Anionic CH⋅⋅⋅X- hydrogen bonds: origin of their strength, geometry, and other properties.

CF3H as a proton donor was paired with a variety of anions, and its properties were assessed by MP2/aug-cc-pVDZ calculations. The binding energy of monoanions halide, NO3(-), formate, acetate, HSO4(-), and H2PO4(-) lie in the 12-17 kcal mol(-1) range, although F(-) is more strongly bound, by 26 kcal mol(-1). Dianions SO4(2-) and HPO4(2-) are bound by 27 kcal mol(-1), and trianion PO4(3-) by 45 kcal mol(-1). When two O atoms are available on the anion, the CH⋅⋅⋅O(-) H-bond (HB) is usually bifurcated, although asymmetrically. The CH bond is elongated and its stretching frequency redshifted in these ionic HBs, but the shift is reduced in the bifurcated structures. Slightly more than half of the binding energy is attributed to Coulombic attraction, with smaller contributions from induction and dispersion. The amount of charge transfer from the anions to the σ*(CH) orbital correlates with many of the other indicators of bond strength, such as binding energy, CH bond stretch, CH redshift, downfield NMR spectroscopic chemical shift of the bridging proton, and density at bond critical points.