GIAO-SCF calculation of the chemical shifts in simple enamines. A comparison of theory with experiment

Methyl-substituted enamines assume overwhelmingly a gauche–out conformation in the absence of steric hinderance. A change of global minimum to the orthogonal–out structure occurs in the presence of steric interactions (Z-substitution pattern). Calculation of the 13C NMR shift at the β-carbon atom as a function of the torsional angle about the C–N bond axis reveals that the experimentally observed differential shielding is directly dependent upon the conformation asssumed by the enamine. The GIAO-SCF method reproduces the observed shift difference at Cβ quite satisfactorily. Furthermore, we show that, at least in the case of methyl-substituted enamines, conformational assignments in solution can be made upon comparison of theoretical with experimental 13C NMR spectra.