2-Benzylindane radical cations in the gas phase (Part I): Substituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes

Article abstract of DOI:10.1016/j.ijms.2016.05.021

In the present and the accompanying article, the unimolecular fragmentation of the radical cations of 2-benzylindane and eleven derivatives bearing meta- or para-substituents at the benzylic moiety has been studied with a special focus on the hydrogen exc

Full text of DOI:10.1016/j.ijms.2016.05.021

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Contents lists available at ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
2-Benzylindane radical cations in the gas phase (Part I): Substituent
effects on a stereoselective McLafferty reaction and related hydrogen
transfer processes
Hans-Friedrich Grützmacher, Dietmar Kuck∗
Department of Chemistry, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present and the accompanying article, the unimolecular fragmentation of the radical cations of 2-
benzylindane and eleven derivatives bearing meta- or para-substituents at the benzylic moiety has been
studied with a special focus on the hydrogen exchange that precedes the McLafferty reaction. Standard
EI mass spectra, low-energy (11 eV) mass spectra and mass-analyzed ion kinetic energy (MIKE) spectra
were recorded to probe the excitation-energy and ion-lifetime dependence of the fragmentation and
the hydrogen exchange. Density-functional theory calculations were used to rationalize the substituent
effects on the competition between the benzylic cleavage and the McLafferty reaction and to clarify
the role of the distonic arenium ion-type intermediate formed by the ␥-hydrogen-transfer step of the
McLafferty reaction. The ipso-protonolysis of the benzyl residue giving rise to the loss of substituted
benzenes specifically from most of the para-isomers was also found to reflect the hydrogen exchange.
© 2016 Elsevier B.V. All rights reserved.
Received 15 February 2016
Accepted 27 May 2016
Available online xxx
In memoriam Nico M. M. Nibbering, a
friend in science and beyond.
Keywords:
McLafferty reaction
Substituent effects
Distonic ions
Hydrogen transfer
Proton affinity
ipso-Protonation
1. Introduction
Hydrogen exchange occurring in the molecular ions, M•+, of
the higher alkylbenzenes and their derivatives was found to be
Hydrogen transfer reactions and hydrogen exchange (“H scram-
bling”) preceding mass spectrometric fragmentation reactions
constitute important elementary processes that are both relevant
for the understanding of the fundamentals of gas-phase ion chem-
istry and the analytical interpretation of mass spectra. Since the
early discoveries by Meyerson and his coworkers in the late 1950s
on scrambling processes occurring in the radical cations of lower
alkylbenzenes [1–3] and later studies by Lightner et al. [4], numer-
ous reports have been published on this subject. The chemical
and physical factors that govern hydrogen transfer and hydrogen
scrambling and their mechanistic implications have been inves-
tigated and reviewed [5–10]. Hydrogen exchange can occur in
various ways: It can be fast or slow with respect to the given life-
time of the ions, leading to “complete” or “incomplete” scrambling,
its extent is lifetime-dependent and mostly increases with increas-
ing ion lifetime (“progressive scrambling”), and it may even take
place by different mechanisms for distinct populations of the same
fragmenting ions (“composite scrambling”) [8].
regioselective [1,4,11–19]. The H atoms of the ␥-position of the
aliphatic chain are involved with high preference and those at
the ortho-positions of the aromatic nucleus participate exclu-
sively. This special feature of the radical cations does not apply
to protonated alkylbenzenes, [M+H]+, both in the regimes of uni-
molecular fragmentation [20–22] and bimolecular ion-molecule
reactions [23], where only the ring hydrogens, but all of them, are
involved in a very fast proton exchange in most cases [8,10,20–23].
In contrast, the hydrogen scrambling preceding the EI-induced
fragmentation of 2-benzylindane (1, Scheme 1) was found to be
particularly clear-cut [24]. The radical cations 1•+ undergo a highly
regioselective hydrogen exchange that involves the four H atoms
at the two cis-positions of the five-membered ring (1-Hcis and 3-
Hcis) and the two ortho-positions of the benzyl group (2ꢀ-H and
6ꢀ-H) prior to fragmentation by McLafferty reaction, giving the 5-
•+
methylenecyclohexa-1,3-diene ion, C7H8 (m/z 92). Such a “4-H
exchange” (or “4-H scrambling”) process has been known for a
long time to occur in the radical cations of 1,3-diphenylpropane
[11,12] and also, in a modified manner, in those of dihydrocinna-
mates [25–27]. Ionized 1,3-diphenylpropane represents a special
case because it undergoes two relatively slow and mutually merged
4-H exchange processes that involve the H␥ and Hortho atoms, on
∗
Corresponding author.
E-mail address: dietmar.kuck@uni-bielefeld.de (D. Kuck).
http://dx.doi.org/10.1016/j.ijms.2016.05.021
1387-3806/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-F. Grützmacher, D. Kuck, 2-Benzylindane radical cations in the gas phase (Part I): Sub-
stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
http://dx.doi.org/10.1016/j.ijms.2016.05.021

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Scheme 1. Major fragmentation reactions of the radical cations of 2-benzylindanes 1–3 and preceding intramolecular hydrogen exchange [24]: (a) benzylic cleavage, (b)
McLafferty reaction.
ꢀ
bilities of the distonic ions 2isom•+ and 3isom•+ depend mainly on the
site-specific (“local”) proton affinity (PA) of the benzylic residues
at their ortho positions and, thus, on the electronic nature and the
the one hand, and the H␣ and Hortho atoms, on the other. The
hydrogen exchange in ionized 1,3-diphenylpropane, as well as in 2-
benzylindane, 1•+, was found to increase with the ions’ lifetime and
to be complete for metastable ions that decompose after 30–50 ␮s
position of the substituent X. Notably, a hydrogen ring walk within
•+
the “protonated” benzylic residues of the intermediates Misom
,
in the 2nd field-free region of a sector-field mass spectrometer.
