NOTES
2225
significant difference in the shifts of phenolic or alcoholic AVOHwith ccl4 from those with aromatic hydrocarbons, say, benzene. This might be ascribed to the fact that H F interacts with CC1, to form an addition complex, though its enthalpy of formation is fairly small. As usual, such oxygen-containing molecules as alcohols, ethers, or ketones are stronger acceptors than nitrile or nitro compounds. It was reported that molecules of this group (the group 3) form a hydrogenbonded molecular complex or an ionized protonated ~ o m p l e x . ~However, the absorption due to the HF2ion which appeared at 2500 to 2600 cm-l lo has not been observed in the present measurement. The dielectric constant of the medium will probably be insufficient to produce ionization of the complex in the present dilute H F solutions. It is interesting to evaluate the enthalpies of complex formation in H F solutions, as they are extremely difficult to measure calorimetrically. Assuming the linear enthalpy-spectral shift correlation17 and the enthalpy and +acetone by Schepvalues given for H F CC1411& kin,llb enthalpy of complex formation is estimated for each group as in the last column of Table 11. According to this evaluation, the value for pyridine is 16.4 kcal/mol. While such a large value would not be in serious error, it is desirable to ascertain this by direct measurement. We are currently engaged in such a study.
+
(17) (a) E. R. Lippincott and R. Schroeder, J . Chem. Phys., 23, 1099 (1955); (b) A. D.Sherry and K. F. Purcell, J . Amer. Chem. Soc., 74, 3535 (1970).
Pressure Dependence of the Cross-Combination Ratio for CF3 and CH3 Radicals by P. C. Kobrinsky, G. 0. Pritchard,* and S. Toby Department of Chemistry, Unizlersity of California, Santa Barbara, California 93106 (Receiwd March 8, 1971) Publication costs borne completely by The Journal o f Physical Chemistry
CF3 and CH3 radicals may be generated by the photolysis of l,l,l-trifluoroacetone' (TFA) or the cophotolysis of acetone (Ac) and hexafluoroacetone (HFA),'r2 and the cross-combination ratio for the two radicals in these systems is given by l a 2
+
+
( R C F ~ C HR C~F ~ = C H , ) / ( R C , F ~ . R C ~ H ~ ) ' ~ ~ where R denotes rate of formation of the various products. At reduced pressures the falloff in the recombination of CH3 radicals will occur CH3 CH3 If C&€e*(+i\I) +CzHs(+LI) =
+
before falloff for the heavier radical pairs,3 so that a study of the pressure dependence of affords a simple method for determining the half quenching pressure, PI,,,when the rate of collisional stabilization of C2H6*is equal to its decomposition rate. Similar studies have recently been carried out for the radical pairs methyl and ethyl and methyl and i~opropy1,~J and methyl and acetonyL6 Below 150" Grotewold, Lissi, et find rapidly decreasing values of PI,*,and at room temperature they report a value which is two orders of magnitude less than the RRKM calculated value, assuming a "loose" activated ~ o m p l e x . ~ We have examined $ as a function of pressure a t 120°, 150", and room temperature, using either TFA or mixtures of Ac HFA as radical sources. Experimental details are given elsewhere. l Low-pressure experiments, down to 0.07 mm, were carried out in a 1321-cm3 cylindrical reaction vessel (5.8 cm in diameter which was fully illuminated) in an Hg-free system.8 The surface to volume ratio was 0.73 cm-'. When TFA was photolyzed a t 120" and 0.07 mm, CF2=CH2 was the major product (reaction times employed produced between and mol of product). Typical relative yields of CF2=CH2, GFa, CFaH, CZH6, and CFaCHa were 100, 50, 10, 5, trace, respectively. At higher pressures, C2H6, CF3H, and CF3CHa formation increased. At room temperature the CZH6 yields became extremely small9 so that HFA mixtures were used. The Ac-HFA ratio Ac was varied between approximately 3:l and 6 : l . A plot of vs. the logarithm of pressure is shown in Figure 1. Collision theory predicts a value of $ = 2.3 for CF, and CH3 radicals, lo and our high-pressure experimental values range from 2.5 to 2.9. These values were obtained in both the present and previous work,'t2 so that it is doubtful that the discrepancy between the experimental values and the simple collision theory calcula$J
+
+
(1) G. 0. Pritchard and ILI. J. Perona, Int. J . Chem. Kinet., 2, 281 (1970). (2) R. D. Giles and E. Whittle, Trans. Faraday Soc., 6 1 , 1425 (1965). (3) S. W. Benson and G. Haugen, J . Phys. Chem., 69, 3898 (1965). (4) J. Grotewold, E. A. Lissi, and M.G. Neumann, J . Chem. SOC.A , 375 (1968). (5) F. Casas, C. Previtali, J. Grotewold, and E. A. Lissi, {bid., 1001 (1970). (6) F. R. Cala and S. Toby, J . Phys. Chem., 75, 837 (1971). (7) B. S. Rabinovitch and D. W. Setser, Adaan. Photochem., 3, 1 (1964). (8) G. 0. Pritchard and J. T. Bryant, J . Phys. Chem., 72, 1603 (1968). (9) This corroborates that the primary split at 3130 A is TFA hv + CFS CHsCO, see E. A. Dawidowics and C. R. Patrick, J . Chem. SOC.London, 4250 (1964). (10) I t is often mistakenly assumed that cross-combination ratios should be exactly equal to 2 in radical-radical reactions, as calculated from collision theory, see J. A. Kerr and A. F. Trotman-Dickenson, Progr. React. Kinet., 1 , 105 (1961). The value of 2.3 in the present case simply arises from the differences in the reduced masses of the three molecules involved. For CHS and CaFi the value is 2.7.
