Natural bond orbital analysis of carbon-13 chemical shieldings in acetylenes

Ab initio calculations of NMR chemical shieldings of alkyne carbon atoms in nine acetylene compounds computed by the gauge-including atomic orbital (GIAO) method are reported. The unusually low chemical shielding anisotropy (159 ppm) found in the terminal alkyne carbon of phenylacetylene is found to be due to the Ramsey diamagnetic component of the shielding tensor rather than the paramagnetic component, which is commonly associated with large changes in shielding, Natural bond orbital analysis of ab initio molecular orbital wave functions has been used to provide insight into the role of electronic delocalization in the chemical shielding.