Multiple linear regression for the estimation of steric effect in cyclic systems from calculation of C-13 chemical shifts and employing parameters derived using DFT method

Abstract

In this work, differences between C-13 chemical shift calculated by the linear addition method (LAM) and the practical values is used as a measure of the steric effects. The C-13NMR chemical shift of a number of saturated and aromatic cyclic organic compounds were selected for achieving this study. The results obtained exhibited high agreement between the calculated and practical values, deviation is only noticed on locations exhibited steric hindrance caused by the substitution of methyl groups on two neighboring carbon atoms. The study proved that the amount of shift resulting from the spatial effect is of an additive character. The amount of shift due steric effect noticed was similar in both aromatic or saturated systems unless other factor is interfered. Density functional theory (DFT) methods is applied to calculate the bond lengths, of the aromatic molecules (toluenes), electronic densities, and the heat of formation using the method B3LYP at basis set 3-21G by (Chem office V.12,2010). Their relationship to space interference were investigated.