Restrained Rietveld refinement of molecular materials followingab-initiostructure solution from powder diffraction data

Molecular dynamics (MD) simulation can be used to predict the behavior of complex crystal structures that contain material confined within nanotubes [G Hummer et al. Nature (2001) 414, 188] and organic compounds. [Y. Pan et al. Molecular Simulation, in press and Y Pan et al. Phys. Rev. B, in press]. The nanotube structures of tryptophylglycine were extensively studied [H. Birkedal et al. Angew. Chem Int. Ed (2002) in press]. MD simulations of this compound were carried out both with and without water within the nanotubes. In the simulation containing water, the system was heated to 450 K and cooled to 40 K starting from the experimental 120 K structure. Phase transitions were observed at 270 K and 370 K. Below 250 K, the system is ordered and water molecules are completely bonded to the tube. Some of the hydrogen bonds, that are connecting water molecules to the walls of the tube, start to break at 250 K. The number of hydrogen bonds decrease to 1/3 and 1/5 from 250 K to 310 K. Water melting is predicted at about 270K and boiling at about 370 K. The water molecules are disordered at temperatures above 270 K and can move large distances along the direction of the tube above 310 K. Another simulation without water carried out from 120 K to 520 K indicates a change from a positive to negative thermal expansion along the c-axis at about 280 K and a phase transition (loss of crystallinity) at 450 K. The simulation results are not only in good agreement with our experimental results but also provide detailed information on the mechanisms responsible for the physical behavior of this nanotube. Our results show that the water molecules are involved in the mechanism of negative thermal expansion in the system containing water.