APPEARANCE POTENTIALS AND MASSSPECTRA OF FLUORINATED ETHYLENES
3731
Appearance Potentials and Mass Spectra of Fluorinated Ethylenes. 11. Heats of Formation of Fluorinated Species and Their Positive Ions1
by Chava Lifshitz and F. A. Long Departntat of Chemistry, Corneli Univerdy, Itham, New York (Recdved April 7, 1966)
Mass spectrally determined appearance potentials of fragment ions from several fluorinated ethylenes have been used to calculate heats of formation of a series of hydrofluorocarbon radicals and ions. Supplementary determinations were carried out on fragments from fluorinated methanes and ethanes. The data lead to maximum values of 9.6, 11.7, and 9.45 for the ionization potentials of CF, CF2,and CH2F, respectively. Some derived heats of formation are: aHf(CF2) 6 -1.6 e.v.; mf(CH2F) = -0.95 f 0.1 e.v.; Nt(C2F3) = - 1.1 f 0.3 e.v. Different ionization potentials are calculated for both CF2K and CFs depending on whether the calculations are from appearance potentials of CF3+ and CFzH+ from the paraffin series or from appearance potentials of the olefin series. These and further disagreements in calculations based upon the paraffin and olefin series are discussed.
Introduction The mass spectra and appearance potentials for a group of fluorinated ethylenes were presented in a previous paper. A few thermochemical calculations were carried out in developing the probable decomposition mechanisms for the parent ions. The number of possible direct calculations is however small since thermochemical data on fluorocarbons and their radicals and ions are quite scarce. Heats of formation of some of the low molecular weight saturated fluoroparaffins are known13 but attempts to obtain thermochemical data for fluorocarbon radical ions, especially CF3+,from appearance potential measurements on fluoroparaffins have not been very successful.4~6The main difficulty appears to be the occurrence of ions with considerable amounts of kinetic energy. The fluorinated ethylenes, however, offer another route since they show yields of ions which are of great thermochemical interest (CF+, CX2+, and CX3+, where X is either F or H). The present paper gives further thermochemical calculations using the available appearance potentials and other available thermochemical data and some newly determined appearance potential values. Experimental Section The mass spectrometer and the techniques used were described in part I which also gives appearance poten-
tials of fragments from fluorinated ethylenes. Table I6-l0 presents results of appearance potential measurements on some paraffins and C3Fe. Perfluoromethane was purchased from Matheson; CHFB, GFs, and C3Fs were kindly furnished by the Jackson Laboratory of the Du Pont Co. CHaCHFz was from the Allied Chemical Co. while CH2F2was from the Columbia Chemicals Co. These were all of high purity (>99%) and were used without further purification. Several appearance potentials of fragments from these (1) Work supported by the Advanced Research Projects Agency through the Cornel1 Materials Science Center. (2) C. Lifshitz and F. A. Long, J. Phys. Chem., 67, 2463 (1963); hereafter called part I. (3) C. R. Patrick, Advan. Fluorins C h m . , 2 , 1 (1961). (4) V. H. Dibeler, R. M. Reese, and F. L. Mohler, J. Res. Natl. Bur. Std., 57, 113 (1956). (5) J. B. Farmer, I. H. S. Henderson, F. P. Lossing, and D. G. H. Marsden, J . C h m . Phy8., 24, 348 (1956). (6) 9. Stokes and A. B. F. Duncan, J. Am. Chem. SOC.,80, 6177 (1968). (7) D. L. Hobrock and R. W. Kiser, J. Phys. Chem., 68,575 (1964). (8) J. W. Warren and J. D. Craggs, Mass Spedromstry, Rept. Conf. Manchester, Engl., 1960,36 (1952). (9) F. M. Matsunaga and K. Watanabe, private communication. (10) V. H. Dibeler, R. M. Reese, and F. L. Mohler, J . Chem. Phys., 20, 761 (1961).
