Reaction of C + ( 2 P) with CH3FPatricia Sullivan Wilson, R. W. Rozett, and W. S. Koski Citation: The Journal of Chemical Physics 53, 1276 (1970); doi: 10.1063/1.1674128 View online: http://dx.doi.org/10.1063/1.1674128 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/53/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Bimolecular reaction of CH3 + CO in solid p-H2: Infrared absorption of acetyl radical (CH3CO) andCH3-CO complex J. Chem. Phys. 140, 244303 (2014); 10.1063/1.4883519 Dynamics of the C/H and C/F exchanges in the reaction of 3P carbon atoms with vinyl fluoride J. Chem. Phys. 139, 064311 (2013); 10.1063/1.4817780 Coupled potential energy surface for the F(2P) + CH4 → HF + CH3 entrance channel andquantum dynamics of the CH4·F− photodetachment J. Chem. Phys. 139, 014309 (2013); 10.1063/1.4812251 Multiphoton ionization spectra of radical products in the F(2 P)+ketene system: Spectralassignments and formation reaction for CH2F, observation of CF and CH J. Chem. Phys. 87, 4546 (1987); 10.1063/1.452867 Rate constants for the reaction of CH3 with N2F4 and NF2 J. Chem. Phys. 59, 4357 (1973); 10.1063/1.1680633

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

Downloaded to IP: 129.22.67.107 On: Sun, 23 Nov 2014 14:19:26

THE JOURNAL OF CHEMICAL PHYSICS VOLUME 53, NUMBER 3 I AUGUST 1970

Reaction of C+ (2p) with CHaF*

PATRICIA SULLIVAN WILSON, R. W. ROZETT,t AND W. S. KOSKI

Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218

(Received 20 April 1970)

The ion-molecule reaction C+(ZP) +CILF has been investigated in the 2-200-eV energy region using a tandem mass spectrometer. All possible ions containing one and two carbon atoms were observed except CzILF+. The behavior of cross section as a function of the kinetic energy of the incident ion was used to infer reaction mechanism. Hydride ion, :fluoride ion, and :fluorine atom pickup were the dominant reactions at low energies.

INTRODUCTION

Recently! we had the occasion to study the reaction of C+ (2P)+CH4 in the 2-200-eV energy region using a tandem mass spectrometer. All possible ions contain­ing one or two carbon atoms were observed except C2H4+. The behavior of the cross section as a function of the kinetic energy of the incident ion was used to infer reaction mechanism. One of the important mecha­nisms proposed was hydride ion transfer to form CH and CH3+. Hydrogen-atom pickup, to form CH+, was also found to be present but with much smaller proba­bility than hydride ion pickup. The CHa transfer to form C2Ha+ was also observed; however, no carbanion (CRa -) transfer was observed since this latter reaction was not energetically favored in the case of CH4. It was decided to carry out a similar study with CHaF since this system would permit an examination of F­and F transfer as well as H- and Hand CHa transfer mechanisms and hence would confirm and extend the mechanistic conclusions arrived at in the CH4 case.

EXPERIMENTAL

The tandem mass spectrometer used in this study has been described previously.2,a It consists of two mag­netic mass spectrometers in series. The first is a 180° magnetic sector instrument with a 1-em radius of curvature. The secondary mass spectrometer, which analyzes the ionic products extracted from a reaction chamber, is a 60° magnetic sector instrument with an 8-in. radius of curvature. The pressure measurements in the reaction chamber were made with an MKS Barytron Type 77H-1 pressure meter. Detection is made with a 17 -stage electron multiplier. Individual ion counting techniques were used. The C+ ions were produced by electron bombardment of CO.I The CHaF and the carbon monoxide that were used in these studies were Matheson's CP products.

RESULTS AND DISCUSSION

ing energy, it was possible to produce the C+ in the ground state (2P).1 So in this study we are concerned with the reaction of C+ (2 P) with methyl fluoride.

The cross sections for ionic products containing one carbon atom are illustrated graphically in Fig. 1. Refer­ring to the curve for CHaF+, the recombination energy of C+ (2P), which is pertinent to this discussion, is 11.26 eV.4 This is considerably lower than the ioniza­tion potential of CHaF (12.84 eV), so on the basis of the adiabatic hypothesis5 the cross section for this reaction would be expected to appear above its thresh­old energy of 2.14 eV (lab) and rise with kinetic energy. Figure 1 shows that the CHaF+ cross section appears just below 6 eV, reaches a maximum of somewhat under two 12 in the region of 140 eV, and then slowly falls with energy. This is the expected behavior of a nonresonant charge-transfer reaction.

