In this work, we combine several recent theoretical advances to better understand the nature of chemical bonds in condensed phases. We take advantage of new methods for developing coordinate-dependent pseudopotentials,[1] new ways of solving the Schrodinger equation at the CISD level with a 108 plane-wave basis set in a time fast enough to allow for molecular dynamics,[2] and a new method for performing umbrella sampling with quantum objects[3] to explore the behavior of the Na2molecule in different environments. In liquid Ar, we find that the bond shortens and its frequency blue-shifts as a result of compression of the bonding electrons by collisions with the nearby solvent atoms. These collisions also induce a large instantaneous dipole since the bonding electrons are pushed off-center, giving rise to an IR absorption.[4] When Na2 is placed in liquid THF, in contrast, the average vibrational frequency is strongly red-shifted and the molecule has a much larger average dipole moment and more intense IR spectrum than in liquid Ar.[5] The reason for this is chelation: the ether solvent chelates both ends of the molecule, pushing the electron density off of the bond axis. The dative bonds formed between the THF oxygen atoms and the Na+ cores are about the same strength as an H-bond in water. As a result, the PMF for chelation of the dimer along a chelation coordination number order parameter shows that each chelated state occupies a local free energy minimum, with barriers of 6 to 8 kBT separating the different chelation states. As a result, each chelation state represents a distinct molecule with its own vibrational frequency, IR and electronic absorption spectrum. Thus, the mere presence of local specific interactions with solvent that are on the same order as an H-bond turns what had been a single Na2 molecule into multiple different molecular species with different properties in equilibrium. Clearly, solvents not only modulate chemical properties, but also play a direct role in solute chemical identity.

Quantum mechanics of how solvents alter the identity of chemical bonds

2019 Fall National Meeting

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