Mass-spectrometric study on ion-molecule reactions of CH5+, C2H5+, and C3H5+ with C9-C19 alkylbenzenes in an ion trap

Article abstract of DOI:10.1246/bcsj.75.241

Chemical ionization of alkylbenzenes (PhC_xH_{2x + 1} = M: x = 3-13) by the CH₅⁺, C₂H₅⁺, and C₃H₅⁺ ions has been studied under a reactant-ion selective mode of an ion-trap type of GC/MS. The dominant product ions for short-chain reagents (x < 7) were [M + H]⁺, [Phil + H]⁺, and C_xH_{2x + 1}⁺ ions, produced through proton-transfer to benzene ring. On the other hand, the dominant product ions for long-chain reagents (x ≥ 7) were C_yH_{2y + 1}⁺ (y < x) and PhC_yH_2y⁺ (y ≤ x) ions. The former ions are produced through the attack of the reactant ions on the alkyl chain and/or the benzene ring, while the latter ones are exclusively formed through the attack of the reactant ions on the alkyl chain. Major formation processes of C_yH_{2y + 1}⁺ and PhC_yH_2y⁺ ions in each reaction were discussed on the basis of observed distributions and calculated thermochemical data.

Full text of DOI:10.1246/bcsj.75.241

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 241–252 (2002)
241
e Chemical
ciety of Ja-
n
+
+
e Chemical
ciety of Ja-
n
Mass-Spectrometric Study on Ion-Molecule Reactions of CH5 , C2H5 ,
+
and C3H5 with C9–C19 Alkylbenzenes in an Ion Trap
Chemical Ionization of Alkylbenzenes
Y. Tanaka et al.
Yuki Tanaka and Masaharu Tsuji†,*
02
1
2
Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences,
Kyushu University, Kasuga, Fukuoka 816-8580
SJA8
09-2673
306
†Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580
(Received August 31, 2001)
01
Chemical ionization of alkylbenzenes (PhCxH2x + 1 = M: x = 3–13) by the CH5+, C2H5+, and C3H5+ ions has been
studied under a reactant-ion selective mode of an ion-trap type of GC/MS. The dominant product ions for short-chain re-
agents (x < 7) were [M + H]+, [PhH + H]+, and CxH2x + 1+ ions, produced through proton-transfer to benzene ring. On
the other hand, the dominant product ions for long-chain reagents (x ꢀ 7) were CyH2y + 1+ (y < x) and PhCyH2y+ (y ꢁ x)
ions. The former ions are produced through the attack of the reactant ions on the alkyl chain and/or the benzene ring,
while the latter ones are exclusively formed through the attack of the reactant ions on the alkyl chain. Major formation
processes of CyH2y + 1+ and PhCyH2y+ ions in each reaction were discussed on the basis of observed distributions and cal-
culated thermochemical data.
01
.2
+
The gas phase ion-molecule reactions of alkylbenzenes in a
methane atmosphere have been extensively studied since the
first chemical ionization (CI) mass spectrometric measure-
ments by Field et al.1–4 They measured CI mass spectra of un-
branched (PhCxH2x + 1 = M: x = 0, 1, 5) and branched alkyl-
benzenes at a medium CH4 pressure of 133 Pa, where domi-
of an ion/neutral complex consisting of CxH2x + 1 and PhH.
Within this complex, the alkyl ions may undergo hydride shifts
to form more stable alkyl ions.
All of previous medium-pressure CH4 CI mass studies on
alkylbenzenes have been carried out without selecting reactant
hydrocarbon ions.1–10 Therefore, the reactivity of dominant
CH5+ and C2H5+ reactant ions for alkylbenzenes has not been
determined. It is known that C3H5+ ion is involved as a minor
reactant ion when CH4 is used as a CI gas. However, no infor-
mation has been obtained on its reactivity for alkylbenzenes.
To the best of our knowledge, no CI mass spectrometric study
has been carried out for alkylbenzenes (PhCxH2x + 1) with a
long alkyl chain (x > 5).
Recently, the ion-trap detector (ITD), which can operate at
much lower CI gas pressures than those in a conventional me-
dium-pressure CI mass spectrometer, has been used as a new,
more sensitive CI mass spectrometer.11–13 Some comparative
studies between medium-pressure CI using magnetic sector in-
struments and low-pressure CI in the ITD have been carried
out.11–13 In general, fragmentation increases and no adduct
ions such as [M + C2H5]+ and [M + C3H5]+ are observed in
the ITD. These facts were attributed to the lack of collisional
stabilization of [M + H]+, [M + C2H5]+, and [M + C3H5]+
ions due to secondary collisions with CH4 and He gases. The
other reason for the higher fragmentation in the ion-trap exper-
iments is higher kinetic energies of reactant ions.
+
nant reactant ions were CH5+ (48%), C2H5+ (40%), and C3H5
(6%).1 They observed abundant [M + H]+ ions, in most cases,
as well as [M + C2H5]+ and [M + C3H5]+ adduct ions charac-
teristic of the molecular weight. The major fragment ions
could be explained by loss of H2 and/or small alkanes from [M
+ H]+, olefin elimination from [M + H]+ and [M + C2H5]+
or, for larger alkyl substituents, formation of alkyl ions. The
loss of H2 and small neutral alkanes has been shown to result
from direct protonation of C–H or C–C bonds; the reactions
are essentially hydride and alkanide ion abstraction reactions.
