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Reactions of CN+ and C2N+ ions
Journal of Muss Spectrometry and Ion Physics, 50 (1983) 179- 187 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
International
REACTIONS
OF CN + AND C,N + IONS
M.J. McEWAN
*, V.G. ANICICH
Jet Propulsion Laboratory,
P.R. KEMPER Department
179
and W.T. HUNTRESS,
Jr.
4800 Oak Grove Drive, Pasadena, CA
91109 (U.S.A.)
and M.T. BOWERS
of Chemistry, University of California, Snnta Barbaru, CA 93106 (U.S.A.)
(First received 12 July 1982; in final form 4 August 1982)
ABSTRACT The rate coefficients and product-ion distributions have been measured at near-thermal energies for reactions of CN + and C, N + with a number of neutral molecules using the ICR technique. Multiple reaction channels were observed including charge transfer, H-atom abstraction and dehydrogenation. The C2N+ structural isomer studied in these reactions may be C-C-N+ instead of C-N-C+.
INTRODUCTION
With the recent identification of HCN in the atmosphere of Titan [l], and the observation of CN and HCN in comets [2] and interstellar clouds [3], it has become necessary to include reactions of CN-derived ions in models of these extraterrestrial systems. The chemistry of these ions must therefore be studied in the laboratory, but few quantitative results of their reactivity have been reported_ We recently published the results of an investigation of the reactions of HCN+ with a number of neutral molecules [4], and we extend the ion chemistry of CN-derived species here with a study of the reactions of CN+ a.nd C,N+. A major difference between this and the previous study is the absence of proton transfer as a potential channe1 for reaction_ The relatively large heats of formation of CN + and C2N+ also mean that for each reaction there are a number of exoergic pathways available.
* On leave from the Department of Chemistry, University of Canterbury, Christchurch, New Zealand. NRC/NASA Senior Research Associate, 1979-80. 0020-7381/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
180 EXPERIMENTAL
The ICR mass spectrometer and the techniques used to obtain rate coefficients and product distributions have been described previously [4,53. The CN+ and C,N+ ions were generated by electron impact on C,N, at electron energies close to their respective appearance energies: 20.5 eV for CN+ and 19.5 eV for C,N+. The CN+ ion was also generated by electron ionization of HCN in a few experiments. Rate coefficients were measured in the trapped-ion mode in which major primary ions not under investigation were ejected from the reaction cell in the first few milliseconds following the electron pulse. Product distributions were measured in the usual way by operating in the drift mode, but in several cases complicating features arose when ions produced directly by electron impact on C,N, had the same m/z ratio as the product ions. These problems were overcome by using the tandem Ion Cyclotron Resonance (ICR) system at the University of California, Santa Barbara (UCSB). The Tandem ICR spectrometer at UCSB is fashioned after the original design of Smith and Futrell [6], and will be described more completely elsewhere [7]. It consists of three main sections: (1) an ion source; (2) an ion optics and mass filter; and (3) a differentially pumped ICR cell, separated from (1) and (2) by a small entrance slit. The reactant ions are formed in the modified mass spectrometer ion source either by electron or chemical ionization. The ions are extracted by a combination extraction-repeller field, accelerated to a high energy (typically 2-4 keV), and mass selected in a 180” Dempster magnetic sector. The ions are then decelerated to ground potential as they pass through the entrance slit into the ICR cell. The entrance slit actually consists of a miniature Wien [S] velocity filter which is used to remove any high energy ions. Once in the ICR cell, the ions react with the desired neutral species and are detected. All three sections (source, mass filter, and ICR cell) are positioned between the pole faces of a 30.5 cm electromagnet. Since the magnetic field must remain fixed, a frequency scanning marginal oscillator [9] is used to observe the ions. Peak heights measured at different frequencies are corrected for changing marginal oscillator sensitivity using a Q-Spoiler [IO] standard signal. In the tandem experiments, the CN + and CZN+ ions were formed by charge transfer from He+ (recombination energy = 24.54 eV) to C,N, or HCN. A source gas mixture of - 1 X lop4 torr of HCN or C,N, in 0.1 torr of He was used. The gases were all of research grade purity except HCN and C,N,. The HCN was purified by vacuum distillation from Fumico (Box 3459, Amarillo, TX 79106) 99% industrial grade HCN, and C,N, was vacuum distilled from the technical grade Matheson (P.O. Box 85, East Rutherter, NJ 07073) product.
181 RESULTS
The reactions studied in this work are summarized in Table 1 (CN + reactions} and Table 2 (C,N + reactions). All products observed with contributions more than 2% of the total product distribution are listed in Tables 1 and 2. The product distributions have been rounded off to the nearest 5%. The two reactant ions in this study, CN+ and C,N+ , have relatively large enthalpies of formation ( 1794 kJ mol- ’ and - 1711 kJ mol- ‘, respectively [ 1 l- 131, thus providing a number of potential exothermic channels, The absence of hydrogen in the reactant ions eliminates proton transfer as a potential reaction channel. Charge transfer, dissociative charge transfer, and H-atom abstraction reactions were observed when thermodynamically allowed. Condensation reactions were not commonly observed as major channels in the reactions studied.
