APPEARANCE POTENTIALS AND
MASS
SPECTRA OF c86, CSF5C1, AND C-CaFe
3741
Appearance Potentials and Mass Spectra of C,F,, C,F,CI, and c-C,F,'
by Chava Lifshitz and F. A. Long Department of Chemietry, Cornell University, Ithaca, New York (Received April 19,1966)
Appearance potentials and mass spectra have been determined for C3Fs, C3F&1, and cC3F6. As with the decomposition of the fluorinated ethylenes, rearrangement processes play prominent roles, notably in the formation of CF3+ and C2F4+. The appearance potential values permit assignment of most of the primary decomposition processes; they also give further information on the thermochemistry of the fluorinated ions and radicals. Ionization efficiency curves for the parent C3F6 and C3F5C1ions, as well as for some of the fragments from C3F6 and C-C~FB, have been determined for several volts above the thresholds. The parent C3F6 ionization curve shows a pronounced break, similar to the one which has previously been observed for C2F4, indicating again the influence of a competitive process. Appearance potential differences of several volts are found for some of the prominent decomposition processes and are discussed qualitatively in terms of the statistical theory of mass spectra.
Introduction Previous papers in this series have dealt with the mass spectral decomposition mechanisms and thermochemistry of fluorinated ethylenes and their positive ions.2ta The present study extends these considerations to CF3CF=CF2, C F 2 C 1 C F 4 F 2 ,and C-CaFe. These compounds were chosen to study further the thermochemistry of the radicals and ions involved. The mass spectra and appearance potentials are presented for these three species, and breakdown patterns are proposed. Ionization efficiency curves are discussed qualitatively in terms of competing processes4 and the statistical theory of mass spectra. The validity of utilizing observed energy differences as activation energies for processes of the type, parent molecule ion --t product ion, is also considered in terms of the statistical theory. A general review of the mass spectra of fluorinated hydrocarbons has recently been p~blished,~ and recent information is also available on the mass spectra of partly fluorinated propanes and propylenes,6 on the mass spectra of cyclic perfluoroalkanes and their hydrogen analogs,' and on appearance potentials for fully fluorinated ethane, propane, and cyclobutane.* Experimental Procedures and Results The experimental procedure for recording the spectra and obtaining appearance potentials is the same as previously described.1a
CFaCF=CF2 was kindly furnished by the Du Pont Jackson Laboratory. c-C3F6 was prepared by Dr. D. C. England of Du Pont by photolysis of hexafluorocyclobutanone and was purified by preparative gas chromatography by Dr. A. Fainberg, Pennsalt Chemicals Corp. CF2C1CF=CF2 w m synthesized by Dr. R. Becker, Cornel1 University, by pyrolysis of CF2CICFClCF2COONaa9 Table I gives the spectra (in percentage yields of the various ions) of the three compounds studied. These were taken at an accelerating voltage of 105 v. compared to the normal operating voltage of 210 v. for the Consolidated 21-401 mass spectrometer. The low voltage permitted this machine to be used for mle values larger than 100. Use of the lower voltage slightly reduces the focusing power of the machine but appears to have no other effect; e.g., spectra of standard compounds taken at 105 and 210 v. were found to be (1) Work supported by the Advanced Research Projects Agency through the Materials Science Center. (2) C. Lifshitz and F. A. Long, J. Phy8. Chem., 67, 2463 (1963). (3) C. Lifshitz and F. A. Long, ibid., 69, 3731, 3737 (1965). (4) C. Lifshitz and F. A. Long, J. Chem. Phys., 41, 2468 (1964). (5) J. R. Majer, Advan. Fluorine Chem., 2 , 55 (1961). (6) W. C. Steele and F. G. A. Stone, J . Am. Chem. Soc., 84, 3460 (1962). (7) P. Natalis, Bull. SOC.Roy. Sci. Liege, 29, 94 (1960). (8) M. M. Bibby and G. Carter, Trans. Faraday SOC., 59, 2465 (1963). (9) P. Reanick, Ph.D. Thesis, Cornell Uljwraity, 1961.