After pioneering investigations by Field et al.,1–4 more detailed
studies have been carried out on CI mass spectra of C8–C11
alkylbenzenes.5–10 It was found that major reaction channels
for both CH5+ and C2H5+ reactions are proton transfers to the
benzene ring, leading to [M + H]+. The [M + H]+ ion de-
composes either by olefin elimination to form protonated ben-
zene (1a) or by benzene elimination to form an alkyl ion (1b):
[PhCxH2x + 1 + H]+ → [PhH + H]+ + CxH2x,
(1a)
(1b)
+
→ PhH + CxH2x + 1
.
We have recently studied CH4 CI mass spectra of n-paraffins
(CxH2x + 2: x = 8–18) and 1-olefins (1-CxH2x: x = 8–18) by the
+
+
+
Isotopic labelling studies have shown that reaction (1a) in-
volves non-random transfer of hydrogen from several different
positions of the alkyl chain rather than specific hydrogen mi-
gration from a single position. There was substantial evidence
that reactions (1a) and (1b) proceed through the intermediacy
CH5 , C2H5 , and C3H5 ions using an ion-trap type of GC/
MS.14,15 In this study, CH4 CI mass spectra of a series of al-
+
+
kylbenzenes (PhCxH2x + 1: x = 3–13) by the CH5 , C2H5 , and
C3H5 ions are measured under a reactant-ion selective mode
of an ion-trap type of GC/MS. The dependence of product-ion
+

242 Bull. Chem. Soc. Jpn., 75, No. 2 (2002)
Chemical Ionization of Alkylbenzenes
distributions on the reaction time was measured in order to ex-
amine the effects of collisional stabilization. The reactivity of
CH5 , C2H5 , and C3H5 for alkylbenzenes was discussed
from the initial product-ion distributions and from thermo-
chemical calculations of heats of reactions.
an excess energy will be partly relaxed by collisions with CH4
and He gases. Therefore, fragmentation will be suppressed in
CI mass spectra obtained at long reaction times. In order to
examine the contribution of collisional stabilization in our CI
conditions, the dependence of product-ion distributions on the
reaction time was measured.
+
+
+
Experimental
For examples, Figs. 1 and 2 show product-ion distributions
of CyH2y + 1+ (y = 3–10) and PhCyH2y+ (y = 1–13) in the reac-
CI mass spectra were obtained using an ion-trap type of Hitachi
M7200 GC/MS under a reactant-ion selective mode. The CI CH4
gas was introduced directly in an ion-trap cell. The electron-im-
+
+
+
tions of CH5 , C2H5 , and C3H5 with typical reagents
(PhCxH2x + 1: x = 8 and 13) at five different reaction times: 0.5,
+
+
2, 10, 20, and 40 ms. The CyH2y + 1+ (y = 3–10) and PhCyH2y
pact ionization on CH4 provides primary CHn (n = 2–4) ions.
One of the reactant CH5+, C2H5+, and C3H5+ ions produced from
(y = 1–13) distributions exhibit single peaks in most cases. It
+
+
+
the subsequent fast ion-molecule reactions of CHn (n = 2–4)
is clear from Figs. 1 and 2 that the CyH2y + 1 and PhCyH2y
with CH4 was selectively trapped as a reactant ion in an ion-trap
cell. The maximum and average kinetic energies of the reactant
ions in our apparatus were evaluated to be 10 and 4.2 eV (1 eV =
96.485 kJ mol−1) for CH5+, 6.0 and 2.4 eV for C2H5+, and 4.3 and
1.7 eV for C3H5+, respectively, using a pseudo-potential well
method.14 These energies are higher than that in the medium-pres-
sure CI experiments, which was estimated to be less than 1 eV.16
The time for storing a reactant ion was kept at a constant time of 5
ms. If reactant ions in vibrationally excited levels are formed,
they will be thermalized by collisions with CH4 and He during
their trapping time in the cell. The ion-trap cell was kept at ꢁ 170
°C. The reagents were diluted in hexane and injected into the GC
with a high-purity carrier He gas. The partial pressures of CH4
and He and in an ion-trap cell were 9 × 10−3 and 7 × 10−3 Pa, re-
spectively. The reaction time corresponding to the residence time
in the ion-trap was varied from 0.5 to 40 ms. The mass spectra
were measured at low reagent concentrations of about 1000–
10000 pg cm−3 in order to reduce secondary ion-molecule reac-
tions. It is known that the number of ions stored in the ion trap de-
pends on the radio frequency voltage.17 Thus, the CI mass spectra
obtained in this study were calibrated against NIST standard da-
ta.18
The operating conditions in the ion-trap cell used in this work
were significantly different from those of the conventional medi-
um-pressure CI mass spectrometer developed by Field et al.1–4 In
the medium-pressure CI measurements, the typical CH4 gas pres-
sure was 133 Pa and the residence time of reactant ions in the ion-
ization-reaction chamber was about 10 µs. Field4 evaluated the to-
tal number of collisions of reactant ions with CH4 during this resi-
dence time to be about 200. In the present low-pressure CI mea-
surements, the total number of collisions of a product ion with
CH4 was estimated to be about 1–100 times within the reaction
time of 0.5–40 ms from a simple gas-kinetic hard-sphere collision
model.
distributions depend on the reaction time in most cases, though
+
the changes in the PhCyH2y distributions are smaller than
those in the CyH2y + 1+ ones. The decreases or increases in the
product-ion distributions with increasing the reaction time are
+
+
shown by arrows in Figs. 1 and 2. In all the six CH5 , C2H5 ,
+
and C3H5 reactions shown in Fig. 1, the branching ratios of
+
CyH2y + 1 decrease with increasing the reaction time, except
+
for the branching ratios of CyH2y + 1+ (y = 9, 10) in the CH5 /
PhC13H27 reaction, for which an inverse relation is observed.
+
The branching ratios of PhCyH2y having small y values de-
crease, while those having large y values increase in many cas-
es.