TABLE
1
Reactions of CN+. Rate coefficients (in units of 1W9 cm3 molecule-’ s-l) ratios for reactions of CN+ at 298 K Neutral reactant
Products
Branching ratio
and branching
-AHa (kJ mol-‘)
kobs This work
Previous work
D2
DCN+
+D
1.0
1.10~0.2
CH,
CH,i HCN+ H&N+
+ HCN +CH, +CH,
0.05 0.35 0.60
1.2f0.2
NH,
NH; NH,+ HCN+ H&N+
+ HCN +CN +NH, +NH
0.05 0.60 0.20 0.15
2.0 + 0.4
350 381 130 477
Hz0
H20+ HCN+ H&N+ HCO+ HNCO+
+CN +OH +0 +HN +H
0.10 0.50 0.15 0.05 0.20
3.2 + 0.6
147 67 362 390 - 315
C,D*
C,DZ DC3N+
+CN +D
0.90 0.10
- 1.5
d +0.3
(1.24) b
129
0.97 =
497 134 396
269 - 306
182 TABLE Neutral reactant
1 (contimed) Products
Branching ratio
This work
C,H,+ HCN+ H&N+
+HCN +CN +C,H, +C,H,
CID: C,D,+ DCN+ D&N+
+ DCN +CN +C,D, +C,D,
C,H: C,HZ C,H,+
+HCN+H, +HCN+H + HCN
C, D3’ C,DZ C,Di+
+DCN+D, +CN+D, + DCN
HCN
HCN+ C,N;
+CN +H
0.80 0.20
CO
co+
+CN
1.0
CO,
co,+ NCO+
+CN -+-CO
C,N,
C,N,f
+CN
N2
no reaction
C*H,
GD4
CZH,
c2Qi
CzH,+
observed
0.10
-AH” (kJ mol-‘)
kobs Previous work
1.6 + 0.3
- 594 354 -135 687
0.55 0.25 0.10 2.0d f0.5
0.15 0.65 0.20
1.9kO.4
- 450 282 - 660
1.9 e +0.4
3.1+0.6
2.8 ’
0.63 f 0.1
< 0.3 > 0.7
1.5*0.3
1.0
2.1+0.4
56 114 19 37 - 56
0.62 ’
78
< 0.01
a. Based on values of AH: given in ref. 11 and proton affinities in P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. b Value for reaction with H, in W.T. Huntress, Astrophys. J. Suppl. Ser., 33 (1977) 495. ’ Ref. 14. d We have assumed similar product channel ratios for C,D,” as for C., H.,. The rate coefficient for C,H, could not be measured due to peak overlap. e C.G. Freeman, P.W. Harland, J.P. Liddy and M.J. McEwan, Aust. J. Chem., 3 I ( 1978) 963. ’ P.M. Inoue and M. Cottin, Adv. Mass Spectrom., 3 (1966) 339: but rate coefficient was measured in a voltage gradient of between 5 and 20 V cm-‘.
183 TABLE 2 Reactions of C,N+. Rate coefficients (in units of 10e9 cm3 molecule-’ ratios for reactions of CCN + at 298 K Neutral reactant
c*, NH3 H2O
C2H2
C2H-4
C2’3,
C2H,
Branching ratio
Products
+ HCN H&N+ +CH, H&N++ H,
C*H,+
kohs
-AH”
(This work)
(Previous work)
-c 0.01 h
(0.0044) c
tkJ
mol-‘)
408 306 ?
H&N + + HCN N2H+ +C2H2
0.90
HCO+ HC2N+
I- HCN +OH
0.75 0.25
0.34 f 0.06
CxH+ HCaN+
+ HCN +H
0.80 0.20
l.O-tO.3
+C,N C,HZ H,CCN+tCC,H, + HCN C,H; H,C,N*+H,
0.10 0.50 0.30 0.10
1.3kO.3
1.9kO.2
(1.8)’
630 444
0.10
+ C,N C*DZ D,CCN ‘+ C, D, + DCN C,D,+ D,C, N ++ D, +CH,CN C,H,+ + HC,N C,Hf +HCN+H, C,H,+ + HCN C,H,+ H&N*+C2H4
s- ‘) and branching
502 d
(0.89) ’
150 ? -182 - 320 461 ?
