A COMBINED STRUCTURAL STUDY USING NMR CHEMICAL-SHIELDING-TENSOR CORRELATION AND NEUTRON-DIFFRACTION IN POLYCRYSTALLINE METHANOL

A structural study by solid-state NMR and neutron diffraction is prese nted for the two crystalline phases of methanol. The temperature depen dence of the cu-phase structure from 1.5 to 145 K shows a variation of the relative orientation of the hydrogen-bonded chains. Results of th e first investigation of the beta-phase by neutron diffraction are giv en. Independent structural information is obtained by spin-diffusion p owder NMR spectroscopy, where the relative orientation of the carbon-1 3 chemical shielding tensors for both phases is determined. By combina tion of the NMR and diffraction results, the orientation of the carbon -13 chemical shielding tenser in a molecular coordinate system can be determined. It is found that the most shielded axis of the tenser is t ilted from the C-O bond direction toward the proton of the hydroxyl gr oup by 7.0 degrees and 5.5 degrees in the alpha-phase and beta-phase, respectively. This tilt fully reconciles the results from NMR and diff raction methods, and the disagreement between NMR and X-ray results in the beta-phase found by Tycko and Dabbagh (Tycko, R,; Dabbagh, G.J. A m. Chem. Sec. 1991, 113, 3592-3593) is resolved. For the alpha-phase, the agreement between NMR and neutron diffraction results is significa ntly improved by taking into account the tilt as well as the temperatu re dependence of the structure.