  The Structural theory of chemistry introduces chemical/molecular structure as
a combination of relative arrangement and bonding patterns of atoms in
molecule. Nowadays, the structure of atoms in molecules is derived from the
topological analysis of the quantum theory of atoms in molecules (QTAIM). In
this context a molecular structure is varied by large geometrical variations
and concomitant reorganization of electronic structure that are usually taking
place in chemical reactions or under extreme hydrostatic pressure. In this
report a new mode of structural variation is introduced within the context of
the newly proposed multi-component QTAIM (MC-QTAIM) that originates from mass
variation of nuclei. Accordingly, XCN and CNX series of species are introduced
where X stands for a quantum particle with a unit of positive charge and a
variable mass that is varied in discrete steps between the masses of proton and
positron. Ab initio non-Born-Oppenheimer (non-BO) calculations are done on both
series of species and the resulting non-BO wavefunctions are used for the
MC-QTAIM analysis revealing a triatomic structure for the proton mass and a
diatomic structure for the positron mass. In both series of species a critical
mass between that of proton and positron mass is discovered where the
transition from triatomic to diatomic structure takes place. This abrupt
structural transformation has a topological nature resembling the usual phase
transitions in thermodynamics. The discovered mass induced structural
transformation is a hidden aspect of the Structural theory which is revealed
only beyond the BO paradigm when nuclei are treated as quantum waves instead of
clamped point charges.
