IQA-embedded fragment attributed molecular system energy change in exploring intramolecular interactions

Abstract

Energy of intramolecular interaction cannot be measured experimentally. Deeply-rooted in the Interacting Quantum Atoms framework, expressions for a fragment attributed molecular system energy change (FAMSEC) are proposed and implemented to quantify energy contribution made by a molecular fragment G ¼ fA; Bg made of interacting atoms. A classical nature of (i) N H (in protonated ethylenediamine, Hen) and O H (in protonated ethanolamine, Hea) and (ii) O O (in eclipsed glycol, gc) was fully recovered and their origin explored; N H and O H stabilize respective molecules locally, local-FAMSEC, and globally, mol-FAMSEC (opposite applies to O O in gc). Higher energy of planar biphenyl (bph) was attributed to (i) C-atoms linking the rings due to an unfavorable change in interactions with all atoms of bph and (ii) increase in self-atomic energies of the remaining C-atoms of the bph bay. Considering ortho-hydrogens, they (i) do not conform to steric clash, (ii) resemble stabilizing interactions in Hen and Hea and (iii) follow changes in physical properties (on interaction formation) found for heteroatoms in Hen and Hea (opposite was found for O-atoms in gc). Moreover, the mol-FAMSEC term (i) accounts to some extent, although indirectly, for the geometric deformation energy of all atoms not involved in the intramolecular interaction, (ii) equally applies to any kind of (de)stabilizing or QTAIM (non)bonded interaction, and (iii) can equally be used for any size of a molecular fragment (e.g. functional groups) as well as for intermolecular interactions.