Model calculations suggested that, in the simpler case of ions 1•
,
+
which is an ubiquitous feature of arenium ions [8,10,21], does not
•+
occur [12,24]. As a consequence, the distonic ion 2isom bearing
where only one 4-H exchange operates, more than seven exchange
cycles, 1•+ → 1isom → 1•+, are necessary to reach the “statisti-
•+
the meta-methoxy substituent is significantly more stable than the
para-isomer 3isom•+. In this and the accompanying paper [30], we
report on an extension of this model system, including a much
larger set of ionized meta- and para-substituted 2-benzylindanes,
•+
cal distribution” (“complete scrambling”) [8,10,24], with 1isom
representing the distonic ion intermediate formed by transfer of
a H␥ atom [28].
4
•+–12•+ (Scheme 2), and their deuterium-labelled isotopologs. In
A meta-methoxy substituent at the benzylic residue of ion-
ized 2-(3-methoxybenzyl)indane, 2•+, strongly accelerates the ␥-H
transfer and the 4-H exchange, whereas a para-methoxy group in
the isomeric 2-(4-methoxybenzyl)indane, 3•+, markedly decreases
these processes in all regimes of the molecular ions’ lifetime [24].
The strongly varying extent of the 4-H exchange is attributed to
the differences of the unimolecular equilibria between the original
molecular ions, M•+, and the corresponding distonic intermediates,
Misom•+, the latter representing more or less stabilized ␴-complexes
containing a remote benzylic radical (Scheme 2) [11,12,24,29]. The
relative stability of the isomeric molecular ions 2•+ and 3•+ is mainly
determined by the ionization energies of the substituted aromatic
moieties and should be similar for both isomers. In contrast, the sta-
the present article, we discuss the fragmentation of ions 1•+–12•+
as a function of the substituents X and the ions’ lifetime, using
density functional theory calculations to substantiate, in particular,
the competition of the McLafferty reaction and the benzylic cleav-
age (Scheme 1). In the accompanying paper, we report in detail on
the effect of the substituents on the relative rate of the reversible
hydrogen transfer and, thus, on the extent of H/D scrambling that
precedes the McLafferty reaction of ions 1•+–12•+. It will be shown
that the extent of H/D exchange in the corresponding isotopologs
also depends markedly on electronic nature and the position of
the substituents X in all lifetime regimes. In addition, it will be
shown that the electronic nature of the substituents also affects the
Scheme 2. Effects of meta- and para-substituents X on the equilibrium between the conventional molecular ions, M•+, and the corresponding distonic ions, Misom•+, of
2-benzylindanes studied in the previous (1–3) [24] and in the present (4–12) work.
Please cite this article in press as: H.-F. Grützmacher, D. Kuck, 2-Benzylindane radical cations in the gas phase (Part I): Sub-
stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
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Scheme 3. Synthesis of the unlabelled 2-benzylindane derivatives 4–12.
stereoselectivity of the hydrogen scrambling process. Finally, ipso-
protonolysis of ions 1•+ and most of its para-substituted derivatives
will be also addressed in view of the preceding hydrogen scram-
bling [30].
2.3. Computational methods
Molecular orbital calculations were performed with the Gaus-
sian 03 suite of programs [37]. The geometry of the stationary
points was optimized at the unrestricted B3LYP level of the density
functional theory (DFT) using the 6-31+G basis set. The transition
state geometries that connect the stationary points were optimized
at the same level. The calculated structures represent local minima
on the reaction energy surface and those of the transition states
exhibit only one imaginary frequency as expected for a saddle point.
The correctness of the transition states was checked by visualiz-
ing the structure using the Gauss View 4.1 [38] program and by
activating the vibration exhibiting the imaginary frequency.
Following the optimization of the geometry, single point cal-
culation of the electronic energy E0 for all species was performed
using BHLYP with the 6-311+G(2d,p) basis set. The combination
with zero point energy (ZPE) correction and thermochemical cor-
2. Experimental and Computations
2.1. Synthesis of compounds
The synthesis of the unlabelled 2-benzylindanes 4–12 was
based on the same procedures used for that of the congeners 1–3
[24,31]. In brief, 1-indanone was condensed with the appropriate
benzaldehydes to give the corresponding 2-benzylidene-1-
indanones (Scheme 3). Catalytic hydrogenation to the dihydrochal-
cones followed by reduction using lithium aluminum hydride
furnished the respective substituted 2-benzyl-1-indanols as mix-
tures of the cis- and trans-isomers. Conversion of these alcohols
to the corresponding 2-benzylindenes was achieved by dehydra-
tion in formic acid under reflux [32] or, more reliably, by heating
in dimethylsulfoxide at 170 ◦C [33]. Undesired cyclodehydration of
electron-rich benzylindanols can be largely excluded by applying
the latter method [34,35]. Hydrogenation of the 2-benzylindenes
was performed by use of hydrogen gas in the presence of Wilkin-
son’s catalyst in benzene, a method that proved to be superior to the
use of palladium-on-charcoal as a catalyst in dipolar or protic sol-
vents in view of regio- and stereoselective deuteration [24,30,31].
All compounds were purified by recrystallization (solids) or by
kugelrohr distillation (oils) and characterized by EI mass spectrom-
etry (70 eV) and 1H NMR spectroscopy (300 MHz, CDCl3).
rections to the enthalpy H◦
at 298 K, respectively, which were
298
obtained from the harmonic vibrational analysis, afforded standard
enthalpy differences shown in the tables and discussed in the text.
