We present two applications of recently developed analysis tools based on absolutely localized molecular orbitals (ALMOs). In the first application, we address the challenge of properly describing intermolecular interactions that involve radicals, where the orientation of the unpaired electrons must be specified. For this purpose, we present an improved unrestricted ALMO-EDA scheme in which fragment spin densities are uniquely aligned in the frozen state. This scheme was then employed to investigate several radical-neutral complexes that are relevant to chemical reactions in the gas phase, such as hydroxyl-water clusters and complexes formed by a free Cl atom and Lewis bases. In the second application, we employed ALMO-based methods to decompose shifts in molecular properties to unravel the origin of “improper” (blue-shifting) H-bonds with the fluoroform molecule (CF<sub>3</sub>H) as the proton donor. It was unambiguously revealed that shortening of the C-H bond and blue shift in its stretch frequency appear already on the frozen surface, where both polarization and charge-transfer (CT) are quenched. The strength of CT, on the other hand, determines whether the hydrogen bond is “improper” in the fully relaxed complex. A further decomposition of forces at the frozen level identifies that Pauli repulsion is the only component that tends to shorten the C-H bond near the equilibrium range, while both permanent electrostatics and dispersion favor a lengthened bond. The interplay of these forces in the medium to long range is also discussed.<br/>

Probing radical-neutral interactions and “improper” hydrogen bonds with analysis tools based on absolutely localized molecular orbitals

2019 Fall National Meeting

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