九州大学大学院総合理工学府報告第 42 巻 第 2 号 1-11 頁 令和 3 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 42, No. 2, pp. 1-11, FEB, 2021

Mass-Spectrometric Study on Ion-Molecule Reactions ofCH5+, C2H5+, and C3H5+ with C8~C18 1-Alkynes
in an Ion Trap
Masaharu TSUJI*1,2† Tsukasa SATO*3 Yuki TANAKA*3 Takeshi ARIKAWA*3
Keiko UTO*1 Jun-Ichiro HAYASHI*1,2 and Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received December 4, 2020, accepted December 15, 2020)
Chemical ionization of 1-alkynes (CxH2x-2: x = 8~18) by the CH5+, C2H 5+, and C3H5+ ions has beenstudied under a reactant-ion selective mode of an ion-trap type of GC/MS. Although no peaks
corresponding to [MW + H]+, [MW – H]+, [MW + C2H5]+, [MW + C3H5]+ ions were detected, fragment
alkenyl CyH2y-1+ (y ≥ 4) and alkynyl CyH2y-3+ (y ≥ 4) ions were observed. Their initial product-ion
distributions were determined by extrapolating the dependence of relative intensities of CyH2y-1+ andCyH2y-3+ ions on the reaction time to zero reaction time. In the CH5+, C2H5+, and C3H5+ reactions, theinitial CyH2y-1+ ion distributions had peak at y = 4 and decrease with increasing y, whereas the initial
CyH2y-3+ ion distributions gave broad peaks in the y = 5~10 range. On the basis of observed initial
distributions and calculated thermochemical data, reaction mechanisms for the formation of alkenyl
CyH2y-1+ and alkynyl CyH2y-3+ ions were discussed. In almost all reactions, total intensities of CyH2y-1+
ions were larger than those of CyH2y-3+ ions, indicating that proton transfer reactions are more favorable
than hydride-ion abstraction reactions.
Key words: Chemical ionization, Hydrocarbon ions, 1-Alkynes, Alkenyl ions, Alkynyl ions, Ion trap type GC/MS,
Collisional stabilization

1.

Introduction

The gas phase ion-molecule reactions in amethane atmosphere have been studied formany reagent molecules since the firstchemical ionization (CI) mass spectrometricmeasurements by Field et al.1-5) about 60 years
ago. Now CH4 CI mass spectrometry has been
established as a standard soft-ionization
method for the determination of molecularweight (MW) and molecular structure ofreagent molecules. Field measured CH4 CI
mass spectra at a medium CH4 pressure of 1
Torr (= 133.33 Pa), where dominant reactant
ions were CH5+ (48%), C2H5+ (40%), and C3H5+(6%).6) In their CH4 CI experiments, thesereactant hydrocarbon ions have not been
*1 Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student*4 Department of Materials Science, Shimane
University

selected, because the most important
information on [MW + H]+ can be easily
obtained from proton transfer reactions
between a mixture of these hydrocarbon ionsand reagent molecules without selectinghydrocarbon ions.
Low-pressure chemical ionization in ion trap
mass spectrometry is a new powerful tool forthe study on the reactivity of each reactant
ion.7) By using a reactant-ion selective CI modeof an ion-trap type of GC/MS, we have made asystematic mass spectrometric study on ionmolecule reactions of CH5+, C2H5+ and C3H5+
ions with many organic molecules such asmonosubstituted benzenes, and C8~C18 nparaffins 1-olefins, and alcohols.8-21) Since theproton affinity of CH4 (5.7 eV) is smaller thanthose of C2H4 (7.1 eV) and C3H4 (8.0 eV),22) more
excess energies are released in the CH5+
reactions. In a general way, CH5+ ionsexclusively induce proton transfer anddissociative proton transfer; this can be
explained by the instability of the [M + CH5]+
adduct ion. The formation of [M + C2H5]+ and[M + C3H5]+ adduct ions in the C2H5+ and C3H5+

Ion-molecule reactions of MH+ (M = CH4, C2H4, C3H4) with 1-alkynes

2reactions is assumed to result from radiative
association.
In our previous study on C8~C18 n-paraffins(CxH2x+2),15) major product ions were parent
alkyl (MW − 1, = CxH2x+1+) ions and fragment
alkyl CyH2y+1+ (y = 3 ~ x–3) ions. In this work,
symbol x represents the total number ofcarbons in reagent hydrocarbons, while symbol
y stands for the total number of carbons infragment alkyl, alkenyl, and alkynyl ions.
Major product ions from 1-olefins (CxH2x) with a
C=C double bond were fragment alkyl CyH2y+1+(y = 3~x) and alkenyl CyH2y-1+ (y = 3~x) ions.16)
For 1-alkynes with a CC triple bond, CH4 CI
mass study has been carried out by Field 23) on
C6 and C8 1-alkynes at a medium CH4 pressureof 1 Torr. The spectra consisted of alkenyl
CyH2y-1+ (y = 4~x) and alkynyl CyH2y-3+ (y = 4~x)
ions. Since the reactant hydrocarbon ions havenot been separated in their CI study, thereactivity of each hydrocarbon ion has not been
determined.
In this study, CH4 CI mass spectra of a seriesof 1-alkynes (CxH2x-2: x = 8~18) by the CH5+,
C2H5+, and C3H5+ ions are measured by using
an ion-trap type of GC/MS under a reactant-ion
selective mode. The dependence of product-ion
distributions on the reaction time wasmeasured in order to examine the effects ofcollisional stabilization. The reactivity of CH5+,
C2H5+, and C3H5+ for C8~C18 1-alkynes was
discussed from the initial product-ion
distributions and thermochemical data for eachreaction. The results obtained were comparedwith our previous results for 1-olefins.16)