The ions CH2F+ and CHa+ have cross sections which are among the highest observed in this study. The shapes of the curves (Fig. 1) are typical of exothermic ion-molecule reactions. Consequently the formation of these ions cannot be the result of dissociative charge transfer reactions which are endothermic by 2.97 and 3.74 eV, respectively (Table I). As in the case of C+ with CH4,l the most likely mechanism for the formation of CH2F+ and CHa+ is hydride and fluoride ion transfer, respectively. The exothermicities of both of these re­actions (Table I) are compatible with the observations. Furthermore, in both of these cases our experimental measurements indicate very little forward momentum for the CH2F+ and CRa+ ions. This is to be expected if anion pickUp is the primary mechanism since the remaining ion which is observed is considered a specta­tor in the terminology used in discussing stripping re­actions. It is of interest to compare the cross sections for the production of CH3+ in the methane and methyl cases. In the case of CHaF the CHa+ can be produced by only one mode, namely, the extraction of F-, and in the case of CH4 the methyl ion can be produced by the extraction of anyone of four possible hydride ions.

By using the proper gas pressure in the primary mass One would expect the observed cross sections for the spectrometer and/or by varying the electron bombard- production of CHa+ in the CHaF and CH4 cases to be

1276

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

Downloaded to IP: 129.22.67.107 On: Sun, 23 Nov 2014 14:19:26

REACTION OF C+ (2P) WITH CHaF 1277

in the ratio of one to four, respectively, if the probabili­ties of F- and H- extraction are the same in the re­spective cases. This appears to be the case since the bond refractions of C-H (1.69 cc) and C-F (1.72 cc) are about the same. A similar comparison can be made of the production cross sections of CH2F+ and CHs+ in the CHsF case. The situation is qualitatively similar, however, the quantitative agreement between the ob­served and expected ratios is not as good as in the CHsF -CIL cases.

Next let us consider the yield of the ions CHF+ and CH2+' The situation here is complex since these ions could be conceivably produced by dissociative charge transfer, H2 and HF pickup, H2- and HF- pickup, or by dissociative hydride ion transfer. The dissociative charge-transfer process can be ruled out by considera­tions similar to that used for CHs+ and CH2F+. Both CH2+ and CHF+ appear below the expected threshold for this highly endothermic reaction. The reactions representing the pickup of H2 or HF, H2- or HF- at low energies are approximately thermoneutral (Table I) if the thermodynamic data are correct, and these mecha­nisms cannot be ruled out on the basis of our data in the low-energy region. One can probably rule out H2 or HF pickup to form CH2+ and CHF+ in the high-

4.0

3.0

2.5

60 80 100 100 150 200

ION ENERGY (eV)

FIG. 1. Cross sections for the production of CH2F+ (0), CHaF+ (.6.), CH3+ (e), CHF+ (D), CH2+ (.), CF+ (0), and CH+ (.) for the reaction C+ (2P) +CilaF.

TABLE I. Heats of reaction" for C+ (2P) with CilaF.

Products

CHaF++C CH2F++CH CH2F++C+H CHa++CF CHa++C+F CH2++CH+F CHF++CH+H CHF++CH2

CH2++CHF CH++CH+HF CH++CH2F CH++C+H2+F CF++CH+H2

CF++CHa CF++C+H2+H C2Ha++F C2H2++HF C2H++H2+F C2++H2+HF C.H2F++H C2HF++H2

C.F++H2+H

Reaction energy

-1.58 +0.54 -2.97 +1.56 -3.74 -5.66 -4.72 -0.32

0.01 -4.89 -1.23 -9.77

(-7.14)b (-2.40)b (+2.2)0

(-10.65) b 3.30 4.84

-2.34 -2.01

2.95 2.39

-2.76

" All heats of reaction are calculated from the heats of formation found In Ref. 6. Positive values are exothermic.

b These values are based on the CF+ bond energy obtained from Ref. 6. o This value based on CF+ bond energy obtained from Ref. 9.

energy region since these processes would have critical energies and the cross sections would be expected to fall with energy above 30 or 40 eV. This is not the case (see Fig. 1). Considerations of simplicity, the parallel behavior of CH2+ and CHF+ cross sections at higher energies, and the analogy with the methane case lead one to postulate a dissociative anion pickup mechanism at the upper end of our energy scale. Another reason to choose dissociative anion pickup over H2 or HF pickup is the higher cross section for CHF+ compared to CH2+' Statistically H2 pickup should be favored over HF pickup, yet the reverse order is observed for the cross sections. On the other hand, since there is more CH2F+ than CHs+, more of the former can dissociate to CHF+ than the latter to CH2+' Continuing the argument, more CF+ would be expected than CH+ and this appears to be the case.