1.6f0.3
0.10 0.25 0.30 0.10 0.25
1.2kO.2
HCN
products
6 0.03 =
N,
no reaction observed
< 0.01 b
co
no reaction observed
< 0.01 b
co2
no reaction observed
< 0.01 b
C2N2
no reaction observed
$0.01
- 410 ? 316 546 - 345 0.3 f
b
a Based on values of AH: given in ref. 11, proton affinities in P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. b No reaction was observed which implies a rate coefficient < 1 X 10-r’ cm3 molecule- ’ s-l. c Value reported in ref. 15 for CIN + produced in reaction 3 (possibly CNC+, see text). d The estimated A Hr”fHC,N ‘) = 1550 kJ mol - ’ derived from photoionization and electron impact techniques (ref. 11) on CH&N would make this channel endothermic by - 120 kJ mol- r. The combined errors for AHP(HC,N+) and AH,!‘(&N+) c&, however, be larger than 120 kJ mol- ‘_ c Products could not be identified. This reaction appears to proceed by a 3-body mechanism and therefore would not be fast at the low pressures in the ICR. f C.G. Freeman, P.W. Harland, J.P. Liddy and M.J. McEwan, Aust. J. Chem., 3 1 (1978) 963.
184
Reactions
ofCN
+
The neutral reaction partners chosen for CNf and listed in Table 1 are those most likely to be present in abundance in interstellar clouds, comets and the atmosphere of Titan. Several of these reactions require further comment, Reaction with CN,
This reaction has also been studied using the selected ion flow technique of Schiff and co-workers [ 141. Although there is relatively good agreement in the rate coefficient for the reaction between the two techniques, Schiff and co-workers did not observe the hydrogen atom abstraction channel leading to HCN+ in their experiment. This channel was observed here to account for 35% of the total reaction. Two further exothermic channels available to this reaction are: CN+
+ CH, + CHZ + CN + 150 kJ mol-’ --, C,Hz
+ NH + - 272 kJ mol-’
(la> (lb)
The charge transfer channel (la) was not observed under normal ICR conditions. Translational excitation of the CN+ ion did however produce a marked enhancement of this channel. The production in reaction (lb), C,HT , has the same m/z ratio as HCN+ . Identification of the product peak at m/z 27 as HCN+ rather than C, Hz is based on the results of experiments on the reaction& of CN+ with CD,. Reactiun with NH,
Product channels were established using both the tandem ICR and the Jet Propulsion Laboratory instruments. Secondary reactions were kept to a minimum by using low reactant pressure ( - 6 x 10m6 torr) and short reaction times (- 1 ms). Contributions from secondary reactions were reduced by extrapolating the product channel ratios to zero reactant pressure. This was done for all the product channel analyses reported here. Reaction
with H,O
In addition to the major channels shown in Table 1, a very small product peak was observed at OH+ from the exothermic channel CN+
+ H,O + OH+ + HCN
+ 129 kJ mol-’
(2a)
The branching ratio foi the charge transfer channel was found to increase with increasing translatiofial energy of the CN+ ion CN+
+ H,O + H,O+
+ CN
(2b)
185
Reaction with C, H6
The main channel was densation products were making up less than 2% leading to the production
found to be the production of C,Hz . No conobserved. Minor exothermic reaction channels of the product distributions were also observed, of C, Hz , H,CN+ , and HCN +.
Reactions of C, N •t
There are several possible structural isomers of the CZN+ species. Double-zeta-plus-polarization Self-consistent field-Configuration Interaction calculations by Haese and Woods [ 131 indicate that the linear CNC’ structure is more stable by 205 kJ mol-’ than the CCN+ structure. The C,N+ species in trapped-ion ICR studies was produced by electron impact of 19.5 V electrons on C,N,, and is most likely to be the CCN+ structural isomer rather than CNC + . Haese and Woods indicate that the CCN+ isomer is produced from C,N, in this way. In addition, they quote a barrier to isomerization of 272 kJ mol- ’ through a state of bent geometry so that it is unlikely that CCN+ once formed will be isomerized to CNC+ . Schiff and Bohme [ 151have reported rate coefficients for several reactions of the C,N+ species using the selected ion flow tube technique in which the C,N + species was produced in reaction (3). C++HCN+C,N++H
(3)
Haese and Woods [ 131 conclude on the basis of theoretical considerations that the production of C2N+ in this reaction leads exclusively to CNC + . Therefore, the C,N+ species examined in this study is likely to be a different structural isomer than that in the study of Schiff and Bohme [ 151. We have indicated this potential difference in Table 2 by including the CNC+ results of Schiff and Bohme in parentheses. Reaction with NH,
The two product ions observed, H,CN+ and N2H+ , require considerable rearrangement within the collision complex for production from the C--C-N+ isomer. The interesting feature of this reaction is that the NZH+ channel was not observed by Schiff and Bohme [ 151, whereas it amounted to 10% of the total reaction in this study. One possible explanation for this difference is that N,H+ is more readily producted from a C-C-N’ - - - NH, encounter than from a C-N-C+ - - - NH, encounter. Alternatively, rapid proton transfer from NZH+ to NH, in the flow tube would convert N2H+ to NH:. Reaction with C, H2
The major channel observed is the production
of C3H+
with a loss of
HCN. There are two possible candidates at m/z 63, namely, HCqN+ and C,Hc. minor product ion as HC,N+ .
