14) Spinning speeds were 1.7 kHz at 40 K and above. Spinning speed dropped slowly as the temperature was lowered and reached 1 kHz at 6 K. This change is reflected in spinning side bands which are identified in the spectra shown in Figure 2. Their presence should not interfere with identification of sidebands indicative of tunneling.( I 5 ) For accounts of the classical vs nonclassical ion controversy, see: (a) Grob, C. A.(8) Trapping of nonequilibrium ratios of (a-(phenylthio)alkyl)ltthium4 and (n-bromoalkyl)lithium reagents3b with in situ carbonyl compounds or Me,SiCI has been reported (9) This compound was prepared in isomerically pure form (>991) by crystallization. This is an important advantage of the 3.5-diphenylcyclohexyl system over the traditional 4-rert-butylcyclohexyl locked cyclohexane.(IO) This effect was discovered by KriePb and has been used to prepare the less stable of two diastereomeric lithium reagents.',& 0002-7863/90/ 15 12-8994$02.50/0 0 1990 American Chemical Society

Chemical shift data obtained from the nmr spectra of several series of 2,4,6-trialkylhalobenzenes enable the assignment of preferred conformation to those ortho alkyl groups which lack threefold symmetry. Application of these conformational preferences to the nmr spectra of a number of alkylfluorobenzenes results in the conclusion that a necessary condition for long-range spin-spin coupling of hydrogen and fluorine which are separated by five bonds is proximate orientation of the nuclei. The magnitude of this long-range spin-spin coupling (JH-&~) is shown to be extremely sensitive to the internuclear separation of the couple but rather insensitive to changes in electron density within the aromatic ring. Thus, these data appear to be compelling experimental evidence in support of a "through-space" mechanism of long-range spin-spin coupling. he observations of spin-spin couplings betweenMyhre, Edmonds, Kruger / Long-Range Spin-Spin Coupling in Alkyl'uorobenzenes ( 5 ) We are grateful to Dr. N. S. Bhacca, Varian Associates, for recording the 100-Mc spectra reported here.

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