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 thermochemical data 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 thermochemical differences 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 decompositions from 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).
POSITIVE IONDECOMPOSITION OF C2F6 AND CaFs IN
THE
these assumptions, has been developed and applied to paraffinic hydrocarbons with moderate success.2, The present study is an attempt to find out whether the theory holds equally well for saturated fluorocarbons. In working with hydrocarbons it has frequently been noted that, in comparison to the breaking of a skeletal C-C bond, the breaking of a C-H bond is a low yield p r o ~ e s s . ~ This ~ * was explained in the early development of the theory2 as well as in later stages316by assuming a rather rigid activated complex for the C-H bond breakage and assuming a relatively nonrigid activated complex for the carbon-carbon breakage, by either reducing some of the vibrational frequencies or assuming free internal rotations. The present study compares the C-F breakage with the C-C breakage in a group of fluoroparaffins. Appearance potentials and relative yields of the processes are presented. Theoretical calculations, based upon statistical theory, are carried ‘out to compute the relative yields of the processes. It will be shown that some unacceptably extreme assumptions concerning the activated complexes have to be made in order to fit theory to experiment. Experimental Section and Results The data were taken on a Consolidated Engineering Corp. mass spectrometer, Model 21-401, which has been modified as described previously.6 The percentage yields of the principal positive ions in the spectra of C2F6 and C3Fs are given in Table I. Listed are also the appearance potentials for the ions resulting from simple carbon-carbon and carbon-fluorine bond breakages in the two molecules. The ap-
Table I : Appearance Potentials and Percentage Yields at 70 E.v. Product species
CF CF2 + CFa + CzFd +
+
CzFs CaF,
+
+
-CzF% at 70 e.v.
5.2 3.2 66.5 0.25 24.9
A.P., e.v.
14.3 16.3
-CsF7% at 70 e.v. A.P., e.v.
4.0 0.8 71.2 3.2 7.5 13.3
MASSSPECTROMETER
3747
Great care was taken to measure the difference in appearance potentials for the two processes (Le., C-C and C-F breakage) precisely. Semilogarithmic plots of the ion current vs. the ionizing voltage were drawn for CF3+ and C ~ F S + from C2F6 and for CF3+ and C3F7+from C3Fs. The curves for each pair of ions were parallel over a twofold change in ion current. The difference in appearance potentials in the case of C&”Fwas found to be 1.96 v., and in the case of C3Hsit was 2.72 v. The present results show a different behavior than the one encountered for hydrocarbons. The C-F bond breakage needs an energy higher by 2.0-2.7 e.v. than the C-C breakage, and yet its yield is fairly high (about 0.4 of the C-C breakage yield in C2Fsand about 0.2 of that yield in C3Fs; Table I). Figures 1 and 2 give the directly recorded dependence of the ionic currents upon ionizing energy. Relatively high yields are observed for the C-F breakages a t the threshold, and no “tailing” is observed at the foot of the ionization efficiency curve for this process. These curves were obtained using an electron beam nonhomogeneous in energy, so that the thermal spread in energies “smears out” any sharp breaks in the ionization efficiency. However, the upward curvature in the CF3+curves from both compounds, which holds over a fairly wide energy range (much wider than the spread in the ionizing electron energy, as shown from a calibrating He+ curve), before the curves get to be linear, is believed to be due to onsets of excited electronic states for the parent ion. (The parent ions are missing in the spectra of both molecules.) This curvature is also believed to be the reason for the higher appearance potentials measured by Bibby and Cartergas compared to ours, since they used the method of “extrapolated linear intercepts,” while we used the “vanishing current method.” The results for a mixed fluorohydrocarbon are given for comparison in Table II.l0 One observes again the relatively low yield, when considering the low appearance potential, for the C-H breakage and the relatively
14.4 15.3 17.1
pearance potentials were measured to rt0.l e.v. There is agreement in the values of the appearance potentials for the CF3+ ion from both compounds with those obtained by Mohler, Dibeler, and Ree~e.~,S There is, however, considerable disagreement with more recent measurements by Bibby and Carter.g
(3) M. Vestal, A. L. Wahrhaftig, and W. H. Johnston, Aeronautical Research Laboratories Report ARL 62-426,Sept. 1962. (4)L. Friedman, F. A. Long, and M. Wolfsberg, J . Chem. Phys., 30, 1605 (1959). (5) M. Wolfsberg, ibid., 36, 1072 (1962). (6) A. B. King and F. A. Long, ibid., 29,374 (1958). (7) F. L. Mohler, V. H. Dibeler, and R. M. Reese, J. Res. Natl. Bur. Std., 49,343 (1952). (8) V. H.Dibeler, R. M. Reese, and F. L. Mohler, Phys. Rev., 87,. 213 (1952),abstract. (9) M. M. Bibby and G. Carter, Trans. Faraday SOC.,59, 2455 ( 1963). (10) C.Lifshitz and F. A. Long, J. Phys. Chem., 67,3731 (1965).