Volume 69,Number 11 November 1966
3732
CHAVALIFSHITZ AND F. A. LONG
Table I: Appearance Potentials (e.v.)" Speciea
CHzFz
CHa + CF+
18.8
CFH+
17.7
CHFa
CF4
CHiCHF:
CzFa
QFa
18.6 20.75 20.2"
C F H ~ + 15.28 CzFHa+ CzFHd+ CF2+
14.8 14.9 20.7
20.2 17.5~
CFZH'
13.11
15.75 16.4"
CFzHz+
12.6 12. 5sbvi
22.2 20.3~ 22.4'
CzFzHz+ CzFzHa GFzHi+
CFIH + CZa+
Table II: 1Wr(g) Values a t 298°K. Used in the Thermochemical Calculations
16.5 12.33 12.68 12. 5g0*'
+
CF, +
13.21
14.42 16.2 14.40' 15.Qd 14.53d 16.0" 14.6f 13, 84b*i
Species
14.2 14. 3h
13.15
Values without superscript are from the present electron impact study. Spectroscopic values from ref. 6. Electron impact values from ref. 7. Electron impact values from ref. 5. * Electron impact values from ref. 4. Electron impact values Electron impact from ref. 8. 0 Photoionization from ref. 9. value from ref. 10. Parent molecule ion.
'
'
gases have been reported previously, and the earlier results are given in Table I for comparison. Mass spectra of molecules CH3CHF2, CFIH, CFzHz, C2F8, and CF4 show little or no parent peak intensity, and measurement of their ionization potentials by electron impact is either impossible or subject to large error. The listed ionization potentials for these species, except the value for CF4, were measured by photoionization by Mr. F. M. Matsunaga and Professor K. Watanabe using samples forwarded by us. If no parent ion is observable in the mass spectrum of a molecule, it is probable that the lowest observed photoionization potential is the appearance potential of the first appearing fragment ion since no mass analysis was employed in the photoionization experiments. In CFsH the lowest photoionization potential agrees very well with the electron impact appearance potential for CF3+ but is distinctly higher than the spectroscopic ionization potential for CF3H (see Table I).
Thermochemical Calculations The appearance potential of an ion may be combined The Journal of Phvsical Chemistry
with other thermochemical data for calculations of a heat of formation subject to the assumption that none of the products of the mass spectral reaction is produced either in an excited state or with excess kinetic energy. The present calculations utilize the appearance potentials of part I and those of Table I of this paper. Various other thermochemical data, available in the literature, are employed in the calculations. The heats of formation of C2F4 and CH2CF2 have been determined experimentally.ll The heats of formation of CHzCHF and CHFCFz are known from estimations which are good to within 5 to 10a/o.12 These values are given in Table II.13-18
CH2CHF CHzCFz CHFCFz CFzCFz CHzFz CHFi CFI CHsCHFz CFsCF=CFz CF
AH!,e.v.
-1.22 -3.36 -4.82 -6.59 -4.60 -7.05 -9.45 -5.10 -11.22 +3.23
Quality'
2 1 2 1 1 1 1 1 1 1
Ref.
12 11 12 11 13 13 11,14 15 16 17
The symbols in this column mean (1) direct measurements or data of good precision and (2) data based on estimations.
Additional heats of formation listed in Table I1 and used in the calculations are those for some of the parent molecules of Table I and for the CF radical; these are reliable values which are obtained by methods other than electron impact,. Heats of formation of hydrocarbons, their radicals, or ions are from ref. 19; any other heats of formation used are from ref. 20 and 21. (11) C.A. Neugebauer and J. L. Margrave, J. Phya. Chem., 60,1318 (1956). (12) P. G.Maslov and Y .P. Maslov, Khim. i Teckhnol. Topliv i Masel, 3,50 (1958); C h m . Abslr., 53, 1910 (1959). (13) C.A. Neugebauer and J. L. Margrave, J. Phya. Chem., 62, 1043 (1958). (14) D. W.Scott, W. D. Good, and G. Waddington, J. Am. Chem. SOC.,77, 245 (1955). (15) I(.R. Lacher, A. Kianpour, F. Oetting, and J. D. Park, Trans. Faraday SOC.,52, 1500 (1956). (16) H.C.Duus, I d . Eng. Chem., 47, 1445 (1955). (17) E.B. Andrews and R. F. Barrow, Nature, 165, 890 (1950). (18) E. B. Andrews and R. F. Barrow, PTOC.Roy. SOC.(London), AM,681 (1951). (19) R. R. Bernecker and F. A. Long, J. Phys. Chem., 65, 1565 (1961).