+
+
In addition to major CyH2y + 1 and PhCyH2y ions, [M +
H]+, [PhH + H]+, [PhC2H5 + H]+, [M + C3H5]+, and [M +
C3H5 − C2H4]+ ions were observed in many reactions. With
increasing reaction time, the branching ratios of [M + H]+,
[M+C3H5]+, and [M + C3H5 − C2H4]+ ions increase, while
those of [PhH + H]+ and [PhC2H5 + H]+ either slightly de-
crease or are nearly constant. These findings indicate that col-
lisonal stabilization with CH4 and He participates in the forma-
tion of the former ions. We found here that collisional stabili-
zation take part in most of all reactions. Thus, the initial prod-
uct distributions were determined by extrapolating the depen-
dence of branching ratios of product ions on the reaction time
to zero reaction time. The results obtained are summarized in
Tables 1–3. The uncertainties of the initial branching-ratios
were estimated to be within 8%. Two major product ions are
+
+
alkyl CyH2y + 1 (y ꢁ x) and PhCyH2y (y ꢁ x) ions. The de-
pendence of their intensity distributions on the chain length x
is shown below for the three reactions.
+
Distribution of Alkyl CyH2y + 1 (y = 3–10: y ꢁ x) Ions:
Figures 3(a)–3(i) show initial product-ion distributions of
CyH2y + 1+ (y = 3–10: y ꢁ x) obtained for short (x = 3–5), me-
dium (x = 6–9), and long (x = 10–13) chain reagents. The
Results and Discussion
+
+
+
CyH2y + 1 (y = 3–10) ions are observed in the CH5 , C2H5 ,
+
Contribution of Collisional Stabilization and Initial
and C3H5 reactions; differences in the intensity distributions
Product-Ion Distributions: When CI mass spectra resulting
are relatively small among the three reactions. Figures 3(a)–
3(i) show that the CyH2y + 1 distributions depend strongly on
+
+
from ion-molecule reactions of CH5 , C2H5 , and C3H5+ with
+
alkylbenzenes (PhCxH2x + 1: x = 3–13) were measured, [M +
the chain length x for short reagents below x < 7, where the
dominant alkyl ions are CxH2x + 1+. On the other hand, the dis-
tributions become similar for long-chain reagents x ꢀ 7, where
+
+
H]+, CyH2y + 1 (y = 3–10), and PhCyH2y (y = 1–13) ions
were observed. Here, x represents the number of carbons of
alkyl groups in a reagent, while y stands for the number of car-
bons of fragment alkyl groups. In addition, the [PhC2H5 +
H]+ ion was observed in the C2H5+ reaction, and adduct [M +
C3H5]+ ions were observed in the C3H5+ reaction. If the colli-
sional stabilization takes part in the formation of product ions,
+
the dominant alkyl ions are CyH2y + 1+ (y = 4–6). In the CH5
+
+
and C2H5 reactions, the CyH2y + 1 distributions peak at y =
3–6 for short (x = 3–6) chain reagents, and at y = 4, 5 for
+
longer (x > 6) chain reagents. In the C3H5 reactions, the
+
CyH2y + 1 distributions peak at y = 3, 4 for short (x = 3–6)

Y. Tanaka et al.
Bull. Chem. Soc. Jpn., 75, No. 2 (2002) 243
Fig. 1. Dependence of product-ion distributions of CyH2y +1+ (y = 3–10) ion on the reaction time in the ion-molecule reactions of
CH5+, C2H5+, and C3H5+ with PhC8H17 and PhC13H27. Reaction time ꢀ: 0.5, ꢀ: 2, ꢁ: 10, ꢂ: 20, and ×: 40 ms. Relative intensi-
ties were branching ratios of each ion for total product ions. Arrows indicate an increase or a decrease in product-ion distribution
with an increasing the reaction time. The line connecting the points in the graphs is the CyH2y + 1+ formed from the same reaction
time.
chain reagents and at y = 4, 5 for medium (x = 7–9) chain re-
0.52 eV.18 Therefore, the C7H7+ ion observed here will be sta-
ble tropylium ion. In the C2H5+ reactions, the second peaks at
y = 3 or 4 become more pronounced for x = 6–13 reagents.
The PhCyH2y distributions in the C3H5 reactions are similar
to those in the C2H5+ reactions, except for the enhancement of
the PhCxH2x+ peaks.
Possible Mechanism and Energetics of Each Ion-Mole-
cule Reaction: In Schemes 1 and 2 are shown possible reac-
tion pathways for the ion-molecule reactions of XH+ (X =
CH4, C2H4, and C3H4) with alkylbenzenes. Reactions (A-1)
and (A-2), shown in Scheme 1, proceed through the attack of
the reactant ions on the benzene ring, while reactions (B-1),
(B-2), and (B-3), shown in Scheme 2, occur through the attack
+
+
agents, as in the cases of the CH5 and C2H5 reactions. It
+
+
should be noted that the CyH2y + 1 distributions in the C3H5
+
+
reactions for long-chain reagents (x ꢀ 7) shift to lower y val-
ues than those in the CH5+ and C2H5+ reactions. This implies
+
that the excess energies released in the C3H5 reactions are
larger than those in the CH5+ and C2H5+ reactions for the for-
mation of CyH2y + 1+ from PhCxH2x + 1 (x ꢀ 7).
