For three series of mono-, di-, and trisubstituted
pyridines, respectively, available incremental methods and
calculation programs for estimating the corresponding 13C
NMR chemical shifts were employed and compared
with the results obtained. The following methods and programs were
used for testing them for their
accuracy: simple pyridine increments, a simplified increment
calculation on the base of benzene increments
(program AROSIM
,
), the calculation method of Fürst
and Pretsch
−
(Carbon-13 module for
ChemWindows),
SPECAL from Specinfo (a database founded calculation
program), CSPEC2,
,
gNMR,
CNMR, and
HyperNMR, respectively.
A wide variety of push–pull alkenes were studied by means of variable‐temperature 1H and 13C NMR spectroscopy with respect to the configuration/conformation and the barriers to rotation about partial CC and CN double bonds. For the assignment of the 13C NMR spectra especially the semi‐selective INEPT pulse sequence and an incremental system for estimating the 13C chemical shift values of aromatic carbon atoms proved useful. The influence of thioether, sulphone and sulphoxide moieties in the acceptor part of the push–pull system on the π‐electron distribution is critically considered.
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