Linear electric field (LEF) effects in 13C NMR. Some model calculations on 5‐chloro‐1‐pentene

Abstract

AbstractThe usefulness and some of the limitations of the linear electric field (LEF) effect in 13C NMR are illustrated by means of model calculations on the 13C NMR chemical shifts of C‐1 and C‐2 in 5‐chloro‐1‐pentene in nine different conformations. Two different concepts are used in order to calculate the electric fields near C‐1 and C‐2 caused by the C‐5Cl dipole. It is also demonstrated that, when using charges calculated by CNDO/2, the influences of the hydrogen atoms at C‐5 cannot be neglected. The use of the dipole–dipole interaction formula is advocated. The consequences of taking into account the additional contributions of the vinylic bonds to the accumulated charges at C‐1 and C‐2 are described. The results with the dipole–dipole formula show that the same signs and similar magnitudes of the LEF effects are calculated in seven out of nine conformations of 5‐chloro‐1‐pentene. This reduces the LEF to a means of confirming, rather than deducing, conformational equilibria in acylic flexible compounds. The usefulness of some recently published applications of the LEF concept to explain some short range substituent effects is critically discussed, mainly in view of the inhomogeneities of the calculated electric fields.