Nov., 1963
APPEARANCE POTENTIALS AND MASSSPECTRA OF FLUORINATED ETHYLENES
2463
APPEARANCE POTENTIALS AND MASS SPECTRA OF FLUORINATED ETHYLENES. I. DECOMPOSITION MECHANISMS AND THEIR ENERGETICS’ BY CHAVALIFSHITZ~ AND F. A. LONG Department of Chemistry, Cornell University, Ithaca, New York Received J u n e 11, 1963 Appearance potentials and mass spectra have been determined for a group of fluorinated olefins. The reactions of the positive ions are similar to those of the hydrocarbon analogs in that there are frequent losses of Hz or H F to form acetylene ions. Rearrangement processes, involving atom migration, occur more frequently in the more highly fluorinated compounds. For the most part the relative rates of the unimolecular decomposition processes vary with the energy demands, but frequency factors are occasionally quite low for rearrangements and for four-center processes leading to acetylenic ions. The observed appearance potentials agree fairly well with values calculated from heats of formation of the species involved, but there are some exceptions. Ionization efficiency curves for some of the fragments from CFZCFH and CzFa show unusual features. Decomposition mechanisms are derived for all the olefins.
htroduction The mass spectra of fluorocarbons are of interest both for themselves and for comparison with the spectra of their hydrocarbon analogs. Comparison reveals that the positive ions of the two series of compounds undergo somewhat different sorts of chemical reactions. There tend to be larger amounts of rearrangements occurring with the fluorocarbons and the rearrangements are of different types. As one e ~ a m p l ethe , ~ species CF3+ is usually observed in high yield from fluorocarbons which themselves do not contain the CF3 group; in contrast CH3+ is not a common rearrangement product. The energetics of the flu0 rocarbon decompositions are also of interest both because they yield data of thermochemical importance and because they permit significant comparisons with theory to be made. I n contrast to the perfluoro paraffins, the mass spectra of the fluorinated olefins show considerable intensity of the parent ion peaks3s4; as a consequence, it is possible to measure the ionization potential of the molecules by electron impact. When data for the decomposition of hydrocarbon ions are analyzed by the statistical theory of mass spectraj6 it is commonly observed that the breaking of a C-H bond occurs with a fsiequency factor which is smaller by several orders of magnitude than that for the breaking of a skeletal C-C bond.6t6 This comparison suffers from the fact that the two sorts of bonds do not occupy equivalent positions in the molecule with, as a consequence, different structures for the activated complexes.’ By studying a partly fluorinated hydrocarbon or by comparing hydrocarbon-fluorocarbon analogs, one can obtain systems where the fluorines and hydrsgens occupy equivalent positions, permitting reactions for breakage of C-H and C-F bonds to be compared more directly. Both for these comparisons and for development of the decomposition patterns for the parent positive ions it is essential to have the energetics of the decomposition processes, i e . , appearance poten(1) This work was supported by the Advanced Research Projects Agency through the Material63 Science Center. ( 2 ) Department of Chemistry, Hebrew University of Jerusalem, Israel. (31 J. R. Maier, “Advances in Fluorine Chemistry,” Vol. 2, M. Stacey. J. C . Tatlow, and A. 0. Sharpe, Ed., Academlc Press, New York, N. Y.,pp. 55-103. (4) E”. L. Mohler, V . H. Dibeler, and R. M. Reese, J. Res. Notl. Bur. Std., 49,343 (1952). (5) H. M. Rosenstock, N. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring. Proc. Natl. Aend. Scz. U.S.,88,667 (1952). (6) L. Friedman, F. A. Long, and M. Wolfsberg, J. Chem. Phys., 80, 1605 (1959). (7) M. Wolfsberg, ehzd., 86, 1072 (1962).