APPEARANCE POTENTIALS AND MASSSPECTRA OF FLUORINATED ETHYLENES
an upper limit since a rearrangement process of this type may involve excess energy. An alternative way of computing AHr(CF+) is from the CF+ appearance potential from CH2F2. This, however, leads to 13.4 e.v., i.e., considerably higher than the above value (Table 111). CF+ is formed in two stages from CH2F2. The formation of CH2F+ in the first stage involves excess energy as may be seen by comparison of AHr(CH2F+) calculated from CHzF+ appearance potentials in CHzCF2and CH2F2(Table 111). Furthermore, all the C-F bond breakages in the methane series appear to involve excess energy as compared to the CH breakages. (Compare the computed heats of formation for CF2H+from CF2H2and from CHF3or those for CF3+ from CFaH and CF4, Table 111.) This is also the reason for the discrepancy between our computed value for AHr(CF+) and the one based upon the appearance potential of CF+ from CHB.' One can estimate the excess energy in the process
Table 111: Calculated AHr(g) Values at 298°K. from Appearance Potentiale Species CF
Reaction
"
CFz CFH CFz
+
or
"
CFzH
CFa
"
-
-
"
CIHZ (CZHZ or CHFC +) CzHF" +
CzHs
+ + + + + + + + + + "+ + + + + "+ + + + " "+ + + + + + + + + + + + + + + CHzCHF CzHzF+ + H C?HzF + F CHzCFz CzFzH" + H CHzCFz CzFzH " + F CFHCFz CZF4 CZFS++ F
CHzCHF -+ CHs CF+ Hz F CHzFz" + CF" CIFP + CzF4" CFZ CHzCFz -+ CHz+ CFz H CFz CHjCHFz + CHa" CFz CFHCFz' -+ C F H + F H CHzFz -L C F H + HF CHzFz -+ CFH C2FIC CFZ+ CFZ CHzFz .-,CFz" 2H H F CHFs- CFz+ CFz 2F CFI CF+ CHzCFsd + CHzF CFzH CF+ CHFCFzd CF CFtCFid -c CFs CHmCFz -+ CHzF CF F CHzFz -c CHzF+ CF CHFCFz -+ CFzH+ H CHzFz -+ CFzH+ F CHFa + CFzH" CHFz+ CHaCHFz -+ CHI CF CFzCFz -* CFa+ CFs+ H CHFa F CF4 .-,CFa" CZHI -+ CZH?+ Hn CHICFZ-+ CzHz" 2F HI CHzCHF -+ CzHF" 2F CHFCFz + CzHF+ H CZHI CzHa+ F CHzCHF -+ CzHs"
-
CHzF CFzH CFa CHzF +
+
"
CaHzF (CHFCF or CHF=CH +) (probably CHFCF ") CzFzH (probably CFFCH +) (CFzCH + or CFHCF +) CzFa +
"
"
CH2Fz +CH2F+
CHzCF2 --+ CH2F+
-+
+
-+
-+
-.c
+
Table I11 summarizes the calculations. The occasional marked discrepanciesin calculated heats of formation for a given species are discussed in the next section.
Discussion AHf(CF+) and I.P.(CF). The formation of CFf from CHzCHF involves a direct rearrangement CH3
+ CF+
(2)
eSS
This cycle is chosen because the metastable transition H2 is observed. * The appearance potential CH2F+ + CF+ of C2F4+ from CsF6is lower than that of any other fragment from CaFs (unpublished results) so that it has to be the product of a direct rearrangement in which CF2 is also formed; for I.P. (c2F4)~see part I. AHr(CF2) = -1.6 e.v. was used in this calculation. aHr(CF+) = +12.82 was used in this calculation.