For large systems, B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) has
been recommended as a reasonable compromise between expected
accuracy and computational economy [39], and this should carry
over to UBHLYP/6-311+G(2d,p)/UBHLYP/6-31+G used here. A spe-
cial problem for the computational study of the 2-benzylindanes
is the presence of a rather large number of conformations of the
molecular ions M•+ and their isomers Misom•+, which differ in
energy only by ∼10 kJ mol−1; this renders it difficult to localize the
global minima on the energy surface of these species in every case.
Therefore, the calcul•a+tions were concentrated to those conform-
ers of M•+ and Misom that are relevant for the hydrogen transfer
between them.
2.2. Mass spectrometry
The standard 70 eV mass spectra were recorded with
a
3. Results and discussion
311 A double-focusing mass spectrometer (Finnigan MAT, Bremen,
Germany) at 3.0 kV accelerating voltage. Other than standard mass
spectra (70 eV and 11 eV) as well as the mass-analyzed ion kinetic
energy (MIKE) spectra were measured with a ZAB-2F double-
focusing instruments (Vacuum Generators, Manchester, UK) at
6.0 kV accelerating voltage and 100 ␮A trap current. The samples
were introduced through the solids probe inlet with slight heat-
ing of the quartz crucible, in part after absorption on silica gel or,
if volatility permitted, via the septum inlet heated to 230 ◦C. The
temperature of the ion source was kept at 200–220 ◦C. For the mea-
surements carried out at 11 eV, the electron energy was adjusted by
correcting the nominal value according to the ionization energy of
benzene (9.24 eV [36]), using the semi-log plot technique. The mass
spectrometric data presented here are average values obtained
from five to eight consecutive runs and most of the measurements
were repeated on other days. Particular attention was paid, where
necessary, to the significant decrease of the partial pressure of the
samples upon evaporation into the ion source.
A tremendous amount of knowledge has been gained about
the local PA values of aromatics in the recent decades both by
experimental [29,36,40–44] and computational studies [45–48].
As a good approximation, the local proton affinities at the ben-
zylic ortho-position C-6ꢀ, in particular, of the meta-isomers 2, 4,
6, 8, and 10, that are oriented para to the substituent X at C-3ꢀ in
each case, should be similar to the experimentally known (“global”)
proton affinities of the corresponding meta-substituted toluenes
[36,40]. However, the local proton affinities at the very same ben-
zylic ortho-positions of the para-isomers 3, 5, 7, 9, and 11 should be
considerably lower and can only be estimated by using increments
obtained by computation.1 Therefore, we performed extensive syn-
1
Note: The local proton affinities of the relevant ortho-positions of meta-
xylene (C-6) and para-xylene (C-2), which can be used as simpler models
Please cite this article in press as: H.-F. Grützmacher, D. Kuck, 2-Benzylindane radical cations in the gas phase (Part I): Sub-
stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
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thesis and mass spectrometric work on a large set of variously
deuterium-labelled isotopologs of the 2-benzylindanes 4–12 [30],
as an extension of the previous studies on 1–3 [24], and combined
this with theoretical computations on the unimolecular hydrogen
transfer and fragmentation reactions of the parent molecular ions,
1
2
•+, and of selected pairs of meta- and para-substituted analoga,
•+/3•+, 6•+/7•+ and 10•+/11•+, employing density functional cal-
culations [37–39]. The fragmentation was determined in three
different lifetime regimes of the fragmenting molecular ions to
gain further information on the effect of the substituents. On this
basis, a rather comprehensive picture of the physical and chemical
factors that govern the fragmentation of the molecular ions of 2-
benzylindanes and the preceding unimolecular hydrogen exchange
will be presented in the present and the accompanying paper [30].
3.1. Fragmentation routes and substituent effects on the
competing McLafferty reaction and benzyl cleavage
The standard EI mass spectra of the 2-benzylindanes inves-
tigated in this work and of those studied previously [24] show
molecular ion peaks of medium relative intensities and essentially
two sets of fragment ions peaks that reflect the charge-retaining
indanyl and benzyl moieties of the original structures (Table 1).
The great majority of the molecular ions decompose by cleavage
of the exocyclic benzylic C2-C␣ bond, with or without hydro-
gen rearrangement (Scheme 1). Similar to the fragmentation of
1,3-diphenylpropane under EI conditions [11,12], the McLafferty
reaction generating C7H7X•+ ions (m/z 92 for X = H) by loss of indene
and the simple benzyl cleavage generating benzyl ions C7H6X+ (m/z
91 for X = H) and the 2-indanyl radical dominate in most cases.
These two fragmentation routes will be discussed in detail below.
The complementary fragmentation channels, that is, the McLafferty
Fig. 1. Enthalpy profile calculated for isomerization and fragmentation of the
molecular radical cations of 2-benzylindane, 1•+. The McLafferty reaction and the
benzylic cleavage leading to ions C7H8 and C7H7+, respectively, are displayed
•+
•+
+
explicitly; the complementary fragmentation paths leading to C9H8 and C9H9
are only specified by the dashed energy levels. TS1 and TS2 indicate the transition
states for the forward and backward H transfer, respectively, and TS3 indicates the
transition state for the (hypothetical) 1,2-H shift in the distonic ion intermediate,
•+
1isom•+, generating the “meta-tautomer” 1isom,meta
.
The enthalpy profiles for the intramolecular isomerization and
the major fragmentation routes of the molecular ions of the parent
2-benzylindane radical cation, 1•+, and of a selected set of isomers,
•+
reaction leading to ionized indene, C9H8 (m/z 116) and neutral
•
•
•
•
•
2+ /3+
, , were determined by density-
6+ /7•+ and 10+ /11+
C7H8, as well as the simple cleavage giving 2- (or possibly 1-)
+
indanyl cation(s), C9H9 (m/z 117) and a benzyl radical, occur with
functional calculations (B3LYP/6–31 + g//B3LYP/6–311 + g(3d,2p)).
