NOTES
3504 (b) the coverage dependence of heat of adsorption, AH (assumed to vary with 0 or s in the same way as AG) for which we define f
1 dAH
CCla
- RT dt9
CClzBr
+
If eq 1is obeyed, then, from eq5, f a = fe 4while from eq 1,f b = fs. If, on the other hand, eq 2 is applicable, then, from eq 7, f a = f a while from eq 15, f b = f s - 4. Thus, if one were to comparef, andfb values for any par4. ticular system, each model predicts that fa = f b The difference between fa andfb therefore cannot be used to ascertain the model which applies to a given surface.
+
Infrared Spectra of CC13 and CClzBr Isolated in an Argon Matrix by Jerry H. Current and Jeremy K. Burdett' Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48104 (Received December 18* 1968)
The infrared spectra of CCla and aCC12Br have been obtained by Andrews using the alkali atom abstraction of a halogen atom from the corresponding tetrahalomethane.2 Possible perturbation of the radical by the resultant alkali halide neighbor was assumed to be small, since the results were independent of the alkali atom used and the abstracted halogen atom. The report by Tan and Pimente13 of methyl alkali halides, formed when alkali atoms are used to abstract a halogen atom from a methyl halide, makes it highly desirable to have an independent radical source in order to investigate the magnitude of the effect of MX perturbations on the radical frequencies. Experimental Section CC13Br is known to dissociate thermally to form .CC13 radical^.^ To obtain high concentrations of radicals, a tubular metal heater was used as the spray-on nozzle inside the helium cold cell. The heated section is approximately 1.5 cm long and is made of 0.0625-in. 0.d. stainless steel tube. The wall thickness is 0.003in., and power is fed to the heater by two 22-gauge platinum wires spot-welded to the tube. Application of about 2 V ac heats the tube to red heat (about goo"), which was the temperature used for this experiment. One of the leads is spot-welded to the very end of the tube. Since the resistance of the platinum wires is not significantly less than that of the tubing, the leads become hot a t the welds. The very end of the tube is therefore almost as hot as any other segment of the heater. The tube is a snug fit into the spray-on line external to the cell SO that essentially all of the sample passes through the hot tube. The Journal of Physical Chemistry
Table I : Observed Frequencies (cm-1) of Radicals This work
Andrews
898.3 i 0 . 3 895.8 i 0 . 3 888.8 i 0 . 5 887.1 =k 0.7 835.6 =k 0 . 7
898.0 i 0 . 5 895.5 k 0 . 5 888.0 i 0 . 5 886.0 =k 0 . 5 835.0 i 0 . 5
The advantages of this resistance heated spray-on line over other furnace designs are (1) very little space is required within the cold cell, (2) no electrical or thermal insulation is used which might cause outgassing problems, (3) the smallest possible hot surface area is inside the cell thus minimizing the heat shielding required (none was actually used), and (4) no heating of the cell exterior is observed due to the low thermal conductivity of the stainless steel. The temperature of the CsI window was monitored with a chrome1 us. constantan thermocouple6 imbedded with Wood's metal into the window. No change in window temperature was observed during the experiment. The 2-1V noise level of the differential voltmeter corresponds to an uncertainty of 1" a t 4.2"K. Results The more intense absorptions of CCl3 and CClzBr were observed within experimental error of Andrews' reported frequencies, as can be seen in Table I. The optical densities of the absorptions are modest compared to those of Andrews (OD 898.3 = 0.65, OD 835.6 = 0.10). The 674-cm-' absorption of aCC13 is not observable in a sample of this size. The low intensities of the .CCI2Br absorpt)ions do not allow an accurate measure of their frequencies. The Beclcman IR-12 spectrometer was calibrated with the NH3 spectrum6 before and after the experiments. Evidence for carbonchlorine bond rupture was not observed by earlier workers4 at temperatures up to 875". The stainless steel surface is apparently catalyzing this fission, and similar side reactions will probably be found as other systems are investigated. The small diameter tubing results in a relatively high pressure (0.6 Torr, calculated') a t the initial hot region of the spray-on line. This is for a normal spray(1) Power Fellow, 1968-1970, Magdalene College, Cambridge, England. (2) L. Andrews, J . Chem. Phys., 48, 972 (1968); J . Phys. Chem., 71, 2761 (1967). (3) L. Y. Tan and G. C. Pimentel, J . Chem. Phys., 48, 5202 (1968). (4) h'l. Rzwarc and A. H. Sehon, ibid., 18, 1685 (1950). (6) L. L. Sparks, R. L. Powell, and W. J. Hall, U. 8. Department of Commerce, National Bureau of Standards, Boulder Laboratories, July 1968. (6) E. K. Plyler, A. Danti, L. R. Blain, and E. D. Tidwell, J. Res. Nut. Bur. Stand., A., 64, 29 (1960). (7) J. Yarwood, "High Vacuum Technique," John Wiley & Sons, Inc., New York, N. Y., 1956.
