Hydrogen Bond Interactions with Sulfur Donors

Calorimetric enthalpy data are reported for the reactions of the acids, 1,1,1,3,3,3-hexafluoro-2-propanol and 2,2,2-trifluoroethanol, with six sulfur donors in CCl₄ solution and four donors in hexane solution. Frequency shift data are also reported for the same two acids reacting with eight sulfur donors. Measurement of the heats of solution of each sulfur donor in both CCl₄ and hexane allows us to estimate hexane enthalpies for the two donors whose enthalpies could not be measured directly in hexane. A comparison of oxygen and sulfur donor ΔH vs. ΔH and Δv vs. Δv equations reveals the greater importance of van der Waals repulsions in the sulfur donor reactions. The change in relative slopes of these equations may also be related to the greater importance of covalent contributions (C_aC_b term) to hydrogen bond formation with sulfur donors. The data adhere nicely to linear enthalpy-frequency shift relations which are considerably different from those reported previously for oxygen donors. A comparison of some analogous oxygen and sulfur donor frequency shifts reveals strong P–S π bonding in a series of phosphine sulfides.