2037
COMMUNICATIONS TO THE EDITOR must be opposite to that in the analogous alkyl substituted radicals. The difference in polarities of the C-H and C-F bonds as well as the presence of lone pair electrons on fluorine might play a part in such a reversal. E. THOMAS STROM
MOBILRESEARCH AND DEVELOPMENT CORPORATION FIELD RESEARCH LABORATORY DALLAS, TEXAS 75221
F 0-
CFj- G-CFj U P = 34.7 G
F' aF= 82.9 G W
n
AAROXL. BLUHM
PIONEERING RESEARCH LABORATORY U. S. ARMYNATICKLABORATORIES NATICK,MASSACHUSETTS 01760
RECEIVED DECEMBER 5, 1969
On the Angular Dependence of @-Fluorine Electron Spin Resonance Hyperfine Coupling in Fluoro-Substituted Nitroxide Radicals'
S i r : I n connection with an esr study of photolysis products of nitrosoperfluoroalkanes, Strom and Bluhm2 found that the magnitude of 0-fluorine coupling in the nitroxide produced increases with substitution. This observation appeared unusual in light of the many known examples where the magnitude of p-hydrogen coupling decreases with ~ u b s t i t u t i o n . ~It was thus suggested that fluorine coupling might be enhanced by p-T overlap which was thought to be favored at larger dihedral angles. This suggestion was based on the assumption that the probable average conformation of the perfluoroalkyl group was the same as for normal alkyl groups. An alternate assumption, however, might have been that the angular dependence of the p-fluorine coupling was normal ( i e . , the same as for hydrogen) but that the average conformation of the perfluoroalkyl group was different from that of normal alkyl group^.^ Table I : Temperature Dependence of Nitrogen and @-Fluorine Hyperfine Coupling in R-NO-C(Me)3" Temp,
= CFs-CFr-
AN'
Ap"
AyF
40 25 -30 -60 -90
11.33 11.25 11.28 11.31 11.32
20.80 21.16 22.27 22.65 23.15
0.39 0.43 0.34 0.22