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 7 × 10-5 and5 × 10-5 Torr, respectively. The reaction timecorresponding to the residence time in the iontrap was varied from 0.5 to 40 ms. The mass
spectra were measured at low reagent
concentrations of about 1000~10000 pg cm -3 inorder to reduce secondary ion-molecule
reactions.
The operating conditions in the ion-trap cellused in this work were significantly different
from those of the conventional mediumpressure CI mass spectrometer developed byField et al.1-6) In the medium-pressure CI
measurements, the typical CH4 gas pressure
was 1 Torr and the residence time of reactant
ions in the ionization-reaction chamber was
about 10 s. Field4) evaluated the total numberof collisions of reactant ions with CH 4 during
this residence time to be about 200. In thepresent low-pressure CI measurements, thetotal 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.
CI mass spectra were measured under theconditions, where concentrations of reactant
ions were much higher than those of productions. Therefore, it was difficult to determine
rate constants from plots of a decay of areactant ion against the reaction time or theconcentration of a reagent.

2.

3.1 CI mass spectrum and effects of collisional
stabilization

CI mass spectra were obtained using an iontrap type of Hitachi M7200 GC/MS under areactant-ion selective mode. The CI CH4 gas
was introduced directly in a trap cell. The
electron-impact ionization on CH4 provides
primary CHn+ (n = 2~4) ions. One of the reactant
CH5+, C2H5+, and C3H5+ ions produced from thesubsequent fast ion-molecule reactions of CHn+(n = 2~4) with CH4 was selectively trapped as areactant ion in an ion-trap cell. The time forstoring a reactant ion was kept at a constanttime of 5 ms. If reactant ions in vibrationallyexcited levels are formed, they will bethermalized by collisions with CH4 and He
during their trapping time in the cell. The iontrap cell was kept at ≤170 oC. The reagentspurchased from Tokyo Chemical Industry Co.,
Ltd. were diluted in hexane, which has a much
shorter retention time than those of 1-alkynes,

Results and discussion

For example, Fig. 1 shows mass spectrum
obtained from the C2H5+/1-C15H28 reaction,

0.35

C4

0.30
0.25

Intensity /a.u.

Experimental

3.

CyH2y-3+0.20

CyH2y-1+

0.15

C5
0.10

C6

C8

C9

C7

0.05

C10
0
50

60

70

80

90

100

110

120

130

140

150

Mass number / m/Z

Fig. 1. CI mass spectrum obtained fromthe C2H5+/1-C15H28 reaction.

令和 2 年度

九州大学大学院総合理工学報告

where alkenyl CyH2y-1+ (y = 4~8) and alkynyl
CyH2y-3+ (y = 4~10) ions are observed. For the
CyH2y-1+ ions, C4 peak is strongest and thenthe intensity rapidly decreases withincreasing the mass number. On the other
hand, a broad intensity distribution isobserved for the CyH2y-3+ ions in the C5~C9
region. When CI mass spectra resulting fromion-molecule reactions of CH5+, C2H5+, andC3H5+ with 1-alkynes (CxH2x-2: x = 8~18) wereobserved, similar alkenyl CyH2y-1+ (y ≥ 4) andalkynyl CyH2y-3+ (y ≥ 4) ions were observed in allcases. No peaks corresponding to [MW + H]+,
[MW – H]+, [MW + C2H5]+, [MW + C3H5]+ ionsare detected, indicating that it is difficult todetermine MW values of 1-alkynes using CH4
CI mass spectrometry at low pressures.
If the collisional stabilization takes part in
the formation of product ions, excess energy
released in the reaction is partly relaxed bycollisions 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, thedependence of product-ion distributions on thereaction time was measured. As typical results
for short, medium, and long carbon chain 1alkynes used in this work, Figs. 2A and 2B
show product-ion distributions of CyH2y-1+ (y =4~x) and CyH2y-3+ (y = 4~x) in the reactions ofCH5+, C2H5+, and C3H5+ with C8, C14, and C18 1alkynes at five different reaction times: 0.5, 2,
10, 20, and 40 ms, respectively. The decreases
or increases in the product-ion distributions
with increasing the reaction time are shown byarrows in Figs. 2A and 2B. In many cases, theproduct-ion distributions of small CyH2y-1+ andCyH2y-3+ ions decrease, while those of large ones
increase with increasing the reaction time.
These results suggest that large ions are often
stabilized by collisions with CH4 and He gases
in our conditions. Without collisional
stabilization at short reaction times, secondary
fragmentation from large CyH2y-1+ ions to small
Cy’H2y’-1+ ions occurs.
CyH2y-1+ (y = large)→ Cy’H2y’-1+ (y’ = small) + Cy-y’H2(y-y’)for C8, C14, and C18 1-alkynes.

