575
ELECTROX I M P A C T STUDIES O F TRIHALOMETHANES
Electron Impact Studies of Some Trihalomethanes: Trichloromethane, Dichlorofluoromethane, Chlorodifluorsmethane, and Trifluoromethanel
by Don L. Hobrock and Robert W. Kiser Department of Chemistry, Kansas State University, Manhattan, Kansas
(Received October 7, 1968)
An electron impact study of the four trihaloniethanes, in which the halogen atoms are chlorine and fluorine, has yielded information concerning the heats of formation of halogencontaining organic ions. Partial mass spectra of these compounds are given and ionization and dissociation processes are assigned based on the experimental energetic data. A heat of formation of the CFZradical, equal to -20 kcal./mole, is derived from the results.
Introduction
Experimental
Several determinations of appearance potentials for various halogenated hydrocarbons have been made. Because of their various unique properties, the fluorocarbons frequently have been the subject of such investigations. In some instances there has been agreement between different reports; in other instances, the discrepancies encountered mere quite serious. Our objectives are: (a) to study mass spectrometrically a fairly wide variety of halogen-containing compounds in an effort to systematize more carefully the coverage of the different possible molecules and ions, and (b) to attempt to correlate this information to give more reliable values for the thermochemical properties of‘ these compounds and their gaseous ions. To initiate tlhese efforts, me have selected the series of trihalomethanes listed in the title of this paper. None of these compounds has been studied in detail by mass spectrometry. However, the ionization potentials for all but dichlorofluoromethane have been determined previously. In this study we have determined the mass spectra and the appearance potentials of the principal positive ions formed under electron bombardment from each of these molecules. Our results are cornpared to data available from other studies wherever possible. In particular, we have noted that the heats of formation of both the CX,+ and the CHX,+ species exhibit a monotonic increase as these ions lbecome more chlorinated (less fluorinated).
The samples of CHF3, CHFzC1, and CHFClt were obtained from a commercial source (Matheson Co., Inc.) The minimum purities of CHF3 and CHFzCl mere specified as 98.0 and 99.9%, respectively. Each was used directly from the cylinder without further purification. The purity of the CFHC1, was listed as 99.90% and was purified further by use of a preparative gas chromatograph. The CHC13 was “spectroquality” reagent grade (Matheson Coleman and Bell) and was used without further purification. The mass spectra and appearance potentials were obtained using a Bendix t,ime-of-flight mass spectrometer. The instrumentation has been described previously. Appearance potentials were determined by interpretation of the ionization efficiency curves using Warren’s extrapolated voltage differences method.3 The values were confirmed by use of the energy compensation t e c h n i q ~ e . ~Xenon ( I = 12.13 e.v.)5 and argon ( I = 15.76 e.v.)Sserved as calibration gases (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract No. AT(ll-1)-751 with Kansas State University. This study is a portion of a dissertation to be presented by D. L. Hobrock t o the Graduat,e School of Kansas State University in partial fulfillment for the requirements for the degree of Doctor of Philosophy. ( 2 ) E. J. Gallegos and R. W. Kiser, J. Am. Chem. Soc.. 83, 773 (1961). ( 3 ) J. W.Warren, S a t w e , 165, 811 (1950). (4) R. W.Kiser and E . J. Gallegos, d . Phys. Chem.. 66, 947 (1962). (5) C. E. Moore, National Bureau of Standards Circular 467, Vol. 3 , V. 9. Gol-t. Printing Office, Washington. D. C., 1958.
Volicme 68, -VVumber S
March, 1964
576
DONL. HOBROCK AND ROBERT W. KISER
Table I : Heats of Formation and Appearance Potentials of the Principal Ions Formed from Trifluoromethane
Ion
Relative abundance a t 70 e.v.
Appearance potential, e.v.
CF CFZ CFzH CF3 +
40 5 11 4 100 0 53 0
202 f O 4 175 f O 3 164 f 0 3 14 67 f 0 20
+
+
+
Probable process
CHF,
+ + + + + +
CFHF F CF2+ H F+ CFzH+ F CF3+ H -+
+
-+
AHf (ion), kcal./mole
350 250
197 124
Table I1 : Heats of Formation and Appearance Potentials of the Principal Ions Formed from Chlorodifluoromethane Relative abundance a t 70 e.v.