for the other product ion observed Reaction with C,D, confirmed the
Reaction with C, H4 / C, D,
Dehydrogenation of C,H, to give H&N+ was observed as the major process, although a substantial fraction of collisions lead to the formation of C,Hl and HCN via the most exothermic channel. The assignment of the peaks at m/z 39 and 40 to C,HT and H2C2N+ was made by comparing the distributions of product ions with C,H, and C,D,. Reaction with C,H, yields peaks at m/z 39 and 40, whereas reaction with C,D, yields only one peak at m/z 42. Reaction with C,H,
The major reaction products are C,HT, H,C,N+, and C,Hc. The ambiguity in the product-ion signal at m/z 39 (C,Hz or HC,N’ ) was resolved by substitution of C, D6 for C, H,. The assignment of C,Hz to the product at m/z 39 is made on the basis of a shift of this peak to m/z 42 and the absence of a peak at m/z 40 in reaction with C, D6. DISCUSSION
The data presented here provide an important addition to the existing data for ion/molecule reactions in astrophysical environments. The reactions of C,N + are significant for the chemistry of C-N compounds in interstellar clouds. This ion is produced in dense clouds by the reaction between C+ and HCN. It appears to be a relatively stable ion with little reactivity towards H, or CO, both of which are major molecular species in dense clouds. The principal loss process in dense clouds is likely to be a reaction with H,O or 0 atoms. The interesting reactions are those between C2N+ and the hydrocarbons, which could contribute to the synthesis of observed interstellar cyanopolyynes by recombination of the resulting HC 2n+ IN+ ions with electrons. The presence of NH, would recycle the CzN+ back to HCN and to N,. The CN+ ion may be important in the atmosphere of Titan as well as in interstellar clouds. In interstellar clouds, the ion will disappear rapidly by reaction with H, to produce HCN+ . In the atmosphere of Titan, the main constituents are N, and CH, so that loss of CN+ occurs principally by reaction with CH,, again to produce HCN+ . In both interstellar clouds and the atmosphere of Titan, the ultimate fate of CN+ is the production of HCN since HCN+ reacts with both H, and CH, to produce H&N+ which reacts in turn to produce HCN both by electron recombination and by proton transfer to NH,. A comparison of the flow
187
tube results of Schiff and Bohme [ 151, with the ICR results obtained here for the reactions of CZN+ with CH,, NH,, and C,H,, shows reasonable agreement for both rate constants and product distributions. This comparison suggests either that there is little difference in reactivity between the C-N-C+ and C-C-N+ structural isomers, or that the methods of production in each study produce the same isomer. Both of these alternatives are in conflict with the theoretical predictions of Haese and Woods [ 131 and clearly deserve further attention_ ACKNOWLEDGEMENTS
This work represents one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract NAS 7-100 sponsored by the National Aeronautics and Space Administration. The work at UCSB was accomplished with support of the National Science Foundation under grant CHE 80-20464. We are grateful for the support of these agencies. REFERENCES 1 R. Hanel, B. Conrath, F.M. Flasar, V. Kunde, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, L. Herath, M. Allison, D. Gruickshank, D. Grantier, P. Gierasch, L. Horn, R. Koppany and C. Pomamperuma, Science, 2 12 ( 198 1) 192. 2 W.F. Huebner, L.E. Snyder and D. Buhl, Icarus, 23 (1974) 580; P-T. Giguere and W.F. Huebner, Astrophys. J., 223 (1978) 638. 3 A. McKeller, Publ. Astron. Sot. Pac., 52 (1940) 187; L.E. Snyder and D. Buhl, Astrophys. J. L&t., 163 (1971) L47. 4 M.J. McEwan, V.G. Anicich and W.T. Huntress, Int. J. Mass Spectrom. Ion Phys., 37 (1981) 273. 5 J.K. Kim, V.G. Anicich and W-T. Huntress, J. Phys. Chem., 81 (1978) 1798. 6 D.L. Smith and J.H. Futrell, Int. J. Mass Spectrom. Ion Phys., 14 (1974) 171. 7 P.R. Kemper and M.T. Bowers, Int. J. Mass Spectrom. Ion Phys., to be submitted. 8 L. Page and N. Adams, Principles of Electricity, D. van Nostrand, Princeton, NJ, 1958, p. 235. 9 P.R. Kemper and M.T. Bowers, Rev. Sci. Instrum., 53 (1982) 989. 10 P.R. Kemper and M.T. Bowers, Rev. Sci. Instrum., 48 (1977) 1477. 11 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (1977) Suppl. No. 1. 12 V.H. Dibeler, R.M. Reese and J.L. Franklin, J. Am. Chem. Sot., 83 (1961) 1813. 13 N.H. .Haese and R.C. Woods, Astrophys. J., 246 (1.98 1) L5 1. 14 H.I. Schiff, G.I. Mackay, G.D. Vlachos and D.K. Bohme, in B. Andrews (Ed.), Proc. I.A.U. Symp. on Interstellar Molecules, No. 87, Reidel, Dordrecht, 1981, p. 307. 15 H.I. Schiff and D.K. Bohme, Astrophys. J., 232 (1979) 740.
