C O M M U N I ~ ATO T ~THE ~ NEDITOR S
440 surements reported here are higher than those obtained using the Eiame incident intensity in a system containing; mercury ~ a p o r .While ~ ~ ~ ((PCO), in the presence of mercury vapor a t 25" has not been measured, the shape of the (PCO plot at 25" in Figure 1 indicates that the value of (PCO = 0.0057 a t a concentration of 8.20 X l.O1* molecules cc-14 may be directly compared to the mercury-free value obtained here, at the same conoentration, of @CO= 0.0092, and that
This value compares favorably with the value of 1.80 predicted on the basis of T~ measurements' and indicates that the quenching reaction a* .$- Ng --+ CFaCOCHa
+ Hg
(5)
provides an eflicieiit path for the degradation of the triplet electronic energy of 1,IJ-TFA.
Experimental Section Apparatus and procedures were almost identical with those used previously. The cylindrical quartz reaction veesel, 20 crn in length, was filled with a parallel beam of light from a dc operated Osram HBO 100-W mercury arc. Li&t was restricted to wavelengths in
the region of 3130 8 by means of a Bausch and Lomb high-intensity monochromator wit a band pass of 100 8. The transmitted light was measured using an RCA 935 photocell and an electrometer. The incident intensity was maintained at 1015quanta sec-1. Acetone was used as an actinometer, @CO at 150" being taken as unity. Quantum yield values were obtained using first-order reflection corrections from the cell and furnace windows.5 The CO was collected a t -210" and was analyzed by means of a gas buret and a copper oxide furnace in the normal manner.6 Two U-traps in seriee were rnaintained indefinitely at -78" so that no contamination of the mercury-free part of the apparatus occurred when analyses were being performed. 1 ,1,l-TFA was purified by bulb-to-b in vucuo. Different samples from K Fluorochem, Ltd., gave identical results. molar extinction coefficient a t 25" was E = 3.95. Acknowledgment. The authors are United Kingdom Science Research Council for a maintenance grant to C. P. (4) R. A. Sieger, iM.Sc. Thesis, The Ohio State University, 1954. (5) G . B. Carter, R. K. Boyd, and K. 0 . Kutschke, Can. J , Chem., 46, 175 (1968).
CATIONS TO THE E D I T
Secondary Unirnolecular Reactions Subsequent to Substitution Reactions by High-Energy
Chlorine-39 Atoms Publication costs assisted by Divisionof Research, U.8. Atomic Enwgy Com.mission
Sir:
The gas-phase substitution reactions of energetic
@I atoms from nucllsar recoil have recently been of renewed intere~tl-~,and offer interesting comparisons with similar reactions of recoil 3H and l*F.KSecondary unimolecular decompositions following high energy substitution reactions have been shown to be quite significant for both irecoil 3€16-13and 18F1a-18 and accurate ineasurexnents of the original primary yields of substitution pToduct,scan only be relied upon after systematic eva,lur-ttionof the secondary reactions in each particular syatem.I2 The percentage depletion of primary yields ranges as high as 100yofor CHzTNC sNC, 9870 for methylcyclobutane-t from T/C€18 in di~nethylcyclobutane,~~ 100% for cThe Jo+olintalof Physkal Chemistry, Vol. 76,No. 8, 1871
CSFPFfrom I8F/Fin c-C3FB,17 and 89% for CFz=CF1*F from l8F/F in CzF4.lo While previous detailed studies (1) L. Spicer and R. Wolfgang, J . Amer. Chem. Soc., 90,2426 (1968). (2) C. M.Wai and F. S. Rowland, ibid., 90,3638(1968). (3) L.Spicer and R. Wolfgang, J . Chem. Phys., 50,3466 (1969). (4) C. M.Wai and F. S. Rowland, J.Phys. Chem., 74,434 (1970). (6) R. Wolfgang,Progr. React. Kinel., 3,97 (1968), (6) E. K. C. Lee and F. S. Rowland, J . Amer. Chem. Soc., 85, 897 (1963). (7) Y.-N. Tang, E. K. C. Lee, and F. S. Rowland, ibid., 86, 1280 (1964). (8) Y.-N. Tang and F. S. Rowland, ibid., 90,574 (1968) (9) Y.-N. Tang and F. S. Rowland, ibid., 90,570(1968). (10) C.T. Ting and F. S. Rowland, J.Phys. Chem., 72,763 (1968). (11) C.T.TingandF.S.Rowland, ibid., 74,445 (1970). (12) Y.-N. Tang, E. K. C. Lee, E. Tachikawa, and F. S. Rowland, submitted for publication in J.Phys. Chem. (13) C. MoKnight, N.J. Parks, and J. Root, ib