Volume 69,Number 11 November 1066
CHAVALIFSHITZ AND F. A. LONG
3742
lonizlng Voltogo (obovo tho I.P. for C3F5Cn
Table I1 summarizes the appearance potentials for the principal ions from the three compounds. Since the main interest in the present paper is in the perfluorocarbon radical ions and since CF2C1CF=CF2 was run only for comparison, appearance potentials from this compound were measured only for the fully fluorinated fragment ions. Table II: Appearance Potentials in Volts Speciea
CF + CF2 + CFa' C2F8+ c2F4+ CaFs C96+ CsFsCl+ +
CFaCF=CFs
18.1 19.8 15.96 19.32 13.15 15.18 11.11 P
CFgClCF=CFz
G-CSFE
17.8
17.3 17.4 15.38 18.3 11.85 14.14 11.3 P'
15.63 1L.22; 11.8" 10.79 P
'
Ionizing Voltogo (obovo tho I.P. for C3Fe) Figure 1. Ionization efficiency curves for the parent ions from CF8CF=CFe and CF&lCF=CF2.
very similar. In the case of CF2C1CF=CF2 the yields of isotopic ions containing Claa and Cia' were added to calculate the total yield of a given ionic species. The observed spectrum of CF3CF=CF2 differs slightly from that reported by Mohler and co-workers,1° small fragments showing relatively lower yields in our machine. _ _ ~ ~
Table I: Mass Spectra (at 75 v.) CFiCF=CW
CF:ClCF=CFa
11.9
10.5 1.2 0.9 1.0 23.2 4.1 0.9 0.3 2.5 1.2 0.2 1.9 2.5 33.5 6.1
1.0 25.0 0.2 3.2 15.2 0.6 23.8
5.5 33.2 0.4 3.3 34.6 0.6 4.4 0.4 P
19.2 P 5.9 P
The JOY& of Physical Ch%m*ru
c-C:FE
16.4
" J. D. Morrison, private communication. This ionization potential is less certain than the others because of the low intensity of the parent ion in the spectrum. Figure 1gives ionization efficiency curves for the parent ions from CFsCF=CF2 and CFtClCF=CF2. Figures 2 and 3 show ionization efficiency curves for the main fragments from CFICF=CF2 and c-C3F6. Each fragment ion curve is drawn together with an Ar+ curve, so that the two curves are parallel a t high voltages. I n this way it is easy to demonstrate the occurrence of curves with unusually slow onset of ionization. Thus, the CaF6+curves from CFaCF=CF2 and c-CIF~demonstrate long "tails," while the similar one from CF2ClCF=CF2 does not. Similarly, the tailing on the CF3+and CF+ curves is much more pronounced for the C3F6 isomers than for C3FsC1. For the fragments which have long tails one would obtain a quite different result for the appearance potential if the semilog matching technique" were used instead of the vanishing current method which has been used in this study.
Discussion Ionization Potentids. There is a marked increase in ionization potential upon changing the three hydrogen atoms in the CHI group of propylene to fluorines and a similar effect on going from c-C3H6 to c - C ~ F ~ . ~ ~ ~ ~ (IO) F. L. Mohler, V. H. Dibeler, and R. M. Reaae, J. Res. Natl. Bur. Std., 49,343 (1962). (11) R. E.Honig, J. Chem. Phye., 16, 105 (1948). (12) F.H. Field and J. L. Frankljm, "Electron Impact Phenomena," Academic Press Ino., New York, N. Y., 1957. (13) K. Watanabe, T. Nakayama, and J. Mottl, "Final Report on Ionization of Molecules by a Photoionisation Method," University of Hawaii, Deo. 1969.
APPEARANCE POTENTIALS AND MASSSPECTRA OF CaF8, C3F6C1, AND C-CsFa
Figure 2. Ionization efficiency curves for the main fragment ions from CFaCF=CFz. (Each curve is drawn together with its calibrating Ar+ curve, so that the two curves are parallel a t high voltages.)