International
REACTIONS
OF CN + AND C,N + IONS
M.J. McEWAN
*, V.G. ANICICH
Jet Propulsion Laboratory,
P.R. KEMPER Department
179
and W.T. HUNTRESS,
Jr.
4800 Oak Grove Drive, Pasadena, CA
91109 (U.S.A.)
and M.T. BOWERS
of Chemistry, University of California, Snnta Barbaru, CA 93106 (U.S.A.)
(First received 12 July 1982; in final form 4 August 1982)
ABSTRACT The rate coefficients and product-ion distributions have been measured at near-thermal energies for reactions of CN + and C, N + with a number of neutral molecules using the ICR technique. Multiple reaction channels were observed including charge transfer, H-atom abstraction and dehydrogenation. The C2N+ structural isomer studied in these reactions may be C-C-N+ instead of C-N-C+.
INTRODUCTION
With the recent identification of HCN in the atmosphere of Titan [l], and the observation of CN and HCN in comets [2] and interstellar clouds [3], it has become necessary to include reactions of CN-derived ions in models of these extraterrestrial systems. The chemistry of these ions must therefore be studied in the laboratory, but few quantitative results of their reactivity have been reported_ We recently published the results of an investigation of the reactions of HCN+ with a number of neutral molecules [4], and we extend the ion chemistry of CN-derived species here with a study of the reactions of CN+ a.nd C,N+. A major difference between this and the previous study is the absence of proton transfer as a potential channe1 for reaction_ The relatively large heats of formation of CN + and C2N+ also mean that for each reaction there are a number of exoergic pathways available.
* On leave from the Department of Chemistry, University of Canterbury, Christchurch, New Zealand. NRC/NASA Senior Research Associate, 1979-80. 0020-7381/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
180 EXPERIMENTAL
The ICR mass spectrometer and the techniques used to obtain rate coefficients and product distributions have been described previously [4,53. The CN+ and C,N+ ions were generated by electron impact on C,N, at electron energies close to their respective appearance energies: 20.5 eV for CN+ and 19.5 eV for C,N+. The CN+ ion was also generated by electron ionization of HCN in a few experiments. Rate coefficients were measured in the trapped-ion mode in which major primary ions not under investigation were ejected from the reaction cell in the first few milliseconds following the electron pulse. Product distributions were measured in the usual way by operating in the drift mode, but in several cases complicating features arose when ions produced directly by electron impact on C,N, had the same m/z ratio as the product ions. These problems were overcome by using the tandem Ion Cyclotron Resonance (ICR) system at the University of California, Santa Barbara (UCSB). The Tandem ICR spectrometer at UCSB is fashioned after the original design of Smith and Futrell [6], and will be described more completely elsewhere [7]. It consists of three main sections: (1) an ion source; (2) an ion optics and mass filter; and (3) a differentially pumped ICR cell, separated from (1) and (2) by a small entrance slit. The reactant ions are formed in the modified mass spectrometer ion source either by electron or chemical ionization. The ions are extracted by a combination extraction-repeller field, accelerated to a high energy (typically 2-4 keV), and mass selected in a 180” Dempster magnetic sector. The ions are then decelerated to ground potential as they pass through the entrance slit into the ICR cell. The entrance slit actually consists of a miniature Wien [S] velocity filter which is used to remove any high energy ions. Once in the ICR cell, the ions react with the desired neutral species and are detected. All three sections (source, mass filter, and ICR cell) are positioned between the pole faces of a 30.5 cm electromagnet. Since the magnetic field must remain fixed, a frequency scanning marginal oscillator [9] is used to observe the ions. Peak heights measured at different frequencies are corrected for changing marginal oscillator sensitivity using a Q-Spoiler [IO] standard signal. In the tandem experiments, the CN + and CZN+ ions were formed by charge transfer from He+ (recombination energy = 24.54 eV) to C,N, or HCN. A source gas mixture of - 1 X lop4 torr of HCN or C,N, in 0.1 torr of He was used. The gases were all of research grade purity except HCN and C,N,. The HCN was purified by vacuum distillation from Fumico (Box 3459, Amarillo, TX 79106) 99% industrial grade HCN, and C,N, was vacuum distilled from the technical grade Matheson (P.O. Box 85, East Rutherter, NJ 07073) product.
181 RESULTS
The reactions studied in this work are summarized in Table 1 (CN + reactions} and Table 2 (C,N + reactions). All products observed with contributions more than 2% of the total product distribution are listed in Tables 1 and 2. The product distributions have been rounded off to the nearest 5%. The two reactant ions in this study, CN+ and C,N+ , have relatively large enthalpies of formation ( 1794 kJ mol- ’ and - 1711 kJ mol- ‘, respectively [ 1 l- 131, thus providing a number of potential exothermic channels, The absence of hydrogen in the reactant ions eliminates proton transfer as a potential reaction channel. Charge transfer, dissociative charge transfer, and H-atom abstraction reactions were observed when thermodynamically allowed. Condensation reactions were not commonly observed as major channels in the reactions studied.