(1)

The initial product-ion distributions were
determined by extrapolating the dependence ofrelative intensities of CyH2y-1+ and CyH2y-3+ ionson the reaction time to zero reaction time, as isshown for the case of the CH5+/1-C16H30

第 42 巻 第 2 号

3reaction in Figs. 3A and 3B. The uncertainties
of the initial branching-ratios were estimatedto be within ±8% in most cases.
3.2 Initial Distributions of alkenyl C yH 2y-1+
and alkynyl C yH 2y-3+ ionsFigures 4A(a)-(i) show initial product-ion
distributions of alkenyl CyH2y-1+ (y = 4~x) ionsobtained for short (x = 8~11), medium (x =12~14), and long (x = 15~18) carbon chain
reagents. In all the reactions, no long chain
CyH2y-1+ alkenyl ions (y >10) are formed. In theCH5+ reactions, the intensity distributions of
CyH2y-1+ ions peak at y = 4 and then rapidly
decrease with increasing y in all cases except
for C10~C14 reagents, for which second peaks
are observed at y = 7 and/or y = 10. In the C2H5+
reactions, the intensity distributions of CyH2y-1+
ions also peak at y = 4 and then decrease withincreasing y in most cases. Small second peaksof CyH2y-1+ are found at y = 6 for C12~C18
reagents. A difference between the CH5+ andC2H5+ reactions is that the distributions of long
chain CyH2y-1+ ions above y = 6 decrease withincreasing y in the cases of C2H5+ reactions. In
the C3H5+ reaction, the intensity distributionsof CyH2y-1+ ions also give peak at y = 4 and they
decrease with increasing y in all cases except
the case of C10 reagent, where a broad
distribution within y = 5~9 is observed.
Figures 4B(a)-(i) show initial product-ion
distributions of alkynyl CyH2y-3+ (y = 4~x) ionsobtained for short (x = 8~11), medium (x =12~14), and long (x = 15~18) chain reagents.
The distributions of large CyH2y-3+ (y = 13~x)
ions are zero in the reactions of C12~C18 1alkynes. The CyH2y-3+ (y = 4~x) ions are
distributed in a wide y range. In the CH5+
reactions, for short and medium chain C8~C14
reagents, the CyH2y-3+ distributions have broad
peaks at y = 5~10, whereas for long-chain
C15~C18 reagents, they have broad peaks at y =5~12. In the C2H5+ reactions, the distributionsof CyH2y-3+ have peaks at y = 6 and weak second
peaks are found at y = 8 or 9 in many cases. In
the
C3H5+
reactions,
broad
CyH2y-3+
distributions are observed in the y = 5~11 range.
It should be noted that, as a whole, the initial
distributions of CyH2y-1+ and CyH2y-3+ ions from
C8~C18 1-alkynes are similar in the three MH+(M = CH4, C2H4, and C3H4) reactions. The
CyH2y-1+ ion distributions have peaks at y = 4
and decrease with increasing y. On the other
hand, the initial distributions of CyH2y-3+ ionshave broad peaks in the y = 5~10 range. It isnoteworthy that these distributions are similarIon-molecule reactions of MH+ (M = CH4, C2H4, C3H4) with 1-alkynes

4CH4(A) CyH2y-1+
0.7
0.6(a) CH5＋/C8H14(b) CH5＋/C14H26(c) CH5＋/C18H34(d) C2H5＋/C8H14(e) C2H5＋/C14H26(f) C2H5＋/C18H34(g) C3H5＋/C8H14(h) C3H5＋/C14H26

0.4
0.2

0
0.7

Relative intensity

0.6
0.4

0.2
0
0.7
0.6

(i) C3H5＋/C18H34

0.4

0.2
0
m/z 55y 4

83
6(CyH2y－1＋)

0.7

111 55
8 4

83
6

111
8

139
10

167
12

195 55
14 4

97
7

139
10

181
13

223 251
16 18(B) CyH2y-3+

0.6

CH5＋/C18H34(a) CH5＋/C8H14(b) CH5＋/C14H26(c)(d) C2H5＋/C8H14(e) C2H5＋/C14H26(f) C2H5＋/C18H34

0.4

0.2

0
0.7

Relative intensity

0.6
0.4

0.2

0
0.7
0.6

(g) C3H5＋/C8H14(h) C3H5＋/C14H26(i) C3H5＋/C18H34

0.4

0.2

0
m/z 53y 4(CyH2y－3＋)

816

109 53
8 4

81
6

109
8

137
10

165
12

193 53
14 4

95
7

137
10

179
13

221
16

Fig. 2. Product-ion distributions of (A) alkenyl CyH2y-1+ (y = 4~x) ions and (B) alkynyl CyH2y-3+(y = 4~x) ions in the reactions of CH5+, C2H5+, and C3H5+ with C8, C14, and C18 1-alkynes at fivedifferent reaction times: ○0.5, □2, ×10, ＊20, and ＋40 ms.