Ion
CF + c1+ CFz CFzH+ CFCl+ CFClH A CFzClH +
+
a
17 7 14 1 54 100 0 2 0
16 5 14
Appearance potential, e.v.
17 30 f 0 205 f 0 161 f 0 12 59 f o 159 4 0 15 11 i 0 12 69 =t0
15 3
Probable process
CFzClH
3
15 3 15 15
+ + + + + + + + + +
CFf HF C1 C1+ CFz H + CFz+ H C1CFzH" C1 -+ CFCl+ H FCFClH+ F -+ CFzClH+ -+
-+
-+
-+
AHf (ion), kcal./mole
323 ( - 20)"
266 150 264 218 181
This is the derived heat of formation of the CFZradical.
Table I11 : Heats of Formation and Appearance Potentials of the Principal Ions Formed from Dichlorofluoromethane
Ion
CF
+
c1+ cc1+ CHCl+ CHFCl+ CHFClz+
Table IV:
Relative abundance a t 70 e.v.
78 77 12 8 64 100 0 56
Appearance potential, e.v.
169 f 0 2 230 1 0 3 183 f O 2 190 f O 2 12 69 f 0 15 12 39 f 0 20
Probable process
+ HC1 + C1 CCI- + HC1 + F CHCl+ + C1 + F CHFCl+ + C1
CClzFH --c C F + -+
+ + -+ -+
c1++ (7) CHFClz+
AHf (ion), kcal./mole
317 359 325 198 219
-
Heats of Formation and Appearance Potentials of the Principal Ions Formed from Trichloromethane
Ion
CH C1+
+
cc1+ CHCl + CHCIzCHC13+
Relative abundance a t 70 e.v.
71 25 7 36 8 15 4
100 0 34
Appearance potential, e.v.
239 1 0 3 220 1 0 3 163 1 0 2 175 f 0 2 11 64 f 0 20 11 39 1 0 12
for the appearance potential determinations. All appearance potentials were determined with synthetic mixtures of the calibrating gas and the compound under investigation. The Joiirnal of Phusical Chemistry
Probable process
CC1,H
+ + + + + +
C H + 3C1 C1+ -J- CH Clz -+ CC1+ HC1 C1 -+ C H C P 2C1 -,CHClz- C1 -t CHCl,+ -+
-+
AHf (ion), kcal./mole
442 340 345 323 216 239
Results The results of the experimental measurements of the mass spectra and the appearance potentials of the major ions are given in Tables I-IV. The partial
577
ELECTRON INPACT STUDIES O F TRIHALOMETHANES
mass spectra we determined at 70 e.v. for these coinpounds compare favorably with those reported in the literature.6-9 The appearance potentials for the various ions, together with the derived heats of forma,tion for these ions and for a few halogen-containing radicals, are discussed below. The error values shown indicate the 2a precision of replicate determinations and do not necessarily reflect the absolute error involved in the measurements. The heats of formation of the gaseous molecules which were employed in the thermochemical calculations with the varjous molecules are as follows: CHF3, - 162.6 kcal./molelO; CHC13, -24 kcal./mole”; CHFCI,, -66.5 kcal,/mole12; and CHF2C1, - 112 kcal./mole.12 The second column of each of the four tables gives the partial mass spectrum of the particular trihalomethane studied. It is noted that the relative intensity of the parent molecule ion tends to decrease with an increase in the number of fluorine atoms in the molecule. I n the case of CHF,, the parent molecule ion is not measurable after the carbon-13 isotope contribution (from CF3+) has been subtracted. The CHXz+ peak IS the largest in the spectrum of these methanes; wherever possible, the elimination of a chlorine atom is apparently favored over the elimination of a fluorine atom in the dissociation processes. Although our experimental procedure does not allow us to detect or study negative ions, the probable processes which we have proposed infer that negative ions are formed along with a hydrogen atom when a CX2+ion is produced. CX3+ Spectes. The only CX3+species for which an appearance potential was obtained was CF3+. The A(CF3+) = 14.67 0.20 e.v. from CHF,, as shown in Table I, gives a AHf(CF3+) = 124 kcal./mole. We note that our determination agrees with the appearance potential of 14.53 f 0.05 e.v. reported by Farmer, et! aZ.13 Values of AHf(CF3+)= 118 kcal./mole14 and 120 kcal./molel0 have been reported previously. However, Lifshitz and Long1* report a much lower value of 86 kcal./mole for AHf(CF3+). Additionally, one may calculate a value of AHf(CF,+) = 119 kcal./mole using the appearance potential of CF3+from CF4, as determined by Warren and Craggs. l7 However, recent experimental results1s from studies of tetrahalomethanes indicate a lower value (of about 110 kcal./mole) for AHt(CF3f). CX2+ Species. Two ions of the general formula CX2+mere studied: CF2+ and CFCl+. The heat of formation of the CFz+ion was determined to be 266 and 250 kea1 ./mole, respectively, from our appearance potential studies of CHFzCl and CHF,, assuming the processes given in Tables I and 11. These values are in
