Tetrahedron Letters Vol. 21, pp 1681 - 1684 @Pergamon Press Ltd. 1980. PrIntedin Great Brltaln TEMPERATURE DEPENDENCE OF IgF CHEMICAL SHIFTS IN SUBSTITUTED HENZYL FLUORIDES. Robert T.C. Brownlee and David J. Craik Dept. of Organic Chemistry, La Trobe University, Bundoora, 3083, Vie. Australia. Abstract- The "F chemrcal shifts of substituted benzyl fluorides are shown to be temperature dependent and this is discussed in terms of conformational effects. It has been known for some time that lg F NM! chemical &ifts in benzyl fluorides (I) are sensitive to the effects of substituents' . The apparent anomaly concerning the direction of response to substituent effects has yet to be resolved2, Thus, the effect of electron donating (withdrawing) substituents is to cause a downfield (upfield) shift in contrast to the expected upfield (downfield) shift. The expected direction is observed in other fluorine serles3r4, including substituted fluorobenzenes3, where explanations of the shifts in Sterms of the electronic effects of the substltuent are well established . One difficulty which has so far prevented delineation of the factors which determine the benzyl fluoride shifts is the non planar nature of the CHZF probe group which prevents zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA a meaningful separation of the sigma and pi components of electron density about the fluorine. The importance of separating these individual components of4electron density has been shown for the fluorobenzenes anti benzoyl fluorides , where pi density changes provide the best fit to the ooserved chemical shifts. Another difficulty concerns the possibility of conformational mobrlity of the CH2F group. In thEi communication we present evidence from a varrable temperature lgF NMR study which suggests that conformational changes have a profound influence on substituent chemical shifts (SCS) in this series. Recent data on a conformationally rigid model system (II) related to the benzyl fluorides in which confirms thas view6. the C-F bond is fixed in the plane of the benzene ring also In that model system, the SCS values were reduced to an extremely small magnitude compared with benzyl fluorides themselves. (I) (II) cB3 (III) (IV) Cl5 orthogonal Chemical shifts were measured at various temperatures using 1000 Hz spectral widths. The data was recorded for dilute (1% w/v) solutions rn d6 acetone and the shifts were measured in Hz directly from the carrier frequency for samples locked to the deuteriun signal of solvent. This proceedure was checked by remeasuring some data relative to internal tetrachlorotetrafluoro cyclobutane 1681