令和 2 年度

九州大学大学院総合理工学報告(A) CyH2y-1
0.3

+

(a-1)

Relative intensity

0.2
y＝5

y＝6

0.1

5

to those of alkyl CyH2y+1+ and alkenyl CyH2y-1+
ions from C8~C18 1-olefins.16) In the MH+ (M =CH4, C2H4, and C3H4) reactions with C8~C18 1alkynes (CxH2x-2) and 1-olefins (CxH2x),
protonation followed by alkene elimination
gives CyH2y-1+ and CyH2y+1+ ions throughprocesses (2A) and (3A), whereas H- abstraction
followed by alkene elimination provide CyH2y-3+
and CyH2y-1+ ions through processes (2B) and(3B), respectively.

y＝4-M
MH+ + CxH2x-2 (1-alkyne) → CyH2y-1+ + alkene
(2A)
-MH2
MH+ + CxH2x-2 (1-alkyne) → CyH2y-3+ + alkene
(2B)

0

(a-2)

y＝7

0.10
y＝8

0.08
0.06

0.04
0.02

y＝9

0

0

20

10

30

40

Reaction time / ms

(b-1)
0.10
y＝8

0.08

y＝7

0.06

y＝6

0.04

0.02
y＝5
0
0.4

(b-2)

0.3
0.2

y＝11

y＝12

y＝9
y＝10

0.1

0
0

10

20

30

-M
MH+ + CxH2x (1-olefin) → CyH2y+1+ + alkene
(3A)
-MH2
MH+ + CxH2x (1-olefin) → CyH2y-1+ + alkene(3B)
It should be noted that similar initial production distributions are observed between CyH2y-1+
in (2A) and CyH2y+1+ in (3A) and between
CyH2y-3+ in (2B) and CyH2y-1+ in (3B). These
results imply that the favorable product ionsformed by protonation and hydride-ion
abstraction reactions are similar in the CH5+,
C2H5+, and C3H5+ reactions with C8~C18 1alkynes and 1-olefins, even though the total
number of H atoms in product ions and reagent
hydrocarbons is different by two due topresence of a C=C or CC bond.

(B) CyH2y-3+

Relative intensity

第 42 巻 第 2 号

40

Reaction time / ms

Fig. 3. Dependence of product-ion
distributions of (A) alkenyl CyH2y-1+ (y =4~9) ions and (B) alkynyl CyH2y-3+ (y = 5~12)
ions on the reaction time in theCH5+/C16H30 reaction.

3.3 Branching ratios of alkenyl and alkynyl
ions in the MH+ (M = CH4, C2H4, and C3H4)
reactions
In Figs. 5a-5c we plot the branching ratios of
all the CyH2y-1+ (y = 4~x) and CyH2y-3+ (y = 4~x)
ions formed from C8~C18 1-alkynes. In most of
all reactions, CyH2y-1+ (y = 4~x) values arelarger than those of CyH2y-3+ values. On thebasis of this finding, protonation products are
more favorable than hydride-ion abstraction
products in the MH+ (M = CH4, C2H4, and C3H4)
reactions with C8~C18 1-alkynes. In the CH5+
and C3H5+ reactions, CyH2y-1+ values increase,
while CyH2y-3+ values decrease with increasing
carbon chain length x. On the other hand, theCyH2y-1+ and CyH2y-3+ values are independentof x in the C2H5+ reactions. Results show thatreaction mechanisms for the formation ofIon-molecule reactions of MH+ (M = CH4, C2H4, C3H4) with 1-alkynes

6(A) CyH2y-1+

0.5

Intensity distributions of alkenyl ion(a) CH 5+/C 8-C11 alkynes
0.6
C8H14
C9H16
C10H18
C11H20

