F. Li et al. / International Journal of Mass Spectrometry 369 (2014) 23–29
27
In comparison, the protonated 2-(o-benzyl) phenylethanimine
favors to undergo NH3 elimination instead of direct decomposition,
due to the terminal amine group. Fragmentation of the ion at m/z
210, originated from dissociation of [1 + H]+, results in a dominant
product ion at m/z 193 via NH3 elimination and the minor benzyl
cation (Fig. 2(a)). The above results undoubtedly demonstrate that
the ion b, formed via the reaction channel of path-b, is the
dominant component of the mixture ions at m/z 210.
Interpreting the formation of the fragment ion at m/z 132 in
Fig. 2(a) will further witness the above result (Scheme 3). The
fragment ion a-P2 (m/z 132), resulting from benzene elimination of
the ion a ([9 + H]+), readily undergoes further decomposition to
generate benzyl cation, indicating a low abundance in the CID
spectrum. Whereas, the benzene elimination product (b-P2, m/z
132) from the ion b is a stable substituted benzyl cation, which is in
agreement with
spectrum.
a relative abundant signal in the CID-MS
Fig. 3. Potential energy diagram for fragmentation of [1 + H]+.
On the basis of the above analysis, we can conclude that benzyl
cation is more facile to transfer to the phenyl ring (path-b) rather
than to the amino N5 (path-a) in the process of decomposition of
[1 + H]+. Interestingly, the ion at m/z 210, derived from dissociation
of [8 + H]+, the protonated N-benzylated 2-phenylethanimine,
shows the much similar CID-MS spectrum with that of the m/z
210 ion from [1 + H]+ (Fig. 2(a,b)), indicating that benzyl cation is
also facile to transfer to the phenyl ring from the amino N5 prior to
the (CO + H2O) elimination of [8 + H]+.
been consolidated by the occurrence of H-scrambling in the NH3
elimination of [1 À2H + 3D]+.
3.3. Theoretical calculation
To gain more detailed insights into the competing benzyl cation
migration reactions, DFT calculations have been preformed at the
B3LYP/6 À 311 + G(2d,p) level of theory for the fragmentation of the
typical ion [1 + H]+. Fig. 3 displays a potential energy diagram for
the dissociation reactions as presented in Scheme 2, and the details
structures of the corresponding species are available as Fig. 4S in
the Supplementary data. The virtual reaction channels might be
more complicated than those in Scheme 2, since the H-scrambling
process occurs effectively according to the D-labeling experimen-
tal results.
3.2. D-labeling experiments
To provide more evidences of the competing benzyl cation
migration reactions, a deuterium labeling experiment has been
carried out for compound 1 (Fig. 1(c)). An abundant deuterated
molecule [1 À2H + 3D]+ at m/z 259 was obtained by the positive ESI
of the methanol-d4 solution of 1.
To begin interpretation of the mechanistic fragmentation of
[1 + H]+, we should first tackle the original protonation site of the
molecule. There are two potential protonation sites for 1, including
(i) the amino N5 (M-1) and (ii) the carbonyl O6 (M-2). The
calculated free energy of M-1 is 74.2 kJ/mol lower than that of M-2,
indicating that the N5 atom is the much more preferred site for
protonation in ESI-MS analysis. The stability of M-1 is attributed to
an intramolecular N5ÀÀH . . . O6 hydrogen bond (1.922 Å), as
available in Fig. 4S in the Supplementary data. Many attempts have
been made to optimize the structure of the O-protonated 1, with
the orientation of the ionizing proton towards the amino group,
but all of the final structures result in M-1. Thus, rotation of the
ester group around the C2ÀÀC3 bond of M-2, with the energy
barrier of 125.1 kJ/mol relative to M-1, can be viewed as the
mechanistic process of the interconversion between these two
isomers.
The product ion d is produced by the simultaneous loss of
(PhCH2OH + CO), triggered by migration of the ionizing proton on
the amino N5. As expected, the mass of the ion d shifts to 122 Da in
the tandem MS spectrum of [1 À2H + 3D]+. No H/D exchange
reaction has been observed for the product ions of d and the benzyl
cation, indicating that the H-scrambling does not occur in M-1
between the amino hydrogen and the phenyl one.
If the (CO + H2O) elimination of [1 + H]+ occurs only via the
channel of path-a, two amino protons in M-1 migrate to the neutral
fragment of H2O, and only one amino proton remains in the
fragment ion a. As a result, there will be only a signal with 1 Da
mass shift for ion a, resulting from dissociation of [1 À2H + 3D]+ via
losing (D2O + CO). Actually, the presence of another deuterated ions
(m/z 212 and m/z 213) in the CID-MS spectrum indicates an
alternative accessible reaction channel of losing (H2O + CO) from
[1 + H]+.
Protonation at the carboxylic O6 in M-2 weakens the O4ÀÀC7
bond, as indicated by the lengthened bond length (1.500 Å in M-1
vs >1.566 Å in M-2, as shown in Fig. 4S in the supporting
information). The disruption of the O4ÀÀC7 bond, upon collisional
activation, leads to the transferring benzyl cation. In the reaction
channel of path-a, migration of the benzyl cation in M-2 to the
amino N5 leads to M-3 with a small energy barrier (TS-2) of
82.9 kJ/mol. Stabilized by an intramolecular NÀÀH . . . O hydrogen
bond (2.000 Å), M-3 is located at the minimum in the potential
energy surface of path-a, which lies 40.6 kJ/mol in free energy
below M-1. The calculated results demonstrate that [8 + H]+ (M-3)
is less unfavorable to undergo fragmentation than [1 + H]+ (M-1),
which is in agreement with the CID-MS experimental results
(Fig.1(a,b)). Due to the electrophilic attack of the benzyl cation, one
of the amino protons (H5) is activated to be transferable in M-3
[19]. The subsequent fragmentation of M-3 occurs via migration of
As for the reaction channel of path-b, migration of the activated
phenyl proton, originating from the electrophilic attack by the
benzyl cation, to the carboxylic hydroxyl leads to dissociation of M-
4 via losing (H2O + CO). Herein, two of amino protons remain in the
fragment ion b, indicating a mass shift of 2 Da for ion b resulting
from dissociation of [1 À2H + 3D]+ via losing (DHO + CO). Alterna-
tively, isomerization of M-4 leads to M-5 through transfer of the
activated phenyl proton to the amino N5, and the subsequent
migration of one of the amino protons to the carboxylic hydroxyl
initiates the decomposition of M-5 to give the product ions at m/z
211 via losing (D2O + CO) and at m/z 212 via losing (DHO + CO),
respectively. The significantly more abundance of the ion at m/z
212 is a consequence of the considerable kinetic isotope effect,
kH/kD [25,26]. Additionally, the presence of the minor product ion
at m/z 213 indicates the occurrence of H-scrambling between
amino proton and the phenyl ring hydrogen in M-5, which has