Journal of the American Chemical Society, Vol.116, No.12, 5315-5323, 1994
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 presented for the two crystalline phases of methanol. The temperature dependence 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 the first investigation of the beta-phase by neutron diffraction are given. Independent structural information is obtained by spin-diffusion powder NMR spectroscopy, where the relative orientation of the carbon-13 chemical shielding tensors for both phases is determined. By combination 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 tilted from the C-O bond direction toward the proton of the hydroxyl group 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 diffraction methods, and the disagreement between NMR and X-ray results in the beta-phase found by Tycko and Dabbagh (Tycko, R,; Dabbagh, G.J. Am. Chem. Sec. 1991, 113, 3592-3593) is resolved. For the alpha-phase, the agreement between NMR and neutron diffraction results is significantly improved by taking into account the tilt as well as the temperature dependence of the structure.
Keywords:MAGNETIC-RESONANCE;CRYSTAL-STRUCTURE;SOLID METHANOL;SPIN-DIFFUSION;SPECTROSCOPY;SPECTRA;DEUTERATION;ANISOTROPY;PHASE;ACID