The CH+ ion may arise from three reaction paths: dissociative charge transfer, H-atom pickup, and dis­sociative hydride ion pickup. The:low cross section of this reaction prevented us from determining the thresh­old which would be expected to be at 1.66 eV if the available thermodynamic data are accurate. The critical energy for H-atom pickup is 63 eV (laboratory) which argues against a stripping mechanism in the high­energy region. Examination of Fig. 1 indicates a striking

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

Downloaded to IP: 129.22.67.107 On: Sun, 23 Nov 2014 14:19:26

1278 WILSON, ROZETT, AND KOSKI

O"(A')

1.0

.10

o

• -. -...

60 100

ION ENERGY (eV)

FIG. 2. Log cross section vs log ion kinetic energy for the production of C2H£: (D), C2H3 + (0), C.H+ (.6.), and C2HF+ (0) for the reactIOn C+('P)+CH3F. The sum of the cross section~ of. all i?ns produc~d for t~is reaction is shown bye, and the sohd hne glves the GIOumouS1S and Stevenson cross section.

similarity between CHs+, CH2+, and CH+ cross sections in the energy region above about 40 eV suggesting a common mechanism. Hydride ion pickup followed by dissociation is a common mechanism which would be compatible with all of the experimental facts. It is, however, not possible to rule out H-atom pickup at low energies.

The case of CF+ is an interesting one. The cross sec­tion falls as the energy increases and shows a typical behavior for an exothermic ion-molecule reaction. This behavior would rule out charge-transfer dissociation and dissociative anion pickup since both of these re­actions would be highly endothermic. It would appear on the basis of NSRDS-NBS 26 data6 that F-atom pickup would also be endothermic (Table I). This heat of reaction was calculated from the ionization potential of carbon (11.26 e V), the bond energy of CF (5.3 e V) , and the ionization potential of CF (13.8 3V).7 These numbers would give a bond energy of 2.78 eV for CF+. This is an unreasonable value since CF+ is isoelectronic with N2• For example the bond energy of BF which is isoelectronic with N2 is 7.9 eV.8 One would expect CF+ to have roughly the same value for the bond energy. Indeed, Hastie and Margrave9 report 5.0, 8.9, and 7.3

for the bond energy of CF, ionization potential of CF, and the bond energy of CF+, respectively. If one uses these data one arrives at a t::.H= +2.2 eV making the reaction exothermic as would be expected from the results obtained in this study. One may therefore con­clude that at low energies CF+ is being produced by F­atom transfer. At higher energies the dissociative hy­dride ion pickup is the most likely mode for the forma­ti.on of CF+ since if this ion were produced by spectator pIckup of the F atoms, its critical energy would be about 8 eV and hence would not be expected to exist at energies higher than this.

It is of interest to compare the low-energy cross sec­tions for the formation of CHs+ and CF+ since these processes correspond to F- transfer and F -atom trans­fer, respectively. The cross section for F- transfer is about 4-5 times higher than that corresponding to F­atom transfer. This is qualitatively compatible with electrical nature of the reaction mechanism. As the C+ ion approaches the F end of CHsF, the high electric fi.eld polarizes the molecule so that transfer of the nega­tive end (F-) to C+ to form CF and CHs+ occurs more readily than the corresponding neutral F transfer.

A similar consideration applies when the C+ ap­proaches the methyl end of CHsF. In this case, the production of C-CHa+, corresponding to an abstraction of a neutral CHs, is favored simply because the reaction corresponding to CHa- abstraction is not energetically feasible:

C+ + CHaF ---l CCHa+ F+, t::.H= -5.28 eV.

Likewise the probability of the production of CH2F+ which results from H- transfer, is very much highe; than the probability of CH+ formation which may cor­respond to H-atom transfer.

The second category of reactions is that containing two carbon atoms with the general formulas, C2H.,,+ (x=3, 2,1,0) and C2H~F+ (x=2, 1,0). The cross sec­tions of CCHa+, CCH2+, and CCHF+ show the typical behavior of exothermic ion-molecule reactions, whereas CCH+ and CC+ give curves indicative of endothermic reactions. The data are illustrated in the log-log form in Fig. 2. The sums of the cross sections of all ions ob­served in the reaction C++ CHsF and the Gioumousis and Stevenson curve are included for comparison.