The profile for ions 1•+ is shown in Fig. 1. According to the calcu-
lations, fragmentation by the McLafferty reaction generating ions
C7H8•+ (m/z 92) requires 134 kJ mol−1, whereas the benzylic cleav-
age is much more energy-demanding (195 kJ mol−1). This is in
agreement with the dominance of m/z 92 peak in the 70 eV mass
spectrum of 1 [24] and the general knowledge on the energetics
of the fragmentation of the molecular ions of alkylbenzenes and
1,3-diphenylpropanes [6,11,36,49]. It is also in line with the fact
that metastable ions 1•+ undergo predominantly fragmentation
lower relative abundance in most cases, with the exception of the
para-fluoro-substituted ion 7•+. These channels have least signifi-
cance in the mass spectra of the electron-rich derivatives [X = OCH3,
OH and N(CH3)2] in accordance with a previous report on methoxy-
•+
substituted 1,3-diphenylalkanes [11]. Along with ions C9H8 and
C9H9+, the formation of the indenyl cation, C9H7 (m/z 115), is
+
observed exclusively in the high-energy (70 eV) mass spectra of
all 2-benzylindanes and is attributed to the secondary fragmenta-
+
tion process C9H9 → C9H7+ + H2. A fragmentation channel of minor
+
general importance but representing a structure-specific feature
is the elimination of benzene or the corresponding arene by ␥-
H transfer to the ipso-position of the benzylic residue, giving rise
by the McLafferty reaction, besides the formation of C9H9 and
C9H8•+, but no benzylic cleavage (Table 2). The complementary
•+
McLafferty-reaction channel, leading to ionized indene, C8H8
,
•+
to ions C10H10 (m/z 130). Electron-donating substituents in the
and (putatively) neutral isotoluene, is more energy-demanding
(173 kJ mol−1) but, notably, the complementary cleavage to the
indanyl ion, C9H9+, and the benzyl radical is about as endothermic
(197 kJ mol−1) as the formation of benzyl cation and the indanyl
radical mentioned above. The finding that metastable ions 1•+ do
not form C7H7+ (m/z 91) but do form abundant ions C9H9+ (m/z 117)
suggests that the latter ions do not have the 2-indanyl but rather
the undoubtedly more stable 1-indanyl structure. Moreover, the
para-position [X = 4ꢀ-OCH3, 4ꢀ-CH3, 4ꢀ-F, 4ꢀ-OH, but not 4ꢀ-N(CH3)2]
enhance this pathway significantly, whereas meta-substituents
rather suppress it. According to the general understanding outlined
above, this fragmentation is attributed to the increased local pro-
ton affinity of the ipso-position of the benzylic residues [29,44,48]
and will be addressed briefly in the last sections of this and the
accompanying paper.
•+
occurrence of ions C9H8 (m/z 116) in the metastable ion spectra
points to the possibility that the C7H8 neutral eliminated upon the
formation of these fragment ions is not the isotoluene tautomer
[36,50–52] but the considerably more stable toluene.2
for the 2-benzylindanes studied here, are known to differ by ꢀPA = PAC−6(m-
Xyl) − PAC−2(p-Xyl) = 18 kJ mol−1, which in this special case corresponds to the
global PA values [41,48]. However, the corresponding difference of the meta-
and para-methylanisoles was calculated to be ꢀPA = PAC−4(3-Me-Ani) − PAC−3(4-
Me-Ani) ≈ 70 kJ mol−1 [44] and that of the meta- and para-fluorotoluenes was
calculated to be ꢀPA = PAC−6(3-F-Tol) − PAC−2(4-F-Tol) = 36.8 kJ mol−1 [45]. For the
meta- and para-cresoles, ꢀPA = PAC−4(3-Cre) − PAC−3(4-Cre) = 66.2 ( 8) kJ mol−1 [44]
was determined by computational approach. While the global protons affinities of
the meta- and para-N,N-dimethyltoluidines are known by experiment [36,40,42],
local PA values are not for either of the isomers, to the best of our knowledge.
2
Note: According to calculations, the dissociation of molecular ion 1•+ into
the indene radical cation and neutral toluene needs a reaction enthalpy of only
29.1 kJ mol−1 and is the thermodynamically most favorable process. However, this
reaction requires the transfer of a hydrogen from the indanyl fragment to the ben-
zyl fragment during the benzylic cleavage either “on the fly” of the departing benzyl
Please cite this article in press as: H.-F. Grützmacher, D. Kuck, 2-Benzylindane radical cations in the gas phase (Part I): Sub-
stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
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Table 1
EI mass spectra (70 eV) of 2-benzylindane (1) and derivatives 2 − 12a.