agreement with the value of 249 kcal./mole calculated from A(CF2+) = 20.3 e.v. from CF4I9using the process CF4 +CF2+
+ F, + e
(1)
and the value of 251 kcal./mole calculated from A (CF2+) = 18.2 e.v. from CFzC1220and assuming the process CF2Cl, +CF2+
+ 2C1 + e
(2)
The higher appearance potentials of CF2+ from CF4, namely, 22.33 e.v.I4and 22.4 allow one to calculate values of AHf(CF2+)= 260 and 262 kcal./mole if, as is reasonable, the higher energy process is CF4 +CF2+
+ 2F + e
(3)
The heat of formation of the CFCl+ ion, as given in Table 11, is 261 kcal./mole. This is in agreement with values of 267 kcal./mole and 278 kcal./mole from CFCla and CF&1, respectively, from the work of Warren and Craggs,” if the processes are taken to be CF’Cl, --+ CFCl+
+ 2C1 + e
(4)
(6) “Mass Spectral Data,” American Petroleum Institute Research Project 44, National Bureau of Standards, Washington, D. C., No. 119, 468, 604, and 691. (7) V. H. Dibeler and R. B. Bernstein, J. Chem. Phys., 19, 404 (1951). (8) “ M a s s Spectral Data,” Manufacturing Chemists Association, Research Project, Chemical Thermodynamics Properties Center, Agricultural and Mechanical College of Texas, College Station, Texas, KO. 134. (9) S. E. Kupriyanov, R. V. Dzhagatspanyan, M. V. Tikhomirov, and E.N. Tunitskii, Zauodsk. Lab., 21, 1182 (1955). (10) C. R . Patrick, “Advances in Fluorine Chemistry,” Vol. 2, M. Stacey, J. C. Tatlow, and A. G. Sharpe, Ed., Butterworths, London, 1961, pp. 1-34. (11) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and 1. Jaffe, “Selected Values of Chemical Thermodynamics Properties,” Circular 500, National Bureau of Standards, Washington, D. C., 1952. (12) S. M. Skuratov and V. P. Kolesov, Zh. Fiz. Khim., 35, 567 (1961). (13) J. B. Farmer, I. H. S. Henderson, F. P. Lossing, and D. G. R . Marsden, J . Chem. Phys., 24, 348 (1956). (14) R. I. Reed and W. Snedden, Trans. Faraday Soc., 54, 301 (1958). (15) W. D. Steele and F. G. A. Stone, J . A m . Chem. Soc., 84, 3450 (1962). (16) C. Lifshita and F. A. Long, “Appearance Potentials and the Mass Spectra of Fluorinated Olefins,” presented before the 10th annual meeting of ASTM Committee E-14 on Mass Spectrometry, New Orleans, La., June 3-8, 1962; “Tinimolecular Decomposition of Positive Ions from Fluorinated Olefins,” presented before the 1963 Summer Symposium, Division of Physical Chemistry, American Chemical Society, Salt Lake City, Utah, July 7-10, 1963. (17) J. W. Warren and J. D. Craggs, “Mass Spectrometry,” Institute of Petroleum, London, 1952, p. 36. (18) D. L. Hobrock and R. 1%’. Kiser, to be submitted for publication. (19) V. H. Dibeler, R . AI. Reese. and F. L. Mohler, J . Res. S a t l . Bur, Std., 57, 113 (1956). (20) R. F. Baker and J. T. Tate, Phys. Rev., 53, 944 (1938)
Volume 68, IVumber 3
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578
DONL. HOBROCK A N D ROBERT W. KISER
and
ions, there is a monotonic increase in the heats of formation of the ions. I n the case of the CHF2Cl molecule, one notes that the ionization potential listed in Table I1 is slightly greater than the appearance potential for the CF2H+ ion. But within experimental error, these two potentials could be reversed, and very likely are. However, even from the relative abundances of the CHF,Cl+ and CHF2+ ions in the mass spectrum of chlorodifluoromethane, one would estimate that the appearance potential of CHFz+ would be only slightly greater than the ionization potential of this molecule. C H X Z +Species. The three possible CHX2- species were observed in this electron impact study. The heat of formation of CHF2+,as listed in Tables I and 11, exhibits