NMR Study of Kaolinite Intercalation Compounds with Formamide and Its Derivatives: 2. Dynamics of Guest Molecules

Dynamics of guest molecules in kaolinite intercalation compounds with formamide (FA), formamide-N, N-d₂ (FA-d₂), N-methylformamide (NMF) and N,N-dimethylformamide (DMF) has been investigated through ²H NMR spectra and ²H and ¹³C spin-lattice relaxations. ²H NMR spectra of kaolinite/FA-d₂ in the temperature range 200�`350 K, which are shown in Fig. 1, are composed of a narrow central peak, a resolved Pake doublet (RD₁) and a distribution of Pake doublets (RD_s). Quadrupole coupling constants and asymmetric factors obtained by fitting the spectral patterns indicate that RD₁ is ascribed to the host hydroxyl group formed through H-D exchange during the synthesis and RD_s to the guest molecules in the interlayer. Temperature dependence of the quadrupole coupling constant reveals that the guest molecules undergo librational motions, and that the librational amplitude increases with temperature. Spin-lattice relaxations of 1H and ²⁹Si spins are due to paramagnetic impurities. ²⁹Si spins relax through direct dipole-dipole interaction with electron spins, and spin diffusion plays a role in the 1H relaxation. Variable-temperature 13C spin-lattice relaxation times T₁ of kaolinite intercalates with FA, FA-d₂, NMF and DMF were recorded at two magnetic fields. T₁ decreases with increasing temperature in all the samples, and increases at the higher magnetic field in the order of FA>FA-d₂>NMF>DMF, where it is almost independent of the field in kaolinite/DMF. ¹³C spins relax through dipole-dipole interactions with ¹H and ¹⁴N by the librational motions of the molecules. The field dependence indicates that the correlation times are of the order of nanoseconds, and that they are in the order of FA>FA-d₂>NMF>DMF. The temperature dependence of the relaxation time is caused by the amplitude change of the librational motions. ²H spins relax mainly through paramagnetic impurities being assisted by spin diffusion at low temperatures, and fluctuation of quadrupole interaction caused by the librational motions contributes to the relaxation at higher temperatures.

Fig. 1. ²H NMR spectra of kaolinite/FA-d₂, measured at n_L = 61.42 MHz. Quadrupole echo pulse sequence was used with a pulse interval of 15 ms.