These increases are in keeping with similar observations on other fluorine compounds where the inductive effect On the other of the fluorine is the dominant hand, upon changing the three other hydrogen atoms of CFaCH=CH2 to fluorines, there is a slight decrease in ionization potential. This is similar to the result from fluorination of ethylene2 and is explained by resonance stabilization due to the fl~0rines.l~Upon replacing the fluorine of monofluorethylene with CFa there is an increase of 0.54 e.v. in ionization potential, and a similar change from C2F4 to C3F6 increases the ionization potential by 0.49 e.v. A similar increase of 0.52 e.v. was also observed in the ionization potentials for the change from C2Fsto i-C3F7 and was ascribed to the electrophilic nature of the CFa group.16 The electron impact ionization potential of C2F4 is the same as the photoionization value2; on the other hand, the electron impact value for CFaCF=CF2 is 0.5 e.v. higher than the photoionization value.lB This different behavior suggests that the nuclear structure of the ionized propylene is considerably different from that of the neutral molecule at the minima of their potential curves, whereas the analogous structures are
3743
Figure 3. Ionization efficiency curves for the main fragment ions from c-CaFe. (The curves are drawn as in Figure 2.)
quite similar for the ethylenes. In fact, a useful way to look at the ion CF&F=CF2+ is to assume a structure which permits a rapid reversible shift of a fluorine atom from the left-hand carbon to the right-hand one. This is consistent with the occurrence of high yields of certain rearrangement ions, e.g., CF3+ from CF2ClCF=CF2+. It is also in keeping with previous suggestions about the structure of propylene ions.” 75-E.v. Spectra. Of the three compounds studied, c-C3Feshows the greatest amount of parent ion decomposition, in agreement with the behavior of other perfluorocyclics in the mass spectrometer.1o CF2C1CF= CF2shows a greater degree of decomposition than CF3CF=CF2. There is a considerably larger degree of decomposition of the C-C1 bond than of the C-F bonds, and perfluorinated fragment ions are more common than the ion fragments containing C1. The ions CFf and CF3+ are abundant in all three spectra. C2F4+is ~~
(14) R. Bralsford, P. V. Harris, and W. C. Price, Proc. Roy. SOC. (London), A258, 459 (1960). (15) I. P. Fisher, J. B. Homer, and F. P. Lossing, J . Am. Chem. SOC.,87, 957 (1965). (16) F. M. Matsunaga and K. Watanabe, private communication, 1962. (17) W. M. McFadden, J. Phye. Chem., 67, 1074 (1963).
Volume 6 g , Number 11 November 1966
3744
CHAVALIFSHITZAND F. A. LONG
abundant in the spectra of the two C3Fe isomers while C3Fs+is abundant for the two olefins. One of the interesting features in the CF2ClCF=CF2 spectrum is the occurrence of CF3+at a high yield, higher than that of the CFzCl+ ion, even though the CF3 group does not appear in the molecule itself, Similar rearrangement ions have been found in other propylenes; e.g., CF3+ and CFzH+are of approximately equal yield from CF3CH=CH2.6 CF3+ from c-C3F6must be also a rearrangement ion; CzF4+ from CF&F=CF2 is probably a rearrangement ion in view of the relatively high yield of C2H3F+from CF3CH=CH2.6 C2F3+ from CF3CF=CFz is a low yield ion in comparison with CF3CH=CH2, where C2H3+ shows the largest yield in the spectrum. Breakdown Patterns. The decomposition mechanisms are mainly established from appearance potentials, but, since very little is known about the thermochemistry of many of the radicals and ions involved, there is some uncertainty, particularly about secondary processes. C3F5+ is surely formed directly from the parent ion by a loss of halogen atom (F or C1 from C3F6 and C3F5C1,respectively). Ion-pair forming processes for C3F5C1were