Volume 69,Number 11 November 1966
CHAVALIFSHITZAND F. A. LONG
3748
Table 111: Appearance Potential of Selected Ions from n-C4Flo Appearance potential, e.v.
14.3 14.6 15.0
1ONlZ1MG
tions. It is, therefore, important to know the energies involved in the transitions from the parent molecule to these two fragment ions. If it is assumed that CF3+, C2F5+, and C3F7+are formed by simple carbon-carbon breakages from nC4Floand if it is further assumed that the two types of C-C bonds in n-CIFlo are of similar strength, then the observed differences in appearance potentials of the three ions from this molecule (Table 111) reflect differences in the ionization potentials of the radicals. The results of Table I11 would predict, under these assumptions, that
VOLTAGE I I R B I T R I R Y ZERO1
Figure 1. Directly recorded ionization efficiency curves for CF3+ and C Z F ~from + CZFB.
+ 0.3 e.v. I.P.(C3F7) = I.P.(CF3) + 0.7 e.v.
I.P.(C2F6) = I.P.(CF3)
I ,
0
I
I
2
3
I
*
I
I
I
5
6
?
.
I
I
I
a
¶
IO
io*IZINC VOLTAM 11111TRARV ZERO1
Figure 2. Directly recorded ionization efficiency curves for CFa+and CaF,+from C ~ F B .
high yield for the C-F breakage, when considering its relatively high A.P.
The ionization potential of CF3is itself still an unsettled matter. Recent results gave I.P.(CF,) = 9.1l0 and showed that excess energy was involved in the formation of CFs+ from fluoroparaffins. Fisher and Lossing estimated recently that I.P.(CF3) = 9.5," and their direct electron impact measuremenk on C2F5 and CaF7 gave" I.P.(C2F5) = 10.0; I.P.(n-C3F7) = 10.0, and I.P.(i-C3F7) = 10.5. Therefore I.P.(i-C3F,)
Table I1 Dissociation process
/-CH3CFz+
CHaCHF2-+
CFzH
\+CHaCHF+
+
++CHa +F
Appearance potential,10e.v.
Percentage yield
12.3
23.0
13.2
48.4
14.9
5.9
In order to get some information concerning the thermochemistry of the CzF5f and C3F7+ radical ions we have measured their appearance potentials from n-C*Flo. The results are presented in Table 111. Some Observations Concerning Thermochemistry. CzF5+and C3F7+are involved in the processes studied in C2Fsand C3F8,which will form the basis for our calculaThe Journal of Physical Chemistry
+ 0.5 e.v. = I.P.(CF3) + 1.0 e.v.
I.P.(C2F6) = I.P.(CF3)
Both sets of results (direct electron impact and appearance potentials from n-C4Flo) show that the following relationship surely exists, namely: I.P.(CF3) < I.P. (C2Fs) 6 IUP.(C3F7). From the above considerations, concerning the ionization potentials of CzFs and C3F7, one can draw a conclusion about the thermochemistry of the processes leading to C2F5+ and C3F7+ in CzFe and C3F8, respectively. From self-consistent estimated heats of formation of the fluorocarbon radicals CF3, C2F5, and C3F7,12one computes that the C-C bond is weaker than the C-F bond in both C2F8and C3F8 by approximately 1.8 e.v. If the ionization potential of CF3 were equal (11) I. P.Fisher,J. B. Homer, and F.P. Lossing,J. Am. Chem. SOC., 87, 957 (1965). (12) W.M.D.Bryant, J. Polyter Sci., 56, 277 (1962).