tials are needed for all of the fragments produced in significant yield. Data are presented here for both the mass spectra and the appearance potentiaIs for the following compounds : CzH4, CHZCHF, CHZCFZ,CHFCF2, CzF4. Experimental Procedure and Results All the fluorinated ethylenes studied were shown by vapor phase chromatography to be better than 99.8% pyre. The compounds CHzCHF and CHzCPz were kindly furnished by Allied Chemical Co.; CFZCFHwas purchased from Penninsular Chem. Research, Inc., and CzFa was Bynthesized from CPzBrCFzBr by debromination with zinc in dioxane. Samples of CFzCFH and CZF4, purified by gas chromatography, were also kindly furnished by A. Fainberg, Pennsalt Chemicals Corp. Argon, neon, and ethylene were the best grade available from Matheson and were used without further purification. The data were taken on a Consolidated Engineering Corp. mass spectrometer, Model 21-401, which had been modified as described previously.* Table I gives the spectra (in terms of percentage yields of the various ions) of several fluoroethylenes. These were obtained with 75-v. electrons, 10-fia. current, and an accelerating voltage of 210 v. The values are rounded off to the nearest O.l%, and peaks below this value are usually omitted. Doubly charged ions are not included. The agreement with previously published results on C2H4, CH2CF2,and CzF4is good, except for a frequent smaller degree of decomposition than observed by other workers. This is especially apparent for CZF4, where CZF3+ is the highest peak in the present spectrum, TABLE
1
MASSSPECTRA OF FLUOROETHYLENES Species
CHp + CH3+ CzH + CzH2 CZHB CzH4+ C F+ CFH + +
+
CHrCHs
CHnCHl?
CHzCFg
0.9 0.1 4.1 24.7 26.8 41 .ap
0.6 0.2 1.8 9.9 11.4
2.6
2.8 0.9 0.1 10.4 24.9 34.5p
13.1
CHFCFp
CFaCFr
15.2 5.7
28.6
1.1
12.4 9.2 16.7 0.9 0.3
4.8 35. 4p
1.8 1.7
10.6
16.0 1.0
0.3
29.7 0.4 26. 2p
1.3 37.3
20.4, (8) A. B. King and F. A. Long, ibid., 29,374 (1958).
2464
CHAVALIFSHITZAKD F. A. LONG
Vol. 67
TABLE I11 Ionizations Potential of Moleclues
Current (arbitrary units) I
Ionizing Electron Energy ( a r b i t r a r y units)
Fig. 1.-Ionization
Molecule
IP, e.v.
Method
Rsf.
Cz& CZH4 CzH, CHzCHF CHzCHF CHzCHF CHzCFz CHzCFz CHzCFz CHFCFz CHFCFz CZF4 CzF4 CZF4
10.51 10.58 10.66 10.36 10.31 10.45 10.33 10.30 10.45 10.14 10.33 10.12 9.3 10.12
Spectroscopy Electron impact Electron impact Photoionization Photoionization Electron impact Photoionization Photoionization Electron impact Photoionization Electron impact Photoionization Electron impact Electron impact
U
efficiency curves of CFf from three fluorinated ethylenes.
whereas CFf is the highest in the spectrum of Dibeler and coworkers.4 Appearance potentials were determined by the vanishing current method, using argon or neon as calibrating gas, in the same way as has been discussed previously.8~9 Table I1 summarizes the appearance potentials for the main peaks of the fluoroethylenes. The values are averages of from three to five runs for each species. The standard deviations are in the order of from 0.05 to 0.1 e.v. for the more abundant ions.
TABLE I1 APPEARANCE POTENTIALS FOR FLUOROETHYLENES IN v OLTS
T T
CH2CH:
CKzCHF
19.0 13.2 14.06 10.66
13.73 14.38 15.43
14.04 14.02 10.45
CHaCFe
CHFCFz
(2F2CFz
17.8 19.78
15.23 15.Og 14.44 14.80
16.67 10.45
15.2 15.38
14.0~
20.0
19.28 14.22 14.83 16.13
15.13
13'54 I6.O0
10.33 10.12 Values for some of the ionization potentials and appearance potentials given in Table I1 have been reported previously. Table I11 gives a comparison of the earlier results with the present ones; agreement is quite good. The present electron impact ionization potentials are all 0.1-0.2 v. higher than the photoionization values, except for CzF4where the photoionization and the electron impact values are the same. h similar behavior has been observed for partially fluorinated benzene^.^ The semilog slope matching technique10.11 for determining appearance potentials was tried on several of the ions and the values obtained by this method usually agreed very well with those given in Table 11. However, it was not possible to match the semilog curves of CFf from either CzF4 or CHFCFi nor that of CF3+ from CzF4 with the respective calibrating Ari curves. Ionization efficiency curves for CF+ from different olefins are shown in Pig. 1. These were drawn so that the slopes of the linear parts are equal in all three cases. The CF+ curve from CHzCFa illustrates the expected behavior, the CF' curve - . ('4) R. R Hernecker and F.4 . Long. J . Phwi. Chem., 66, 1585 (1'831) (10) R. E. Honig, d . Ckem. Phys., 16, 105 (11)48). (11) S.N. Foner and R. L. IIudson, %but.. 26, 802 (1956).
