Pr. Seidl et Kz. Leal, Steric contributions to carbon-13 chemical shifts of 1-and 2-methyl adamantanes by DFT/GIAO, J MOL ST-TH, 539, 2001, pp. 159-162
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.