Ion-Molecule Reactions in Methyl Fluoride and Methyl Chloride

Transcription

1 Ion-Molecule Reactions in Methyl Fluoride and Methyl Chloride A. A. HEROD,' A. G. HARRISON: AND N. A. MCASKILL~ Department of Chemistry, University of Toronto, Toronto 181, Ontario Received January 22, 1971 The reactions of the molecular ion have been studied as a function of the ion kinetic energy for methyl fluoride and methyl chloride. The following reactions are observed For methyl fluoride (X = F) reactions c and d have kinetic energy thresholds and become significant at high ion energies. For CH3C1 (X = C1) reaction a is not observed and reactions c and d are of only minor importance at high ion energies. Rate coefficients for the molecular ions and a number of fragment ions as well as rate coefficients for further reaction of CH4X+ are reported. On a ttudie les reactions de I'ion moleculaire des fluorure et chlorure de mkthyle en fonction de l'tnergie cinktique de I'ion. On a observe les reactions suivantes Les reactions c et d, dans le cas du fluorure de mkthyle (X = F), ont des energies cinetiques minima et ne deviennent importantes qu'a hautes energies des ions. Dans le cas du chlorure de methyl (X = C1) on n'observe pas la reaction a et I'importance des reactions c et dest negligeable a hautes energies des ions. On rapporte les coefficients de vitesse des ions moleculaires et d'un certain nombre d'ions fragmentes ainsi que les coefficients de vitesse d'autres reactions de I'ion CH4X+. Canadian Journal of Chemistry, 49, 2217 (1971) Introduction The ion-molecule reactions forming CH,CI+ in CH3Cl have been studied previously at low electron energies both by the zero-field pulsing technique (1) and by pressure variation (2). More recently McAskill (3) has reported a detailed study of the ionic reactions in methyl chloride at ev ionizing energy and ion exit energies ranging from 0.2 to 2.0 ev. In this pressure study the major reactions occurring have been elucidated and the energy dependence of the reactions investigated. A similar, but briefer, study of the methyl fluoride system also has been reported by McAskill (4), while the reactions of the molecular ion have been studied at thermal ion energies using ion cyclotron resonance techniques by Marshall and Buttrill (5). The present study of the ionic reactions in methyl fluoride and methyl chloride can be divided into three parts. Pressure studies at ev electron energy have been carried out; the results from these studies confirm the earlier results of McAskill (3, 4). In each system the reactions of the molecular ion have been studied in detail at low electron energies and the product distribution determined as a function of ion energy. Finally, the ionic reactions have been investigated using the ion-trapping technique reported - - recently (6). Experimental 'Present address: British Coal Utilization Research The pressure studies, both at low and high electron Association, Leatherhead, Surrey, England. energies, were carried out using the medium pressure 'To whom correspondence should be addressed. instrument described previously (2, 7). The ionizing 3Present address: Australian Atomic Energy Com- electron beam was pulsed and ion source residence times mission, Lucas Heights, NWS, Australia. were measured directly by the deflection technique (2).

2 2218 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 At low electron energies, pressure studies were made at various repeller voltages covering the range of ion exit energies detailed in the following. In each run the electron energy was adjusted to give the maximum possible fractional yield of the molecular ion at low pressures. For methyl chloride CH3CI+ was > 95% of the total ionization at low pressures, however, for methyl fluoride the CH2F+ fragment ion amounted to -20% of the total ionization at low pressures since a.p.(chzf+) = ev (8) is relatively close to i.p.(ch3f) = ev (9) (where a.p. = appearance potential, i.p. = ionization potential). The remaining pressure studies were carried out at 50 or 70 ev electron energy and ion exit energies as detailed in the following. Source pressures were measured directly using an MKS Baratron micromanometer. The ion-trapping studies were carried out using the instrument and techniques previously described (6) and source concentrations in the region of 1 x IOl3 molecule cm-3. The source pressure was not measured directly but was related to the measured sample system inlet pressure through a study of the reaction CH4+ + CH, in methane. Consequently the rate coefficients reported for the iontrapping experiments are relative to the value of 1.20 x cm3 molecule-' s-' (2) for the methane reaction. Methyl chloride was obtained from Matheson and Co. while methyl fluoride was obtained from Columbia Organic Chemical Co. In each case samples were distilled bulb-to-bulb in vacuo and a middle fraction retained for use. The mass spectra showed no detectable impurities. Results and Discussion Methyl Fluoride A typical pressure plot obtained at 3.3 ev ion exit energy and low electron energy is shown in Fig. 1. It is evident that at this ion energy CH,', CH2F+, CH4F+, and C2H4F+ are products of the reaction of CH,F+ while C2H,F+ is a higher-order product originating from the reaction The reaction of CH,F+ to produce both C2H4F+ and CH4F+ as well as the subsequent reaction of CH4F+ by reaction 1 have been reported previously (4, 5). At higher conversions some of the CH2F+ also may originate from reaction of CH4F+ as suggested previously (4). From A plots (7, 10) we have obtained the initial fractional yields at ion exit energies ranging from 0.73 to 3.3 ev. These fractional vields are plotted in Fig. 2 as a function of the average ion energy B = Eel, (11) where E, is the ion exit energy. Also shown in Fig. 2 are the fractional yields at thermal energie~re~orted by Marshall and Buttrill (5) and the yields obtained from the ion-trapping studies at low electron energies (see below). The yields of CH,F+ and C2H4F+, both PRESSURE (rntorr) FIG. 1. Pressure plot for CH3F at low electron energy. formed in presumably exothermic reactions, decrease with increasing ion energy while the yields of CH2F+ and CH,' increase with ion energy and have an apparent kinetic energy threshold (ill-defined for CH2F+) consistent with formation of these products by endothermic reactions. Reactions 2 to 4 are possible reactions for formation of CH2F+ Although the energy threshold for formation of CH2F+ is low this product is not observed at thermal ion energies indicating that the reaction must have a significant energy barrier and suggesting reaction 2 as the most probable reaction. This reaction is analogous to the collisioninduced decomposition of CH4+ responsible for CH,' formation in methane at higher ion kinetic energies (12).