b C
d e C
d
e C
e C
e
f C
Appearance Potentials of Fragments Parent molecule
Ion
Present AP, e.v.
Previous AP, e.v.
Ref.
19.0 19.3 g CZH4 CHz 13.2 13.5 g CZH4 CzHz 14.06 14.06 9 CZH4 C2H3 15.13 15.2 f CZF4 CFz+ W. C. Price and W. T. Tutte, Proc. Roy. SOC.(London), A174, 207 (1940). See ref. 9. Present study; results from Table 11. F. M. Matsunaga and K. Watanabe, private communication. e See ref. 13. f J. L. Margrave, J. Chem. Phys., 31, 1432 (1969). g See ref. 20. +
+
from CzF4 shows a break, while the CF+ curve from CFHCFZ shows a long tail. The appearance potentials of the ions showing "abnormal" ionization efficiency curves, namely CF-, CF3+(CzF4),and CF+(CHFCF2) were determined in the follow'ing way: the pressure of the gas studied and that of the argon were matched so that the Jirst parts of the ionization efficiency curves for the ion and for Sr+ coincided (contrary to the usual procedure of matching the linear partsa). The values thus obtained are the ones given in Table 11. Appearance potentials for these CF+ and CF3' ions are uncertain to a greater extent than is sho-cvnby the experimental spread because of the abnormal shapes of the ionization efficiency curves. A 60" sector mass spectrometer was used to look for negative ions in the CzF4 spectrum. The only detected species was F-. The ionization efficiency curve of F- indicated that an ion-pair process was occurring with an appearance potential of 14 f 1 e.v., where the energy scale was calibrated by means of 0- from the ion-pair process in CO. Independent experiments on the negative ions from CzF4 were carried out by Morrison and Dorman,12 who have verified the occurrence of an ion-pair process forming F-, the observed appearance potential being 14.3 e.v. They also observed three resonance capture processes forming F a t 2.7. 5.8. and 11.2 e.v. Two other negative ions were reported by them, Fz- and CzF-, both in very small yield. Mass spectral patterns of CzF4 and CHFCFz were determined at low electron voltages as a function of the ionizing voltage (Fig. 2 and 3). In Fig. 2 (V, - P ) is the ionizing voltage minus the ionization potential of the parent; in the case of CHFCFz the ionizing voltage was not calibrated. The ionization efficiency curves obtained for the fluoroolefins may be classified according to three types as previously explained.8 The parent ions (not shown in Fig. 2 and 3) all belong t o the type I curve, CF3- (from CZF4) and CFzH- (from CFHCFz) belong to type 11, while all the other fragments give type I11 behavior.
