Study On The Raman Non-coincidence Effect And Aggregation Structure Of C=O Stretching Mode

Intermolecular interactions play an important role in many areas of science,such as biochemistry,crystal engineering and supramolecular chemistry.Intermolecular interactions such as induced dipole interactions may determine what is the possible conformations of solute molecules in solvents,and hydrogen bonding play a fundamental role in determination of the possible conformations and the secondary structure of a protein in vivo.Thus,over the past several decades,intermolecular interactions were turned to be the focus of the research.Raman spectroscopy has been widely used in studying the molecular interactions in terms of their vibrational frequency shifts disturbed by environmental changes.In the binary mixture,during the dilution process the solute-solvent interaction weakens the transition dipole–transition dipole（TD-TD）interaction,which lead to the frequencies shift of both the isotropic(ν_iso)and anisotropic(ν_aniso)Raman components of the reference vibrational mode of solute.The vibrational frequency difference is known as Raman noncoincidence effect（NCE）.Consequently theoretical works have been well established in order to describe the NCE phenomenon.In our work,the Raman spectroscopy and density functional theory（DFT）calculations were used to study the Raman NCE of the C=O band of theβ-propiolactone,γ-Caprolactone and NMP at different concentrations.1.The optimizedβ-propiolactone and its dimer structures as well as their vibrational frequencies and depolarization ratio were calculated using the DFT/PCM calculations at the hybrid B3LYP-D3/6-311G（d,p）.By intermolecular interaction,β-propiolactone molecules can form dimers and trimer.According to the calculated results,we have proposed aggregated model to explain the C=O vibration NCE phenomenon and its concentration effect,we found it largely consistent with our experimental findings.In brief,only those pairs that with prominent vibrational frequency difference and depolarization ratio difference exists NCE effect.To investigate the NCE solvent polarity dependent properties and demonstrate the rationality of dimer model,we collected the isotropic and anisotropic Raman spectra ofγ-Caprolactone in a series of solvent with different static dielectric constant.It illustrates that the stronger polar solvent will weaken the aggregated structure ofβ-propiolactone in the mixtures.2.Its frequency difference between the FT-Raman spectrum and FT-IR spectrum ofγ-Caprolactone is noticeable(4cm^-1 frequency difference).This frequency difference is one key character for intermolecular interactions.The Raman NCE of the C=O band of theγ-Caprolactone in CCl₄ has been analyzed and theΔν_NCE has been measured at different concentrations.We found that the Raman peak frequencies of both the isotropic(ν_iso)and anisotropic(ν_aniso)show an increase in wavenumber and the NCE get smaller and disappear with the decrease of solute concentrations.It is because that the solvent molecules diffuse towards the reference molecules and break its short-range orientational order（dimer）structure at high dilution,there-by weakening the dipole–dipole interaction of the solute molecules.We calculated the optimized geometry ofγ-Caprolactone monomer and its dimer and the corresponding vibrational frequencies using the DFT calculations at the B3LYP-D3/6-311G（d,p）level of theory.It illustrates that with the decrease ofγ-Caprolactone concentrations the Raman spectra gradually transformed from dimer character to monomer character thus blue shift the C=O stretching.Therefore,the DFT calculation manifested the phenomenon.The NCE of the C=O stretching mode in theγ-Caprolactone/DMSO mixtures exhibits a downward（concave）curvature,in contrast to that in theγ-Caprolactone/CCl₄ mixtures,which shows an upward（convex）curvature.The reduction and enhancement of the dimer structure（short-range orientational order）ofγ-Caprolactone in theγ-Caprolactone/DMSO andγ-Caprolactone/CCl₄mixtures respectively may play a major role in shifting of peak frequencies.Solvent dependent experiment show the value of NCE declined with the increase of the solvent dielectric constant under the same condition.3.The NCE of the C=O stretching mode of the NMP molecule has been investigated bying Raman spectroscopy and DFT calculations.In addition,these DFT calculations provided a satisfactory result and fit well with the experimental findings.Solvent dependent experiment show the NCE character is largely determined by dipole moment of solvent.In aprotic solvents the shape of the graph of NCE vs concentration depends on the relative values of the dielectric constants:the slope increases with increasing concentration when the solvent has a higher dielectric constant than the solute,and decreases in the reverse case.We calculated the optimized geometry of NMP-H₂O complex,and the bond length between the H atom of NMP and O atom of H₂O is 1.81?.It’s conform to the properties of classical O-H…O hydrogen bonding.We found that both theν_iso andν_anisoniso of C=O decrease with the dilution of NMP.It is because that the chemical dilution may weaken the dipole–dipole interaction of the solute molecules,and the hydrogen bonding will dominate in the binary mixture thus redshift the frequencies of C=O stretching.