Engineering Sciences Reports, Kyushu University
Vol. 44, No. 1, pp. 12-16, SEP, 2022

九州大学大学院総合理工学府報告第 44 巻 第 1 号 12-16 頁 令和 4 年 9 月

Mass-Spectroscopic Studies on Ion-Molecule Reactions ofAr2+, N3+, and N4+ Cluster Ions withSimple Aliphatic Hydrocarbons at Thermal Energy
Masaharu TSUJI*1,2† Hiroyuki Kouno*3 Keiko UTO*1
Jun-Ichiro HAYASHI*1 Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received May 31, 2022, accepted June 7, 2022)
Ion-molecule reactions of Ar2+, N3+, and N4+ cluster ions with CH4, C2H2, C2H4, and C2H6 have beenstudied by using a thermal ion-beam apparatus. Rate constants were determined and compared withthose obtained from Langevin theory. Rate constants of the Ar2+ + C2H2, Ar2+ + C2H4, and N3+ + C2H6
reactions, which have not been measured, were determined to be 1.4, 8.1, and 3.6 × 10-10 cm3 s-1,respectively. These values correspond to 14%, 74%, and 28% of calculated rate constants from Langevintheory, respectively.
Key words: Ar2+ ion, N3+ ion, N4+ ion, Aliphatic hydrocarbon, Thermal ion-beam apparatus, Rate constant,Langevin theory

1.

Introduction

Gas phase ion-molecule reactions at thermalenergy have been studied extensively by using
flowing afterglow (FA), selected ion flow tube(SIFT), and ion trapping methods for use inmodeling the chemistry of gas discharge
plasma,planetaryatmospheres,andinterstellar clouds. Anicich1) complied production distributions and reaction rate constants ofbimolecular ion-molecule reactions over 2,300
references in 2003.1)
We have previously made a systematic massspectroscopic studies on ion-molecule reactionsof Ar+, ArN2+, and CO2+ with simple aliphatichydrocarbons using a thermal ion-beamapparatus.2-8) Reaction mechanisms werediscussed from product ion distributions andreaction rate constants.
In this study, ion-molecule reactions of Ar2+,
N3+, and N4+ cluster ions with such simple
aliphatic hydrocarbons as CH4, C2H2, C2H4, andC2H6 are investigated by using a thermal ionbeam apparatus. Total reaction rate constants*1

Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Molecular Science and Technology*3 Department of Molecular Science and Technology,
Graduate Student*4 Department of Materials Science, Shimane
University

are determined and compared with thoseobtained from Langevin theory.

2.

Experimental

The thermal ion-beam apparatus used in thisstudy was essentially the same as that reported
previously.2,7) The apparatus consisted of a FA
ion source, a low-pressure reaction chamber,and a quadrupole mass spectrometer. The
ground-state Ar+(2P3/2) ions and metastable(Ar+)* ions were generated by a 2.45 GHz
microwave discharge of high purity Ar gas in aquartz flow tube, and N2 was added about 10
cm downstream from the center of thedischarge. The product ions were thenexpanded into an interaction chamber througha nozzle centered on the flow tube axis. At low
Ar buffer gas pressures, Ar+ and N2+ were foundas reactant ions, whereas besides these ions,
Ar2+, ArN2+, N3+, and N4+ cluster ions wereobserved at high Ar buffer gas pressures by thefollowing bi-molecular and ter-molecular
reactions.
Ar+ + Ar + Ar → Ar2+ + Ar,
Ar+ + Ar + N2 → Ar2+ + N2,
N+ + N2 + Ar → N3+ + Ar,
N+ + N2 + N2 → N3+ + N2,
N2+ + N2 + Ar → N4+ + Ar,
N2+ + N2 + N2 → N4+ + N2,

(1)
(2)
(3)
(4)
(5)
(6)

令和 4 年度

九州大学大学院総合理工学報告

N4+ + Ar ⇌ ArN2+ + N2,
Ar2+ + N2 ⇌ ArN2+ + Ar.

(7)
(8)

Since the reactions of Ar+, N2+, and ArN2+ withaliphatic hydrocarbons at thermal energy havealready been studied,1,4,5,7,8) the present study
focuses on the reactions of Ar2+, N3+, and N4+cluster ions with aliphatic hydrocarbons.
The sample gas was kept at a constant mass
flow and injected into the reaction chamberfrom a stainless-steel orifice placed 5 cm
downstream from the nozzle. The reactant andproduct ions were sampled through amolybdenum orifice (2 mm in diameter) placed
3 cm further downstream and analyzed using aquadrupole mass spectrometer. The mass
spectra were averaged using a digital storage
oscilloscope and the data were stored in a
microcomputer. Typical operating pressures
were 1.5 Torr (1 Torr = 133.3 Pa) in the FA
ion-source

第 44 巻 第 1 号

13

ion-source chamber, 3 × 10-3 Torr in thereaction chamber, and 2 × 10-5 Torr in the mass
analyzing chamber. The partial pressures ofthe sample gases in the reaction chamber wereless than 1 × 10-5 Torr.

3.

Results and Discussion

3.1 Mass spectra obtained before and after
addition of reagent aliphatic hydrocarbons
Figures 1(a)–4(a) show typical mass spectra
observed before addition of reagent gases,
where N2+, Ar+, N3+, N4+, ArN2+, and Ar2+ ionsare observed. The relative concentrations of[N2+], [Ar+], and [ArN2+] ions are much higher
than those of [N3+], [N4+] and [Ar2+] ions in ourconditions. Figures 1(b)–4(b) show mass
spectra obtained after addition of CH4, C2H2,C2H4, and C2H6, respectively, where thefollowing primary and secondary product ions00

Fig. 1. Mass spectra obtained (a) beforeand (b) after CH4 addition.