Discussion one goes from ethylene to perfluoroethylene (Table I), successively substituting the hydrogens by fluorines, the general features are: a gradual decrease in yields of acetylene ions (CL&+, C2HF+,or C2F2+); an increase in the yields of the rearrangeinelit products CH3+, CH.F+, arid CFd+; a r ~
The 75-v. Spectra.-As
(12) Personal cornrriuiiication fioni J. Y. hIorrison
Nov., 1963
h P P E A R A K C E P O T E N T I A L S AND
MASSS P E C T R A
increase in the yield of CF+. The compounds C H Z C F ~ and CHFCF2show low yields of the ion of the parentminus-one-hydrogen-atom ; a similar behavior has been observed in CeF4H2and CeF6H.3 There is also a gradual decrease in parent ion yield with increase in fluorine content and an increase in the yields of methyleneions, e.g., CH2+, CHF+, and CF2+. Ionization and Appearance Potentials.-The ionization potentials of the ethylenes decrease upon successive fluorination (Tables I1 and 111). This has previously been explained13as being the resultant of two opposing effects of the fluorine atom-an inductive effect which tends to increase the ionization potential of the molecule and a slightly larger resonance effect which stabilizes the molecular ion and thus tends to decrease the ionization potential. If this explanation for the trend in ionization potentials i s correct, then the structure of the molecule-ion in its ground state is planar with some double-bond character in the C-C bond. The ionization potentials measured by photoionization are the (adiabatic ionization potentials, while the ones measured by electron impact need not be and may more nearly reflect vertical “Franck-Condon” ionizations. The ionization potential of C2F4 is the same according to either photoionization or the present electron impact meaaurement (Table III), which suggests that the spatial configurations of the C2F4 molecule and molecule-ion are the same at the minima of their potential curve8. Usually electron impact measurements of ionization potentials give slightly higher values than photoionization and this is the case for the other members of the ethylene series. The ions of lowest, appearance potentials for the hydrogenated end of the ethylene series (Table 11) are the acetylene ions, formed from the parent by the split-off of HZor HF. For the more fluorinated members, the rearrangement, ions CFzH+, CF3+, as m7ell as CF+ lead to the lowest appearance potentials. The energy demand for the production of the CX2+ ions (CH2+, CHF+, and CFt+) decreases with fluorine content. On the other hand, t,he energy which is needed for the production of parent,-minus-one species increases with fluorine content; this again is similar to the trend observed for fluorobenzenes. The next section illustrates these features further. Mechanisms for Production of Mass Spectra of Fluoroethy1enes.-The proposed mechanisms for the breakdown of the ethylenes are based solely on energetic considerations. No metastable transitions have been observed and for these molecules isotopic labeling cannot add any further information. The acetylene ions C&Z+, where X is either H or F, usually have lower (appearance potentials than the parent-minus-one ions, so that they must be formed by a direct H2 or HF split from the parent. The ions CX3+ have also lower appearance potentials than the parentminus-one ions, so that they too appear to be formed by a direct rearrangement from the parent, a proposal which is confirmed by the low energy patterns (Fig. 2 and 3). The choice between a single-step and a twostep mechanism for the formation of CX+ or CX2+from the parent ion is based on similar considerations, supplemented by comparisons with available thermochemi(13) R. Rralsford, P. V. Harris, and W. C. Price, Proc. Roy. SOC.(London), A2S8,459 (1960).
O F FLUORIXATED
ETHYLENES
2465
%.
t
24
16 I4
-
Fig. 2.-Dependence
volts v.- 1‘. of fragment-ion yields on ionizing electron energy in CzF4.
%.
18
I o n i z i n g E l e c t r o n Energy ( o r b i t r o r y u n i t s ).
Fig. 3.-Dependence
of fragment-ion yields on ionizing electron energy in CHFCF,.
cal data on the fragments. Table IV summarizes some thermochemical calculations for processes which may lead to the observed CX2+ and other ions; the results are compared with the experimental energies. h few calculations are included for fragments which are actually absent from the spectrum. For instance, CHa+ (C2H4) comprises only 0.1% of the spectrum at 75 v. and its appearance potential could not be measured, although it can be calculated. The calculation is of interest for comparison since with the highly fluorinated ethylenes there is abundant rearrangement to form CX3+ ions. The heats of formation of hydrocarbons and their positive ions, used in the calculations of Table IV, are from a recent compilation.9 The heats of formation of C2F4,14CH2CFe,14CHFCH2,I5 CFHCF2,15CF,16 and C F P are from the recent literature. Other heats of formation come from the compilation of Rossini, et aZ.’* The recently measured IP (CH2) = 10.3919 was utilized for calculations involving the CH2radical. (14) C. 4. Neugebauer and J. I,. Margrave, J . Phys. Chem., 60, 1318 (1956). (15) P. G. Maslov and Y o P. Maslov, Khim. i Tekhnol. Topliva i Masel, 3, 50 (1958); Chem. Abstr., 53, 1910 (1959). (16) E. B. Andrew and R. F. Barrow, Nature, 166, 890 (1950); Proc. Roy. Soc. (London), A64, 681 (1951). (17) L. Brewer, J. L. Margrave, R. F. Porter, and K. Wieland, J . Phis. Chem., 66, 1913 (1901). (18) F. D. Rossini, D. D. Wagman, W. H. Evans, W. H. Levine, and I. Jaffe, Natl. Bur. Std. Giro. No. 500, U. S. Govt. Printing Office, Washington, n.c.,1952. (19) G. Herzberg, Can. J. PhUs., 39, 1511 (1961).
