COMMUNICATIONS TO THE EDITOR
1326 For such a situation, division of the reaction into hot and thermal processes is especially arbitrary. It is not clear yet whether real “scavengers” exist for nearthermal 18Fatom systems-it seems likely that several reactions (abstraction of H by 18F; addition to multiple bonds; perhaps 18F-for-I)have sufficiently high reaction probabilities that no trace ingredient can successfully compete with such reactions with a major component. The 02 or H I scavenger molecules serve very well in reacting preferentially with radical intermediates; whether they function with any success as scavengers for 18Fis a moot point. The incremental absolute yield of CH318Fdetected by comparison of scavenger experiments with H I VS. O2 is little changed from CH3F to CH3I and is therefore fractionally less important in the overall CH318F yields for the heavier halogen atoms. While this “decomposition” yield has not yet been distributed between CH318F* and CH218FX* decompositions, the indication is that substitution for X in molecules with weaker C-X bonds leaves less excitation energy with the CH318F*product, on the average, and is consistent with the hypothesis that these reactions are often initiated by lSFatoms quite near the thermal energy range. Other factors, including collision radii, may also contribute to the variations in substitution yields, and more detailed elaborations of the bond energy dependence can be suggested. However, the overall conclusion still remains that substitution reactions initiated by l8F atoms are influenced strongly by the same chemical factors which are reflected in changes in the bond strengths of C-X bonds. DEPARTMENT O F CHEMISTRY UNIVERSITY OF CALIFORNIA IRVINE, CALIFORNIA92664
THOMAS SMAIL R. SUBRAMONIA IYER F. S. ROWLAND*
have found the vapor to be stable up to a t least 200”. The colorless liquid was distilled in a vacuum system with the middle fraGtion being retained; no special precautions were found to be needed and no impurities were detected by gas chromatography. In our preliminary photochemical experiment~~ with the imine the decadic extinction coefficients at 254 and 185 nm were found to be 87 and 3.7 1. mol-l cm-l a t 25”. Thus 100 Torr in a 5-cm long Suprasil cell will transmit 7901, of the incident 185-nm radiation, but only 0.5% at 254 nm. Hexafluoroacetone imine is photochemically stable, presumably due to very efficient self-q~enching.~The most abundant product of the photolysis at 254 nm is CF3CN, and its quantum yield a t 25” and 100 Torr is HBr was used as an actinometer. The imine filter is thus very photostable and only occasionally requires replacement, depending on the intensity of the light used.
Acknowledgment. The authors thank the National Science Foundation for support of this work. S. T. thanks Rutgers University for a Faculty Fellowship. (1) L. C. Glasgow and J. E. Willard, J. Phys. Chem., 74, 4290 (1970). (2) For properties see W. J. Middleton and C. G. Krespan, J . Org. Chem., 30, 1398 (1965). (3) S. Toby and G. 0. Pritchard, to be submitted for publication.
DEPARTMENTS OF CHEMISTRY RUTGERS UNIVERSITY NEWBRUNSWICK, NEWJERSEY 08903 UNIVERSITY OF CALIFORNIA SANTABARBARA, CALIFORNIA93106
SIDNEY TOBY*
GLYN0. PRITCHARD
RECEIVED JANUARY 28, 1971
RECEIVED JANUARY 4, 1971
Kinetics of Isopropyl Alcohol Radicals by Electron Spin Resonance-Flow Techniques Hexafluoroacetone Imine Filter for 185 nm
Publication costs assisted by Texas A & M University
Publication cost8 assisted by the National Science Foundation
Sir: The isolation of the 185-nm line from the 254-nm line when using low-pressure mercury arcs has long proved a troublesome photochemical problem. Glasgow and Willard1 have most recently developed a complex filter utilizing a flow of ozonized oxygen for this purpose. The ratio of the extinction coefficients of ozone for 254: 185 nm is found1 to be about 10: 1. We wish to report on a very simple chemical filter consisting of a cell of gaseous hexafluoroacetone imine (hexafluoroisopropylidenimine) . Hexafluoroacetone imine has a lower extinction coefficient than ozone at 185 nm, and the 254: 185 nm extinction coefficient ratio is 23: 1. The imine2 (purchased from Hynes Chemical Research Corp., Durham, N. C.) boils a t 16” and we T h e Journal of Physical Chemistry, Vol. 76, N o . 9, 1071
Sir: The interpretation of the kinetic behavior of radicals R1 and Rz from 2-propanol offered by Burchill in a previous communication1 cannot be a suitable alternative. If the decays of substrate radicals are clearly determined by reaction 1, then a case analogous to transient equilibrium would hold and both R1 and Rz would decay with the same specific rates. The decays of both R1 and R2 would parallel the decay of Ti(II1) on first-order plots, with [R1]/ [R2]remaining constant after equilibrium had been established. This is clearly not the case, as shown by Figures 2 4 and the data in Table I of our previous paper.2 (1) C. E. Burchill, J. Phys. Chem., 75, 167 (1971). (2) R. E. James and F. Sicilio, ibid., 74, 1166 (1970).