NOTES
3505
on rate of 2000 cc cm/hr. This pressure gives efficient activation of the molecules and resulted in a 10% conversion of reactant into radicals. We assume the extinction coefficients are approximately equal. The high pressure, however, increases the probability of radical recombination. Toward the end of one experiment a small absorption a t 683 cm-l was observed. This may have been from CzCls formed by mCC1, combination. The other CzC16 absorptions were covered by parent molecule absorptions. Apparently the dilution of the reactant (1/400) with argon and the fact that only 10% of the reactant was dissociated is sufficient to prevent significant radical combinations. We do think that higher pressures should be avoided. Since the observed vibrational frequencies and relative intensities in this work agree with those reported by Andrews, we have further evidence that ’CC1, and .CC12Br do not form strong complexes with alkali halides. Andrews’ estimate of the geometry (tetrahedral) of .CCl, is not altered. The dramatic variations in the geometry with halogen substitution on methyl radicals are discussed elsewhere.8 Qualitative consideration of (1) isovalent hybridization and (2) the mixing in of resonance structures forms a satisfactory model to rationalize the observed results. Radical-Alkali Halide Complexes. Tan and Pimente13have carefully documented the formation of methyl alkali halide complexes when alkali atoms are used to abstract halogens from the methyl halides. Using a three-center molecular orbital model for these complexes, they predicted the stability of these species to be maximized when the electronegativities of the terminal groups of the three center bonds are the same. The 3Iulliken scale electronegativity of cc13 and .CH8 is calculated to be 2.0 from data found in ref 9. The orbil a1 electronegativity scale of JaffB, et al.,1° indicates xCC4 > xCH3. Thus, CCla. MX should be the more stable complex. In any case there is not a sufficient difference in the electronegativities of the two radicals to cause one complex to be stable and the other to be not stable. Difficulties in correlating bond stabilities and electronegativityll have been discussed by others, and this one parameter does not seem adequate to explain the lack of spectral evidence for CCla. ”tX. Analogy with BX3. I n their complex formation properties, substituted methyl radicals should behave similarly to the substituted boranes.12 I n both systems (assuming the three-center bond mode1l3 is valid for CH3. .MX) electron density is contributed to a lowlying unfilled orbital localized on the central atom. However, in the known substituted borane complexes the borane group is pyramidal (despite its strong tendency to be planar as a free molecule) whereas the methyl group is still planar in the alkali halide complexes14(despite the ease with which it becomes pyramidal). This indicates that the electron density increase e
-
-
on the methyl is very small and that these complexes are thermodynamically quite unstable relative to the complexes in the borane system. Thus, the .CCL complexes may form with approximately the same stability as with .CHa, but .CH3 is in a tenuously stable configuration which is highly sensitive to small increases in electron density. This is reflected in changes in the observed vibrational frequency (v2) in the complexes. However, small increases in the electron density on the carbon atom, in the already pyramidal aCCI3, can hardly alter the hybridization of the bonding orbitals of the carbon atom.15 No observable frequency shifts result in v 3 or v ~ . ~ An independent verification of some of the observed frequencies of .CCI, and -CCl2Brhas been made indicating that the formation of radical-alkali halide complexes does not affect the observed frequencies and the analysis of Andrews. This result is interpreted not as evidence that CX,. *NIXcomplexes do not form but as a special sensitivity in v 2 of the methyl radical to weak complex formation. Acknowledgmenl. Grateful acknowledgment is made to the National Science Foundation; to the donors of the Petroleum Research Fund, administered by the American Chemical Society; and to the Faculty Research Fund of the Horace H. Rackham School of Graduate Studies of the University of Michigan for their partial support of this research. (8) J. H. Current and J. K. Burdett, J . P h y s . Chem., 73,3505 (1969). (9) V. I. Vedeneyev, L. V. Gurvich, V. N. Kondratiyev, V. A.
Medvedev, and Ye. L. Frankevich, “Bond Energies Ionization Potentials and Electron Affinities,” Edward Arnold Ltd., London, 1966. (10) J. Hinze, M.A. Whitehead, and H. H. Jaff6, J . Amer. Chem. SOC.,85, 148 (1963). (11) R. 6 . Neale, J. Phya. Chem., 68, 143 (1964). (12) T. D. Coyle and F. G. A. Stone, “Progress in Boron Chemistry,” Vol. 1, Pergamon Press, New York, N. Y., 1964, p 83. (13) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” W. H. Freeman and Co., San Francisco, Calif., 1960, p 236. (14) L. Andrews and G. C. Pimentel, J . Chem. Phys., 47, 3637 (1967). (15) Andrews has also shown that the infrared absorptions of
.CBr3 do not exhibit effects of alkali halids complex formation; L. Andrews, ibid., 49, 896 (1968).
The Structure and Bonding in Methyl and Substituted Methyl Radicals by Jerry H. Current and Jeremy K. Burdettl Department of Chemistry, University of Michigan, Ann Arbor, Michigan @lo.!+ (Received December 17, 1968)
Walsh’s molecular orbital discussion2of the geometry and electronic states of small molecules has been extremely useful. However, no clear choice for the geomVolume 73, Number 10
October 1069