2466
CHAVALIFSHITLAND F. A. LONG
VoI. 67
TABLE IV Process
+ +
Thermochemical cycle
+
+
1. CZH4 CzHd CzHz Hz 4 CZHZ' H2 CZ&+ HZ 2. CzH4 4 CH,' 2CHs - 2H + 2CH2'4 CHe' CHz CHz CZH, 3. CzHs 4 CH2' CH f H CH H C2H4 4 CHz' 4. C2H4 CH,+ CH CzH, 4 CHg CH CHs+ CH CHzCHF --+. CzHz 5. CHzCHF CzHz' HF HF CzHz+ H F 6. CH2CHF C2H2' H F CHzCHF CiH2 H f F CzH2' H F 7. CHzCHF 4CzH3' F CHzCHF 4CzHd - H F -+a CzH,+ +F CH CH~CHF--+.CHzCFz+H-F-+'CHzF'+CF + H - F + C H z F + + C H 8. CHzCHF CHzF' 9. CHzCFz 4 CHz+ CFz CHZCF2 3 CHz' f CF2 10. CHzCFz CHz+ CF + F CHzCFz CHz' CF F CHzCFZ CHFCFz H - F4 ' CRF2' CF H F 11. CH2CFz 4CH CHFz' CH CHF2' 12. CHFCFs CH CF3' CHFCFz CzF4 H - F -+a CF8+ CF H - F *a CF3' CH 13. CHFCFz CzFa' H CHFCFn -+ CzF4 H - F +a C&'s' H The energy changes involved in these transitions are from Table 11. +
+ +
-+
-+
-+
-+
-+
-+
-+
-+
+
+
+
+
+ + + + + + + + +
-+
-+
-+
-+
+ +
+
-+
-+
+
-+
+ + + +
+
+
+ +
+
+
+
+
+
+ -
+
+
Calcd., AP, e.v.
Obsd. AP. e.v.
13.21 16.36 22.28 16.85 12.19 18.05 14.38 15.87 16.3 21.81 15.69 14.88 15.67
13.2 19.0 19.0
..
13.73 13.73 14.38
..
17.8 17.8
..
.. ..
HC1 from CZHaCl+ to give CZHZ+, it appears that the introduction of a halogen atom substantially modifies the character of the activated complex. Further indication of some unusual features of the transition states comes from the observation that production of C2HF+slightly exceeds that of C2H2+ even though the energy requirement for the former is 0.3 v. higher. It is conceivable that the production of CzHz+involves formation of Fion and indeed the energy requirement for formation from CH&HF,of C2H2+ H F- is very close to the observed appearance potential but there are difficulties. The process cannot go by initial decomposition t o CZH3+ F- since this would give a much lower appearance potential for CzHsf than is observed. The other alternative is simultaneous formation of the three products, a most unusual sort of reaction. Formation of CF+ must involve a rearrangement with simultaneous production of CH3 a t least a t the threshold, since a two-step process for its production necessarily requires improbably low values for the heat of formation of CFf, much lower than reported by previous workers.21122 The simple bond rupture processes whereby CH2CHF+loses either an H or an F atom give relatively larger amounts of decomposition in terms of the energy requirements than do the rearrangement or four center processes. This is the expected result; so is the fact that production of the lower energy species CzHzF+ is larger than that of C2H3+. However, this last result does indicate that the frequency factors for breaking C-H and C-F bonds in CH2CHF+ are rather similar, a somewhat unexpected result in view of the anomalously low factor for breaking C-H bonds in saturated hydrocarbon ions. CH2CF2.--n'eutral radicals are assumed to be formed along with the positive ions, since no negative ions mere observed for CH2CF~.23For this species, yields from four-center processes to give acetylenic ions are diminished relative to the more hydrogenated olefins (Fig. 6). So also are yields of simple decompositions to give parent-minus-one species, e.g., C2HzF+ and C2HF2+. Rearrangements to give CHzFf plus C F and to give CF+ plus CHzFare, however, much more probable, evidently due to lower energy demands. Further-
+ +
+
41.3 C2H4' AP = 10.66
3.40
26.8 + CzH3++ H
8.3
0.9
(21) R. I. Reed and W. Snedden, Trans. Faraday Soc., 64, 301 (1958). (22) J. W. C. Johns and R. F. Barrow, Proc. Roy. Soe., (London), 8 7 1 , 476 (1858). (23) R. RI. Reese, V. H. Dibeler, and F. L. Mohlei-, J. Res. N d . Bur. Std., 61, 367 (1956).
