Abstract:
A method designed to investigate, on a fundamental level, the
origin of relative stability of molecular systems using BeII complexes
with nitrilotriacetic acid (NTA) and nitrilotri-3-propionic
acid (NTPA) is described. It makes use of the primary and
molecular fragment energy terms as defined in the IQA/F
(Interacting Quantum Atoms/Fragments) framework. An extensive
classical-type investigation, focused on single descriptors
(bond length, density at critical point, the size of metal ion or
coordination ring, interaction energy between BeII and a donor
atom, etc.) showed that it is not possible to explain the experimental
trend. The proposed methodology is fundamentally different
in that it accounts for the total energy contributions
coming from all atoms of selected molecular fragments, and
monitors changes in defined energy terms (e.g., fragment
deformation, inter- and intra-fragment interaction) on complex
formation. By decomposing combined energy terms we identified
the origin of relative stability of BeII(NTA) and BeII(NTPA)
complexes. We found that the sum of coordination bonds’
strength, as measured by interaction energies between BeII ion
and donor atoms, favours BeII(NTA) but the binding energy of
BeII ion to the entire ligand correlates well with experimental
trend. Surprisingly, the origin of BeII(NTPA) being more stable is
due to less severe repulsive interactions with the backbone of
NTPA (C and H-atoms). This general purpose protocol can be
employed not only to investigate the origin of relative stability
of any molecular system (e.g., metal complexes) but, in principle,
can be used as a predictive tool for, e.g., explaining reaction
mechanism.