a range of 150-197 kcal./mole. The lower value is in good agreement with AHf(CHFZ+) = 142 kcaI.,/mole, reported by Lifshitz and Long.’O The heats of formation determined for the CHFClf ion are 198 and 218 kca!./mole. Again, we believe that the lower value is the one which is more nearly correct. The AHf(CHCl,+) = 216 kcal./mole, as determined in our laboratory, compares favorably with Af3f (CHCl2+) = 215 f 3 kcal./mole, as reported by Harrison and Shannon.2 3 C H X + Xpecies. The values of H f ( C H C l + ) , as shon n in Tables I11 and IT’, are 325 and 323 kcal./mole, according to the assigned processes. The C H F + ion a t mle = 32 could not be studied, unfortunately, because of an 02+ background interference. However, through a series of comparisons and correlations, we have estimated that the value of AHf(CHF+) will be about 263 kcal. /mole. CF, Radical. From the processes assigned and heats of formation for these ions, we find it possible to obtain a value for the heat of formation of the CFZ radical. If the process for the formation of the chlorine positive ion is
CF3C1 --+ CFC1+
+ 2F + e
(5)
analogous to reactions given in eq. 2 and 3. C X + Speciea. As shown in Tables 1-111, the heat of formation of CF+ ion falls in the range of 317-350 kcal./mole, from the molecules we hare studied and the process assignments made. This range of values does not agree with the value of 296 kcal./mole given in the preliminary reports by Lifshitz and Long.lG From the appearance potential for CF+ from CF,, as reported by Dibeler, et a1.,l9 one may calculate AHr(CF+) = 363 kcal./mole, assuming the ionization and dissociation process to be
CF, --+ C F +
+ Fz + F-
(6)
If, instead of this latter value, an appearance potential of 27.2 e.v. for this ion from CF, is empl0yed,~7and the process chosen is eq. 7 CF, + CFf
+ 3F + e
(7)
AHf(CF+) = 352 kcal./mole. Similarly, one calculates from A ( C F + ) =21.7 e.v. from CF2C12, as determined by Warren and Craggs,17 that AHf(CF+) = 313 kcal./mole if the process occurring under electron impact is CF,Cl2 +C F +
+ 2C1 + F + e
(8)
The heat of formation of CCl +, as given in Tables 111 and IT’, is 358 and 345 kcal./mole, respectively. These agree with a value of 340 kcal./mole listed by Field and Franklin.21 A(CC1T) = 19.3 e.v. from CCh by Warren and Craggs17 gives AHf(CCl+) = 344, if the process is CC1,
--j
CCl+
+ 3C1 + e
(9)
Also, A(CCI+) = 21.2 e.v. from CCl3Fl7leads to AHE (CCl+) = 347 kcal./mole, assuming neutral fragments of 2C1 F. From this, we conclude that a “best value” for the heat of formation of CCl+ is 350 f 9 kcal. /mole. C H X 3 +Species. Three ions of the general formula CHX3+were observed and studied in this investigation. These are shown in Tables II-ITT, which also indicate the processes assigned which lead to AHf(CHF&l-) = 181 kcal./mole, aHI(CHFClz+)= 219 kcal.,’mole, and AHf(CHCI3+) = 239 kcal./mole. We were unable to observe- any significant quantity of CHF3+ from Buoroform, as already noted; however, in the spectroscopic determination by Stokes and Duncan,22 the ionization potential of fluoroform was measured and found to be 13.84 e.\-. From this, we calculate AHf (CHF3+) = 157 kcal./mole. Again, we note that as chlorine atoms replace fluorine atoms in this class of
+
The Journal of Phwical Chemistry
CF,CIH ----f Cl+
+ CF2 + H + e
(10)
then we calculate AHf(CF2) to be -20 kcal./mole. This value is in good agreement with values of -17 kcal. /molez4 and - 18 kcal. imole. 26 However, other (21) F. H . Field and J. L. Franklin, “Electron Impact Phenomena and the Properties of Gaseous Ions,” Academic Press, Inc., Ken; York, N. Y., 1957, p. 289. (22) S.Stokes and A. B. F. Duncan, J. Am. Chem. Soc., 80, 6177 (1958). (23) A. G. Harrison and T. 1%’. Shannon, Can. J. Chem., 20, 1730 (1962). (24) R . W . Potocki and D. E. Mann, Sational Bureau of Standards Report, KO,1439, U. S. Govt. Printing Office, Washington, D. C . , February 15, 1962. (25) J. Reed and B. Rabinovitch, J . Phys. Chem., 61, 598 (1967).
