Study On The Aggregated Structure And Raman Non-coincidence Effect Of Amides

Intermolecular interactions have always been the focus of research in the field of chemical science.There are many forms of intermolecular interactions,such as the dipole-dipole interaction between C=O vibration modes and the typical hydrogen bond interaction.Intermolecular interactions is the main research content of solution system.Solvent effect is the interaction between solvent and solute molecules.Solvent effects are of great importance in industrial application,biological and chemical processes,and vibrational spectroscopy can reveal the nature of such interactions in solutions.In the spectral study of solutions,organic molecules containing C=O stretching vibration modes can be used to evaluate the intermolecular interaction by observing the change of C=O frequency and intensity with the solution concentration and solution in different solvents.The red shift and blue shift of C=O vibration mode frequency can be observed in Raman spectra of different carbonyl compounds.It is well known that the difference between the isotropic and anisotropic Raman frequencies of the C=O vibrational modes of liquid carbonyl compounds occurs,which is usually called the Raman noncoincidence effect(NCE).NCE can be seen as a valuable tool for probing the structural configuration of liquids at the molecular level and for monitoring structural changes caused by the presence of solvent molecules in mixtures.The rapid development of quantum chemical calculation can further clarify the nature and laws of chemical phenomena,because of its wide range of applications,involving many fields and become the majority of chemists research tools.In this paper,Raman noncoincidence effect and aggregation structures of C=O modes in Nethylmaleimide,N-ethylsuccinimide,N-methylformamide and N-methylacetamide are investigated by means of Raman spectroscopy and Density functional theory.1.Using DFT theoretical calculation and experimental FT-IR/Raman spectroscopy to determine the state of aggregation structurethe of N-methylethylamide which can form eight yuan ring dimers.The C=O vibrational mode Raman spectra of N-methylacetamide in different concentrations of CCl4,CH3 OH,CH3CN and H2 O solvents were tested to explore the aggregation state structure of N-methylacetamide in different solvents.In CH3 OH and CH3 CN solvents,the isotropic and anisotropic Raman frequency of C=O as the reduce of concentration move to the high frequency,??NCE also gradually reduced.The results showed that N-methylacetamide gradually changed from dimer to monomer structure.The isotropic and anisotropic Raman frequency of C=O and ??NCE remain relatively stable in CCl4 solvent with gradually dilution,which means that N-methylethylamide remained eight yuan ring dimers configurations.In solvent H2 O,isotropic and anisotropic Raman frequency of C=O vibration mode move to low wavenumber region,??NCE first increases and then decreases to 3 cm-1with the solution concentration dilutionto.According to the experimental phenomena in aqueous solvents,different aggregation state models are proposed,which are consistent with the theoretical results.The aggregation state configuration of each monomolecular Nmethylacetamide in the dimer N-methylacetamide is hydrogen bonded with one H2 O molecule and three H2 O molecules respectively2.The aggregation structure of N-methylformamide was optimized and calculated with the method of DFT theory calculation,and the stable dimer configuration was obtained,which was consistent with the NCE effect in the experimental C=O vibration of N-methylformamide molecule.In order to explore the influence of different solvents on the aggregation structure of N-methylformamide molecules,we respectively collected a series of Raman spectras of N-methylformamide with different concentrations in CH3 Cl and CH3 OH solvents,and found the NCE in CH3 Cl solvent at dilute solution is still the larger value,suggesting that N-methylformamide is still a dimer configuration with C=O coupling between molecules and is verified by theoretical calculation.In CH3 OH solvent,with the reduction of N-methylformamide concentration,the isotropic and anisotropic Raman frequency of C=O move to the the high frequency,??NCE also gradually reduced.These phenomena indicate that in CH3 OH,with the decrease of solution concentration,the dimer with hydrogen bond will be separated and eventually exist in the form of monomolecular N-methylformyl.3.The FT-IR/Raman spectra of N-ethylmaleimide and N-ethylsuccinimide were respectively collected and compared with the spectra obtained by DFT theory calculation.The configuration optimization calculation was repeatedly adjusted,so as to determine the aggregation structure of the dimer of N-ethylmaleimide and Nethylsuccinimide.In order to further illustrate the influence of NCE effect and solvent effect,tested the Raman spectrums of C=O vibration modes of N-ethylmaleicimide and N-ethylsuccinimide respectively in different volume fraction in CCl4 and CH3 CN solvent mixture,found that the concentration of the solution and the polarity of the solvent would affect the state of aggregation structure of N-ethylmaleicimide and Nethylsuccinimide,namely the dilute solution concentration and the increase of polarity of the solvent would weaken the coupling effect between the C = O vibration modes.