Magnetic Shielding Analysis of Bonding in [1.1.1]Propellane

The bonding in [1.1.1]propellane,
bicyclo[1.1.0]butane,
bicyclo[1.1.1]pentane,
tetrahedrane, and cyclopropane is investigated by analyzing changes
in the off-nucleus isotropic magnetic shielding within the space surrounding
each of these molecules and, for [1.1.1]propellane, by examining also
the diamagnetic and paramagnetic contributions to this shielding.
Any shielding arising from the two “exo” sp3-like hybrid atomic orbitals on the bridgehead carbon atoms that
have been used to support the idea of an inverted bond between these
two atoms is found to be almost entirely contained within the [1.1.1]propellane
cage and to contribute to a strongly shielded central region. This
strongly shielded region suggests the establishment of a mainly covalent
bonding interaction involving all carbon atoms that cannot be straightforwardly
decomposed into contributions from individual carbon–carbon
bonds. The emergence of the strongly shielding central region is traced
by comparing the shielding variations in and around molecules with
one three-membered carbon ring (cyclopropane), two fused three-membered
carbon rings (bicyclo[1.1.0]butane), and three fused three-membered
carbon rings ([1.1.1]propellane).