+
+
The Journal of Physical Chemistry, Val. 7 6 , No. 14, 1971
2226
NOTES Approximate values were 2.0 X and 8.5 X 10-8 moll/z cc - 1 / 1 sec-'/* at the respective pressures. This strongly suggests that an adsorbed phase reaction between CFs radicals and TFA to produce CFaH does (CH&OCH3)ad, + not occur. The reaction CHa CH4 (CHaCOCH2)ad, has recently been established.laJ4 The reader is referred to Konstantatos and Quinn's paperla for a detailed discussion of the variwith ation in the comparable function RCH,/RC~H~~/' varying surface to volume ratios. We see from Figure 1 that PI/,is close to 1 mm (-0.8 f 0.2 mm) a t room temperature and 120'. This value correlates well with published data obtained by other methods" and is close to the RRKM calculated values in the temperature range, based on a "loose" activated c ~ m p l e x . ~In their studies below 150°, Grotewold, Lissi, et U Z . , ~ + ~ find an increasing discrepancy with Rabinovitch and Setser's calc~lation,~ and a t 15" report that Pl12= mm from their cross-combination s t ~ d y . The ~ data are interpreted in terms of a more "rigid" activated complex, where the free rotations of the methyl groups are replaced by a low-frequency bending. Our data would seem to dispose of misgivings concerning the validity of the "loose" complex model for CH, recombination, a conclusion arrived at by Hole and Mulcahy.ll
+
Pressure, Torr. Figure 1. Semilog plot of $-z vs. pressure. In the case of mixed bath gases the pressures are taken as additive. Although deactivating efficiencies may vary, as long as some of the excess vibrational energy of C2Ha*is transferred on the initial HFA mixtures: 0, collision, it will not redissociate. Ac room temperature. All data at 70 mm, work of Giles and Whittle;% 0,157"; e, overlapping points from runs at 23, 63, and 103'. TFA: 0, -120', numbers refer to overlapping points. Point a t 30 mm overlapping points of runs a t 90, 130, and 150'; 0, reduced light beam: C), -150'; 8, wall 1 mm c-CeFlz. 0, two points reactor; C), 0.07 mm TFA at 220'; B, 330".
+
+
tion is due solely to analytical errors, and thus the difference is real, at least below 150". A few experiments were carried out at higher temperatures but the possibility of increasing olefin loss due to radical addition reactions1*2cannot be discounted. is found experimentally1 to be 2 at 300" and 30 mm, presumably, in part at least, due to this cause. Also propane becomes an increasing minor product, which renders the mechanism less clear-cut. A trend toward larger values of PI,, is apparent, in agreement with experiment and t h e ~ r y . ~ , ~ ) ~ ~ Some experiments were also carried out with an incident light beam reduced in diameter to 1.9 cm, so that only the central portion of the reactor was illuminated, and it was found that J, was unaffected. I n addition, a reaction vessel was constructed with three quartz tubes of different diameters mounted axially and running nearly the length of the reactor; the volume was 892 cma, with a surface to volume ratio of 3.1 cm-l. With this vessel the value of J, obtained at 0.07 mm agreed reasonably well with the other values, as seen in Figure 1. However, between 1 and 1.5 mm, $ values of -2.2 were obtained at 120"; some unassessed heterogeneous effect may be involved. l 2 The homogeneous pressure-independent value of J, is clearly >2.5 in Figure 1. Interestingly enough, the value of the function RCFaH/RC2Ba1'2 at 120" was independent of the reactor employed and the diameter of the incident light beam at both 0.07 and 1.0 mm. The Journal of Physical Chemistry, Vol. 76,N o . 14, 1971
+
Acknowledgments. G. 0. Pritchard thanks The National Science Foundation for support and S. Toby thanks Rutgers University for a Faculty Fellowship. (11) K. J. Hole and M. F. R. Mulcahy, J . Phys. Chem., 73, 177 (1969). (12) K. M. Maloney, ibid., 74, 4177 (1970). (13) J. Konstantatos and C. P. Quinn, Trans. Faraday Soc., 65, 2693 (1969). (14) H.Shaw and 5. Toby, J . Phvs. Chem., 72, 2337 (1968).
The Gas-Phase Acidities of Alcohols' by Mary Jane McAdams and Larry I. Bone* Department of Chemistry, East Texas State University, Commerce, Texas 76@8 (Received January 16, 1971) Publication coats assisted bv the Robert A. Welch Foundation
Brauman and Blair2,*have reported a scale of the relative acidities of alcohols in the gas phase. Using ion cyclotron resonance techniques, they Rere able to show that as the size of the alkyl group increases on an (1) This work is supported by a Faculty Research Grant, East Texas State University, and the Robert A. Welch Foundation. (2) J. I. Brauman and L. K. Blair, J. Amer. Chem. &c., 92, 6987 (1970). (3) J. I. Brauman and L. K. Blair, ibid., 90,6561 (1968).
