Nuclear magnetic resonance relaxation, permittivity, viscosity and ultrasonic velocity measurements in binary mixtures of methanol and tetrahydrofuran

Abstract

Proton NMR relaxation rates of CD₃OH and proton and oxygen-17 NMR relaxation rates of CD₃¹⁷OH with 6, 11.7 and 26.8%¹⁷O enrichment have been measured in binary mixtures with deuteriated tetrahydrofuran ([²H]THF) containing 10, 20, 30, 50, 60, 75, 85 and 95 mol% methanol at 288, 298 and 308 K. Deuteron NMR relaxation rates of CH₃OD have also been measured in tetrahydrofuran (THF) at the same compositions and temperatures. From the measured proton relaxation rates, the ¹⁷O-induced intramolecular proton relaxation rates have been extracted. Using these values, the rotational correlation time (τ^OH₂) of the OH bond of methanol has been determined at various compositions. The variation of τ^OH₂ with composition indicates that the hydrogen bonding (self-association) of methanol begins to break up after the addition of ca. 25 mol%[²H]THF and at >40 mol%[²H]THF the structure breaking of methanol becomes much more pronounced. Negative values of the excess isentropic compressibility (K^E_s), excess permittivity (ε^E), excess viscosity (η^E) and excess volume (V^E) of CH₃OH–THF mixtures over the whole composition range also indicate strong structure breaking, especially in the range 30–60 mol% THF, where all these excess parameters pass through a minimum. Proton NMR measurements also confirm that structure breaking of CH₃OH by THF is stronger at >40 mol% THF. The deuteron and ¹⁷O quadrupole coupling constants of methanol in various CH₃OD–THF and CD₃¹⁷OH–[²H]THF mixtures have been evaluated and a strong variation of the deuteron quadrupole coupling constant with composition and temperature as compared to that of ¹⁷O is observed.