CH2CHF
+F
If one assumes the rearrangement energy in the proc-
-
-+
3733
(1)
and one computes AHr(CF+) 6 12.82 e.v. Since Mr(CF) = 3.23 e.v. (Table II), this implies I.P. (CF) 6 9.6 e.v. The heat of formation computed from process 1 is
+ CF
(3)
to be negligible, then the excess energy involved in (2) is 9.86 - 8.49 = 1.37 e.v. Using this, one calculates the heat of formation of CF+ from its appearance potential in CHpFz as AHf(CF+) = 13.4 - 1.4 = 12.0 e.v. and I.P.(CF) = 8.8 e.v. These are lower by 0.8 e.v. than the values computed from process 1. Even so, in view of the uncertainties involved in this second calculation, we shall tentatively adopt the higher values, as given above, for both AH(CF+) and I.P.(CF). There are several conflicting values for the ionization potential of CF, namely, 13.81 e.v.22(by direct electron ~ on spectroimpact on the CF radical), 11 e . ~ . 2(based scopic evaluations of the dissociation energies of CF . ~a ~ and CF+ in their ground states), and 8.9 e . ~ (by similar spectroscopic evaluation). The values from spectroscopy are based on linear extrapolations and are not highly reliable. The present value of 9.6 e.v. agrees best with the latest spectroscopic value of 8.9 e.v.,24 especially in view of the previously discussed (20) F. D. Rossini, D. D. Wagman, W. H. Evans, W. H. Levine, and I. Jaffe, National Bureau of Standards Circular No. 500, U. 6. Government Printing Office, Washington, D. C., 1952. (21) "Janaf Interim Thermochemical Tables," The Dow Chemical Co., Midland, Mich., Dec. 31, 1960-June 30, 1961. (22) R. I. Reed and W. Snedden, Trans. Faraday SOC.,54, 301 (1958). (23) G. Herrberg, private communication. (24) J. W. C. Johna and R. F. Barrow, Proc. Roy. SOC.(London), A71,476 (1958).
Volume 68,Number 11 November 1966
3734
CHAVA LIFSHITZAND F. A. LONG
possibility of excess rearrangement energy in process 1. The relatively low ionization potential of CF, compared to 11.1 e.v. for CH,25is explained by Price and coworkers26as being due to an increased sharing of the nonbonding p.rr electrons of fluorine with carbon in the ion relative to the radical. No explanation has been given for why direct electron impact leads to an ionization potential of CF which is higher by several electron volts. aHf(CF2). Two recent groups of workers give AHf(CF2) from appearance potential calculations as close to - 1.6 e.v.,27t28 similar to the estimation from onset bands from CF4 C of - 1.5 f 0.5 e . ~ . ~ ~ Our data permit a similar calculation from the appearance potential of C2F4+ from C3F8 for which the mocess is
+
C3Fs
+ e = CoF4++ CF2 + 2e
+ e = CF2+ + CF2 + 2e
A.P.(CF+)
- A.P.(CHZF+) =
(5)
the appearance potential of 15.13 =t 0.1 e.v. leads to Uf(CF2+) = 10.1 e.v. and I.P.(CF2) = 11.7 e.v. This excellent agreement between direct measurement and indirect calculation from olefin mass spectral data contrasts favorably with the situation for fluoro paraffins and aromatics where agreement is much poorer.32 (x= H,)’ Radicals* Heats Of Formation Of the The heats of formation of CHtF, CF2H, and CF3 are obtained from rearrangement processes of fluorinated olefins as listed in Table 111. The assumption made in the calculations for CF2H and CF3 is that CF+ from the olefins is produced at its threshold by a rearrangement process rather than by an ion-pair process. In view of the experiments of part I this seems justified. Previous values, from methods other than electron impact, are available only for AHt(CF3). The best value is the one calculated from an analysis of kinetic data by Rabinovitch and Reed,33 AHf(CF3) = -5.2 e.v., which agrees very well with our electron impact value of - 5.35. The Table I11 values for the other two radi-
