Application Of Scattering Methodology In The Research Of Molecular Clusters

As a class of nanometer-scaled and well-defined atomic or molecular assembly,molecular clusters(MCs)have diverse sizes,structures and components,which exhibit good prospects on both fundamental science and applications.These features make MCs a suitable template system for fundamental research.On the other hand,there are still unsolved problems in MCs.At present,there are many characterization techniques successfully applied in MCs studies,including electron microscopy,light spectroscopy,nuclear magnetic resonance spectroscopy,mass spectrometry,X-ray diffraction etc.Although these methods can provide useful structural information,they can not fully meet some specific requirements.Therefore,aiming at demands in MCs studies,suitable methods are urgently needed.Among numerous structural characterization techniques,As an important category,scattering method has been extensively applied in microstructure characterization.Compared with other methods,scattering method has its advantage on adaptation and flexibility,which has low requirement on sample status and thus is easily to realize in situ and operando characterization.These advantages make scattering method a valuable technique in the structural characterization of MCs,especially in situ characterization.The difficulty in data analysis of scattering method limits its application in MCs studies to some extent.This raises the urgent demands for scattering methodologies and data analysis methods targeted at specific fields.Therefore,studying MCs by scattering methods not only gives information,evidences and conclusions of MCs related scientific questions,but also is a development on scattering methodologyIn this thesis,we focus on 2 scattering methods suitable for MCs studies,small angle scattering(SAS)and dynamic light scattering(DLS).First,we conduct research on scattering theory and develop new scattering methodologies and tools for data analysis.Then scattering methods are applied in the studies of MCs,where general protocols are proposed for MCs’ scattering characterization.And solution structure,formation mechanism,solvated conformation and deformed structure are studied in depth,which gives evidences and conclusions on some unsolved scientific questions based on scattering results.The details are as follows:(1)Due to the lack of corresponding scattering models,there are great difficulties concerning the SAS data’s analysis and model fitting of nano-particles with specific shapes.To solve this problem,we propose a method for calculating theoretical SAS curves from arbitrary shape models.Based on M(?)ller-Trumbore algorithm and spherical harmonics expansion,theoretical derivation is conducted and a utility software Model2 SAS is developed.By applying this method,we analyze the theoretical SAS curves of porous spherical shell model,which hasn’t had scattering functions yet.(2)For the existing problems in DLS characterization of polydisperse systems,we propose a multi-angle DLS theory and data analysis method.Based on Mie’s scattering theory,scattering of different angles are connected and included in a same system,where the theoretical principle is derived and demonstrated.Under this theory,machine learning method and framework are introduced.Regularization method and Adam algorithm are applied to the optimization of multi-angle DLS data analysis,which is implemented as program named Mdls NN.By analyzing the simulated and experimental DLS data by CONTIN and Mdls NN seperately and comparing their results,Mdls NN gives more accurate polydisperse distribution than the most used CONTIN algorithm.(3)For a special class of MCs,polyoxometalates(POMs),applicable small-angle X-ray scattering(SAXS)characterization and data analysis general methods and protocols are proposed.If atomic structure available,theoretical SAXS curves calculated from atomic structures can be used to identify POM species in solution by comparing and fitting with experimental SAXS data.If no prior information available,ab initio shape determination method based on Monte Carlo simulated annealing algorithm can be used to obtain possible solution structure and identify POM species.Using these methods,the dimer structure of {Ce20W100} in solution is confirmed.And POM species in Na2 Mo O4solution with different p H values are explored.(4)Solution structure and formation mechanism of molybdenum blue(MB)clusters are studied by SAXS.First,a custom carved ellipsoid model is proposed based on the structural features of MB,and its scattering function is derived,which shows better result when fitting with SAXS data of MBs.Then the synthesis process of MB is studied in depth.Through a number of experiments and species identification by SAXS,a formation path and mechanism of{Mo154} and {Mo176} is proposed: {Mo154} is product controlled by kinetics and {Mo176} is controlled by thermodynamics,where the important effects of counter ions,concentration and reaction time are revealed for the first time.(5)Using multiple scattering methods including SAXS,small-angle neutron scattering(SANS)and neutron reflectometry(NR),a comprehensive study on the solution structure,solvated conformation,interface structure and deformed behavior of star-like metal-organic polyhedra(MOPs)is conducted.MOP core of star-like MOPs is proved to be intact in good solvent.Peripheral grafted chains exhibit overall extended conformation but experience different level of confined effect.Solvent molecules can penetrate grafted chains layer into the cavity of MOP core.Additionally,the deformed structure of star-like MOP is produced at water-air interface using Langmuir-Blodgett film preparation technique.This deformed structure is characterized and proved by NR for the first time.NR results of spin-coated films of star-like MOPs indicate the films’ smoothness and homogeneity,which shows good processability.