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

✍ Scribed by Hans-Jörg Schneider; Wolfgang Freitag; Volker Hoppen

• Publisher: John Wiley and Sons
• Year: 1980
• Tongue: English
• Weight: 406 KB
• Volume: 13
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270130409

✦ Synopsis

Abstract

Intrinsic temperature dependencies of ^13^C 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 ^13^C 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, δ__H__° to the time averaged shifts yields δ__H__° 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 δ__G__° values; inclusion of shifts above coalescence, however, yields unreliable δ__H__° and δ__S__° parameters. This can only partially be remedied by application of temperature shift corrections obtained from parent t‐butylcyclohexyl compounds.