3 HEROD ET AL.: ION-MOLECULE REACTIONS IN CH3F AND CHsCl 2219 TABLE 1. Rate coefficients in methyl fluoride system Average ion energy (ev) C + CH+ Rate coefficient (cm3 molecule-' s-') x lo9 CH2+ CF+ CHF+ CH3F+ CH4F+ Reference This work This work..- This work , This work This work ' ' 2.20 This work , 2J1 1.7, , This work 4.45* 0.3, 1.1, 1. g This work Thermal *Trapped-ion experiments. L ' ~ ' l ' l ' l 1 I ' l AVERAGE ION ENERGY (ev) FIG. 2. Fractional yield us. ion energy for CH,F system. Reactions 5 to 7 are possible reactions leading to CH,' formation. Although reaction 7 is essentially thermoneutral, the threshold behavior indicates that there is a significant energy barrier in the reaction forming CH,+. No clear decision concerning the reaction occurring can be made. The rate coefficients for disappearance of the CH3Ff primary ion obtained from the pressure studies at low electron energy are recorded as the first five entries in Table 1. Pressure studies were also carried out at 70 ev electron energy and ion exit energies of 0.78 and 1.9, ev. The results obtained with respect to the reactions occurring were in agreement with the conclusions reached by McAskill (4). The disappearance rate coefficients for the molecular ion and several fragment ions are listed in Table 1. Figure 3 shows typical results obtained in the study of the methyl fluoride system at low electron energies using the ion-trapping technique. The fractional yields of the products, with the exception of CH,+, are shown in Fig. 2 as the data points at 0.45 ev ion energy. Because of mass discrimination effects the low yield of CH,+ could not be determined with any accuracy. The CH4F+ ion reacts quite rapidly, primarily to produce C2H,Ff but may also produce CH2F+ as suggested by McAskill (4). Both C2H,Ff and C2H4F+ are unreactive with CH3F at the pressures used while CH2Ff reacts only very slowly to produce CHF,' and C2H,F2+. Difficulties were encountered in studying the reactions of fragment ions by the trapping technique because it was observed that several of the fragment ions (Cf, CHf, CH,', CH,') were not completely trapped, presumably because they are formed with initial kinetic energy. Consistent with this reasoning was the observation that these ions also had broad shallow ion source residence time profiles on the medium pressure instrument. The kinetic data which could be obtained reliably from the trapping experiments is summarized as entry 8 in Table 1.

4 2220 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 change (i.e. no reaction or charge transfer). The increase of the disappearance rate coefficient undoubtedly reflects the effect of the ion kinetic energy on the competition between the reactive and the non-reactive or charge-transfer reaction channels. Several of the fragment ions, notably C', CH', and CH,', also appear to have rate coefficients which are energy dependent. DELAY TIME (rns) FIG. 3. Normalized ion intensities us. reaction time for CH3F system. The final two entries in Table 1 present the kinetic data obtained by McAskill(4) from pressure studies at 70 ev electron energy and the thermal energy results of Marshall and Buttrill (5). The agreement between the present results and the earlier data is quite satisfactory; it is particularly encouraging to note the excellent agreement between the trapped-ion results and the i.c.r. results for reaction of CH4F+. The rate coefficient for disappearance of CH3F+ clearly increases with increasing ion energy in contrast to the results for many simple systems (2) where the rate coefficient is independent of energy and in contrast to theoretical predictions. For an ion-induced dipole interaction as for an ion-averaged dipole interaction the collision rate coefficient should be independent of ion energy, while for an ion "lockedin" dipole interaction the collision rate coefficient should decrease with increasing ion energy (2). It should be noted that the experimental rate coefficient is considerably less than the calculated rate constant of 3.3 x lop9 cm3 molecule-' s-' (4) based on ion-averaged dipole interaction model (3), indicating that an appreciable fraction of the collisions lead to no detectable chemical Methyl Chloride Figure 4 shows a typical pressure plot obtained at low electron energy and 3.3 ev ion exit energy. The major reaction of CH3Cl+ results in formation of CH4Cl+ which subsequently reacts to form C2H,Cl+. CH2C1+ and CH3+ are formed in low yield only in contrast to the methyl fluoride system at the same ion exit energy (cf. Fig. 1) where the analogous products are formed in substantial yield. No C2H4Cl+ product, comparable to C2H,Ff in methyl fluoride, was observed. Table 2 records the fractional yields obtained from A plots as a function of the average reactant ion energy. Comparison of the results with those obtained for methyl fluoride (Fig. 2) clearly shows the much lower yield of the CH2X+ and CH3+ products. This undoubtedly results from the fact that the possible reactions producing these species in the methyl chloride system, reactions 8 to 13, Are much more endothermic than the analogous reactions 2 to 7 in the methyl fluoride system. TABLE 2. Fractional yields for reaction of CH3CI+ Average ion energy (ev) CH, + CH2CI+ Fractional yield CH,CI +