TABLE
1
Reactions of CN+. Rate coefficients (in units of 1W9 cm3 molecule-’ s-l) ratios for reactions of CN+ at 298 K Neutral reactant
Products
Branching ratio
and branching
-AHa (kJ mol-‘)
kobs This work
Previous work
D2
DCN+
+D
1.0
1.10~0.2
CH,
CH,i HCN+ H&N+
+ HCN +CH, +CH,
0.05 0.35 0.60
1.2f0.2
NH,
NH; NH,+ HCN+ H&N+
+ HCN +CN +NH, +NH
0.05 0.60 0.20 0.15
2.0 + 0.4
350 381 130 477
Hz0
H20+ HCN+ H&N+ HCO+ HNCO+
+CN +OH +0 +HN +H
0.10 0.50 0.15 0.05 0.20
3.2 + 0.6
147 67 362 390 - 315
C,D*
C,DZ DC3N+
+CN +D
0.90 0.10
- 1.5
d +0.3
(1.24) b
129
0.97 =
497 134 396
269 - 306
182 TABLE Neutral reactant
1 (contimed) Products
Branching ratio
This work
C,H,+ HCN+ H&N+
+HCN +CN +C,H, +C,H,
CID: C,D,+ DCN+ D&N+
+ DCN +CN +C,D, +C,D,
C,H: C,HZ C,H,+
+HCN+H, +HCN+H + HCN
C, D3’ C,DZ C,Di+
+DCN+D, +CN+D, + DCN
HCN
HCN+ C,N;
+CN +H
0.80 0.20
CO
co+
+CN
1.0
CO,
co,+ NCO+
+CN -+-CO
C,N,
C,N,f
+CN
N2
no reaction
C*H,
GD4
CZH,
c2Qi
CzH,+
observed
0.10
-AH” (kJ mol-‘)
kobs Previous work
1.6 + 0.3
- 594 354 -135 687
0.55 0.25 0.10 2.0d f0.5
0.15 0.65 0.20
1.9kO.4
- 450 282 - 660
1.9 e +0.4
3.1+0.6
2.8 ’
0.63 f 0.1
< 0.3 > 0.7
1.5*0.3
1.0
2.1+0.4
56 114 19 37 - 56
0.62 ’
78
< 0.01
a. Based on values of AH: given in ref. 11 and proton affinities in P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. b Value for reaction with H, in W.T. Huntress, Astrophys. J. Suppl. Ser., 33 (1977) 495. ’ Ref. 14. d We have assumed similar product channel ratios for C,D,” as for C., H.,. The rate coefficient for C,H, could not be measured due to peak overlap. e C.G. Freeman, P.W. Harland, J.P. Liddy and M.J. McEwan, Aust. J. Chem., 3 I ( 1978) 963. ’ P.M. Inoue and M. Cottin, Adv. Mass Spectrom., 3 (1966) 339: but rate coefficient was measured in a voltage gradient of between 5 and 20 V cm-‘.
183 TABLE 2 Reactions of C,N+. Rate coefficients (in units of 10e9 cm3 molecule-’ ratios for reactions of CCN + at 298 K Neutral reactant
c*, NH3 H2O
C2H2
C2H-4
C2’3,
C2H,
Branching ratio
Products
+ HCN H&N+ +CH, H&N++ H,
C*H,+
kohs
-AH”
(This work)
(Previous work)
-c 0.01 h
(0.0044) c
tkJ
mol-‘)
408 306 ?
H&N + + HCN N2H+ +C2H2
0.90
HCO+ HC2N+
I- HCN +OH
0.75 0.25
0.34 f 0.06
CxH+ HCaN+
+ HCN +H
0.80 0.20
l.O-tO.3
+C,N C,HZ H,CCN+tCC,H, + HCN C,H; H,C,N*+H,
0.10 0.50 0.30 0.10
1.3kO.3
1.9kO.2
(1.8)’
630 444
0.10
+ C,N C*DZ D,CCN ‘+ C, D, + DCN C,D,+ D,C, N ++ D, +CH,CN C,H,+ + HC,N C,Hf +HCN+H, C,H,+ + HCN C,H,+ H&N*+C2H4
s- ‘) and branching
502 d
(0.89) ’
150 ? -182 - 320 461 ?
