Molecular Dynamics Study of Hydrated Imogolite. 1. Vibrational Dynamics of the Nanotube

An atomistic model of imogolite was defined and tested in molecular dynamics simulations. The model is based on the structure by Cradwick et al. (Nature, Phys. Sci. 1972, 240, 187). The interatomic interactions were defined by a generalized valence force field. As the experimental samples are mostly hydrated, water molecules were also included into the model. X-ray diffraction pattern computed from molecular dynamics trajectory well corresponds to the experimental one, thus confirming the validity of structural model. Calculation of the vibrational dynamics of imogolite nanotube shows a good overall agreement between the experimental and the calculated infrared and Raman spectra. The detailed analysis of the vibrational spectra leads to the conclusion that there exist strong couplings between internal coordinates preventing from the complete assignment of the observed bands to a particular coordinate or structural unit. These strong couplings also lead to the appearance of breathing vibrations involving in-phase displacements of all atoms at low frequencies. Origins of the Raman activity of some peaks are analyzed in detail.