Ion/comp’d
1b
2 (3ꢀ-OMe)b
3(4ꢀ-OMe)b
4 (3ꢀ-Me)
5 (4ꢀ-Me)
m/z
208
130
117
116
115
92
rel. int. (%)
m/z
238
rel. int. (%)
m/z
rel. int. (%)
M•+
C10H10
32.7
1.5
10.3
<0.2
3.3
26.6
10.8
7.0
222
130
117
116
115
106
105
91
29.7
1.6
36.4
5.9
•+c
+a
130
117
116
115
122
121
91
C9H9•+a
52.4
35.0
48.6
100.0
53.4
18.7
11.0
22.0
100.0
11.0
36.3
44.1
20.3
28.3
100.0
26.3
24.6
C9H8
5.7
6.8
+
C9H7
13.1
100.0
8.5
12.1
39.7
100.0
9.8
C7H7X•+a
C7H6X+
91
91
+d
C7H7
15.4
Ion/comp’d
6e (3ꢀ-F)
7e (4ꢀ-F)
8 (3ꢀ-OH)
9 (4ꢀ-OH)
10f (3ꢀ-NMe2)
11 (4ꢀ-NMe2)
12 [3ꢀ,5ꢀ-(OMe)2]
m/z
226
rel. int. (%)
m/z
rel. int. (%)
m/z
rel. int. (%)
m/z
rel. int. (%)
M•+
C10H10
30.3
1.3
46.0
8.5
224
130
117
116
115
108
107
91
28.2
0.4
10.8
9.4
13.4
100.0
11.0
6.7
60.2
31.7
46.6
26.3
5.9
67.8
100.0
3.1
251
130
117
116
115
135
134
91
24.0
3.0
3.1
3.7
8.2
100.0
8.2
12.7
19.3
1.2
1.3
1.7
3.7
17.1
100.0
4.0
268
130
117
116
115
152
151
91
5.3
0.1
1.9
3.6
6.7
100.0
2.9
7.2
•+c
+a
130
117
116
115
110
109
91
C9H9•+a
41.8
43.4
47.8
100.0
30.6
20.4
100.0
80.6
68.5
51.3
66.1
20.8
C9H8
+
C9H7
C7H7X•+a
C7H6X+
+d
C7H7
a
Intensities of adjacent peaks are not corrected for isotope contributions.
Data taken from Ref. [24].
b
c
d
e
f
Ions [M − C6H5X]•+ formed by H transfer to ipso-position (C-1ꢀ).
+
Ions C7H7 originating from the indanyl moiety.
Additional characteristic fragment ions: C6H5F•+ (m/z 96) for 6 (3.1%) and 7 (4.8%).
Besides the peak at m/z 135, a peak at m/z 136 (26.1%) was observed.
Table 2
Relative abundances of the fragment ions C7H7 and C7H8 and their substituted derivatives formed from 2-benzylindanes 1–12 in three energy regimes.a,b,c
+
•+
Ions
m/z
70 eV
11 eV
m*
70 eV
11 eV
m*
M•+(X = H)
208
91
92
1•+
1.2
98.8
+
C7H7•+
29.3
70.7
(<0.3)
100.0
C7H8
M•+(X = OMe)
C8H9O+
238
121
122
2•+(X meta)
0.3
3•+(X para)
51.1
48.9
7.9
92.1
0.0
100.0
80.7
19.3
33.9
66.1
C8H10O•+
99.7
M•+(X = Me)
222
105
106
4•+(X meta)
0.3
5•+(X = para)
1.5
98.5
+
C8H9
9.9
90.1
0.0
100.0
28
72
0.0
100
•+
C8H10
99.7
M•+(X = F)
C7H6F+
226
109
110
6•+(X meta)
0.6
7•+(X para)
4.2
95.8
23
77
0.0
100.0
59.3
40.7
2.6
97.4
C7H7F•+
99.4
M•+(X = OH)
C7H7O+
224
107
108
8•+(X meta)
0.1
9•+(X para)
28.7
71.3
15.4
84.6
0.0
100.0
71.9
28.1
8.3
91.7
C7H8O•+
99.9
M•+(X = NMe2)
251
133
134
135
136
10•+(X meta)
11•+(X para)
4.5
17.4
75.1
2.9
2.0
84.8
12.4
0.8
0.0
100.0
C9H12N+
1.1
94.5
4.4
0.0
100.0
90.5
7.5
2.0
C9H13N•+
M•+[X = 3’,5’-(OMe)2]
268
151
152
12•+
+
C9H11O2•+
4.3
95.7
(≤0.2)
0.0
100.0
C9H12O2
100.0
a
Given as%ꢁ; corrected for natural contributions of 13C1- and 13C2-containing isotopologs.
b
c
Data for compounds 1−3 taken from reference [24]. They slightly deviate from the corresponding data given in Table 1 due to different instrument parameters.
m*: Metastable ion data originate from mass-analyzed ion kinetic energy (MIKE) spectra obtained at 70 eV.
of the distonic ion 1isom•+, which themselves differ by 3 kJ mol−1
with the same relative tendency. Interestingly, the energies of the
two respective transition states for the forward and backward H
transfer differ more strongly, namely by 11 kJ mol−1. In view of
the relative low energy barrier of the hydrogen exchange in ions
1•+ as compared to the calculated reaction enthalpy of the McLaf-
ferty reaction (ꢀrH = 64 kJ mol−1), the extent of the H/D exchange
observed experimentally for the deuterated isotopologs of ions 1•+
The intramolecular 1,5-hydrogen transfer from C-1cis or C-3cis to
one of the ortho-positions of the benzyl residue can start from two
distinct conformers of ions 1•+ (Fig. 2). They differ only by 5 kJ mol−1
and the 1,5-H transfer leads to two likewise distinct conformers
fragment or in an ion-neutral complex of both fragments. It was not possible to
detect any of these fragmentation routes by the computational methods used.
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Fig. 2. Calculated structures of the molecular ions of 2-benzylidane, 1•+, the distonic ions, 1isom•+, and the transition states of the respective forward and backward H transfer,
TS1 and TS2 (cf. Fig. 1). The full sequence 1•+ (conformer 1) → 1•+ (conformer 2) constitutes one hydrogen exchange cycle (see text). Note that conformer 1, being more stable
than conformer 2 in the case of ions 1•+, is the less stable form in some of the substituted molecular ions.
Fig. 3. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(3-methoxybenzyl)indane, 2•+, by McLafferty reaction
and benzylic cleavage.