Distribution of PhCyH2y+ (y = 1–13: y ꢁ x) Ions:
Fig-
ures 4(a)–4(i) show initial product-ion distributions of
PhCyH2y+ (y = 1–13: y ꢁ x) obtained for short (x = 3–5), me-
dium (x = 6–9), and long (x = 10–13) chain reagents. The
+
+
+
PhCyH2y (y = 1–13) ions are observed in the CH5 , C2H5 ,
and C3H5+ reactions. Differences in the intensity distributions
among the three reactions are larger than those observed for
CyH2y + 1+. The PhCxH2x+ = [M − H]+ ion peaks increase in
of the reactant ions on the alkyl chain. Protonation to benzene
ring (A-1) gives three products: [M + H]+, CxH2x +
and
+
1
[PhH + H]+, while addition to benzene ring (A-2) provides [M
+ XH]+, [M + XH − C2H4]+, and [PhH + XH]+ ions. Proto-
nation either to C–H or C–C bond (B-1) leads to [M + H]+,
[M−H]+, CyH2y + 1+, and PhCyH2y+, while hydride-ion ab-
+
+
+
the order of the CH5 , C2H5 , and C3H5 reactions. The
+
PhCyH2y (y < x) distributions depend on the chain length x
for short-chain reagents x < 7, while they become similar for
+
+
long-chain reagents x ꢀ 7. In the CH5 reactions, the
straction (B-2) gives the [M−H]+, CyH2y + 1+, and PhCyH2y
PhCyH2y+ distributions peak at y = 1 for x = 3–9 reagents, and
second peaks appear at y = 3 for x = 10–13 reagents. Al-
ions. The PhCzH2z or CzH2z + 1 ion abstraction (B-3) pro-
−
−
+
+
vides CyH2y + 1 or PhCyH2y ions, respectively. In our previ-
+
+
+
+
though benzyl ion and tropylium ion are possible as PhCyH2y
ous study on the reactions of CH5 , C2H5 , and C3H5 reac-
(y = 1) ion, the latter ion is more stable than the former one by
tions with n-paraffins, [M + H]+ ions could not be observed

244 Bull. Chem. Soc. Jpn., 75, No. 2 (2002)
Chemical Ionization of Alkylbenzenes
Fig. 2. Dependence of product-ion distributions of PhCyH2y+ (y = 1–13) ion on the reaction time in the ion-molecule reactions of
CH5+, C2H5+, and C3H5+ with PhC8H17 and PhC13H27. Reaction time ꢀ: 0.5, ꢀ: 2, ꢁ: 10, ꢂ: 20, and ×: 40 ms. Relative intensi-
ties were branching ratios of each ion for total product ions. Arrows indicate an increase or a decrease in product-ion distribution
with an increasing the reaction time. The line connecting the points in the graphs is the PhCyH2y+ formed from the same reaction
time.
Table 1. Branching Ratios of Each Product Channel in Reactions of CH5+ with PhCxH2x + 1 [x = 3–13]a)
Reagents [x (PhCxH2x + 1)]
3
4
5
6
7
8
9
10
11
12
13
Product ions
Attack of the reactant on the
benzene ring
[M + H]+
0.00
0.86
0.13
0.00
0.88
0.038
0.00
0.74
0.00
0.0057 0.00 0.00
0.00
0.00
0.00
0.00 0.00
0.00 0.00
0.00 0.00
0.0067 0.0080
+
CxH2x + 1
0.57
0.00
0.00 0.00
0.00 0.00
0.00
0.00
0.00
0.00
+
C6H7
Attack of the reactant on the
benzene ring or alkyl chain
+
CyH2y + 1
0.00
0.00
0.0063 0.19
0.31
0.00
0.87 0.87
0.86
0.85 0.84
0.80
0.75
Attack of the reactant on
alkyl chain
PhCxH2x+ = [M − H]+
0.00
0.00
0.00 0.0054 0.0058 0.00 0.018 0.0079 0.00
0.13 0.12 0.13 0.15 0.14 0.18 0.23
+
PhCyH2y
0.011 0.080
0.079 0.12
a) Uncertainties are within 8%.
+ 14
due to complete decomposition of [M + H]+ into CyH2y + 1
.
The heats of reactions of each process for the formation of
dominant CyH2y + 1+ and PhCyH2y+ ions were calculated using
reported thermochemical data.18 The results obtained are
shown in Figs. 5(A)–5(F) for the case of x = 13 reagent. Sim-
It is therefore reasonable to assume that all [M + H]+ ions ob-
served in this study are ring-protonated ions produced through
reaction (A-1).

Y. Tanaka et al.
Bull. Chem. Soc. Jpn., 75, No. 2 (2002) 245
Table 2. Branching Ratios of Each Product Channel in Reactions of C2H5+ with PhCxH2x + 1 [x = 3–13]a)
Reagents [x (PhCxH2x + 1)]
3
4
5
6
7
8
9
10
11
12
13
Product ions
Attack of the reactant on the
benzene ring
[M + H]+
0.20
0.28
0.065 0.011
0.18
0.42
0.12
0.40
0.00
0.12
0.34
0.00
0.045 0.058 0.026
0.043 0.051 0.059 0.042
+
CxH2x + 1
0.022 0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
+
C6H7
0.00
0.00
Attack of the reactant on the
benzene ring or alkyl chain
+
CyH2y + 1
0.00
0.15
0.0053 0.15
0.23
0.55
0.48
0.48
0.49
0.49
0.44
0.43
Attack of the reactant on
alkyl chain
PhCxH2x+ = [M − H]+
0.11
0.058 0.043 0.029 0.029 0.041
0.16 0.21 0.30 0.39 0.43
0.028 0.050 0.064 0.061
0.43
+
PhCyH2y
0.056 0.12
0.40
0.43
0.46
Addition to the benzene ring
[PhC2H5 + H]+
0.17
0.13
0.061 0.032 0.020 0.015 0.0061 0.00
0.00
0.00
0.00
a) Uncertainties are within 8%.