0.5
0.4
0.3
0.2
0.1
0.0

C12H22

0.4

C13H24
C14H26

0.3
0.2
0.1
0.0

4

5

6

7

8

9

10

6

8

10

12

Intensity distributions of alkenyl ion(d) C2H 5+/C8-C11 alkynes

C8H14
C9H16
C10H18
C11H20

0.4
0.3
0.2
0.1
0.0
4

5

6

7

8

9

10

(e) C 2H5+/C12-C14 alkynes
0.6
C12H22

0.5

C13H24
C14H26

0.4
0.3
0.2
0.1

6

8

10

12

0.5
0.4
0.3
0.2
0.1
0.0
5

6

7

8

y in CyH2y-1

9

10

11

0.05
6

8

10

12

14

16

18

16

18

16

18

(f) C 2H5+/C15-C18 alkynes
0.5
C15H280.4

C16H30
C17H32

0.3

C18H34

0.2
0.1

4

6

8

10

12

14

0.6

(h) C 3H5+/C 12-C14 alkynes
0.5
C12H22

0.4

C13H24
C14H26

0.3
0.2
0.1
0.0

4

0.10

14

Intensity distributions of alkenyl ion

C8H14
C9H16
C10H18
C11H20

C18H34

0.15

0.0

4
Intensity distributions of alkenyl ion

Intensity distributions of alkenyl ion(g) C3H 5+/C8-C11 alkynes

0.6

C17H32

0.20

4

0.6
0.7

C16H30

0.25

0.6

0.0

11

C15H28

0.30

14

0.7

0.5

(c) CH5+/C15-C18 alkynes

0.35

0.00
4

11

0.6

Intensity distributions of alkenyl ion

0.40(b) CH5+/C12-C14 alkynes

Intensity distributions of alkenyl ion

Intensity distributions of alkenyl ion

0.7

Intensity distributions of alkenyl ion

alkenyl(i) C3H5+/C15-C18 alkynes
0.5

C15H 28
C16H 30

0.4

C17H 32
C18H 34

0.3
0.2
0.1
0.0

4

6

+

8

10

y in CyH2y-1

12

14

4

6

8

10

+

12

14

y in CyH2y-1+(B) CyH2y-3+
C8H14
C9H16
C10H18
C11H20

0.5
0.4
0.3
0.2
0.1

0.30

C12H22

0.25

C13H24
C14H26

0.20
0.15
0.10
0.05

5

6

7

8

9

10

4

11

6

8

10

12

C8H14
C9H16
C10H18
C11H20

0.3
0.2
0.1

(e) C2H5+/C12-C14 alkynes

0.35

C12H22

0.30

C13H24

0.25

C14H26

0.20
0.15
0.10
0.05

5

6

7

8

9

10

(g) C3H 5+/C8-C 11 alkynes

0.35

Intensity distributions of alkynyl ion

0.40
C8H14
C9H16
C10H18
C11H20

0.30
0.25
0.20
0.15
0.10
0.05
0.00

6

8

10

12

5

6

7

8

y in CyH2y-3

9

+

10

11

0.10
0.05

6

8

10

12

14

16

18

(f) C2H 5+/C15-C18 alkynes
0.30

C15H28
C16H300.25

C17H32

0.20

C18H34

0.15
0.10
0.05

4

14

6

8

10

12

14

16

18

0.30

0.25

(h) C3H5+ /C12-C14 alkynes

C12H22
C13H24

0.20

C14H26

0.15
0.10
0.05

(i) C3H5+/C15-C18 alkynes
0.25

C15H28
C16H300.20

C17H32
C18H34

0.15
0.10
0.05
0.00

0.00
4

C18H340.00
411

Intensity distributions of alkynyl ion

4

C17H32

0.15

4

0.00

0.0

C16H300.35

Intensity distributions of alkynyl ion

Intensity distributions of alkynyl ion(d) C2H 5+/C8-C 11 alkynes
0.4

C15H28

0.20

14

0.40

0.5

(c) CH5+ /C15-C18 alkynes
0.25

0.00

0.00
4

Intensity distributions of alkynyl ion(b) CH 5+/C12-C 14 alkynes

0.35

Intensity distributions of alkynyl ion

Intensity distributions of alkynyl ion

Intensity distributions of alkynyl ion(a) CH5+/C 8-C11 alkynes

0.6

0.0

Intensity distributions of alkynyl ion

0.30

0.40

0.7

4

6

8

10

y in CyH2y-3+

12

14

4

6

8

10

12

14

16

18

y in CyH2y-3+

Fig. 4. Initial product-ion distributions of (A) alkenyl CyH2y-1+ (y = 4~x) ions and (B) alkynyl
CyH2y-3+ (y = 4~x) ions in the reactions of CH5+, C2H5+, and C3H5+ with C8~C18 1-alkynes.

令和 2 年度

九州大学大学院総合理工学報告

0.8(a) CH5+

Branching ratio / %

0.7
0.6
0.5
0.4
0.3
Alkenyl ion

0.2

Alkynyl ion

0.1
0.0

0.7

(b) C2H5+

Branching ratio / %

0.6
0.5

7

The energetics for the formation of CyH2y-1+
and CyH2y-3+ from (1a), (1b), (2), (4a) and (4b)was calculated using reported thermochemical
data.22) The Ho values obtained are shown in
Figs. 6A and 6B for the cases of C8, C14, and C18
reagents. Similar energy relationships are
obtained for the reactions of other 1-alkynes.
For example, ∆Ho values in Figs. 6A (a) and 6B(a) represent the heats of reaction of thefollowing CH5+/1-C8H16 reactions leading tovarious CyH2y-1+ and CyH2y-3+ ions:
CH5+ + 1-C8H14 → CyH2y-1+ + C8-yH2(8-y) + CH4,
(process 1a in Scheme 1)(4a)CH5+ + 1-C8H14→ CyH2y-3+ + C8-yH2(8-y) + CH4 + H2,(process 1b in Scheme 1).
(4b)