5 , HEROD ET AL.: ION-MOLECULE REACTIONS IN CH3F AND CH3Cl 2221 TABLE 3. Rate coefficients in methyl chloride system Rate coefficient (cm3 molecule-' s-l) x lo9 Average ion energy (ev) C+ CH+ CH2+ CH3+ C1+ HCI+ CCl+ CHCI+ CH3CI+ CH4CI i 1.5i o i * *Trapped-ion experiments. PRESSURE (mtorr) FIG. 4. Pressure plot for CH3Cl at low electron energy CH3+ + Cl2 + CH3 AH = + 73 kcal/mol CH3+ + HCI + CHZCI AH = +49 kcal/mol Pressure plots were also carried out at 50 ev electron energy and ion exit energies in the range 0.63 to 3.3 ev. The general features of these plots and the reaction assignments are in agreement with those reported earlier by McAskill (3). The disappearance rate coefficients obtained for a number of ions are summarized in Table 3. The rate coefficient for disappearance of CH,Cl+ was found to be independent of ion energy, in contrast to the CH,F results, and yielded an average value of 1.5, x cm3 molecule-' s-' in good agreement with the CH,CI+ decay rate of 1.66 x cm3 molecule-' s-' obtained by McAskill 0. 1 I l l l l l l l l l l DELAY TIME (ms) FIG. 5. Normalized ion intensities us. reaction time for CH3CI system. (3). The low electron energy pressure plots yielded rate constants in agreement with the high electron energy data. As in the methyl fluoride system we observed that several of the low mass ions were formed with initial kinetic energy and were not completely trapped in the electron space charge. Consequently rate coefficients for disappearance of only a few ions could be determined. Figure 5 shows a typical plot obtained at 12 ev nominal electron energy, while the rate coefficients obtained from the trapping experiments are shown as the final entry in Table 3. Several points should be noted concerning the

6 2222 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 methyl chloride results. At the ion energies of the 1. A. G. HARRISON and J. C. J. THYNNE. Trans. the yield of Faraday Soc. 64, 1287 (1968). CH2C1f is very 2. S. K. GumA, E. G. JONES, A. G. HARRBoN, and low and can be for J. J. MYHER. Can. J. Chem. 45, 3107 (1967). reaction of CH,'. This indicates that the reaction 3. N. A. MCASKILL. AUS~. J. Chern. 22, 2275 (1969). 4. N. A. MCASKILL. Aust. J. Chem. 23, 2301 (1970). [14] CH,Cl+ f CH3Cl + CH,Cl+ f HCl f CH4 5. A. G. MARSHALL and S. E. BUTTRILL. JR. J. Chern. suggested by McAskill (3) is not significant at Phys. 52, 2752 (1970). 6. A. A. HEROD and A. G. HARRISON. Int. J. Mass these energies. McAskill has presented evidence Spectrom. Ion Phys. 4, 415 (1970). that reaction 14 is endothermic with the rate 7. A. A. HEROD and A. G. HARRISON. J. Phys. Chem. increasing with ion energy. Our A plots at high 73, 3189 (1969). ion energies indicated a tertiary formation of 8. R. H. MARTIN. F. W. LAMPE. and R. W. TAFT. J. Am. Chem. Sbc. 88, 1353 (1966). CH2Clf which supports this conclusion. Neither 9. D. C. FROST and C. A. MCDOWELL. Proc. Roy. in the pressure studies nor the trapped-ion Soc. London, A241, 194 (1957). studies was there any evidence that CH2Clf 10. J. J. MYHER and A. G. HARRISON. J. Phys. Chern. reacted to form CH,Clf. This result is in agree- 72, 1905 (1968). ment with McAskill but in disagreement with 11. P. WARNECK. J. Chern. Phys. 46, 513 (1967). 12. F. P. ABRAMSON and J. H. FUTRELL. J. Chern. earlier ratio plot studies (1) in this laboratory Phys. 45, 1925 (1966). which must be considered erroneous. The authors are indebted to the National Research Council of Canada for continued financial support.