Unusually Large Effects of Charge-assisted C-H⋠⋠⋠F Hydrogen Bonds to Anionic Fluorine in Organic Solvents: Computational Study of (19) F NMR Shifts versus Thermochemistry.

A comparison of computed (19) F†NMR chemical shifts and experiment provides evidence for large specific solvent effects for fluoride-type anions interacting with the σ*(C-H) orbitals in organic solvents like MeCN or CH(2) Cl(2) . We show this for systems ranging from the fluoride ion and the bifluoride ion [FHF](-) to polyhalogen anions [ClF(x) ](-) . Discrepancies between computed and experimental shifts when using continuum solvent models like COSMO or force-field-based descriptions like the 3D-RISM-SCF model show specific orbital interactions that require a quantum-mechanical treatment of the solvent molecules. This is confirmed by orbital analyses of the shielding constants, while less negatively charged fluorine atoms (e. g., in [EF(4) ](-) ) do not require such quantum-mechanical treatments to achieve reasonable accuracy. The larger (19) F solvent shift of fluoride in MeCN compared to water is due to the larger coordination number in the former. These observations are due to unusually strong charge-assisted C-H⋠⋠⋠F(-) hydrogen bonds, which manifest beyond some threshold negative natural charge on fluorine of ca. < -0.6†e. The interactions are accompanied by sizable free energies of solvation, in the order F(-) ≫[FHF](-) >[ClF(2) ](-) >[ClF(4) ](-) . COSMO-RS solvation free energies tend to moderately underestimate those from the micro-solvated cluster treatment. Red-shifted and intense vibrational C-H stretching bands, potentially accessible in bulk solution, are further spectroscopic finger prints.