Hydrogen Bond Donation Properties of Organic Solvents and Their Aqueous Mixtures from ¹³C NMR Data of Pyridine-N-Oxide

Measurements of the ¹³C nmr chemical shifts δ of pyridine-N-oxide as a probe in 18 solvents, with literature values for others, established that the differences d₂₄ and d₃₄ of the δ of carbons 2 and 3 with respect to that of carbon 4 depend solely on the hydrogen bond donation ability of the solvents, as measured by the Taft-Kamlet α parameter. This substance and method can, therefore, be used as an effective probe for α, without the need to invoke other solvent properties, such as their polarities/polarizabilities, measured by π*, required by other probes. The probe was applied also to aqueous mixtures of acetone, acetonitrile, and methanol (values of π* and ß for acetone, besides those of α, being reported here too) with similar results. The two aqueous aprotic solvent mixtures exhibit microheterogeneity, as previously established for aqueous acetonitrile from other results.