Nuclear magnetic resonance relaxation investigation of tetrahydrofuran and methyl iodide clathrates

Abstract

Proton and deuteron spin–lattice relaxation rates of the guest molecules have been measured as a function of temperature in methyl iodide and tetrahydrofuran clathrates. By use of the isotopic-dilution method in proton n.m.r., i.e. employing the compounds (CH₃I)_x·(CD₃I)_1–x·17D₂O and (C₄H₈O)_x·(C₄D₈O)_1–x·17D₂O with varying mole fractions x, intramolecular and intermolecular contributions to the proton relaxation rate may be separated. Deuteron relaxation rates have been measured in the compounds CD₃I·17H₂O and C₄D₈O·17H₂O. The maximum temperature range covered was 0 to –120 °C.

The proton relaxation rates contain rather large intermolecular contributions, ca. 45% for methyl iodide and 23% for tetrahydrofuran. The correlation times obtained from the intramolecular contributions lie in the range 1-12 and 0.5-1 ps for methyl iodide and tetrahydrofuran, respectively, which is the range typical for liquids. This confirms the well known fact that the enclathrated guest molecules perform very fast motion. The evaluation of the deuteron relaxation rates leads to correlation times which are comparable to those derived from the intramolecular proton rate for tetrahydrofuran, but differ in the case of methyl iodide. This indicates anisotropic rotational motion of the methyl iodide molecule in the clathrate cage.