A Combined Structural Study Using NMR Chemical-Shielding-Tensor Correlation and Neutron Diffraction in Polycrystalline Methanol

Abstract

A structural study by solid-state NMR and neutron diffraction is presented for the two crystalline phases of methanol. The temperature dependence of the α-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 β-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 tensor in a molecular coordinate system can be determined. It is found that the most shielded axis of the tensor is tilted from the C-O bond direction toward the proton of the hydroxyl group by 7.0° and 5.5° in the α-phase and β-phase, respectively. This tilt fully reconciles the results from NMR and diffraction methods, and the disagreement between NMR and X-ray results in the β-phase found by Tycko and Dabbagh (Tycko, R.; Dabbagh, G. <em>J. Am. Chem. Soc.</em> <em>1991</em>, <em>113</em>, 3592-3593) is resolved. For the α-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.