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

Abstract

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 are composed of a narrow central peak, a resolved Pake doublet (RD1), and a distribution of Pake doublets (RDs). 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 RDs 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 nano-seconds, 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.