0.4
0.3
Alkenyl ion
Alkynyl ion

0.2

第 42 巻 第 2 号

0.1
0.0
0.8

(c) C3H5+

Branching ratio / %

0.7
0.6
0.5
0.4
0.3
0.2

Alkenyl ion
Alkynyl ion

0.1
0.0
8

10

12

14

16

18

x in CxH 2x-2

Fig. 5. The dependence of total alkenyl(CyH2y-1+) and alkynyl (CyH2y-3+) ion
intensities on the reagent carbon chain
length x in the reactions of CH5+, C2H5+, andC3H5+ with C8~C18 1-alkynes.
alkenyl and/or alkynyl ions in the C2H5+
reaction are different from those in the CH5+
and C3H5+ reactions.
3.4 Energetics and formation mechanisms ofalkenyl and alkynyl ions from the ion-molecule
reactions of MH+ (M = CH4, C2H4, and C3H4)
ions with 1-alkynes
Scheme 1 shows possible formation processesof alkenyl CyH2y-1+ and alkynyl CyH2y-3+ ionsfrom the MH+ (M = CH4, C2H4, and C3H4)
reactions with 1-alkynes. Alkenyl CyH2y-1+ ionscan be formed via proton transfer to an alkynyl(1a) and adduct-ion formation (4a). On theother hand, alkynyl CyH2y-3+ ions are produced
via proton transfer to an alkyl group followed
by loss of H2 (1b), hydride-ion abstraction (2),alkanide-ion (Cx-yH2(x-y)+1-) abstraction (3), andadduct-ion formation (4b). The rearrangement
shown in Scheme 1 represents hydride andalkyl sifts leading to more stable isomers (e.g.
primary → secondary → tertiary ions).

There are three possible C3H5+ isomers, whose∆Ho values are 946, 969, and 1069 kJ mol-1 for
CH2=CHCH2+, CH3C=CH2+, and protonatedcyclopropene ion, respectively.22) Since the moststable CH2=CHCH2+ isomer is a significant ion
produced
from
CH4
CI
gas,2)
allthermochemical calculations for C3H5+ arecarried out using the above ∆Ho value of
CH2=CHCH2+.
It is clear from Figs. 6A and 6B that the Hovalues for the formation of alkenyl CyH2y-1+ ionsand alkynyl CyH2y-3+ ions decrease withincreasing the product-ion chain length y,
except for the highly endoergic processes at y =x-1, x+1, or x+2, where unstable CH2 radicals areformed as a neutral byproduct. The Ho values
for the formation of CyH2y-1+ from protonation
process (1a) given in Figs. 6A increase in theorder of CH5+, C2H5+, and C3H5+ reactions due
to an increase in the proton affinity (PA) in theorder of CH4 (PA = 543.5 kJ mol-1), C2H4 (680.5
kJ mol-1), and C3H4 (775.3 kJ mol-1).22)
Therefore, the formation of CyH2y-1+ (y = 4) ionsslightly endoergic in the C2H5+, and C3H5+
reactions. These Ho values are calculated forthe formation of primary ions. Actually, an
intramolecular rearrangement leading to morestable secondary (s-CyH2y-1+) and tertiary (tCyH2y-1+) ions occurs. Since Ho values ofsecondary and tertiary carbocations are lower
than that of primary ones by 0.87 and 1.3 eV,
respectively,2) such endoergic processes canalso occur energetically. The formation of
CyH2y-1+ from C2H5+ addition (4a) isenergetically more favorable than that fromproton transfer (1a) by about 1 eV, and they areexoergic for y = 4 ~ x-2. There are no significant
differences in the Ho values for the formation

Ion-molecule reactions of MH+ (M = CH4, C2H4, C3H4) with 1-alkynes

8

of

of

Scheme 1. Possible reaction mechanisms for the formation of alkenyl CyH2y-1+ and alkynyl CyH2y-3+
ions in the reactions of CH5+, C2H5+, and C3H5+ with C8~C18 1-alkynes.