579
COMBUSTION CA4LORIMETRYO F SILICON AXD ORGb?;OSLLICON COMPOUKDS
values of - 10,14-30 f 20,26 -36,?' -37,16 and -35 i 10 28 kcal. /mole have been reported in the literature. It is obvious from these studies, and others, that additional iiivestigstioiis of the various halogen-containing ions and radicals are required. We hope that investigatioiis currently being conducted in our laboratories will aid in resolving these remaining problems.
Acknowledgment. We wish to thank Mr. K. W. Tatkina for his aid in some of the preliminary gas chromatographic analyses.
-
(26) J. L. Margrave, J . Chem. Phys., 31, 1432 (1969). (27) L. Brewer, J. L. Margrave, R. F. Porter, and K. Wieland, J . Phys. them., 65, 1913 (1961). (28) J. L. Margrave, S a t w e , 197, 376 (1963).
A New Approach to the Combustion Calorimetry of Silicon and Organosilicon Compounds.
Heats of Formation of
Quartz, Fluorosilicic Acid,, and Hexamethyldisiloxanel
by W. D. Good, J. L. Lacina, H. L. DePrater, and J. P. McCullough Contribution .\To. 1279 from the Thermodynamics Laboratory of the Bartlesville Petroleum Research Center, Bureau of -Wines, C. S. Department of the Interior, Bartlesaille, Oklahoma (Received October 7 , 1969)
A rotating-bomb method was developed for precision conibustion calorimetry of crystalline silicon and oigaiiosilicon compounds. Mixtures of pure, crystalline silicon and vinylidene fluoride polymer, (CH2CF2),,were burned in oxygen in the presence of water (or an aqueous €IF solution) to produce homogeneous solutions of H2SiF, in an excess of aqueous HF. The heat of formation of an aqueous fluorosilicic acid solution was determined from these measurements. Also, the results of these measurements and a literature value of the heat of solution of quartz in aqueous HF were used to compute the heat of formation of SiOz(c, quartz). Solutions of hexaniethyldisiloxane, (CH3)3SiOSi(CA3)3,in benzotrifluoride, C6H5CF,,were burned to yield the same Hz8iF6 solutions, and the heat of formation of hexamethyldisiloxane was derived. This value of the heat of formation TT as used with other thermodynamic property values for hexaniethyldisiloxane obtained in earlier research of this laboratory to calculate various chemical thermodynamic properties for the liquid state a t 298.15'K. and the ideal gas state at selected temperatures betiveen 0 and 1500°K. The heats of (.ombustion and formation of the vinylidene fluoride polymer also were determined, and heats of combustion and formation of benzotrifluoride were redetermined.
product in an ill-defined state that is largely amorphous but may be partly low cristobalite. The same problem arises in combustmioncalorimetry of organosiljcon compounds and, in addition, carbon is occluded
Research Projects Agency and by the Air Force Office of Scientific nesearcl1 u,lder contract xo, c ~ o - j ~;iR~2Ak ) - ~ , order 24-59, Task 3. Since February 1. 1962, this work has been sponsored solely by the Advanced Research Projects A4gency. ( 2 ) G. L. Humphrey and E. G. King, J . Am. Chem. Soc., 74, 2041 (1952).
Volume 68, S u m b e r 9 March, 1964