AHf(CF+)
+
- AH t (CHzF+) - AH (CF) AHr(CF) + I.P.(CF) + AHr(CH2F) AHr(CH2F)
AHt(CH2F)
(4)
(A conceivable alternative is production of CF2-, but direct search for this species in the negative ion spectrum of C3F6 was unsuccessful which is consistent with the presumed small electron affinity of C F 2 . 9 From process 4 and the observed appearance potential of C2F4+ from C3F6 of 13.15 e.v., one computes AHf(CF2) = - 1 . 6 0 . ~ ~This is in excellent agreement with the results of the Majer and Lossing groups. Ur(CFz+) and I.P.(CF2). Fisher, Homer, and Lossing give the ionization potential of CF2 as 11.7 by direct measurement.28 Our calculation from the appearance potential of CF2+ from C2F4 fully confirms this. Assuming the process C2F4
cals are Ut(CF2H) = -2.67 and hHf(CFH2) = -0.95 e.v. For comparison AHf(CH3) = +1.39 e.v. AHt(CH2F+) and I.P.(CH2F). AHf(CH2F+)= 8.5 e.v. if excess rearrangement energy in process 3 is negligible. I.P.(CH2F) = AHr(CH2F+) - AHf(CH2F) = 8.49 - (-0.95) = 9.4 e.v. Actually the preferred procedure in calculating I.P. (CH2F) from our results does not involve the heat of formation of the radicals but goes as follows: CH2F+and CF+ are both formed from CH2CF2, and the difference in their appearance potentials is
= I.P.(CF)
f
- I.P.(CHaF) - AHf(CF)
- I.P.(CH,F)
Using for I.P.(CF) its appearance potential in CH2CHF, we calculate I.P.(CH2F) = 9.4 f 0.1 e.v. The value of I.P.(CH2F) determined by direct electron impact is 9.35 e . ~ The . ~ agreement ~ between our result and this direct electron impact value leads to a greater degree of confidence in our value of I.P.(CF). Furthermore, it suggests that the rearrangement energy in reactions such as (1) and (3) is small (perhaps not more than 0.1 e.v.). aHr(CF2H+)and I.P.(CFtH). The appearance potentials of CF2H+ from CHFCF2 and CH2F2 lead to the same value for AHt(CF2H+), namely, 6.2 e.v. The appearance potentials of CF2H+from CF3H and CF2HCH3lead to U t ( C F 2 H + )values higher by 1.6 and 0.5 e.v., respectively. We shall assume that the lowest value (6.2 e.v.) is the correct one. From the relationship
(25)A. E.Douglas and G . Herzberg, Can. J . Res., A20,71 (1942). (26) R. Brdsford, P. V. Harris, and W. C. Price, Proc. Roy. Soc. (London), A258,459 (1960). (27) J. R. Majer and C. R. Patrick, Nature, 201, 1022 (1964). (28)I. P. Fisher, J. B. Homer, and F. P. Lossing, J . Am. Chem. SOC., 87,957 (1965). (29) L. Brewer, J. L. Margrave, R. F. Porter, and J. Wieland, J. Phye. chm., 65, 1913 (1961). (30) J. L. Margrave, J . Chem. Phys., 31, 1432 (1959). (31) This was earlier reported in a paper by C. Lifshitz and F. A. Long to the 10th ASTM Conference on Mass Spectroscopy, New Orleans, La., June 1962. (32) See Table I1 of ref. 28. (33) B. 8. Rabinovitch and J. F . Reed, J . Chem. Phye., 22, 2092 (19M). (34) F. P. Lossing, P. Kebarle, and J. B. Desousa, “Advances in Mass Spectrometry,” J. 0. Waldron, Ed., Pergamon Press Ltd London, 1958,p. 439.