Assuming that CCHs+ has the same heat of formation as the mass-27 ion formed by photoionization of ethyl­ene, one may write the equation for its production as

t::.H=3.30 eV.

Similarly the reaction to form CCH2+ is

C+ (2P) + CHsF---lC-CH2++HF, t::.H=4.84 eV,

if the mass-26 ion has the same heat of formation as the

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

Downloaded to IP: 129.22.67.107 On: Sun, 23 Nov 2014 14:19:26

REACTION OF C+ (2P) WITH CHaF 1279

same mass ion produced by electron bombardment of acetylene and if HF is also formed. If a stripping mecha­nism is assumed and if it is further assumed that all of the excitation energy goes into the carbon-carbon bond, both of these reactions have a critical energy of about 2 eV. At energies exceeding this value the product ion might be expected to decompose into CHa+, CH2+, etc. However, it is more likely that the excitation energy is distributed over all of'the bonds of the ionic product. This would permit the ionic products to persist to higher projectile ion kinetic energies. Other factors that may contribute to the presence of ions at kinetic energies higher than expected on the basis of the strip­ping picture are binary collision mechanisms of the type suggested by Bates.1O Instrumental factors such as a spread in beam energy could also contribute to an ap­parent behavior of this type. At any rate if the excita­tion energy is spread over all of the bonds in an ion such as C-CHa+, a likely mode for its further frag­mentation, as the internal energy increases, is the breaking of hydrogen bonds to produce ions such as CCH2+, CCH+, etc. This suggests that this category of two carbon ions may be related through such a dis­sociative process.

The reaction to produce CCH+ is

AH= -2.34 eV,

assuming that the CCH+ ion has a structure like the ion with corresponding mass produced from electron bombardment of acetylene. These calculated exo- and endothermicities are in harmony with experimental ob­servations.

Finally the family of ions, C2H2F+, C2HF+, and C2F+ was observed corresponding to the ions C2Ha+, C2H 2+, and C2H+ observed in the reactions of C+~ with CIL and CHaF. The most prominent ion in this series is

C2HF+ and its cross section plot is-shown in Fig. 2. The reaction represents an exothermic process closely parallel to the analogous reaction for the formation of C2H2+. The intensities of the other ions in this series, i.e., C2H2F+ and C2F+ were considerably lower than that of C2HF+ and were not plotted in Fig. 2.

The predictions of the Gioumousis andfStevensonll

ion-induced dipole mechanism is shown as a straight line in Fig. 2. The sum of the cross sections for all of the ions observed for the reaction c+ (2 P) +CHaF is also shown in the same figure. Just as in the methane case the agreement between the GS cross sections and the sum of all cross sections is surprisingly good at low energies. The agreement between the slopes is probably indicative that the 1/r4 potential is approximated in this case at low energies.

* This work was done under the auspices of the U.S. Atomic Energy Commission.

t Present address: Department of Chemistry, Fordham Univer­sity, Bronx, N.Y.

1 P. S. Wilson, R. W. Rozett, and W. S. Koski, J. Chern. Phys. 52, 5321 (1970).

2 E. R. Weiner, G. R. Hertel, and W. S. Koski, J. Am. Chern. Soc. 86,788 (1964).

3 R. W. Rozett and W. S. Koski, J. Chern. Phys. 49, 2691 (1968).

• E. Lindholm, Advan. Chern. Ser. 58,1 (1966). 6 H. S. Massey and E. S. Burhop, Electronic and Ionic Impact

Phenomena (Oxford U. P., Oxford, England, 1952). 6 J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron,

K. Drazl, and F. H. Field, Nat!. Std. Ref. Data Ser., Nat!. Bur. Std. (U.S.) 26, (1969).

7 R. 1. Reed and W. Smedden, Trans. Faraday Soc. 54, 301 (1958) .

8 D. L. Hildenbrand and E. Murad, J. Chern. Phys. 43, 1400 (1965) .

9 J. W. Hastie and J. L. Margrave, Fluorine Chern. Rev. 2, 77 (1968) .

10 D. R. Bates, C. J. Cook, and F. J. Smith, Proc. Phys. Soc. (London) 83,49 (1964).

11 G. Giournousis and D. P. Stevenson, J. Chern. Phys. 29, 294 (1958).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

Downloaded to IP: 129.22.67.107 On: Sun, 23 Nov 2014 14:19:26