1.6f0.3
0.10 0.25 0.30 0.10 0.25
1.2kO.2
HCN
products
6 0.03 =
N,
no reaction observed
< 0.01 b
co
no reaction observed
< 0.01 b
co2
no reaction observed
< 0.01 b
C2N2
no reaction observed
$0.01
- 410 ? 316 546 - 345 0.3 f
b
a Based on values of AH: given in ref. 11, proton affinities in P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. b No reaction was observed which implies a rate coefficient < 1 X 10-r’ cm3 molecule- ’ s-l. c Value reported in ref. 15 for CIN + produced in reaction 3 (possibly CNC+, see text). d The estimated A Hr”fHC,N ‘) = 1550 kJ mol - ’ derived from photoionization and electron impact techniques (ref. 11) on CH&N would make this channel endothermic by - 120 kJ mol- r. The combined errors for AHP(HC,N+) and AH,!‘(&N+) c&, however, be larger than 120 kJ mol- ‘_ c Products could not be identified. This reaction appears to proceed by a 3-body mechanism and therefore would not be fast at the low pressures in the ICR. f C.G. Freeman, P.W. Harland, J.P. Liddy and M.J. McEwan, Aust. J. Chem., 3 1 (1978) 963.
184
Reactions
ofCN
+
The neutral reaction partners chosen for CNf and listed in Table 1 are those most likely to be present in abundance in interstellar clouds, comets and the atmosphere of Titan. Several of these reactions require further comment, Reaction with CN,
This reaction has also been studied using the selected ion flow technique of Schiff and co-workers [ 141. Although there is relatively good agreement in the rate coefficient for the reaction between the two techniques, Schiff and co-workers did not observe the hydrogen atom abstraction channel leading to HCN+ in their experiment. This channel was observed here to account for 35% of the total reaction. Two further exothermic channels available to this reaction are: CN+
+ CH, + CHZ + CN + 150 kJ mol-’ --, C,Hz
+ NH + - 272 kJ mol-’
(la> (lb)
The charge transfer channel (la) was not observed under normal ICR conditions. Translational excitation of the CN+ ion did however produce a marked enhancement of this channel. The production in reaction (lb), C,HT , has the same m/z ratio as HCN+ . Identification of the product peak at m/z 27 as HCN+ rather than C, Hz is based on the results of experiments on the reaction& of CN+ with CD,. Reactiun with NH,
Product channels were established using both the tandem ICR and the Jet Propulsion Laboratory instruments. Secondary reactions were kept to a minimum by using low reactant pressure ( - 6 x 10m6 torr) and short reaction times (- 1 ms). Contributions from secondary reactions were reduced by extrapolating the product channel ratios to zero reactant pressure. This was done for all the product channel analyses reported here. Reaction
with H,O
In addition to the major channels shown in Table 1, a very small product peak was observed at OH+ from the exothermic channel CN+
+ H,O + OH+ + HCN
+ 129 kJ mol-’
(2a)
The branching ratio foi the charge transfer channel was found to increase with increasing translatiofial energy of the CN+ ion CN+
+ H,O + H,O+
+ CN
(2b)
185
Reaction with C, H6
The main channel was densation products were making up less than 2% leading to the production
found to be the production of C,Hz . No conobserved. Minor exothermic reaction channels of the product distributions were also observed, of C, Hz , H,CN+ , and HCN +.
Reactions of C, N •t
There are several possible structural isomers of the CZN+ species. Double-zeta-plus-polarization Self-consistent field-Configuration Interaction calculations by Haese and Woods [ 131 indicate that the linear CNC’ structure is more stable by 205 kJ mol-’ than the CCN+ structure. The C,N+ species in trapped-ion ICR studies was produced by electron impact of 19.5 V electrons on C,N,, and is most likely to be the CCN+ structural isomer rather than CNC + . Haese and Woods indicate that the CCN+ isomer is produced from C,N, in this way. In addition, they quote a barrier to isomerization of 272 kJ mol- ’ through a state of bent geometry so that it is unlikely that CCN+ once formed will be isomerized to CNC+ . Schiff and Bohme [ 151have reported rate coefficients for several reactions of the C,N+ species using the selected ion flow tube technique in which the C,N + species was produced in reaction (3). C++HCN+C,N++H
(3)
Haese and Woods [ 131 conclude on the basis of theoretical considerations that the production of C2N+ in this reaction leads exclusively to CNC + . Therefore, the C,N+ species examined in this study is likely to be a different structural isomer than that in the study of Schiff and Bohme [ 151. We have indicated this potential difference in Table 2 by including the CNC+ results of Schiff and Bohme in parentheses. Reaction with NH,
The two product ions observed, H,CN+ and N2H+ , require considerable rearrangement within the collision complex for production from the C--C-N+ isomer. The interesting feature of this reaction is that the NZH+ channel was not observed by Schiff and Bohme [ 151, whereas it amounted to 10% of the total reaction in this study. One possible explanation for this difference is that N,H+ is more readily producted from a C-C-N’ - - - NH, encounter than from a C-N-C+ - - - NH, encounter. Alternatively, rapid proton transfer from NZH+ to NH, in the flow tube would convert N2H+ to NH:. Reaction with C, H2
The major channel observed is the production
of C3H+
with a loss of
HCN. There are two possible candidates at m/z 63, namely, HCqN+ and C,Hc. minor product ion as HC,N+ .
