Abstract:
We strongly advocate distinguishing cooperativity from cooperativity-induced
effects. From the MOWeD-based approach, the origin of all-body cooperativity is
synonymous with physics- and quantum-based processes of electron (e) delocalization
throughout water clusters. To this effect, over 10 atom-pairs contribute to the
total e-density at a BCP(H,O) between water molecules in a tetramer. Intermolecular
all-body e-delocalization, that is, cooperativity, is an energy-minimizing process that
fully explains non-additive increase in stability of a water molecule in clusters with an
increase in their size. A non-linear change in cooperativity and cooperativity-induced
effects, such as (i) structural (e.g., a change in d(O,O)) or topological intra- and intermolecular
properties in water clusters (e.g., electron density or potential energy density
at bond critical points) is theoretically reproduced by the proposed expression. It
predicted the limiting value of delocalized electrons by a H2O molecule in homodromic
cyclic clusters to be 1.58e. O-atoms provide the vast majority of electrons that
“travel throughout a cluster predominantly on a privileged exchange quantum density
highway” ( O–H O–H O–H ) using Bader's classical bond paths as density bridges
linking water molecules. There are, however, additional electron exchange channels
that are not seen on molecular graphs as bond paths. A 3D visual representation
of the “privileged” and “additional” exchange channels as well as detailed intra- and inter-molecular patterns of e-sharing and (de)localizing is presented. The energy stabilizing
contribution made by three O-atoms of neighboring water molecules was
found to be large ( 597 kcal/mol in cyclic hexamer) and 5 times more significant
than that of a classical O–H O intermolecular H-bond.