令和 2 年度

九州大学大学院総合理工学報告第 42 巻 第 2 号(A) CyH2y-1+

2.0

2.0

2.0

(c) CH5+ / C18H34(b) CH5+ / C14H26(a) CH5+ / C8H14

1.0

1.0

1.0

0.0

H+ addition (1a)

-1.0

-2.0

Energy / eV

H+ addition (1a)Energy / eV

Energy / eV

0.0

0.0

-1.0
-2.0

-2.0

-3.0

-3.0

-4.0

-4.0

-4.0

-5.0

2

3

4

5

6

7

-5.0

1

8

2

4

6

8

10

12

14

4.0

4.0

(d) C2H5+ / C8H14

4

6

8

10

12

14

16

18

(f) C2H5+ / C18H34

H+ addition (1a)3.0

C2H5+ addition (4a)

1 2

4.0

(e) C2H5+ / C14H26

H+ addition (1a)3.0

H+ addition (1a)

-1.0

-3.0

1

9

3.0

C2H5+ addition (4a)

2.0

2.0

2.0

1.0

1.0

1.0

0.0
-1.0

-1.0
-2.0

-3.0

-3.0

2

4

6

8

-4.0

10

-2.0
-3.0
-4.0

1

4.0

3.0

3.0

10

15

Energy / eV

1.0

1.0

H+ addition (1a)

0.0

-2.0

y in CyH2y-1

6

7

8

1

2

4

+

6

8

10

y in CyH2y-1(B) CyH2y-3+

6.0

12

-2.0

14

2

-4.0

-4.0

-6.0

-6.0

14

16

18

16

18

16

18

2.0

+

H addition (1b)

0.0
-2.0

12

(c) CH5+ / C18H34

2.0

-2.0

10

4.0

Energy / eV

Energy / eV

0.0

8

6.0

(b) CH5+ / C14H26

H+ addition (1b)6y in CyH2y-1+

4.0

2.0

4

+

6.0

(a) CH5+ / C8H14
4.0

20

1.0

-1.0

5

16

2.0

0.0

4

12

3.0
H+ addition (1a)

2.0

-1.0

3

8

4.0

0.0

2

4

(i) C3H5+ / C18H34

-1.0

1

1

5.0

H+ addition (1a)-2.0Energy / eV5(h) C3H5+ / C14H26

4.0

2.0

-1.0

5.0

(g) C3H5+ / C8H14

C2H5+ addition (4a)0.0Energy / eV1

5.0

Energy / eV

0.0

-2.0

-4.0

Energy / eV

Energy / eV

Energy / eV

H+ addition (1a)0.0

H+ addition (1b)

-2.0
-4.0

2

3

4

5

6

7

8

-6.0

2

4

6

8

10

12

-8.0

14

2

6.0

6.0

Alkanide abstraction (3)

0.0

-2.0

-2.0

-4.0

-4.0

-6.0

-6.0

2

3

4

5

6

7

14

H- abstraction (2)0.0Alkanide abstraction (3)
-2.0

-6.0
-8.0

2

8

4

6

8

10

12

14

C3H5+ addition (4b)

2.0

0.0

C3H5+ addition (4b)

0.0
-2.0

-2.0
-4.0

2

4

6

8

y in CyH2y-3+

10

10

12

14

C3H5+ addition (4b)
0.0
-2.0
-4.0

-6.0

-6.0
-8.0

2

5

10

y in CyH2y-3+

15

H- absraction (2)Alkanide abstraction (3)

2.0

-4.0

-8.0

8

4.0

H- absraction (2)Alkanide abstraction (3)Energy / eV

4.0

Energy / eV

2.0

H- absraction (2)Alkanide abstraction (3)

6

6.0

6.0

4.0

4

(i) C3H5+ / C18H34(h) C3H5+ / C14H26

6.0

2

8.0

8.0

(g) C3H5+ / C8H14

-6.0

12

-4.0

8.0

Energy / eV

H- abstraction (2)Energy / eV

Energy / eV

Energy / eV

0.0

10

2.0

2.0

Alkanide abstraction (3)84.0

H- abstraction (2)2.0

6(f) C2H5+ / C18H34

4.0

4.0

4

6.0

(e) C2H5+ / C14H26(d) C2H5+ / C8H14

2

4

8

12

16

20

y in CyH2y-3+

Fig. 6. The Ho values obtained for the formation of (A) alkenyl CyH2y-1+ ions and (B) alkynyl
CyH2y-3+ ions from processes (1a), (1b), (2), (4a) and (4b) in Scheme 1.