searched for but not detected.18 The energy needed to form C3F5+from the parent ions, as measured by the appearance potential differences AAP., is only 0.4 e.v. for CF2C1CF=CF2, but is 4.1 e.v. for CF3CF=CFz, and this presumably reports the difference between a C-C1 bond break and a C-F break. For both isomers of C3Fs the ion formed at lowest energy is C2F4+. Hence, this must be formed in a single-step decomposition along with C R . For c-C~FG this involves breaking two C-C bonds. For CF3CF= CF2 it involves a C-C bond break and a simultaneous fluorine transfer. The high-yield ions CF3+ and CF+ may be formed by single-bond breaks. For example, CF3CF=CF2+ may C2F3 and alternatively decompose t o give CF3+ CF+ C2F5. However, decompositions in which two C-C .bonds are simultaneously broken have been suggested for the propylene^'^ and in other cases also.20 Thus, CF3+from C3F6+ may be formed along with CF and CF2. It is conceivable but not plausible that CF3+ arises from secondary decomposition of C3F5+ since, if this were so, its appearance potential for each of the three parent compounds should differ by a constant value from that for the species C3F5+,and this is not observed. I t is also known2 that CF4f is not formed in significant yield from C2F4+. I t is notable that the appearance potential differences for CF+ and CF3’ are virtually the same for the three parent molec u b implying that similar bond breaks are involved. The strong implication of these results is that CF3+is
+
The Journal of Physical Chemistry
+
formed by a primary decomposition. This is possible although less sure for the formation of CF+ since in this case also several of the conceivable secondary processes to form it, as for example formation of CF+ from CF3+,21 turn out to be quite implausible. Secondary processes may be involved in the production of C2F3+ and of CF2+,but the yields of both are small. In fact, a significant comment for all three of the parent molecules is that primary decomposition processes of the molecule ions appear to be the source of the high yield products even for bombardment by 75-e.v. electrons. Thermochemical Calculations. The thermochemical calculations were carried out in a similar way to those for radicals and ions from the fluorinated ethylenes.2t3 Table I11 summarizes some thermochemical calculati0ns22-2~for the processes which can lead to the observed ions and compares the calculated appearance potentials with the experimental values. Table 111
Process
+ + + + + + + + +
(1) CFaCF=CFz + CFa+ CzFa‘ (1’) CFaCF=CFz 4 CFa+ CF (2) CF&F=CFz + C F + C2FC (2’) CFaCF=CFz -* C F + CFz (2”) CFaCF=CFz 4 C F + CzF4 (2’”) CFaCF=CF2 + C F + 2CFz (3) CFaCF=CF2 + CzFa+ CFz (4) CF&F=CFz + CFz 2CFz6 (4’) CFaCF=CFz 4 CFz+ CF (10) ~ 4 9 48 CZF4+ C Z F ~ ’ +
+
+ CF;
+ CFs +F +F
+ Fd + CFa
Cslod. A.P., e.v.
Obsd. A.P.,
15.2 17.96 15.0 17.4 18.2 21.6 19.03 18.12 19.6 12.3
15.96 15.96 18.1 18.1 18.1 18.1 19.32 19.8 19.8 12.3
e.v.
a AHr(CFa+) used in this calculation is based upon the appearance potential of CFa+in CFaH3 and is about 0.4 e.v. lower than the value based on the appearance potential in CF&zHa.E The heat of formation of CzFa is estimated to be -1.1 e.v.a * AHf(CFa+)as in a; AHf(CF) and AHf(CF2)as reported previous1y.a A H ~ ( C F + )as reported previouslya; aHr(C2F6) i0 based upon an estimated C2F6-F bond energyzzand is -9 e.v. Using the appearance potential of CzFa+ in CzF4.2 e Using the appearance potential of CFz+ in CzFa.2 Based upon the ~ (CZF4). ~ ~ ~ ~ ~ heat of reaction: c-C4F* 4 ~ C ZandF I. P.
’
(18) See footnote a , Table 11. (19) H R. Harlem, ASTM E-14 Mass Spectrometry Conference, chican;, - 111., 1961, (20) R. R. Bernecker and F. A. Long, J . Chem. Phys., 39, 253 (1963). ~ & H ; ~ ~ $ i ~ Reese* ~ ) Mand * F* L* Mohlers Res.
J.