Fig. 4. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(4-methoxybenzyl)indane, 3•+, by McLafferty reaction
and benzylic cleavage.
[24,30] and its completeness in the metastable ions’ energy regime
is in accordance with the enthalpy profile of ions 1•+. We also cal-
culated the energy requirements for the hypothetical 1,2-H shift in
the distonic ion intermediate 1isom•+. Notably, such a process has
never been observed in either the molecular radical cations of sim-
ple alkylbenzenes [4,6] or 1,3-diphenylpropane [12] and it does
also not occur in any of the 2-benzylindane ions 1•+ − 12•+ stud-
ied in the present work [30] and previously [24]. As also shown
in Fig. 1, the barrier for a 1,2-H shift in the “ortho-protonated” ion
much higher than that toward the 1,2-proton shift in simple ring-
protonated arenes (arenium ions, 32 − 40 kJ mol−1) [8,10,21,53,54]
and probably reflects the radical character of the molecular ions of
2-benzylindane, 1, and its derivatives 2−12.
Different from the hydrogen exchange occurring in the radical
cations of open-chain alkylbenzenes and ␣,␻-diphenylalkanes, a
complete 4-H exchange cycle in 2-benzylindane ions comprises
two sets of different conformations both for the molecular ions
1
•+ and their distonic ions, 1isom•+. Therefore, the transitions states
for the forward and the backward hydrogen transfer steps, TS1
and TS2, are also different (Figs. 1 and 2). In the case of the par-
ent hydrocarbon, the enthalpies of these transitions states were
•+
•+
1isom leading to the “meta-tautomer”, 1isom,meta was calculated
to be exceedingly high, 93 − 96 kJ mol−1 above the level of the con-
formers of distonic ion intermediate 1isom•+. This barrier is clearly
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Fig. 5. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(3-fluorobenzyl)indane, 6•+, by McLafferty reaction
and benzylic cleavage.
Fig. 6. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(4-fluorobenzyl)indane, 7•+, by McLafferty reaction
and benzylic cleavage.
calculated to lie both well below the final step for the McLaf-
ferty reaction. Therefore, the finding that the 2-benzylindane ions
1•+ undergo a complete 4-H exchange prior to the fragmentation
ing [24] that the 4-H exchange in the para-isomer 3•+ is much
slower than in the meta-isomer 2•+ (see accompanying paper [30]).
The presence of a methyl group in the meta- or para-position of
the benzyl residues of the molecular ions 4•+ and 5•+, respectively,
has only a moderate effect on the competition of the fragmenta-
tion channels. A significant relative acceleration of the McLafferty
•+
to C7H8 is in accordance with the enthalpy profile obtained by
computation [24]. The structures of the starting and fin•a+l confor-
mations of the molecular ions 1•+, the distonic ions 1isom and the
transition states TS1 and TS2 are visualized in Fig. 2. Similar calcu-
lations have been performed for the pairs of methoxy-, fluoro- and
dimethylamino-substituted molecular ions 2•+/3•+, 6•+/7•+ and
10•+/11•+, respectively. The relative stabilities of the conformers
within a given pair of isomers were found to depend on the sub-
stituent. In the following discussion, the most stable conformer is
defined as the starting conformer from which the forward H trans-
fer takes place, notwithstanding its individual geometry.
•+
reaction leading to ion C7H7CH3 (m/z 106) is found for the meta-
isomer, 4•+. The direct benzylic cleavage of 4•+, giving rise to ion
+
C7H6CH3 (m/z 105), is decreased to some extent. The relative
•+
increase of the ions C10H10 (m/z 130) formed by loss of toluene
from 5•+ through ipso-protonation is remarkable (see below). Cal-
culations were not performed because of the minor substituent
effect of the methyl substitutents on the fragmentation of ions 4•+
and 5•+
.
The energy profiles calculated for the methoxy-substituted
derivatives 2•+ and 3•+ are depicted in Figs. 3 and 4, respectively.
As compared to the parent ions 1•+, the benzyIic cleavage is more
The mass spectra of the two isomeric fluorobenzylindanes 6
and 7 also reflect the positional effect of the fluoro substituents.
The McLafferty reaction leading to ions C7H7F•+ (m/z 110) dom-
inates the fragmentation of the meta-substituted molecular io•n+s
energy-demanding for the meta-isomer 2•+ (ꢀE = +19 kJ mol−1
)
and markedly less energy-demanding for the para-isomer 3•+
(ꢀE = −32 kJ mol−1). The reverse is found for the McLafferty reac-
tion, which requires considerably less energy in the case of 2•+
(ꢀE = −30 kJ mol−1) but slightly more for 3•+ (ꢀE = +6 kJ mol−1).
In agreement with the relevant local proton affinity differences
of meta- and para-methylanisole discussed above, the stabil-
6•+ to about the same extent as in the case of the parent ions 1
.
In contrast, the para-fluoro substituent in the molecular ions 7•+
gives rise to a decrease of both the McLafferty reaction and the
simple benzylic cleavage to ions C7H6F+ (m/z 109) in favor of the
formation of fragment ions that originate from the indanyl moiety.
+
Formation of the indanyl cation C9H9 (m/z 117) gives rise to the
•+
•+
•+
ities of the distonic ions 2isom
and 3isom
differ strongly
base peak in this case. The arene elimination giving ions C10H10
(ꢀE = 51 2 kJ mol−1) in favor of the former isomer. As argued pre-
viously, the stability of the ␴-complexes formed by the hydrogen
transfer step is a key factor of the McLafferty reaction of the meta-
methoxy-substituted alkylbenzenes under EI conditions [11,55,56].
