Detection of oriented association via NMR dilution shifts in tetramethylsilane solvent. 4. Temperature coefficients for perfluoroalkyl halide systems

Temperature coefficients, Fθ and Fθ∘, of 19F deshielding values, Fδ and Fδ∘, and Sticking indices, FSV, at 30 and 40°C, are presented for a variety of perfluoro(n-octyl) and (trans-4-ethylcyclohexyl) halides, and for their weak associations with various donor and acceptor reagents. The distant CF3 group resembles CCl3F as an internal reference, but apparently is less affected by added reagents; perhaps neither is ideal. A few measurements are reported on I(CF2)4I and n-C5H11F. The temperature coefficients are interpreted as detecting energetic associations and/or conformational changes; preferred packings due to shape factors of semi-rigid structures should show little or no temperature dependence. Conclusions are generally in agreement with, and amplify, those derived from isothermal dilution studies. Careful but essentially routine NMR spectroscopy is adequate, provided that good temperature control and measurement is used. The temperature coefficients will be useful whenever it is intended to measure exact Fδ∘ values. “NMR thermometers” containing deuterium isotopomers as D-lock additives are proposed for convenience in performing temperature measurements. Corrections are made to compensate for the logarithmic character of the familiar δ-scale.