ISSN:
0030-4921
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1270130409
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