Accordingly, the transitions states TS1 and TS2 lie much lower in
energy for the meta-isomer as compared to the para-isomer. Thus,
based on the calculations, it appears that the McLafferty reaction of
the meta-methoxy-substituted ions 2•+ samples significantly less
excited ions. In addition, the energy barriers toward the hydro-
(m/z 130) by ipso-protonation is triggered by the para-fluoro sub-
stituent of 7•+ (see below). Thus, both the characteristic inductive
(electron-withdrawing) and mesomeric (electron-releasing) sub-
stituent effects of the fluoro substituent are reflected by the
fragmentation of both isomers 6•+ and 7•+
.
The energy profiles calculated for the 2-(fluorobenzyl)indanes
6•+ and 7•+ are shown in Figs. 5 and 6, respectively. In line
with observation, the energy demand for the benzylic cleavage is
increased in both cases by the presence of the fluoro substitutents
as compared to the parent ions 1•+. In contrast, the McLafferty reac-
tion of the meta-isomer 6•+ is scarcely affected in comparison to
•+
gen transfer in the distonic ions 2isom lie significantly deeper
(ꢀE ≥ 43 kJ mol−1) with respect to the energy required for the C •C+
bond cleavage, as compared to the case of the distonic ions 3isom
(ꢀE ≥ 31 kJ mol−1). In total, the calculated enthalpy profiles for the
methoxy-substituted ions 2•+ and 3•+ are in good agreement with
the observed fragmentation and the marked differences between
the two isomers. They also rationalize the previously reported find-
ions 1• (ꢀE = −2 kJ mol−1), whereas it requires considerable more
+
energy in the para-isomer 7•+ (ꢀE = +17 kJ mol−1). This parallels
the trend found for the methoxy analogs 2•+ and 3•+ discussed
above. The energies of the transition states TS1 and TS2 and the
distonic ions 6isom•+ and 7isom•+ are markedly lower than the ener-
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stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
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TS
Fig. 7. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(3-N,N-dimethylaminobenzyl)indane, 10•+, by McLaf-
ferty reaction and benzylic cleavage.
Fig. 8. Enthalpy profile calculated for the isomerization and fragmentation of the
molecular radical cations of 2-(4-N,N-dimethylaminobenzyl)indane, 11•+, by McLaf-
ferty reaction and benzylic cleavage.
gies required for C C bond cleavage of the intermediates but the
11•+. In contrast, the McLafferty reaction, giving dimethylamino-
substituted 5-methylenecyclohexa-1,3-diene ions (m/z 135), is
particularly thermodynamically favorable in the case of the meta-
isomer 10•+. This channel requires significantly less energy as
compared to that of the corresponding channel in parent ions
1•+ (ꢀꢀrH = − 27 kJ mol−1). In contrast, the McLafferty reaction of
the isomeric para-substituted ions 11•+ is calculated to be slightly
more endothermic as compared to ions 1•+ (ꢀꢀrH = +15 kJ mol−1).
Obviously, the strongly stabilizing effect of the dimethylamino
group is much more efficient if this substituent is placed at the
C-3 position of ionized 5-methylenecyclohexa-1,3-diene than at
differences are very similar (ꢀrH = 67 − 80 kJ mol−1) in both cases.
•+
The marked stabilization of ions 6isom•+ as compared to ions 7isom
(ꢀE = 16 − 20 kJ mol−1) is in agreement with the local proton affin-
ity difference of meta- and para-fluorotoluene mentioned above.
While the calculations are only in moderate agreement with the
observed differences in the 70 eV mass spectra of 6 and 7, they
clearly show that the ␥-H transfer generating the distonic ions
and the overall McLafferty reaction is the energetically preferred
reaction channel for the meta-isomer.
The fragmentation behavior of the two isomeric 2-
(hydroxybenzyl)indanes 8 and 9 is similar to that of the methoxy
analogs 2 and 3. The McLafferty reaction strongly dominates the
mass spectrum of the meta-isomer 8, giving rise to ions C7H7OH•+
(m/z 108), whereas the simple benzylic cleavage leading to ions
C7H6OH•+ (m/z 107) generates the base peak in the spectrum of
the para-isomer 9. Again, the para-hydroxy group in the molecular
ions 8•+ exerts a particular strong accelerating effect on the
C-2. Thus, ions 3-(CH3)2NC6H5CH2 (m/z 135) from 10•+ were
•+
calculated to be by 42•k+J mol−1 more stable than the isomeric
ions 2-(CH3)2NC6H5CH2 from 11•+. This difference exceeds that
determined for the isomeric methoxy-substituted ana•lo+gs, 3-
CH3OC6H5CH2 (m/z 122, from 2•+) and 2-CH3OC6H5CH2 (from
•+
3
•+) discussed above (36 kJ mol−1). Most interestingly, however,
•+
is the finding that the transition states TS1 and TS2 calculated
for the ␥-H transfer in the meta-isomer 10•+ lie surprisingly high
at 75–83 kJ mol−1 and only 24–32 kJ mol−1 below the threshold
− much higher than those calculated for corresponding meta-
methoxy analog 3•+ (43–53 kJ below the threshold, cf. Fig. 3). This
difference also nicely explains the diverging experimental obser-
vations made on the 4-H exchange in ions 10•+ and 3•+ [30].
Finally, the energy profile calculated for the para-
dimethylamino-substituted ions 11•+ is worth being commented
(Fig. 8). It rationalizes the observation that the simple benzylic
ipso-protonation channel, giving rise to ions C10H10 (m/z 130)
whereas, once again, this process is practically absent in the case of
the meta-isomer (see below). Because of the similarity of the frag-
mentation of the respective hydroxy- and methoxy-substituted
2-benzylindanes ions, calculations were not performed for the
phenolic derivatives.