10

Ion-molecule reactions of MH+ (M = CH4, C2H4, C3H4) with 1-alkynesof CyH2y-3+ ions from H- abstraction (2) in theCH5+, C2H5+, and C3H5+ reactions, and theformation of CyH2y-3+ (y = 4 ~ x-2) ions areenergetically possible without occurrence of
intramolecular rearrangement. The formationof CyH2y-3+ ions from alkanide-ion abstraction(3) and C2H5+ and C3H5+ addition (4b) areenergetically more favorable than that from Habstraction (2) by 0.84~0.86 eV.
CyH2y-1+ ions are formed via proton transfer
reaction (1a) in the CH5+ and C3H5+ reactions,whereas they can be formed not only via protontransfer reaction (1a) but also via C2H5+
addition (4a) in the C2H5+ reactions. The
CyH2y-1+ and CyH2y-3+ values increase anddecrease with increasing the chain length in
the CH5+ and C3H5+ reactions. In general, theprobability of collisions of a reactant ion withan alkyl group increases, whereas that with a
CC group decreases with increasing thecarbon chain length of reagent (x). It may
therefore be reasonable to assume that major
active sites for the formation of CyH2y-1+ andCyH2y-3+ are the alkyl chain and the CC triplebond, respectively. This prediction isinconsistent
with
the
major
reactionmechanisms for the formation of alkenyl andalkynyl ions, assuming that the alkenyl andalkynyl ions are dominantly formed throughproton transfer (1) and H- abstraction (1b) in
Scheme 1, respectively.
According to ICR studies on the CH5+/nparaffins reactions using deuterium, alkyl
CyH2y+1+ ions are formed by the attack of H+ on
a C-H or C-C bond of the alkane leading to
extremely unstable carbonium ions; thesubsequent decomposition by loss of H 2 orsmaller alkane molecules yields CxH2x+1+ andvarious fragment alkyl CyH2y+1+ ions,
respectively.24-27) On the basis of above facts,
the attack of H+ on an alkyl part of 1-alkyne
leading finally to CyH2y-1+ by loss of alkene from
CxH2x-1+ may also occur in the CH5+/1-alkynes
reactions studied here. If such proton transfer
reactions to alkyl group take part in theformation of CyH2y-1+ ions, the increase in theCyH2y-1+ values with increasing reagent
chain length in the CH5+ and C3H5+ reactions
can be explained. This process is not describedin Scheme 1 for the sake of clarity.
In the C2H5+ reactions, the CyH2y-1+ andCyH2y-3+ values are constant. This may be dueto the occurrence of attach of C2H5+ not only on
an alkyl chain ((1a) in Scheme 1) but also on aCC triple bond ((4a) in Scheme 1) for theformation of CyH2y-1+.
CyH2y-3+ ions can be formed via protontransfer followed by loss of H2 (1b) in the CH5+
reaction. They can be formed via H- abstraction(2) and alkanide-ion abstraction (3) in the
C2H5+ reactions, whereas in addition to theabove two processes, C3H5+ addition process(4b) can contribute to the formation of CyH2y-3+
ions in the C3H5+ reactions. The decrease inCyH2y-3+ values with increasing the chain
length in the C3H5+ reaction may arise from thecontribution of C3H5+ addition process (4b)because the probability of attach of C3H5+ to aCC triple bond decreases with increasing
chain length.

4.

Summary and Conclusion

The gas-phase ion-molecule reactions of CH5+,
C2H5+, and C3H5+ with 1-alkynes (CxH2x-2: x =8~18) have been studied in an ion-trap type of
GC/MS by separating each reactant ion.
Although no peaks corresponding to [MW + H]+,
[MW – H]+, [MW + C2H5]+, [MW + C3H5]+ ionsare detected, fragment alkenyl CyH2y-1+ andalkynyl CyH2y-3+ ions were observed. The
dependence of the relative intensities of
CyH2y-1+ and CyH2y-3+ ions on the reaction times
indicated that collisional stabilization took partin the formation of some product ions. As awhole, no significant differences were observedin the initial product-ion distributions of
CyH2y-1+ and CyH2y-3+ ions for the threereactant hydrocarbon ions. The CyH2y-1+
distributions peaked at the lowest C4 ion anddecreased with increasing y. On the other hand,
the CyH2y-3+ distributions gave broad
distributions in the y = 5~10 range. The
CyH2y-1+ and CyH2y-3+ distributions were similar
to the CyH2y+1+ and CyH2y-1 ones produced from
1-olefins, respectively.16) These results suggestthat product ions formed from protonation (+H+) and hydride abstraction (– H-) gave similar
distributions
betweentwo
unsaturated
aliphatic hydrocarbons (1-alkynes and 1olefins). The total alkenyl-ion intensities werelarger than those of total alkynyl-ion ones,
indicating that protonation processes are morefavorable than H- abstraction processes in the
MH+ (M = CH4, C2H4, and C3H4) reactions with
1-alkynes. The total alkenyl and alkynyl ion
intensities increased and decreased withincreasing the reagent chain-length in the CH5+
and C3H5+ reactions, while they wereindependent of the chain length in the C2H5+
reactions. These results suggested that major
active sites for the formation of CyH2y-1+ andCyH2y-3+ in the CH5+ and C3H5+ reactions are analkyl chain and a CC triple bond, respectively.

令和 2 年度

九州大学大学院総合理工学報告

The attack of CH5+ and C3H5+ ions on alkyl
parts may contribute to the formation of
CyH2y-1+ ions, though further detailed
experimental studies using deuterium arenecessary to confirm this prediction. The
different dependence in the C2H5+ reactions
may be due to the contribution of adduct-ion
production process for the formation of CyH2y-1+
ions. The present results provide new
information on the reactivity of hydrocarbon
ions for 1-alkynes; they are important inunderstandingorganic
reactionsbycarbocations.

8)

Acknowledgments

16)This work was supported by the Mitsubishi
foundation (1996) and JSPS KAKENHI Grant
number 09440201 (1997-2000).

17)

9)
10)
11)
12)
13)
14)
15)

18)
19)

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