Raman Spectroscopy Study On Non-coincidence Effect Of C=O System And Steric Hindrance Effect On It

For decades,dipole interactions and hydrogen bonding in intermolecular forces have been widely concerned by scientists,so that the research methods for dipole interactions and hydrogen bonding are endless.The non-coincidence（NCE）effect has been the focus of scientists who research on intermolecular forces since Fini et al.proposed it firstly in 1974.With the rapid development of quantum chemical calculations,there are increasing theoretical simulation methods have been applied to the study of non-coincidence effect mechanisms,such as Monte Carlo（MC）,molecular dynamics（MD）,Logan and so on.However,these theoretical simulation methods are neither convenient nor inefficient.Therefore it’s an urgent problem to be solved that finding out a theoretical method that studies the non-coincidence effect simplely and efficiently.According to the previous research of the research group,we applied the theory of molecular aggregation state to the study of non-coincidence effect by combined density functional theory calculation（DFT）with Raman spectroscopy.Density functional theory calculation method is more accurate and faster than other calculation methods.In this paper.The NCE effect,concentration and solvent effect of C=O vibration mode in dimethyl carbonate are studied by using Raman spectroscopy combined density functional theory（DFT）with Polarizable Continuum Model（PCM）.The effects of steric hindrance on the NCE effect in ketones,amide systems and ester systems were discussed for the first time.This gives us a deeper understanding of the NCE effects of carbonyls.1.It was detected firstly that dimethyl carbonate（DMC）has an NCE effect in the text.The Raman spectra of C=O vibration modes of dimethyl carbonate in different volume fractions of cyclohexane solution were collected by micro-Raman spectroscopy.We have found that the Raman vibration frequency of the C=O vibration mode in dimethyl carbonate are shifted to high wave numbers,while theΔν_NCE value is gradually reduced.The DFT/PCM solvent model was used to obtain the most stable geometric configuration of dimethyl carbonate mono-and poly-polymers at the B3LYP-D3/6-311G（d,p）calculation level.The calculated results are consistent with the experimental phenomena.Then we collected the Raman spectrum of dimethyl carbonate in different volume fractions of acetone.By comparison,it was found that the change of the C=O vibration mode in DMC was different from the dimethyl carbonate-cyclohexane solution with the dilution of the dimethyl carbonate-acetone solution.We suspect that this is related to the polarity of the solvent,so we collected the isotropic and anisotropic Raman spectra of dimethyl carbonate in the same volume fraction of different polar solvents.Both the experimental data and calculated results show that the polarity of the solvent will affect the self-association between the solute molecules directly.In this paper,we compared the NCE values of pure acetone,dimethyl carbonate,acetophenone and benzophenone to discusse the effect of steric hindrance on non-coincidence effect.2.Microscopic Raman spectra of formamide,N,N-dimethylformamide（DMF）,N,N-dimethylacetamide（DMA）in different volume fractions of ethanol were collected by micro-Raman spectrometer.The experimental data shows that the steric hindrance of the formation of polymer structures between adjacent molecules,which makes it more difficult for the molecules with more methyl groups to form aggregated molecules than those with less methyl groups,resulting in the NCE of C=O vibration mode being smaller.By comparing the NCE of C=O vibration mode in formamide,DMF and DMA with the change of volume fraction in ethanol,we found that the effect of steric hindrance on the non-coincidence effect increases firstly and then decreases when the solute is at a low concentration,but the steric hindrance effect still make the NCE value of the dipole down.Then we collect the Raman spectra of DMA in water.The experimental analysis shows that the Raman frequency of C=O vibration mode gradually shifts to low wavenumber with dilution in the solution of DMA-H2O.At the B3LYP-D3/6-311G（d,p）calculation level,we obtained the stable cyclic poly-configuration of DMA-H₂O for the first time.It’s a powerful theoretical support for experimental phenomena.3.We obtained microscopic Raman spectra of different volume fractions of methyl acetate and methyl benzoate in carbon tetrachloride by micro-Raman spectroscopy.By comparing theΔν_NCE of the C=O vibration mode of methyl acetate and methyl benzoate in tetrachloride,it is found that with the addition of phenyl,the steric hindrance of phenyl makes theΔν_NCE of C=O decrease.In other words,the steric hindrance will prevent the coupling self-polymerization of C=O in the solute.