Solid-State Nuclear Magnetic Resonance Study Of The Structure And Dynamics In Small Molecular Crystals

The knowledge of structures and dynamics of crystalline materials in molecular level is key important for us to understand the macroscopic properties of the materials.In this dissertation,crystal structures and molecular motions in several small molecular crystals were investigated by using a series of advanced solid-state NMR techniques,including static 2H NMR and 13C chemical-shift anisotropy(CSA)lineshape analysis and so on.The relationship between the micro behaviors and the macro-performance of these materials was discussed.The main contents of the dissertation are as follows:1.The dynamics of caged imidazolium cation in metal-organic framework(MOF)material(HIm)2[KCo(CN)6](HIm=imidazolium cation)was investigated.The 2H NMR revealed the different dynamics of the Him cation in multiple phase transition.The high resolution solid state 1H NMR unveiled the presence of the association/dissociattion process of the N-1H proton of HIm cation.Based on the NMR results,the molecular mechanism of the dielectric transition of the material was discussed.2.The changes of the molecular dynamics of the(2-chloroethyl)ammonium(BCEA)cation in the molecular crystal,bis(2-chloroethyl)amine hydrochloride,were studied.The 2H NMR revealed that the BCEA cation undergoes dynamic changes from the static to rotating state.The rotation axis was confirmed later by the deliberately designed combination of solid state 2H NMR and 13C CSA lineshape analysis.The switching dielectric property of the material was discussed based on the observed change of the rotational dynamics of the BCEA cation.3.The dynamics of the HPy cation in MOF material,(HPy)2[Na(H2O)2Co(CN)6](HPy=pyridinium cation),was studied.The motion of the HPy cation was studied by using solid state 2H NMR and the 13C CSA lineshape analysis.The obtained results unveiled the role of hydrogen-bonding interactions on the motion of the HPy cation and in turn,the dielectric switch of the material.4.The molecular dynamics of the MA cation in three MOF materials,(MA)2[B'Co(CN)6](MA=methylammonium cation,B'=Na,K,Rb),was studied by solid state NMR in great detail.The lattice size influences on the motion of the MA cation was unveiled.The regulation of phase transition by modifying the lattice size of the materials was discussed.5.The solid-state phase transition of the small molecular crystal,[CH6N3]2[FeK2(H2O)6CN6]([CH6N3]=guanidinium cation),was studied.The reversible dehydration-rehydration process in the crystalline material was investigated by in-situ 1H NMR detection.Based on the NMR results,the molecular mechanism of the phase transition of the material was discussed.In summary,in this dissertation,the molecular dynamics present in a series of small molecular crystals was explored through the combined use of solid-state 2H NMR,13C CSA NMR and in-situ NMR techniques.The relationship between the molecular motions and the macroscopic properties in the materials was discussed.The works in this dissertation demonstrate the unique advantage of solid-state NMR on the characterization of the structure and molecular dynamics in the small molecular crystal systems.The results yielded from the works in this dissertation will be helpful to deepen our understanding of the origin of the macroscopic properties of the materials in molecular level,and at the same time,might provide a promising direction of the structure design of the new functional materials.