APPEARANCE POTENTIALS AND MASSSPECTRA OF FLUORINATED ETHYLENES
A.P.(CF~H+)C,F,H - A.P.(CF+)C,F,E= I.P.(CFeH)
- I.P.(CF)
one calculates I.P.(CF2H) = 8.9 f 0.2 e.v. Owing to the unusual shape of the CF+ ionization efficiency curve from CF2CFH2there is some uncertainty in the value of I.P.(CF2H). A direct electron impact measurement of I.P.(CF2H) gave 9.45 e . ~ . in, ~good ~ agreement with the value of I.P.(CF,H) = 9.4 e.v. calculated from AHf(CF2H) = -2.67 e.v. (Table 111) and AHf(CF2H+) = 6.72 e.v. (Table I11 based on CF2H+ appearance potential from CF2HCH3). Mf(CH3+) and I.P.(CF3). AHr(CF3+) = 3.72 e.v. if the CF radical is formed with CFa+ from C2F4 and if the excess rearrangement energy involved in the process is negligible. Curiously enough, neither the H loss nor the F loss from the methanes gives a similar value for Mr(CF3+). The formation of CF3+ from CF4 involves approximately 0.8 e.v. excess kinetic en erg^,^^^ and if this is taken into account, then the appearance potentials of CF3+ from CF3H and CF4 give the same value for Mf(CHs+),namely, 5.1 e.v. From the relationship A.P.(CF~+)C,F,- A.P.(CFf)c,p, = I.P.(CFa)
- I.P.(CF)
one calculates I.P.(CF3) = 9.1 f 0.2 e.v. if neither CF3+nor CF+ is formed by an ion-pair process. A direct calculation of I.P.(CF3) based on the appearance potential of CF3+from C2F4 and on the heat of formation of CFaa3(-5.2 e.v.) gives I.P.(CFa) = 8.9 f 0.2 e.v. Similar calculations based on the CF3+ appearance potentials from CF3C1,CF3Br, and cF3I lead to a value of 9.3 0.2 e.v.4 for the ionization potential of the CF3 radical. Direct electron impact by Lossing and co-workersa4gave I.P.(CF3) = 10.1 e.v. in good agreement with the CF3+ appearance potentials from the paraffins CF3H, CF4, CF3CH3,and C2Fe (see also ref. 5). However, Fisher, Homer, and LossingB recently re-estimated the value of I.P.(CF3) to be 9.5-9.6 e.v. Price2shas pointed out that the decrease in the ionization potential of ethylene upon fluorination can be attributed to the large resonance stabilization of the planar fluoroethylene ions which more than compensates for the inductive effect of the fluorine atoms. Price also points out that, since the fluoromethyl radical ions are planar in the ground state, a similar lowering of the ionization potential of the methyl radical should result upon fluorination. The present results do show an over-all decrease in ionization potential upon fluorination (CH, 9.85 e.v.; CH2F, 9.45 e.v.;
*
3735
CHF2, 8.85 e.v.; CF3, 9.1 e.v.) although there is an in crease from CHF2 to CF3. Lossing's direct electron impact values34show a decrease from CH3to CH2F,but then an increase: CF2H, 9.45 e.v.; and CF3, 10.1 e.v. On the other hand, I.P.(CHs) = 9.85, I.P.(CH2C1) = 9.32, I.P.(CHC&) = 9.30, I.P.(CCl,) = 8.78.34 In all of the above cases the ionization potentials for fluorinated methyl radicals as calculated from mass spectral results from the paraffin series are much larger than the values which result from the data for the olefin series. Direct measurements on ionization of the radicals lead to the larger values. There is, however, good reason to believe that both the direct resultsZRand the indirect results for the methyl ion paraffins involve formation of CX3+ ions in excited states. Lossing, et d., suggest28that one plausible explanation for the discrepant direct results is that CF3+ is planar in the ground state and C R nonplanar so that direct ionization of the latter leads to an excited CF3+. The CX3+ ions from the olefins result from decomposition of an olefin molecule ion, a relatively slow process which should permit formation of the CX3+ in its ground state. With the fluoroparaffins, however, no parent molecule ions are observed, and there is a good possibility that the electron impact leads directly to a dissociative state and rapid formation of a CX3+ ion which thus has a greater likelihood of retaining its tetrahedral structure, i.e., of being formed in an excited state. AHf(C2X2+) (X = H , F ) . The heat of formation of C2H2+calculated from the H2 loss from ethylene is equal to values found by direct measurements on C2H2+lswhereas the value based on HF loss from CH2CHF is much higher.2 A similar effect is observed for CzFH+.2 If H2 abstraction from any ethylene involves no excess energy (by analogy with C2H4),then AHt(C2HF+) = 12.8 e.v. is a reliable value. Excess energy was encountered in the H F losses from the ethylene series. A similar behavior is found for CH3CHF2. The CH2CHF+ appearance potential from this molecule (see Table I) is 14.8 e.v., in comparison with the value calculated from the CH2CFH heat of formation and ionization potential, namely, 11.6 e.v. AHr(C2X3+) (X = H , 8'). These ions are formed by a single bond rupture from the parent molecules so that their calculated heats of formation should be reliable. There is some uncertainty with the heats of formation of the more highly fluorinated members of the series because of the possible entrance of excess activation energies.2 From part I one calculates for the processes C2H4
CzH3
+H A C2H3+
+ H a + b = 14.06 e.v.