for the other product ion observed Reaction with C,D, confirmed the
Reaction with C, H4 / C, D,
Dehydrogenation of C,H, to give H&N+ was observed as the major process, although a substantial fraction of collisions lead to the formation of C,Hl and HCN via the most exothermic channel. The assignment of the peaks at m/z 39 and 40 to C,HT and H2C2N+ was made by comparing the distributions of product ions with C,H, and C,D,. Reaction with C,H, yields peaks at m/z 39 and 40, whereas reaction with C,D, yields only one peak at m/z 42. Reaction with C,H,
The major reaction products are C,HT, H,C,N+, and C,Hc. The ambiguity in the product-ion signal at m/z 39 (C,Hz or HC,N’ ) was resolved by substitution of C, D6 for C, H,. The assignment of C,Hz to the product at m/z 39 is made on the basis of a shift of this peak to m/z 42 and the absence of a peak at m/z 40 in reaction with C, D6. DISCUSSION
The data presented here provide an important addition to the existing data for ion/molecule reactions in astrophysical environments. The reactions of C,N + are significant for the chemistry of C-N compounds in interstellar clouds. This ion is produced in dense clouds by the reaction between C+ and HCN. It appears to be a relatively stable ion with little reactivity towards H, or CO, both of which are major molecular species in dense clouds. The principal loss process in dense clouds is likely to be a reaction with H,O or 0 atoms. The interesting reactions are those between C2N+ and the hydrocarbons, which could contribute to the synthesis of observed interstellar cyanopolyynes by recombination of the resulting HC 2n+ IN+ ions with electrons. The presence of NH, would recycle the CzN+ back to HCN and to N,. The CN+ ion may be important in the atmosphere of Titan as well as in interstellar clouds. In interstellar clouds, the ion will disappear rapidly by reaction with H, to produce HCN+ . In the atmosphere of Titan, the main constituents are N, and CH, so that loss of CN+ occurs principally by reaction with CH,, again to produce HCN+ . In both interstellar clouds and the atmosphere of Titan, the ultimate fate of CN+ is the production of HCN since HCN+ reacts with both H, and CH, to produce H&N+ which reacts in turn to produce HCN both by electron recombination and by proton transfer to NH,. A comparison of the flow
187
tube results of Schiff and Bohme [ 151, with the ICR results obtained here for the reactions of CZN+ with CH,, NH,, and C,H,, shows reasonable agreement for both rate constants and product distributions. This comparison suggests either that there is little difference in reactivity between the C-N-C+ and C-C-N+ structural isomers, or that the methods of production in each study produce the same isomer. Both of these alternatives are in conflict with the theoretical predictions of Haese and Woods [ 131 and clearly deserve further attention_ ACKNOWLEDGEMENTS
This work represents one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract NAS 7-100 sponsored by the National Aeronautics and Space Administration. The work at UCSB was accomplished with support of the National Science Foundation under grant CHE 80-20464. We are grateful for the support of these agencies. REFERENCES 1 R. Hanel, B. Conrath, F.M. Flasar, V. Kunde, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, L. Herath, M. Allison, D. Gruickshank, D. Grantier, P. Gierasch, L. Horn, R. Koppany and C. Pomamperuma, Science, 2 12 ( 198 1) 192. 2 W.F. Huebner, L.E. Snyder and D. Buhl, Icarus, 23 (1974) 580; P-T. Giguere and W.F. Huebner, Astrophys. J., 223 (1978) 638. 3 A. McKeller, Publ. Astron. Sot. Pac., 52 (1940) 187; L.E. Snyder and D. Buhl, Astrophys. J. L&t., 163 (1971) L47. 4 M.J. McEwan, V.G. Anicich and W.T. Huntress, Int. J. Mass Spectrom. Ion Phys., 37 (1981) 273. 5 J.K. Kim, V.G. Anicich and W-T. Huntress, J. Phys. Chem., 81 (1978) 1798. 6 D.L. Smith and J.H. Futrell, Int. J. Mass Spectrom. Ion Phys., 14 (1974) 171. 7 P.R. Kemper and M.T. Bowers, Int. J. Mass Spectrom. Ion Phys., to be submitted. 8 L. Page and N. Adams, Principles of Electricity, D. van Nostrand, Princeton, NJ, 1958, p. 235. 9 P.R. Kemper and M.T. Bowers, Rev. Sci. Instrum., 53 (1982) 989. 10 P.R. Kemper and M.T. Bowers, Rev. Sci. Instrum., 48 (1977) 1477. 11 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (1977) Suppl. No. 1. 12 V.H. Dibeler, R.M. Reese and J.L. Franklin, J. Am. Chem. Sot., 83 (1961) 1813. 13 N.H. .Haese and R.C. Woods, Astrophys. J., 246 (1.98 1) L5 1. 14 H.I. Schiff, G.I. Mackay, G.D. Vlachos and D.K. Bohme, in B. Andrews (Ed.), Proc. I.A.U. Symp. on Interstellar Molecules, No. 87, Reidel, Dordrecht, 1981, p. 307. 15 H.I. Schiff and D.K. Bohme, Astrophys. J., 232 (1979) 740.