Fig. 2. Mass spectra obtained (a) before and(b) after C2H2 addition.

Fig. 3. Mass spectra obtained (a) beforeand (b) after C2H4 addition.

Fig. 4. Mass spectra obtained (a) before and(b) after C2H6 addition.

14

Ion-molecule reactions of Ar2+, N3+, and N4+ with simple aliphatic hydrocarbons at thermal energy

are detected for each reagent:
CH4: CH3+, CH4+, CH5+, C2H5+
C2H2: C2H2+
C2H4: C2H2+, C2H3+, C2H4+, C2H5+
C2H6: C2H2+, C2H3+, C2H4+, C2H5+

These products ions are dominantly formedby the ion-molecule reactions of major N2+,Ar+, and ArN2+ ions with the above fouraliphatic hydrocarbons, denoted as CmHn.
3.2 Rate constants of ion-molecule reactionsof Ar2+, N3+, and N4+ cluster ions with simple
aliphatic hydrocarbons
Totalrateconstants𝑘𝑅
aredetermined from the decay of reactant ions,which is governed by the pseudo-first-order
rate law.
𝐼 𝑅𝐼 𝑅exp

𝑘𝑅

C H 𝑡(9)Here, 𝐼 𝑅
represents the initial R+ ion
current and t is the reaction time. Because ofthe difficulty in evaluating the accurate t
value, the 𝑘𝑅values are evaluated byreference to known rate constants of the Ar+ +CmHn and N2+ + CmHn reactions, 𝑘Ar
and 𝑘N .1)
Figures 5–8 show the decay of ArN2+, Ar2+,N2+, Ar+, N3+, and N4+ upon addition of CmHn.
Satisfactory linearities are found, indicatingthat relation (9) holds for all cases. The rate
constants obtained from slopes are summarizedin Table 1. The accuracy of the present value

Fig. 5. Variation in the reactant
ion currents with CH4 flow rate.

was estimated to be ±25%. The kobs values
obtained in this study are in reasonableagreement with reported data except for thekobs values for the reactions of N3+ and N4+ withC2H2. The kobs values for the reactions of N3+and N4+ with C2H2 obtained in this study at lowpressures (3 × 10-3 Torr) are smaller than thoseof SIFT data1,9) at a He buffer gas pressure of
0.47 Torr by factors of 5.5 and 7.1, respectively.
The stabilization of intermediates by collisions
with third-body He atoms may enhance thereaction rate in the SIFT experiment. The kobs
values of the Ar2+ + C2H2, Ar2+ + C2H4, N3+ +C2H6 reactions, which have not been measured,are determined to be 1.4, 8.1, and 3.6 × 10-10
cm3 s-1, respectively. It should be noted that thekobs value for the N3+ + CH4 reaction, 4.5 × 10-11
cm3 s-1 is small. Since the recombination energyof N3+ (11.06 eV) is lower than the ionizationenergy of CH4 (12.61 eV), as shown in Table 1,
the charge-transfer channel is energeticallyclosed for the N3+ + CH4 reaction. Therefore,
only the following N+ insertion productchannels are open.1)
N3+ + CH4 → HCNH+ + N2 + H2 (95%)
CH2NH2+ + N2 (5%)

(10a)
(10b)

The small rate constant probably arises froman existence of relatively high energy barrier inthe reaction pathway leading to the major
HCNH+ ion from the (N3 + CH4)+ intermediate.
Total rate constants of thermal-energy ionmolecule reactions have been evaluated byusing Langevin theory for nonpolar molecules

Fig. 6. Variation in the reactant
ion currents with C2H2 flow rate.

令和 4 年度

九州大学大学院総合理工学報告

Fig. 7. Variation in the reactant
ion currents with C2H4 flow rate.

15Fig. 8. Variation in the reactant
ion currents with C2H6 flow rate.

with small dipole moments,10)kL = 2πe ( /)1/2

第 44 巻 第 1 号(11)where e, the elementary charge, , thepolarizability of the reagent, and, the reduced
mass of the ion-reagent pair. The  values foraliphatic hydrocarbons used here are the sameas reported previously.2) The ratio of theobserved and calculated rate constants serves
as a measure for the efficiency of a reaction.
The kobs/kcalc ratios are 0.52–1.3 in most cases,
indicating that the ion-molecule reactions ofAr2+, N3+, and N4+ cluster ions with CH4, C2H2,C2H4, and C2H6 occur efficiently. The kobs/kcalc
ratio for the N3+ + CH4 reaction is exceptionallysmall, 0.041. It may be due to the existence ofsome energy barrier along N+ insertion andrearrangement reaction pathway.

4.

Summary and Conclusion

Ion-molecule reactions of Ar2+, N3+, and N4+
cluster ions with CH4, C2H2, C2H4, and C2H6
have been studied by using a thermal ion-beamapparatus. Rate constants were determined
and compared with those obtained fromTable 1. Rate constants of ion-molecule reactions of Ar2+, N3+, and N4+ cluster ions withCH4, C2H2, C2H4, and C2H6 at thermal energy

16Ion-molecule reactions of Ar2+, N3+, and N4+ with simple aliphatic hydrocarbons at thermal energy

Langevin theory. Rate constants of the Ar2+ +C2H2, Ar2+ + C2H4, and N3+ + C2H6 reactions,which have not been measured, weredetermined. The kobs/kcalc ratios are 0.52–1.3 inmost cases, indicating that the ion-molecule
reactions of Ar2+, N3+, and N4+ cluster ions withCH4, C2H2, C2H4, and C2H6 occur efficiently.
Acknowledgments
This paper is dedicated to Emeritus Prof.
Teiichiro Ogawa, who deceased on March 15,2022, for his attractive introduction of gasphase elementary reactions to one of theauthors (MT) about 50 years ago.
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