Solution Structures And Molecular Interactions In Binary Association Solution Systems

Molecular interaction is one of the fundamental questions in physical chemistry.To investigate molecular interactions and solution structure of liquid mixtures with association characteristics is one of the most challenging tasks.In this work,solution structure and molecular interactions in a number of small molecular systems and ionic liquid–solvent systems were investigated by employing spectroscopic methods and quantum chemical calculations.Particularly,the hydrogen bonding interactions,the existing forms of molecules/ions in the studied solutions,and their transformation in the process of dilution were studied in detail.First,a new method is developed to identify molecules with two different existing forms in a binary liquid solution based on the theory of excess spectroscopy.In this so-called two-state situation,the excess band of a vibration mode of the molecule will show fixed peak positions and zero-intensity frequencies in the concentration range if the other component has no absorption in the region.Otherwise,these positions will be concentration-dependent.One application of the theory on the binary system,C6F5I–cyclo-C6H12,suggests that C6F5 I exists in two forms,namely C6F5 I monomer and dimer.Second,a few binary systems containing small neutral molecules were studied.In C₂H₅OH–CH3CN system,a number of species,namely multimer,trimmer,and dimer of C₂H₅ OH,and C₂H₅OH-CD₃ CN complex,were identified experimentally.Synchronous moving-window 2D NIR correlation spectra suggest that the dissociation of the multimers into C₂H₅OH-CD₃ CN complexes proceeds through the intermediate species upon dilution by CH3 CN.In NMF-DMSO and NMA-DMSO systems,the excess infrared spectra of ??N-H?show opposite properties.This is interpreted as that NMA,but not NMF,can form long chain-type structure connected by resonance-assisted hydrogen bonds?RAHBs?.By quantum chemical calculation,the chain-type RAHBs were characterized from the perspective of interaction energies,hydrogen bond length,bond order,and NBO charge.In DMF-DMSO system,lacking of N-H bond,the hydrogen bonding interaction is very weak.Third,an ionic liquid-molecular solvent system was investigated.Excess infrared spectra show that C?N stretching vibration in CD₃ CN is more sensitive to micro-environment than other vibration modes of functional groups in the mid-infrared regions in the binary system of [Emim][Tf2N] and CD₃ CN.Thus,it can be used as a good probe to detect the microenvironments of different action sites in ionic liquid.A possible reason as why the nitrile group is so powerful over other functional groups could be that C?N interacts with proton donor neither too weak nor too strong.Other than this,it was found that,when the concentration of acetonitrile is low,anions and cations tend to stay together due to strong electrostatic interactions.When the mole fraction of acetonitrile reaches to 0.9,the cations and anions are separated entropically,forming solvated ions.