NATURAL BOND ORBITAL ANALYSIS OF C-13 CHEMICAL SHIELDINGS IN ACETYLENES

Ab initio calculations of NMR chemical shieldings of alkyne carbon ato ms in nine acetylene compounds computed by the gauge-including atomic orbital (GIAO) method are reported. The unusually low chemical shieldi ng anisotropy (159 ppm) found in the terminal alkyne carbon of phenyla cetylene is found to be due to the Ramsey diamagnetic component of the shielding tensor rather than the paramagnetic component, which is com monly 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 c hemical shielding.