Table 3. Branching Ratios of Each Product Channel in Reactions of C3H5+ with PhCxH2x + 1 [x = 3–13]a)
Reagents [x (PhCxH2x + 1)]
3
4
5
6
7
8
9
10
11
12
13
Product ions
Attack of the reactant on the
benzene ring
[M + H]+
0.073 0.080 0.073 0.11
0.065 0.11
0.084 0.10
0.12
0.00
0.00
0.14
0.00
0.00
0.10
0.00
0.00
+
CxH2x + 1
0.56
0.00
0.39
0.00
0.22
0.00
0.16
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
+
C6H7
Attack of the reactant on the
benzene ring or alkyl chain
+
CyH2y + 1
0.00
0.17
0.022 0.37
0.44
0.09
0.61
0.50
0.50
0.48
0.59
0.41
0.37
Attack of the reactant on
alkyl chain
PhCxH2x+ = [M − H]+
0.20
0.13
0.09
0.18
0.10
0.27
0.09
0.30
0.13
0.28
0.09
0.13
0.13
0.23
0.16
0.29
+
PhCyH2y
0.020 0.11
0.087 0.11
Addition to the benzene ring
[M + C3H5]+
0.13
0.13
0.060 0.020 0.00
0.00
0.00
0.00
0.00
0.00
0.00
[M + C3H5 − C2H4]+
0.022 0.046 0.040 0.059 0.050 0.021 0.00
0.0055 0.0094 0.025 0.025
a) Uncertainties are within 8%.
ilar energy relationships are obtained for the reactions of other
alkylbenzenes. For example, ∆Ho values in Fig. 5(A) represent
the heats of reaction of the following reactions, leading to vari-
ous CyH2y + 1+ (y ꢁ x) ions:
There are three possible C3H5+ isomers, whose ∆Ho values are
946, 969, and 1069 kJ mol−1 for CH2wCHCH2 , CH3CwCH2 ,
and protonated cyclopropene ion, respectively.18 Since the
most stable CH2wCHCH2+ isomer is a significant ion produced
+
+
+
from CH4 CI gas,19 all thermochemical calculations for C3H5
CH5+ + PhC13H27 → CxH2x + 1+ + PhH + CH4,
→ CyH2y + 1+ + PhC13−yH2(13−y) + 1 + CH4,
(Reaction B-1 for x = 13 in Scheme 2),
are carried out using the above ∆Ho value of CH2wCHCH2 .
+
The ∆Ho values in Figs. 5(A)–5(F) are shown only for the for-
mation of unstable –CH2 (= CyH2y + 1+) and –CHwCH+ (=
+
CyH2y−1+) ions, n-paraffins, and 1-olefins as ionic and neutral
products, for the sake of clarity. However, more stable isomers
having secondary and tertiary alkyl groups will also be formed.
Since the ∆Ho values of secondary and tertiary carbocations
are lower than the value of primary ones by about 0.87 and 1.3
CH5+ + PhC13H27 → CyH2y + 1+ + PhC13−yH2(13−y)−1
+ CH4 + H2,
(Reaction B-2 for x = 13 in Scheme 2).

246 Bull. Chem. Soc. Jpn., 75, No. 2 (2002)
Chemical Ionization of Alkylbenzenes
Fig. 3. Initial product-ion distributions of CyH2y + 1+ (y = 3–10) ion in the ion-molecule reactions of CH5+, C2H5+, and C3H5+ with
C9–C19 alkylbenzenes. (a), (d), (g) ꢀ: PhC3H7, ꢀ: PhC4H9, and ꢁ: PhC5H11, (b), (e), (h) ꢀ: PhC6H13, ꢀ: PhC7H15, ꢁ: PhC8H17,
and ꢂ: PhC9H19, (c), (f), (i) ꢀ: PhC10H21, ꢀ: PhC11H23, ꢁ: PhC12H25, and ꢂ: PhC13H27. The largest CyH2y + 1+ ions correspond to
the CxH2x + 1+ ions. The line connecting the points in the graphs is the alkyl CyH2y + 1+ (y = 3–x) ion formed from the same alkyl-
benzene.
eV, respectively,18 many endoergic processes change to exthoe-
endothermic in most cases. Thus, chain protonation transfer
(B-1) is most favorable on the basis of thermochemical stabili-
ty of the final products. It is shown that the formation of CyH2y
+
+
rgic ones for the formation of large CyH2y + 1 and PhCyH2y
ions via stabilization due to isomerization. The ∆Ho values for
+
+
proton transfer channels, (A-1) and (B-1), increase in the order
ions in the CH5 reactions dominantly proceeds through
+ 1
+
+
+
of CH5 , C2H5 , and C3H5 reactions due to increase in the
proton affinity in the order of CH4(PA = 5.7 eV), C2H4 (7.1
eV) and C3H4 (8.0 eV).18 On the other hand, there are no sig-
nificant differences in the ∆Ho values among the hydride-ion
proton-transfer to C–H or C–C alkyl chain for n-paraffins and
proton-transfer to CwC bond and H− abstraction from the alkyl
portion for 1-olefins.14,15 Since there are more CwC double
bonds in alkylbenzenes than in 1-olefin, the probability of the
attack of the reactant ions to a CwC bond in ring will become
larger than that to alkyl chain. It is therefore highly likely that
−
−
abstraction (B-2) and PhCzH2z or CzH2z + 1 ion abstraction
+
+
reactions (B-3). Although the CyH2y + 1 and PhCyH2y ions
can competitively be produced through reactions (B-1), (B-2),
and (B-3), the formation of the PhCyH2y+ ions is energetically
more favorable than that of CyH2y + 1+ ones on the basis of the
energetics. It is clear from Figs. 5(A)–5(F) that the ∆Ho values
are essentially independent of chain length of fragment ions y
for the formation of CyH2y + 1+, while they generally decrease
with increasing y for the formation of PhCyH2y+ without taking
isomerization into account.
