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Manipulation of Diatomic Molecules with 'Oriented External Electric Fields': Linear Correlations in Atomic Properties Lead to Non-Linear Molecular Responses

'Oriented external electric fields (OEEFs)' have been shown to have great potential in being able to provide unprecedented control of chemical reactions, catalysis and selectivity with applications ranging from H2 storage to molecular machines. We report a theoretical study of the atomic origins of molecular changes due to OEEFs; understanding the characteristics of OEEF-induced couplings between atomic and molecular properties is an important step toward comprehensive understanding of the effects of strong external fields on molecular structure, stability, and reactivity. We focus on the atomic and molecular (bond) properties of a set of homo- (H2, N2, O2, F2, and Cl2) and hetero-diatomic (HF, HCl, CO, and NO) molecules under intense external electric fields in the context of quantum theory of atoms in molecules (QTAIM). It is shown that atomic properties (atomic charges and energies, and localization index) correlate linearly with the field strengths, but molecular properties (bond length, electron density at bond critical point, bond length, and electron delocalization index) exhibit non-linear responses to the imposed fields. In particular, the changes in the electron density distribution alter the shapes and locations of the zero-flux surfaces, atomic volumes, atomic electron population, and localization/delocalization indices. At the molecular level, the topography and topology of the molecular electrostatic potential undergo dramatic changes. The external fields also perturb the covalent-polar-ionic characteristic of the studied chemical bonds, hallmarking the impact of electric fields on the stability and reactivity of chemical compounds. The findings are well-rationalized within the framework of the quantum theory of atoms in molecules and form a coherent conceptual understanding of these effects in prototypical molecules such as diatomics.