Development Of The Generalized Energy-Based Fragmentation Method And Its Applications For Ionic Liquids And Emission Properties Of Molecular Crystals

A major objective of theoretical and computational chemistry is the accurate calculation of the energies and properties of molecules,so that chemical reactivity and material properties can be understood from first principles.For small systems,their ground-state energies and thermodynamic properties can now be evaluated within chemical accuracy(±1 kcal/mol).However,the computational cost of the conventional quantum chemistry methods increases rapidly with the system size.In order to extend quantum chemistry calculations to large systems,theoretical chemists have developed many linear scaling quantum chemistry methods.As one of the most efficient linear scaling quantum chemistry methods,the generalized energy-based fragmentation(GEBF)approach has been demonstrated as a powerful tool for computing ground-state energies,optimized structures,and vibrational spectra of a wide range of large systems including molecular clusters,biomolecules and condensed-phase systems.Within the GEBF approach,the ground-state energy(or properties)of a large system is evaluated as a linear combination of ground-state energies(or properties)of a series of small electrostatically embedded subsystems.This feature allows quantum chemistry calculations of large systems to be computationally feasible.In this thesis,we have focused on the development and application of the GEBF approach.The main contributions of this thesis is that we proposed the ion-pair-based fragmentation for the GEBF approach.With this fragmentation,the GEBF approach has been developed to facilitate ab initio calculations of ground-state energies,structures and vibrational spectra,NMR properties of general ionic liquid(IL)clusters,ionic crystals,and ionic liquids(ILs).Meanwhile,the GEBF method was applied to understand the emission properties of molecular crystals.We have developed a refined QM/MM approach for understanding the luminescence mechanism of molecular crystals.Main contributions of the present dissertation can be summarized as follows:In chapter 3,the GEBF approach has been developed to facilitate ab initio calculations of ground-state energies,structures and vibrational spectra of general IL clusters.For selected IL clusters,the accuracy of the GEBF approach with two different fragmentation schemes(ion-pair-based fragmentation and ion-based fragmentation)was evaluated with the conventional quantum chemistry calculations.Our results demonstrated that for selected IL clusters,the GEBF approach with the ion-pair-based fragmentation scheme can provide much more accurate descriptions than that with the ion-based fragmentation scheme.The main reason for these results is that the non-integer charge behavior of each ion in IL systems may induce significant errors for the GEBF approach with the ion-based fragmentation scheme,in which every ion is assumed to have an integer charge.However,this problem can be avoided by the ion-pair-based fragmentation scheme,in which each ion pair is assumed to be electrically neutral.Hence,for IL cluster,the ion-pair-based fragmentation scheme is recommended for GEBF calculations.With this strategy,the GEBF approach can provide accurate descriptions on the ground-state energies,structures,vibrational spectra,as well as other properties of general IL clusters.In chapter 4,based on the proposed ion-pair-based fragmentation,the periodic GEBF approach has been developed and then applied for predicting ground-state energies,structures and spectra properties of general ionic crystals and ILs.Firstly,for the ground-state energies,optimized structures,and vibrational spectra of selected IL crystals,we found that the calculated results from the GEBF approach with the ion-pair-based fragmentation scheme are in good agreement with those from the standard periodic electronic structure methods.Then,the GEBF approach was applied to predict the structure and lattice energy of LiCl,NaCl and KCl crystals,structures and Raman spectra of the[n-C4mim][Cl]polymorphs,and 1H NMR chemical shifts of[C2mim][BF4]ILs.Our results demonstrated that the GEBF approach with a suitable theory can provide satisfactory descriptions of optimized structures,vibrational spectra and NMR spectra of ionic crystals and ILs.Hence,the GEBF approach,combined with advanced quantum chemistry methods,may become an effective tool in predicting the structure and properties of ionic crystals and ILs.In chapter 5,the GEBF approach has been applied to understand the emission properties of molecular crystals.Upon the optimized crystal structures with the GEBF approach,we have developed a refined QM/MM approach for investigating the emission spectra of molecular crystals.In this approach,we proposed a simple and effective way of automatically building cluster models for the general QM/MM approach to deal with the excited states of a molecule in the crystalline environment.Our results show that the monomer model for QM/MM calculations is suitable for describing the emission spectra of crystals without ?…? stacking interactions,while the most stable trimer model or at least dimer model(with the highest binding energy)should be used for accurately describing the emission spectra of crystals with the notable ?…? stacking interactions.The effect of the notable ?… stacking interactions on the emission properties of crystals can be understood by the fact that the intermolecular ?…? interactions can reduce the gap between frontier molecular orbitals.This approach was then applied to understand the emission properties of two kinds of organic polymorphs.The refined QM/MM strategy is expected to be an efficient tool for understanding the absorption and emission properties of molecular crystals,and should be helpful for experimental scientists to design novel luminescent materials.