  The proton between the two oxygen atoms of the malonaldehyde molecule
experiences an effective double-well potential in which the proton wavefunction
is delocalized between the two wells. Herein we employed the state-of-the-art
multi-component quantum theory of atoms in molecules partitioning scheme to
obtain the molecular structure, i.e. atoms in molecules and bonding network,
from the superposed ab initio wavefunctions of malonaldehyde. In contrast to
the familiar clamped-proton portrayal of malonaldehyde, in which the proton
forms a hydrogen basin, for the superposed states the hydrogen basin disappears
and two novel hybrid oxygen-hydrogen basins appear instead, with an even
distribution of the proton population between the two basins. The interaction
between the hybrid basins is stabilizing thanks to an unprecedented mechanism.
This involves the stabilizing classical Coulomb interaction of the one-proton
density in one of the basins with the one-electron density in the other basin.
This stabilizing mechanism yields a bond foreign to the known bonding modes in
chemistry.