POSITIVE IONDECOMPOSITION OF CZF6 AND C3Fs IN
THE
to those of C2F6 and C3F7, this would lead to an observed difference of -1.8 e.v. for the C-C breakage and C-F breakage in either C2F6 or C3F8, to give the respective radical ions. The observed differences are higher than 1.8 e.v. in both compounds, since the ionization potentials of CZF6 and C3F7are higher than that of CF3. On the basis of the above given differences in ionization potentials of CF3, C2Fs, n-C3F7, and i-C3F7, one can compute the thermochemical differences. The observed difference of 2.0 e.v. coincides within 0.2 e.v. with the expected thermochemical difference for CzFa. It is hard to decide whether the C3F7+ radical ion formed from C3Fs is n-C3F7+ a t threshold energies. However, AHt(C3F7+) and AHt(i-C,F?+) are approximately the same since i-C3F7 is more stable than n-C3F,,12 but its ionization potential is higher." Thus, again the observed difference of 2.7 e.v. in the case of CaFs coincides within 0.2 e.v. with the expected thermochemical difference. In conclusion, no appreciable kinetic shift is observed in the appearance potentials of CzFs+from CzFs and of C3F7+from C3F8. The experimentally observed appearance potentials are considered to coincide with the expected values from thermochemistry. Some Observations Concerning Negative Ions. In the present study neutral fragments are assumed to be formed along with the positive ions; i.e., ion-pair forming processes are excluded. Since the parent molecule ions are absent in the spectra of C2F6 and C3F8, even at low ionizing voltages one might have expected ion-pair forming processes. Negative ions were observed in both s p e ~ t r a . ~These were, however, found to be due to electron-capture processes rather than ionpair forming processes. Theoretical Calculations. Calculations were carried out for the relative yields of carbon-carbon and carbonfluorine breakages in C2F6and C3F8. The differences in appearance potentials (2.0 and 2.7 e.v., respectively) were taken as differences in activation energies of the processes involved. Since no parent ion is observed in either molecule, the plausible assumption to make, in terms of the theory, is that the activation energy for CF3+ formation (the lowest appearing and most abundant ion in either case) is, in fact, zero. The activation energies for the C-F breakage are then 2.0 and 2.7 e.v. in C2F6 and C3F8, respectively. [The unimolecular rate constant a t the threshold energy is given by6si3IC = l / h N * ( E ) , where h is Planck's constant and N*(E) is the density of states of the active molecule a t internal energy E. This expression forms the basis for our assumptions that, if the activation energy for a process in the mass spectrometer is zero, no parent ion will be observed. If the activation energy is zero, k =
MASSSPECTROMETER
3749
l/hN*(O); now from known dependences of sums of vibrational states upon internal energy, one can estimate a maximum value for N*(O) of approximately 100 e.v.-'. Thus, the minimum value of IC at threshold is 2.4 X 10l2see.-'; k a t any higher energy E will certainly be higher. The fraction of parent ions remaining at the sec. is then less than lO-'O', i.e., negligibly end of small. ] The basis for the calculations is the unimolecular reaction rate theory as developed by Marcus.13 The relative yields of the C-C and C-F breakages are computed as a function of the internal energy E of the active molecule.s,la Let Ea(C-C) and Ea(C-F) be the activation energies for the C-C and C-F breakages, respectively, and E+ (C-C) = E - Es(C-C) and E+(C-F) = E - Es(CF) be the internal energies of the two activated complexes. Let Y(C-C) and Y(C-F) be the yields of carbon-carbon and carbon-fluorine breakages, respectively. Then (Y(C-C)/Y(C-F))E equals the sum of states for the C-C activated complex a t E+(C-C) over the sum of states for the C-F complex a t E+(C-F). The sum of vibrational-rotational states for energy E+ is given by the formulaI3
+
P(E,)[(rr/2 l)]-l X ( S ~ ~ / / h 9 X >'/~ Ev