Nov., 1963
--I APPEARANCE POTENTIAL^
3.28
9.9
+
CzHz+
AND
+ HF
*
4.35
11.4 CzH8+
A P = 34.5 CHzCIIF+ 10.45
+F g G F + + Hz
24.9 CzHZF+
4.98 L
Fig. 3.--Mechanism
-+
2467
MASSSPECTRAOF FLUORINATED ETHYLENES
+H
16.7 4.98 CsH*F++ F----t 1.1 CzHz+ 4- F 4.8 CzFzH+ H
6.22
35.4 CHZCFpf -A P = 10.45
2.8 CF+-+CHI
for CH&XF spectrum.
more, the yields from the rearrangement processes which give CEzF+ and CF+ are both higher than for that to give C&HF+ plus HF even though the energy requirement is smaller for this last process. This can be reconciled with statistical theor:y if one makes the plausible assumption that the transition state HzC-CF \/
CtHF+fHF
+
4.63
12.4 CHzF++CF
4.78
13.1 CF++CHzF
*
7.35
2.6 > CH2+
+ CF2
F
is less rigid than the one which leads to loss of the species HF. Qualitatively, the yields of the other principal products are also consistent with expectation from theory if one assumes that a t high energies considerable CF+ is produced by secondary processes. This is possible but does not appear very likely. Rearrangement to give CHFzf plus CH apparently fails to occur because of a high energy requirement (Table IV). As Table IV shows, formation of CHz+ must, a t least near its threshold, be a primary product. However, the observed appearance potential indicates that considerable exceris energy is involved, just as was found for production of CHtf from CzH4. I n both cases, the thermochemically calculated activation energy is close to 6 v. and it is possible that decomposition processes with such high energy requirements will not occur at sufficient rates t o give measurable yields in a conventional mass spectrometer, for energies near the threshold.’ Detailed calculations are now in progress to investigate this point and to see what amount of excess energy may be needed to produce measurable decomposition. CHFCF2.--By far the most probable process for decomposition of CHITCFz+is loss of an F atom to give CzFzH+ (Fig. 7). This occurs in large yield even though the energy requirement is 5.8 v., the largest observed for any of the primary processes. It is a t least partly understandable that the products CFzH-I-,CF+, and C2F2+are formed in lower yields since they all result from rearrangement or four-center processes. The energy terms favoring the rearrangements are, however, rather large, almost 2 v. for the case of CFzH+. Furthermore CF+ is produced in virtually the same yield as is CF2Af even though the threshold energy to form CF+ is 1 v. higher. This apparent discrepancy can perhaps be explained by assuming that at higher energies most of the CF+ actually results from a secondary decomposition. The observed shapes of the ionization efficiency curves are consistent with this. At the lowest energies CFzH+ is actually produced in larger yield than is CF+. Furthermore the ionization
1
26.2 5.80 CHFCFz+--p+ A P = 10.33
[tl 30.5 CpFzH+
-4.5
1.0 CzF2+
5.05
5.3 CFH+
-
+F
3.15 +
f;+C:H
+ HF
+ CF2
CzFH
+F
CHAVALIFSHITZAND F. A. LONG
2468
quency factors, since, for decompositions of CHzCFz+ and CHzCHF+, the intrinsic probabilities of H and F loss appear to be about equal, i.e., there are no obvious differences in frequency factors. It is, of course, possible that the assumed heat of formation of C2F3+is wrong. Wolfsberg's recent calculations7suggest that for a molecule of 12 oscillators such as ethylene processes with true thermochemical activation energies of up to 4 v. will still occur normally in a mass spectrometer and hence give appearance potentials in agreement with t h e ~ r y . However, ~ for processes with energy requirements as large as 6 v., observation in a mass spectrometer may require the presence of excess energy in the parent species so that appearance potentials would not reflect the thermochemistry. For the present studies this suggests that the appearance potentials for C2F2H+ from CHzCF2 and CHFCF2, and also for CzF3+ from CzF4,may be considerably higher than thermochemically required. This point merits further study both for these cases and for the previously mentioned case of CHz+ from C2H4and CH2CF2. CzR.-The spectrum for this species results from a simple C-F bond split, a C-C double bond break, and two rearrangement processes leading to the species CF+ and CF3+ (Fig. 8). No acetylenic ion product is ob3.42 r
