Study On Non-coincidence Effect And Aggregated Structure Of Ketones And Anhydrides

Intermolecular interactions exist widely between organisms and common molecules,Studying intermolecular interactions plays an important role in understanding the changes in the structure and properties of matter in physics,chemistry,and biology.Molecular vibrational spectroscopy can directly or indirectly reflect the structure and properties of molecules,and are affected by various forms of intermolecular interactions in liquid phase.For example,in the process of the solid-liquid phase transition,we known the existence form and ways of the intermolecular weak interaction,by observing the intensity of spectral band,the shape and frequency position of the band maximum caused,and in some cases the band splitting change.Carbonyl compounds have long been focus of studying molecular aggregation states and weak interactions between molecules.When the dipole-dipole interaction exists between adjacent molecules,the intermolecular resonance energy shifts,promote the formation of a long-range or short-range ordered structure between molecules,and the anisotropic and isotropic raman spectra do not coincide,it is called as"non-coincidence effect".This paper uses a combination of microscopic Raman spectroscopy and density functional theory（DFT）calculation,it is systematically studied that the effects such as concentration and solvent on the non-coincidence effect（NCE）of"C=O stretching vibration mode"in 2,5-hexanedione,propionic anhydride,isobutyric anhydride and some similar materials,us the"aggregate state theory"to reasonably explain the aggregation state of the compound.This study helps us to have a deeper understanding of the mechanisms of intermolecular interactions and NCE effects.1.We used DFT and PCM（Polarizable Continuum Model）and Raman spectroscopy to study the NCE effect of C=O stretching vibration mode in 2,5-hexanedione molecule,using the B3LYP-D3/6-311G（d,p）basis group to optimize the most stable geometry of 2,5-hexanedione monomolecular and dimer,calculated the depolarization ratio and vibration frequency of the vibration mode,zero point corrects the total energy of the free energy.The theoretical calculation results show When the frequency of the C=O vibration mode gradually shifts to a high wave number,the 2,5-hexanedione molecule is converted from a dimer to a single molecule structure.In order to further observe the concentration effect,we collected anisotropic and isotropic Raman spectra of different concentrations of 2,5-hexanedione in carbon tetrachloride.The results show that with continuous dilution,the peak frequency of the C=O vibration mode is blue-shifted,and the peak-to-frequency difference between the anisotropic and isotropic raman spectra is gradually reduced.In addition,we collected anisotropic and isotropic raman spectroscopy of the same concentration of 2.5-hexanedione molecules in solvents of different polarities.It is found that the polarity of the solvent has an influence on the size of NCE,and the solvent with greater polarity has a greater influence on the value of NCE,this result is consistent with our PCM theoretical model spectrum.When investigating the effect of the spacing between carbonyl groups on the NCE,it was found thatΔν_NCE gradually decreased with the decrease of the intramolecular spacing,because the coupling of the carbonyl group in the molecule increased with the decrease of the distance,inhibit of dipole-dipole interactions of intermolecular carbonyl groups.2.Using DFT and PCM theoretical calculations and raman spectroscopy,we studied the“non-coincidence effect”of the Propionic anhydride C=O vibration mode,collected raman spectroscopy of propionic anhydride at different concentrations and different solvents and based on molecular aggregation theory,we have already analysis the effect of concentration change onΔν_NCE,The results show that as the solvent continues to dilute,the nature of the spectrum changes,Propionic anhydride molecules are converted from dimers to single molecules.the isotropic and anisotropic vibration frequencies of the C=O vibration mode are blue-shifted,Δν_NCE gradually decreases until it approaches a critical value,its size depends on the structure and properties of the solute;When analyzing the polarity of the solvent affects onΔν_NCE,we found that theΔν_NCECE value became smaller as the polarity of the solvent increased,this is because a solvent with a large polarity can weaken the short-range ordered structure of the Propionic anhydride molecule.3.For the first time,this paper systematically studied the relationship between the"non-coincidence effect",the concentration effect and the solvent effect of the C=O stretching vibration mode in isobutyric anhydride molecules,and use"aggregate state theory model"to explain the aggregation state of liquid molecules.The results show:that when carbon tetrachloride is used as a solvent,with constant dilution,the Raman spectrum peak gradually shifts to a high wave number and theΔν_NCE value decreases.When DMSO is used as a solvent,with constant dilution,Anisotropic and isotropic raman spectral peak shifts to low wavenumber andΔν_NCE value appears to decrease.In order to explain different experimental phenomenas,we use the B3LYP/6-311G（d,p）basis group calculation to optimize the monomer and dimerization structure of isobutyric anhydride,theoretical calculations are consistent with experimental results,the dimeric structure of Isobutyric anhydride can provide favorable evidence for explaining different experimental phenomena.When studying the"solvent effect",we use several different polar substances as solvents,found that with the polarity of the solvent increases,the value ofΔν_NCE decreases.Similarly,we use the DFT theory to calculate the theoretical raman spectrum in different solvents in combining with Polarization continuum model（PCM model）.It was found that the weaker the solvent,the calculated spectrum is closer to the Isobutyral anhydride raman spectrum of the gas phase.