W.M. D. Bryant, J. polyner &i., 56, 277 (1962). (23)J. N.Butler, J. Am. C h m . sot., 84, 1393 (1962). (24)H. c. DUUS, I&. ~ n gChem., . 47, 1445 (1955).
(22)
APPEARANCE POTENTIA-
AND
MASSSPECTRA OF C36, CaF5C1, AND C-CsF,
The heat vf formation of CF3CF=CFz is known to be -11.22 e . ~ . and , ~ ~from the appearance potential of C$4+ from this compound the heat of formation of CF2 was previously calculated to be - 1.6 e.17,~Useful calculations can also be made for production of CF3+, CF+, CzF3+, and CF2+. If only two fragments (one positive and one natural) are formed in a process (e.g., process 1) and there is agreement between the computed and observed values for the appearance potential of the positive fragment, then this is probably the process which takes place at the threshold energy. The species CF3+ is probably formed by a simple primary bond split which additionally gives CZF3. If three fragments are formed in a process (e.g., process 3), then, even if there is agreement between the computed and observed appearance potentials, it is still not certain whether the process takes place the way it is written, i e . , in a single step. Production of CF+ appears to come from process 2” of Table 111. On the basis of the calculations of Table I11 the most likely process for formation of CzF3+ is (3) and that for formation of CFz+is (4’).
Not enough information is available on heats of formation of c-C3F6and C8F5C1 to permit detailed calculations for reactions of these species. However, comparison of their behavior with that of C3F6 permits a few useful points to be made. Since the following two processes take place CFsCF=CFz
-+-
+ CF2 A.P.(CzF4+) = 13.15 c-CaF6 +CzF4+ + CFz AnP.(C2F4+)= 11.85 CzF*+
(5)
(6)
one calculates AHt(c-CaF6) - AHr(CF3CF=CF) = 1.30 e.v. Since AH~(CF~CF=CFZ)= -11.22 e.v., this implies AHf(c-CsFe) = -9.9 e.v. If the C3F5+ ion has the same structure whether formed from CF3CF=CF2 or C-C~FB,then a slightly smaller value of 1.0 e.v. is calculated for the difference in heats of formation, of these two compounds. On the other hand, the possibility cannot be excluded that at threshold energies C3F5+from C-CQFG is the cyclopropyl radical ion and not the allyl ion. The similar difficulty with the C3H5+ion from c-C& has been noted before.25 A further anomaly concerning the C3F5+ ion is observed. If the C3F5+ions formed from CF3CF=CFz and CF2C1CF=CF2 are the same and if no excess energy is involved in either process, then the difference in appearance potentials of C3F6+from these two compounds should be equal to the difference in the C3F5-F and C3F5-Cl bond energies. CFzCF=CFz +C3F5
+ F +C3Frj+ + F
(7)
3745
CFzClCF=CF2 +C3Frj
+ C1
---t
C3F5+
+ C1
(8)
Thus A.P. [C3F,+(CF3CF=CFz)] A.P. [C~F~+(CFZCICF=CFZ)] = D(C3F6-F) D(C3Fb-Cl) = 3.96 e.V.