1.3 --+ CF3+
+ CF
28.6 + CF++CF3
'
20.4 CF2CF2+ AP = 10.12
[t ] [I .7 I
5.88
I
'
---j
5.01
L
Fig. S.-Mechanism
37.3 C2F3+
10.6
+ CFz+ for CFzCFz spectrum.
+F
+ CFz
served. Production of C2F3+ is the process of highest yield even though the required activation energy is also the largest observed. As usual, the low yields for the rearrangement processes can formally be explained as due to the requirement of a more rigid cyclic transition state for this formation. However, this explanation will not do for the low yield of CF2+which requires only a simple bond split and whose yield therefore remains anomalously low. There is an apparent anomaly for the rearrangements in that CF3+, which is formed a t a lower threshold energy, is found a t 70 v. in much lower yield than CF+. However, the latter can also be formed by a secondary
Vol. 67
decomposition of CzF3+,and a t low energies CF3+ is indeed formed in higher yield than is CF+. The ionization efficiency curves for the ions from CzF4 are decidedly unusual and have been discussed in detail in another p ~ b l i c a t i o n . ~However, ~ the curve for CF+ is consistent with a large secondary production of this species from C2F3+. A less likely but conceivable interpretation is that there is an ion-pair process involved in the low energy formation of CF+. CzF4 +CF+
+ CF2 + F-
This is consistent with the appearance of F- ions at about 14 v. The higher yield production of CF+, in this view, could perhaps be due to the process C,F, +C F +
+ CF2 + F
I n addition to problems with the rearrangement processes, the anomalous yields of C2F3+and CF2+ suggest that simple statistical theory may not be adequate to explain their yields. If the appearance potential for CzF3+ formation is found to be in accord with thermochemistry, its formation a t the thermochemical threshold will probably have to be explained by decomposition from an excited state of the parent ion which does not cross the lower electronic state. I n view of the fact that we are dealing with a relatively small molecule, in which the spacing between the various electronic levels is quite large, this may very well be the case; calculations on this point are in progress. Summary Remarks.-The over-all trends in going from the fully hydrogenated ethylene to the perfluoroolefins seem reasonable and at least roughly consistent with the changes in stabilities of the various product types. Sone of the results leads to inescapable contradictions with the statistical theory for mass spectra. Thus for similar energy requirements, rearrangement and four-center products usually have lower yields than do simple rupture processes, and for a given type of reaction, processes with lower energy requirements usually lead to higher yields. There are, however, a number of unusual results indicating that a more quantitative analysis may show disagreement between observation and prediction from simple theory. A point of particular interest is that several of the processes have such large energy requirements as to indicate that considerable excess energy may be needed to obtain decomposition rates high enough to give a measurable yield. Acknowledgments.-We wish to thank Drs. J. D. I\Iorrison, F. H. Dorman, and V. H. Dibeler for vaiuable discussions and for measurements of appearance potentials of negative ions from CzF4. (24) C. Lifshitz and F. A. Long, submitted for publication.