+
the dominant formation process of CyH2y + 1 from alkylben-
+
zenes is (A-1). In reaction (A-1), the CyH2y + 1 ions are pro-
+
duced through the decomposition of CxH2x + 1+ into CyH2y + 1
+ olefins. Although the ∆Ho values for the formation of the
+
CxH2x + 1 + PhH are the same for reactions (A-1) and (B-1),
those for the formation of CyH2y + 1+ + PhH + olefin through
reaction (A-1) are higher than those for the formation of Cy-
H2y + 1+ + PhCx−yH2(x−y) + 1 through reaction (B-1) by about 1
In the CH5+ reactions, [M + H]+, CyH2y + 1+ (y ꢁ x), [PhH
eV. The preferential formation of CyH2y + 1 through reaction
+
+
+ H]+, and PhCyH2y (y ꢁ x) ions are observed, as shown in
(A-1), even though it is energetically less favorable than reac-
tion (B-1), is explained by the stability of precursor ring-proto-
nated ions being higher than that of chain protonated ones. We
Table 1. Among them, [M + H]+, CxH2x + 1+, and [PhH + H]+
are produced via proton transfer to benzene ring (A-1). The
+
+
CyH2y + 1 ions can be formed via reactions (A-1), (B-1), and
found that no CxH2x + 1 ions are formed for molecules x > 7
(B-2), while the PhCyH2y+ ions can be formed via reactions (B-
1) and (B-2). It is clear from Fig. 5(A) and Fig. 5(D) that reac-
tion (B-1) is exothermic, while reactions (A-1) and (B-2) are
and that major product alkyl ions are CyH2y + 1 (y = 4–6).
+
This indicates that significant stabilization of CyH2y + 1+ occurs
by isomerization before decomposition of CxH2x + 1+, as ob-

Y. Tanaka et al.
Bull. Chem. Soc. Jpn., 75, No. 2 (2002) 247
Fig. 4. Initial product-ion distributions of PhCyH2y+ (y = 1–13) ion in the ion-molecule reactions of CH5+, C2H5+, and C3H5+ with
C9–C19 alkylbenzenes. (a), (d), (g) ꢀ: PhC3H7, ꢀ: PhC4H9, and ꢁ: PhC5H11, (b), (e), (h) ꢀ: PhC6H13, ꢀ: PhC7H15, ꢁ: PhC8H17,
and ꢂ: PhC9H19, (c), (f), (i) ꢀ: PhC10H21, ꢀ: PhC11H23, ꢁ: PhC12H25, and ꢂ: PhC13H27. The largest PhCyH2y+ ions correspond to
the PhCxH2x+ ions. The line connecting the points in the graphs is the PhCyH2y+ (y = 1–13) ion formed from same alkylbenzene.
served by Harrison et al. for the formation of CxH2x + 1+ in re-
action (A-1).7,8
actions. It is therefore concluded that (B-1) is responsible for
+
+
the formation of PhCyH2y in the CH5 reactions, and (B-1)
+
+
The branching ratios of PhCxH2x = [M − H]+ increase
and/or (B-3) are responsible for the formation of PhCyH2y in
+
+
+
+
greatly in the order of CH5 , C2H5 , and C3H5+ reactions, indi-
cating that the fragmentation is suppressed in the same order
due to a decrease in the excess energy. The PhCxH2x+ ions can
the C2H5 and C3H5 reactions. The relative contribution of
(B-1) and (B-3) in the latter reactions cannot be determined in
+
the present study. The preferential formation of C7H7 in the
+
be produced through (B-1) and (B-2) in the CH5 reactions,
CH5+ reactions indicates that stable tropylium ion is favorably
produced when a sufficient excess energy is supplied to
PhCyH2y+ (y ꢁ x).
+
+
and through (B-1), (B-2), and (B-3) in the C2H5 and C3H5
reactions. Although no significant changes are found for the
∆Ho values among reactions (B-2) and (B-3), such a tendency
is found among the ∆Ho values for reaction (B-1). It is there-
fore reasonable to assume that proton transfer to alkyl chain,
followed by H2 loss (B-1), is dominantly responsible for the
formation of PhCxH2x+ in the three reactions.
+
+
In the C2H5 reactions, the [M + H]+, CyH2y + 1 (y ꢁ x),
[PhH + H]+, PhCyH2y+ (y ꢁ x), and [PhC2H5 + H]+ ions are
observed, as shown in Table 2. Among them, [M + H]+,
CxH2x+1+, and [PhH + H]+ are produced via proton transfer to
benzene ring (A-1). The [PhC2H5 + H]+ ion is produced
through addition of C2H5+ to the benzene ring (B-3). In addi-
tion to reactions (A-1), (B-1), and (B-2), PhCzH2z− ion abstrac-
+
The PhCyH2y (x < y) ions can also be produced through
(B-1) and (B-2) in the CH5+ reactions, and through (B-1), (B-
2), and (B-3) reactions in the C2H5+ and C3H5+ reactions. The
+
tion (B-3) is possible for the formation of CyH2y + 1 in the
+
+
+
extent of fragmentation of PhCyH2y in the CH5 reactions is
higher than the extents in the C2H5+ and C3H5+ reactions. This
suggests that the ∆Ho values for the formation of PhCyH2y+ in
C2H5+ reactions. It is shown that the formation of the CyH2y + 1
+
ions in the C2H5 reactions dominantly proceeds through via
hydride-ion abstraction for n-paraffins and proton transfer to
+
+
+
the CH5 reactions are much higher than those in the C2H5
CwC bond in the reactions of C2H5 with 1-olefins.14,15 It is
+
−
and C3H5 reactions. Since the ∆Ho values for the formation
of PhCyH2y+ through hydride-ion abstraction (B-2) are similar
among the three reactions, it can be excluded as a major pro-
cess. On the other hand, the ∆Ho values for the formation of
PhCyH2y+ through (B-1) in the CH5+ reactions are much lower
than those through (B-1) and (B-3) in the C2H5+ and C3H5+ re-
clear from Fig. 5(B) that PhCzH2z ion abstraction reactions
(B-3) are more favorable than reactions (A-1), (B-1), and (B-2)
+
on the basis of energetics. The CyH2y + 1 distributions in the
C2H5+ reactions are similar to those in the CH5+ reactions, in-
dicating that excess energies released in CyH2y + 1+ are similar
to those in the CH5+ reactions. There are no significant differ-

248 Bull. Chem. Soc. Jpn., 75, No. 2 (2002)
Chemical Ionization of Alkylbenzenes
Scheme 1. Reaction scheme of the ion-molecule reactions of XH+ (X = CH4, C2H4, and C3H4) with alkylbenzenes for attack of the
reactant ion on the benzene ring.