Volume 6Q,Number 11 Nove1nb6r 1966
3736
CHAVALIFSHITZAND F. A. LONG
2C2H3 + F 5
CZH~F
+F
C2H3+
a'
+ b'
= 14.38 e.v.
Since b equals b', a' - a = 0.32 e.v.; i.e., the carbonfluorine bond energy in C2H3F is 0.32 e.v. higher than the carbon-hydrogen bond energy in CzH4. In the series C2H3+, CzHzF+, and C2F2H+, formed by an H atom loss from the parent, the appearance potentials are 14.06, 14.02, and 16.6, respectively (Table 11, part I).2 Each appearance potential is equal to the C-H bond energy in the parent plus the ionization potential of CZX~.Thus, in this series either the bond energy or the ionization potential or both of these quantities increase. A similar behavior is encountered for the series CZ&+ formed by an F atom loss. From the first resonance capture process forming Fin C2F4at 2.7 e.v. (observed by J. D. Morrison and F. H. Dorman, see part 12)and assuming the following capture process for it C2F4 +C2F3
+ F-
one computes the C2F3-F bond energy to be 6.3 e.v. and AH,(C2F3) = -1.1 e.v. From Mf(C2F3+) = 8.6 e.v. (Table 111) one computes I.P.(C2F8) = 9.7 e.v. The ionization potential of C2H3 is 9.45 e.v.19 This increase in ionization potential upon fluorination is contrary to the trend in the ethylene molecules and may be due to a relatively small resonance stabilization of the ion radical C2F3+. On the other hand, the possibility of excess activation energy in the process forming C2F3+ cannot be excluded. Table IV summarizes the heats of formation of some simple fluorocarbon molecules, radicals, and ions. It contains those values from the present study which are
The J O U Tof ~P h y d Chemistry
considered reliable, as well as selected literature values for comparison. This table is of the same format and is based on the same fundamental constants as the similar table of Bernecker and Long.'$ Table IV: AHt(g) Values for Neutral Species and Their Positive Ions Species CF
-----Un-ionized AHf, e.v. Qual." +3.23
S
---Ref.b 17, 18
X CHz CHF CFz CHs CHzF
$4.0 +0.36 -1.60 +1.39 -0.95
CFzH
-2.67
CF3
3, 3 2 1 2
S
19, c P P, 28 20 P
X 2
P
-5.35
2
P
X +2.350
1
19
+2.83 +1.15
2 3
19 e
-1.1
2
P
AHt, e.v.
Ionized---Qual.a Ref.b
4-12.8
2
X 4- 8 . 9 4-14.4 4-12.16 f10.10 +11.23 +8.49 x 9.45 + 6 . 1 7 (olefin) + 6 . 7 2 (paraffin) X 8 . 8 5 (olefin) + 3 . 7 2 (olefin) + 5 . 1 1 (paraffin) X 9 . 1 (olefin) +13.75 +12.8
S
+
3 3 2 S 2
P 24 ID P P, 28 d P
2
P
+
2 2 2 2 2 2
+12.28 +10.65 +10.8 +8.59
2 2 2 2 2
P P P P P P 19 P 19 P P P
+
X
CzHz CzFH CzHs CzHzF CzFzH CzFa
----
s
a Quality standards as in ref. 19. P refers to present data. G . Herzberg, Can. J . Phys., 39, 1511 (1961). G. Herzberg and J. Shoosmith, ibid., 34, 523 (1956). e W. M. D. Bryant, J. POlYT?lt?Tsci., 56, 277 (1962).
Acknowledgments. We thank Dr. J. D. Morrison and Dr. F. H. Dorman for their measurements of appearance potentials of negative ions from C2F4, as well as for very helpful discussions.
