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

Substituent effects are widely used to investigate the factors affecting ca rbon-13 chemical shifts. Among the most important of these are the steric e ffects used in structure determination. Recent investigations have shown th at they may originate from carbon-hydrogen bond polarization or geometric f actors 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 proper ties due to substitution. The absence of ring strain as well as other effec ts due to electric fields, anisotropy, hyperconjugation, etc, permits compa risons based almost exclusively on steric factors. Calculations of NMR chemical shifts are becoming affordable and accurate en ough 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 ada mantane and its 1- and 2-methyl derivatives by Gauge Including Atomic Orbit al (GIAO) methods using B3LYP/6-31G*(d) basis sets. Although agreement betw een calculated and observed chemical shifts may be off by up to 2 ppm for c arbon, substituent effects due to bond polarization caused by steric intera ctions can be clearly distinguished. (C) 2001 Elsevier Science B.V. All rig hts reserved.