Steric contributions to carbon-13 chemical shifts of 1- and 2-methyl adamantanes by DFT/GIAO

Abstract

Substituent effects are widely used to investigate the factors affecting carbon-13 chemical shifts. Among the most important of these are the steric effects used in structure determination. Recent investigations have shown that they may originate from carbon–hydrogen bond polarization or geometric factors such as bond distances, angles and dihedral angles.Adamantane and its 1- and 2-methyl derivatives are well suited to probe the relative contribution of these factors. These molecules’ symmetry renders several positions equivalent, allowing for comparisons of changes in properties due to substitution. The absence of ring strain as well as other effects due to electric fields, anisotropy, hyperconjugation, etc. permits comparisons based almost exclusively on steric factors.Calculations of NMR chemical shifts are becoming affordable and accurate enough to be used in interpretation of high field spectra. Whether they also can be employed in probing the subtleties of chemical structure depends on how well they reflect small adjustments in structure due to steric effects. In this respect, we calculated carbon and hydrogen chemical shifts for adamantane and its 1- and 2-methyl derivatives by Gauge Including Atomic Orbital (GIAO) methods using B3LYP/6-31G∗(d) basis sets. Although agreement between calculated and observed chemical shifts may be off by up to 2ppm for carbon, substituent effects due to bond polarization caused by steric interactions can be clearly distinguished.