The fragmentation of the N,N-dimethylamino-substituted 2-
benzylindanes 10 and 11, as well as that of the doubly
meta-methoxy-substituted congener 12, also fit into the overall
picture. However, the electronic effects are even more pronounced
in all of these cases. The McLafferty reactio•n+, giving rise to ions
+
cleavage leading to ions 4-(CH3)2NC6H4CH2 (m/z 134) is the only
•+
fragmentation channel of the metastable ions 11•+. The transition
state for the McLafferty reaction lies only 13 kJ mol−1 below the
enthalpy required for the direct benzylic cleavage and at about the
same level as the exit for the C C bond cleavage of the distonic
ions 11isom•+. Moreover, this enthalpy profile is also in line with
the finding that the para-dimethylaminobenzyl ions formed by
fragmentation of the appropriate deuterium-labelled isotopologs
do not give any indication for a preceding intramolecular hydrogen
exchange [30].
C7H7N(CH3)2 (m/z 135) and C7H6(OCH3)2 (m/z 152), respec-
tively, governs the fragmentation of the molecular ions 10•+ and
12•+, whereas the benzylic cleavage, leading to the particularly
+
stable para-dimethylaminobenzyl cations, CH2C6H4N(CH3)2 (m/z
134), strongly dominates the fragmentation of ions 11•+. Never-
theless, the particularly high electron-releasing character of the
dimethylamino group does not completely suppress the McLafferty
reaction in the latter case.
The energy profiles for the fragmentation of the dimethylamino
derivatives 10•+ and 11•+ (Fig. 7 and 8, respectively) are in line
with the experimental findings. The enthalpy requirements for the
3.2. Arene elimination via ipso-protonolysis
benzylic cleavage reactions differ strongly (ꢀꢀrH = 86 kJ mol−1
)
in favor of the facile formation of the highly stabilized para-
dimethylaminobenzyl cations (m/z 134) from the para-isomer,
As mentioned above on several occasions, loss of benzene from
the molecular ions of the parent hydrocarbon, 1•+, and of the
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Scheme 4. Elimination of C6H5X by ipso-protonolysis of ions 1•+ and the para-substituted derivatives 3•+, 5•+, 7•+ and 9•+, after incomplete 4-H exchange. The structure
•+
suggested for ions C10H10 (m/z 130) and the insinuated 1,2-C shift are hypothetical.
corresponding arenes from the molecular ions of several para-
substituted 2-benzylindanes, 3•+ (X = OCH3), 5•+ (X = CH3), 7•+
(X = F) and 9•+ (X = OH) occurs as an additional fragmentation
path in most cases. However, this ipso-protonolysis, a common
fragmentation channel of protonated arenes [8,21,29], gives rise
to a negligibly small peak in the mass spectrum of the para-
dimethylamino-substituted congener 11 and only very minor
peaks in the mass spectra of all meta-substituted 2-benzylindanes
alkoxy)-substituted alkylbenzene derivatives under EI conditions
[58].
4. Conclusion
The experimentally observed substituent effects on the
fragmentation of the radical cations of various substituted
2-benzylindanes, as probed for three different energy and ion-
lifetime regimes, have been rationalized by means of theoretical
calculations of the corresponding enthalpy profiles. The energy
requirements for the McLafferty reaction and the benzylic cleav-
age depend strongly on the position and the electronic nature of
the substituent in each case, and the dominance of the McLafferty
reaction for the meta-substituted isomers agrees with the energet-
ically favorable formation of the distonic ion intermediates formed
by the ␥-hydrogen transfer step. As will be shown in the accom-
panying paper [30], the extent of progression of the 4-H exchange
process in the various molecular ions also agrees qualitatively with
the depth of the “isomerization valley” relative to the fragmenta-
tion threshold.
(Table 1). These trends are even more pronounced in the MIKE
•
spectra: The [M − C6H5X]•+ ions (C10H10 +, m/z 130) are formed
in varying relative abundances, but it is telling to note that
they represent the dominant peak in the MIKE spectrum of the
para-fluoro-substituted ions, 7•+, whereas they are absent in the
spectrum of the para-dimethylamino congener, 11•+. Again, this
points to the role of the charge distribution and the character of
this hydrogen rearrangement route as a proton transfer rather than
a hydrogen atom transfer path. Except for the case of ions 11•+, it
is obvious that the local proton affinity at the ipso-position of the
benzyl residues in ions 3•+, 5•+, 7•+ and 9•+ plays an important
role in this process, representing the para-position relative to the
respective electron-releasing substituent. As mentioned above, the
local proton affinities for many aromatic compounds are known by
computational work and can be estimated employing the additivity
rule [41,44–48]. Also, ipso-protonation has been discussed as a frag-
mentation route that can compete with the McLafferty reaction of
suitably substituted alkylbenzenes, if steric and/or electronic con-
ditions are unfavorable for the latter reaction [57]. A mechanism for
the loss of arenes from the molecular ions is suggested in Scheme 4.
The ipso-protonolysis channel provides further, but less gen-
eral, insight into the same hydrogen exchange processes that
occur prior to the McLafferty reaction of the molecular ions of
the 2-benzylindanes, as will be shown in the accompanying paper
[30]. In a more general view, this may be expecially relevant
for understanding the fragmentation of para-hydroxy- (and para-
Appendix A. Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.ijms.2016.05.021.
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Please cite this article in press as: H.-F. Grützmacher, D. Kuck, 2-Benzylindane radical cations in the gas phase (Part I): Sub-
stituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes, Int. J. Mass Spectrom. (2016),
http://dx.doi.org/10.1016/j.ijms.2016.05.021

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MASPEC-15618; No. of Pages11
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