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The temperature dependence of 13 C NMR shifts in polar compounds and its role for the determination of conformational equilibria
Author(s) -
Schneider HansJörg,
Freitag Wolfgang,
Hoppen Volker
Publication year - 1980
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270130409
Subject(s) - chemistry , polar , conformational isomerism , chemical shift , chemical polarity , halide , solvatochromism , crystallography , coalescence (physics) , computational chemistry , solvent , thermodynamics , molecule , organic chemistry , physics , astronomy , astrobiology
Intrinsic temperature dependencies of 13 C NMR shifts in alkanes bearing one polar C‐αX 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 CX 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.