the ionizing proton, and the key step in path-c is the migration
of the activated amino proton to the ester O8 via TS-c.
Figure 3. Plot of In[(RC11H13N+)/(C5H8NO+2)] versus the Hammett substit-
uent constants sp+ for the fragmentation of protonated substituted-benzyl
esters of proline.
Figure 4. Potential energy diagram for fragmentation of [1 + H]+.
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F. Li et al
Table 2. The calculated energy barriers of the fragmentation channels for the protonated proline acid substituted-benzyl esters at the B3LYP/6-
311g (2d,p) level
Compound
R
Path-a ΔGa (kJ/mol)
Path-b Eab (kJ/mol)
Path-c Eac (kJ/mol)
Eac-Eab (kJ/mol)
1
2
3
4
5
6
7
–H
138.2
75.2
151.3
120.1
138.7
148.6
147.4
147.7
177.8
159.1
158.8
159.7
163.3
161.9
163.5
163.5
7.8
38.7
21
–OCH3
–CH3
–F
112.1
132.1
128.9
128.4
182.2
14.7
14.5
15.8
ꢁ14.3
–Cl
–Br
–NO2
As shown in Fig. 4, the energy barrier for the hydride transfer
in path-b is 7.8 kJ/mol less than that of path-c, indicating a
kinetically more favorable process of path-b. In contrast, the
sum energy of P-b and toluene lies 9.4 kJ/mol above that of P-c
and (CO + H2O), representing a thermodynamically less preferred
process of path-b. Thereby, P-b is more abundant than P-c under
a low collision energy in the CID mass spectra and becomes less
abundant than P-c with increasing the collision energy (Fig. S3).
Additionally, the sum of free energy (138.2 kJ/mol) of the direct
decomposition products, P-a and proline, is significantly less than
the calculated energy barrier for path-b or path-c, which agrees
with the CID experimental results that P-a is the most abundant
product ion under various CID conditions.
Given that the stability of the benzyl cation depends on the
electronic nature of the substituents, the calculated ΔG value in
the channel path-a is significantly affected by the different
substituent (Table 2), which is in well agreement with the experi-
mental results (Table 1). For the occurrence of the competing
channels between path-b and path-c, the relative abundances of
the two corresponding product ions are also affected by the nature
of the substituents. DFT calculations indicate that the electronic
nature of the substituents has a slight influence on the energy
barrier (Ea-c) of path-c (Table 2), because the reaction center
disconnects to the benzyl group. In contrast, the electronic nature
of the substituents has a significant impact on the energy barrier
(Ea-b) of path-b. The presence of an electron-donating group leads
to a reduction in Ea-b, whereas the presence of an electron-
withdrawing group leads to an increase in Ea-b. These results reveal
that the reaction channel of losing (CO + H2O) is almost immune to
the electronic effects of the substituents on the benzyl group,
whereas the nature of the substituents distinctively impact on
Ea-b and then the distribution of the two competing product
ions (Figure S5 in the Supporting Information).
D-labeling experiments and theoretical calculations. The reaction
channel of losing (CO + H2O) is almost immune to the nature of
the substituents on the phenyl ring. In contrast, the nature of the
substituents changes the energy barrier of the hydride transfer
and then the distribution of the two competing product ions. The
results presented in this work will provide a better understanding
of the role of the transferring benzyl cation in the fragmentation
of the analogous species in the CID MS spectra.
Acknowledgements
The authors gratefully acknowledge financial support from the
National Science Foundation of China (No. 21205025) and the
National Science Foundation of Zhejiang Province (Y4100020).
Computer time was made available on the SGI Aktix 450 sever
at Computational Center for Molecular Design of Organosilicon
Compounds, Hangzhou Normal University.
Supporting information
Supporting information may be found in the online version of
this article.
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Conclusion
An interesting benzyl cation transfer, accompanied with a hydride
transfer, has been obtained and proved in the dissociation
reactions of the [M+ H]+ ions of O-benzylated proline derivatives.
The dissociation of the protonated molecules leads to the
generation of an INC of [benzyl cation+ proline]+, which then
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J. Mass Spectrom. 2013, 48, 423–429

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