NMR Study of Dynamics of Dimethyl Sulfoxide Molecules in Kaolinite/Dimethyl Sulfoxide Intercalation Compound

S. Hayashi
[J. Phys. Chem., Vol.99, pp.7120-7129, 1995]

We have studied molecular motions in kaolinite/dimethyl sulfoxide (DMSO) compounds by means of solid-state NMR. Quantitative analysis of thermogravimetric analysis and ¹³C and 29Si magic-angle-spinning (MAS) NMR spectra reveal the formation of Al₂Si₂O₅(OH)₄¥(CH₃)₂SO. ¹³C MAS NMR spectra show two inequivalent methyl carbons for interlayer DMSO molecules over the temperature range ~ 170 - 330 K (See Fig. 1), demonstrating that no exchange between the two methyl sites take place. We have measured ¹³C spin-lattice relaxation times T₁ for kaolinite/DMSO, where ¹³C spins relax by rotation of the CH₃ group around the C₃ axis. Activation energies for the methyl rotation have been estimated for the two sites, 13.0 and 16.5 kJ/mol for the keyed and unkeyed methyl groups, respectively. ²H spectra have been measured for kaolinite/DMSO-d₆ between 160 and 330 K, which are composed of resolved doublet (RD) and narrow central (NC) components. NC is a surface-adsorbed species, while RD contains surface and interlayer molecules. Line shapes indicate that RC undergoes rotation of the CD₃ group around the C₃ axis and NC does isotropic rotation. ²H T₁ has been measured also in the above temperature range, and ²H spins are relaxed by the CD₃ rotation around the C₃ axis. 29Si spins in the kaolinite/DMSO compound relax by the CH₃ rotation in addition to the relaxation by paramagnetic inpurities, indicating the contribution of ¹³C - ¹H dipolar interaction between the guest molecule and the host. The ¹H chemical shift of hydroxyl groups in the host was determined by the CRAMPS technique, which is not changed by the intercalation.

Fig. 1. Temperature dependence of ¹³C CP/MAS NMR spectra of the kaolinite/DMSO compound (sample II), measured at vL = 100.61 MHz. The spinning rate was about 1.7 kHz ; the contact time was 2 ms.