The temperature dependence of 13C NMR shifts in polar compounds and its role for the determination of conformational equilibria

Intrinsic temperature dependencies of ¹³C NMR shifts in alkanes bearing one polar C-α? X bond are determined with neopentyl and 4-tert-butylcyclohexyl derivatives as conformationally homogeneous model compounds. The increased shiedling for C-α at higher temperatures can be related to a C-α—X bond length increase and, for less polarizable C? X bonds, essentially to a decrease of solvent polarity on raising the temperature. The use of temperature dependent ¹³C shifts in conformationally mixed compounds for the determination of the equilibrium constants, K, is evaluated with n-propyl halides; the computer fit of the unknown conformer shifts and the conformational enthalphy difference, δH° to the time averaged shifts yields δH° values which, although converging rather broadly, are in general agreement with literature data. In compounds with higher conformational barriers, such as methoxy- and bromocyclohexane, low temperature signal integration yields accurate δG° values; inclusion of shifts above coalescence, however, yields unreliable δH° and δS° parameters. This can only partially be remedied by application of temperature shift corrections obtained from parent t-butylcyclohexyl compounds.