+
ences in the ∆Ho values for the formation of CyH2y + 1 be-
tween (B-3) in the C2H5+ reactions and (B-1) in the CH5+ reac-
tions. It is therefore reasonable to assume that CyH2y + 1+ ions
Reactions (A-1), (B-1), (B-2), and (B-3) are possible for the
+
+
formation of CyH2y + 1 ions in the C3H5 reactions. It has
been demonstrated that the formation of the CyH2y + 1+ ions in
the C3H5+ reactions dominantly proceeds through via alkanide
ion abstraction and hydride-ion abstraction for n-paraffins and
addition to CwC bond for 1-olefins.14,15 It is clear from Fig.
5(C) that PhCzH2z− ion abstraction reactions (B-3) are more fa-
vorable than reactions (A-1), (B-1), and (B-2). The ∆Ho values
−
are dominantly formed via PhCzH2z ion abstraction reactions
(B-3) in the C2H5+ reactions.
In the C3H5 reactions, the [M + H]+, CyH2y + 1 (y ꢁ x),
[PhH + H]+, PhCyH2y+ (y ꢁ x), [M + C3H5]+, and [M + C3H5
− C2H4]+ ions are observed, as shown in Table 3. There are no
doubts that [M + H]+, CxH2x + 1+, and [PhH + H]+ are pro-
duced through reactions (A-1), while [M + C3H5]+ and [M +
C3H5 − C2H4]+ are formed via reactions (A-2). Although the
[PhC3H5 + H]+ ion can be formed through reaction (A-2), its
+
+
+
for the formation of CyH2y + 1 from reaction (B-3) in the
C3H5+ reactions are similar to those of (B-1) in the CH5+ reac-
tions, and there are no more low-energy reaction pathways
+
+
leading to CyH2y + 1 in the C3H5 reactions. The extents of
+
+
+
m/z value (119) is identical with that of PhCyH2y (y = 3).
fragmentation of CyH2y + 1 in the C3H5 reactions are either
higher or similar to those in the CH5+ reactions, indicating that
similar or higher excess energies are released in the C3H5+ re-
Therefore, it is difficult to distinguish whether m/z = 119
+
peaks are [PhC3H5 + H]+ or PhC3H6 . However, no signifi-
+
cant enhancement of the m/z = 119 peak was found in the
C3H5+ reactions. Therefore, m/z = 119 peaks observed in this
study will be PhCyH2y+ (y = 3).
actions. It is therefore reasonable to assume that CyH2y + 1
−
ions are dominantly formed via lowest energy PhCzH2z ion
abstraction reactions (B-3) in the C3H5+ reactions.

Y. Tanaka et al.
Bull. Chem. Soc. Jpn., 75, No. 2 (2002) 249
Scheme 2. Reaction scheme of the ion-molecule reactions of XH+ (X = CH4, C2H4, and C3H4) with alkylbenzenes for attack of the
reactant ion on the alkyl chain.
Changes in Reaction Mechanism with the Chain Length:
In Figs. 6(a-1), 6(a-2), 6(a-3), 6(b-1), 6(b-2), and 6(b-3), we
plot the dependence of branching ratios of major products on
attributed to the fact that isomerization to secondary and tertia-
+
ry alkyl groups occurs significantly for CyH2y
and
+
1
PhCyH2y+. Therefore, the CxH2x + 1+ and PhCxH2x+ ions are de-
+
+
+
+
+
the alkyl chain x in PhCxH2x + 1 in the CH5 , C2H5 , and C3H5
composed into smaller stable CyH2y + 1 and PhCyH2y ions
with a secondary or tertiary alkyl group, when they are pro-
duced. The low branching ratios of CyH2y + 1+ for x < 7 can be
explained by the absence of or the low probability of such a
reactions. Figures 6(a-1), 6(a-2), and 6(a-3) shows results for
the three product ions, [M + H]+, CxH2x + 1+, [PhH + H]+,
produced through the attack of the reactant ions on the benzene
ring. Figures 6(b-1), 6(b-2), and 6(b-3) shows the results for
+
low-energy decomposition channel for short alkyl CxH2x + 1
Σ([M+H]+, CxH2x + 1+, [PhH+H]+)denotedbyA, ΣCyH2y + 1
ions. Therefore, the CxH2x + 1 ions become dominant alkyl
+
+
(y < x) denoted by B, and ΣPhCyH2y + 1+ (y ꢁ x) denoted by C,
[PhC2H5 + H]+, and Σ([M + C3H5]+ and [M + C3H5 −
C2H4]+). In all the three reactions, dominant ions are the [M +
H]+, CxH2x + 1+, and [PhH + H]+ ions, produced through reac-
tion (A-1) for short-chain reagents (x < 7). The branching ra-
tios of [M + H]+ in the CH5+ reactions are smaller than those
ions for x < 7.