If the value A.P. [C3F5+(CF2ClCF=CFz)] = 11.818 is used, the calculated bond energy difference is 3.4 e.v. A computed difference of 3.5 to 4 e.v. in the C-F and C-C1 bond energies is quite out of line with the normally observed differences of 1 to 2 e.v. When the C2F3+ appearance potentials from CzF4 and CzF3Clwere compared, the difference between the values agreed with the difference in bond energies (1.2 e . ~ . ) . ~ The appearance potential of C3F5+from c-C4F8 was measured in order to get some further information concerning the heat of formation of this ion. The appearance potential of the other major ion in the c-C4Fsspectrum, namely, CZF4+, was measured as well. The value obtained in either case was 12.3 f 0.1 e.v. in excellent agreement with other recent measurements.8 These values correspond, presumably, to the reactions c-C~F + ~ C3F5+ f CF3
(9)
+ CzF4
(10)
c-C~F + ~ C2F4+
The appearance potential for process 10 is calculated in Table I11 and agrees very well with the observed value. If no excess energy is involved in process 9, one computes AHt(CaF5+)= +2.2 e.v. If this value is used to recalculate an appearance potential for reaction 7, one arrives at an observed excess energy for reaction 7 of -1 e.v. This question will be discussed further in the next section on ionization efficiency curves. Ionization Eficiency Curves. The ionization efficiency curve of C3F6+ (Figure 1) resembles closely the CZF4+ curve from CzF4 which has been discussed in a previous paper.4 The sharp break in the C3Fs+ curve occurs about 4 e.v. above the threshold, at the energy at which C3F5+starts to appear; the analogous break in the CzF4+ curve occurred about 6 e.v. above threshold, a value which corresponded to the energy at which CZF3+ started to appear. CzF4+from CF&F=CFz appears at an energy lower than the energy at which the break in the C3F6+ curve occurs and exhibits normal behavior at the threshold of its appearance; ie., it is parallel to the Ar+ curve (Figure 2). On the other hand, all the other curves (C3F5+,CF3+, and CF+) exhibit long tails since in the energy region where they appear the parent ion concentration remains practically (25) R. F. Pottie, A. G . Harrison, and F. SOC.,83, 3204 (1961).
P. Lossing, J. Am. Chem.
Volume 69,Number 11 November 1966
CHAVALIFSHITZAND F. A. LONG
3746
constant. The most obvious explanation for these results is that there is competition from decomposition into fragments4 and that this affects the ionization efficiency curves of both parent and fragment ions. In the absence of more complete thermochemicaldata it is not possible to determine whether statistical theory will account for the observed mass spectra. There is considerable uncertainty whether the AA.P. values represent true thermochemicaldifferences and hence are truly activation energies for the decompositions. In several cases the AA.P. values for parent ion and primary product are large enough to indicate activation
energies well over 3 e.v. I n such cases decomposition rates should be very small for energies close to threshold.a This would invalidate the use of AA.P. as the activation energy and might also be the source of the long “tails” of the ionization efficiency curves. However, these and other uncertainties must await for resolution the development of more extensive thermochemical data.
Acknowle~ments. We wish to thank Dr. J. D. Morrison and Dr. F. H. Dorman for parallel appearance potential measurements of ions from C3FsCI.
Some Observations Concerning the Positive Ion Decomposition of C,F, and C,F, in the Mass Spectrometer’
by Chava Lifshitz and F. A. Long Department of Chemistry, CorneU University, Ithaca, New York (Received April 19, 1966)
Appearance potentials were measured for the positive ions formed by carbon-carbon and carbon-fluorine bond ruptures in CzFaf and CsFsf. For these ions the carbonfluorine bond breakage occurs at an energy higher by 2.0 and 2.7 e.v., respectively, than the carbon-carbon breakage. I n terms of the statistical theory of mass spectra, these values represent differences in activation energies for the unimolecular decomposition reactions. Relative rates of the two processes have been computed for the two compounds, for various internal excitation energies. I n order to fit theory to experiment, one has to make extreme assumptions about the structures for the activated complexes of the two processes. Since the assumptions are not reasonable, we conclude that the statistical theory is not applicable to fluorocarbon decomposition. Most probably, these reactions involve direct decompositionsfrom repulsive electronic states.
Introduction The statistical theory for the production of positive ion mass spectra2 assumes: (a) there is a large nllmber of densely spaced electronic states for a polyatomic molecule ion:, (b) if the Franck-Condon ionization of a molecule leads to an excited electronic state, the elec\
goes any chemical reaction; (c) there is complete equilibration of the excitation energy among the various internal degrees of freedom of the molecule ion before decomPosition OCCUrS. A consistent theory, based on
,
is transformed into energy Of the lowest electronic state before the molecule ion underThs Journal of Physical Chemkdry
(1) Work supported by the Advanced Research Projects Agency through the Materials Science Center. (2) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H.
Eyring, Proc. ~ a t iA&. .
s~i. u. s., 38,667 (1962).
