NMR Chemical Shifts. 3. A Comparison of Acetylene, Allene, and the Higher Cumulenes

Abstract

The 13C NMR shieldings for acetylene, allene, and some higher cumulenes have been calculated at the MPW1PW91/6-311+G(2d,p) level using both the GIAO and IGAIM procedures. The calculations satisfactorily reproduce the tensor components of the shielding for both acetylene and allene, including the remarkably large difference in the components about the long axes of these molecules. A linear molecule such as acetylene has no paramagnetic shielding about the symmetry axis. The strong diamagnetic shielding about this axis for acetylene results in part from the energetically and spatially degenerate π-orbitals. With allene, the π-orbitals are energetically degenerate, but are not spatially degenerate, and as a result they cannot be mixed and cannot give a shielding diamagnetic term. Instead, in the presence of a magnetic field, they mix with virtual orbitals, leading to a downfield paramagnetic term. The shieldings for all the compounds are analyzed on an MO basis.