The branching ratios of CyH2y + 1+ in the CH5+ reactions are
larger than those in the C2H5+ and C3H5+ ones. One reason for
this is higher excess energies of [M + H]+ and CxH2x + 1+ ions
in the CH5+ reactions due to the highest acidity, which induces
higher probability of fragmentation of these ions into smaller
CyH2y + 1+ ions. It should be noted that the [M + H]+ ions are
+
+
in the C2H5 and C3H5 reactions. This indicates that ring-
+
+
protonated [M + H]+ ions are decomposed into CxH2x + 1
,
observed in the C2H5 , and C3H5+ reactions for long-chain re-
when sufficient excess energies are supplied to the precursor
[M + H]+ ions in reaction (A-1). For long-chain reagents (x ꢀ
7), major product ions are CyH2y + 1+ and PhCyH2y+. Dominant
agents (x > 7) and that their branching ratios are nearly con-
stant. Although the branching ratios of [M + H]+ ions, pro-
duced through the attack of the reactant ions on benzene ring,
will decrease with increasing the chain length, they are essen-
tially independent of x. One reason for this will be an effective
occurrence of intramolecular proton transfer from chain proto-
+
+
CyH2y + 1 and PhCyH2y ions produced from x ꢀ 7 reagents
+
are y = 4–6 fragment ions. The lack of CxH2x + 1 and high
branching ratios of CyH2y + 1+ and PhCyH2y+ ions for x ꢀ 7 are

250 Bull. Chem. Soc. Jpn., 75, No. 2 (2002)
Chemical Ionization of Alkylbenzenes
+
+
Fig. 5. Energy relations in the ion-molecule reactions of CH5+, C2H5+, and C3H5 with PhC13H27 leading to 1-CyH2y + 1 and
PhCyH2y+. ꢀ: (1a) Proton transfer, ꢀ: (2a) hydride-ion abstraction, ꢁ: (3a) PhCzH2z− ion abstraction. The largest CyH2y + 1+ ions
correspond to the CxH2x + 1+ ions. ꢀ: (1b) Proton transfer, ꢀ: (2b) hydride-ion abstraction, ꢁ: (3b) CzH2z + 1− ion abstraction. The
+
+
+
largest PhCyH2y ions correspond to the PhCxH2x ions. The line connecting the points in the graphs is the CyH2y + 1 and
PhCyH2y+ formed from the same reaction pathway. ∆Ho values are shown in eV units (1 eV = 96.485 kJ mol−1).
nated ions to more stable ring ones, though this mechanism is
not described in Scheme 2 for the sake of clarity.
products. On the basis of the product-ion distributions and the
energetics, major reaction mechanisms for the formation of
+
+
Concluding Remarks:
The gas-phase ion-molecule
CyH2y + 1 and PhCyH2y ions are discussed. The dominant
+
+
+
+
reactions of CH5 , C2H5 , and C3H5 with alkylbenzenes
(PhCxH2x + 1: x = 3–13) have been studied in an ion-trap type
of GC/MS by separating each reactant ion. In all the reactions,
formation processes of CyH2y + 1 ions are proton transfer to
benzene ring in the CH5+ reactions, and PhCzH2z− ion abstrac-
tion in the C2H5+ and C3H5+ reactions. On the other hand, the
processes of PhCyH2y+ ions are proton transfer to alkyl chain in
+
+
CyH2y + 1 (y = 3–10), PhCyH2y (y = 1–13), and [M + H]+
ions were observed. In addition, [PhC2H5 + H]+ ions were
the CH5 reactions, and proton transfer and/or CzH2z ion ab-
+
−
observed in the C2H5 reactions, while adduct [M + C3H5]+
straction in the C2H5 and C3H5 reactions. Further detailed
experimental and theoretical studies are required to confirm
the above prediction.
+
+
+
and [M + C3H5 − C2H4]+ ions were observed in the C3H5+ re-
actions. The dependence of the relative intensities of product
ions on the reaction times indicated that collisional stabiliza-
tion took part in the formation of some product ions. For
short-chain reagents (x < 7), dominant product ions were the
[PhH + H]+, CxH2x + 1+, or [PhH + H]+ ions, produced by
proton transfer to benzene ring. For long-chain reagents (x ꢀ
The authors acknowledge financial support from a Grant-in-
Aid for Scientific Research No. 13558056 from the Ministry of
Education, Science, Sports and Culture, and from Kyushu Uni-
versity Interdisciplinary Programs in Education and Projects in
Research Development (2001).
+
+
7), the CyH2y + 1 and PhCyH2y ions were observed as major

Y. Tanaka et al.
Bull. Chem. Soc. Jpn., 75, No. 2 (2002) 251
+
Fig. 6. Dependence of the branching ratios of product ions on the alkyl chain x in PhCxH2x + 1. (a) CH5+, (b) C2H5+, and (c) C3H5
.
ꢃ: [M + H]+, ꢄ: CxH2x + 1+, ꢅ: [PhH + H]+, ꢀ: (A) = Σ[M + H]+, CxH2x + 1+, and [PhH + H]+), ꢀ: (B) = CyH2y + 1+ (y < x), and
ꢁ: (C) = PhCyH2y+ (y ꢁ x). ꢂ: [PhC2H5 + H]+, ꢆ: Σ ([M + C3H5]+ and [M + C3H5 − C2H4]+).
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