Effect Of Hydrogen Bond On The Fermi Resonance

Fermi resonance (FR) is a very common phenomenon in complex molecules. It is the result of large perturbations between near-degenerate vibrational levels produced by anharmonicity, as first recognized by Fermi in the case of CO2. Phenomenologically, this perturbation is revealed by the occurrence of two strong Raman lines of about equal intensities in a wavenumber region where only one strong line, or possibly one strong line and one very weak line, are expected. FR plays an important role in molecular spectra, energy transfer and internal conversion, and reaction dynamics. Furthermore, FR can also cause an accelerated rate of population or energy transfer between the vibrational modes, thus influencing the dynamical behaviours of molecules and the fate of chemical reactions. Because of its importance in spectrum analysis, the studies of FR remain an active area of research, both experimentally and theoretically.In this thesis, according to the FR theory, I will mainly focus on using Raman spectroscopy and the solution concentration variation method to study:the effect of the FR on the Raman scattering cross sections (RSCSs) of the Fermi doublet v1and2v2of the liquid CS2diluted with C6H6at different volume concentrations; the effect of the weak hydrogen bond on the v1-2v2FR of the liquid CS2when diluted with CHCl3and CH2Cl2to different volume concentrations respectively; the effect of the anti-hydrogen bond on the v1-v12FR of pyridine at different volume concentrations in the presence of carbon tetrachloride, or formamide, or water. The main results and the creative points are as follows.(1) We have studied the effect of the FR on the RSCSs of the Fermi doublet v1and2v2of CS2in C6H6using the solution volume concentration variation method. We have calculated the RSCSs of the Fermi doublet v1and2v2of CS2in C6H6at different volume concentrations using Onsager’s theory with the992cm-1Raman line of C6H6as the internal standard. It was found that the RSCS of the v1Raman line decreased, while that of the2v2Raman line unexpectedly increased with decreasing the volume concentration of CS2. Then, we calculated the absolute changes of the RSCSs of the Fermi doublet v1and2v2due to the effect of the solvent effect and the FR, respectively. It was analyzed that this result was mainly caused by the FR. In particular, we have also analyzed the effect of the FR on the RSCSs of the Fermi doublet v1and2v2of CS2in C6H6at different volume concentrations. Additionally, we also give a reasonable explanation to the experimental results deviating from the theoretical results of the Raman scattering coefficients of CS2in solvent C6H6which was mentioned by G. Fini et al.(2) The effects of the weak hydrogen bond on the v1-2v2FR of CS2in CH2C12, CHCl3and C6H6at different volume concentrations were studied respectively. We measured the Raman spectra of CS2dilution with CH2C12and CHCl3at different volume concentrations, respectively. It can be found that the Fermi doublet v1and2v2of CS2had an asymmetric wavenumber shift phenomenon and C-H symmetric stretching modes of CH2C12and CHCl3had a blue shift with decreasing the volume concentration of CS2, which could be entirely attributed to the weak hydrogen bond formed between CS2and diluents. The slopes of both plots of the C-H symmetric stretching mode of CH2C12and CHCl3versus volume concentration were almost identical, which indicated that the strengths of weak hydrogen bond CS2/CH2C12and CS2/CHCl3would change to the same extent when changing the volume concentration within the same degree. In addition, we also calculated the v1-2v2FR parameters of CS2in CH2C12and CHCl3at different volume concentrations using the FR theory. With decreasing the volume concentration of CS2, the v1-2v2FR of CS2became stronger. We found that not only the slopes of both plots of the Fermi coupling coefficient W of the v1and2v2FR of CS2in CH2C12and CHCl3versus volume concentration were identical, but also the slopes of both plots of the Raman shift2v2of CS2in CH2Cl2and CHCl3versus volume concentration were identical, which implied that no matter using CH2Cl2or CHCl3to dilute CS2for the same concentration, the effect of weak hydrogen bond on the v1-2v2FR of CS2were the same.When CS2were diluted with C6H6to different concentrations, we also found the Fermi doublet v1and2v2of CS2had an asymmetric wavenumber shift phenomenon and C-H symmetric stretching mode of C6H6had a blue shift with decreasing the volume concentration of CS2. The FR coupling coefficient W of CS2in C6H6at different volume concentrations were calculated based on the FR theory. With the volume concentrations of CS2decreasing, the FR coupling coefficient W gradually increased, which indicated that FR between the v1and2v2became stronger.Since Fermi coupling coefficient W represents the degree of resonance coupling between the two modes through anharmonic terms in the Hamiltonian, the obvious interpretation of this increase is that the sulphide atom’s potential function becomes more anharmonic with decreasing the volume concentration of CS2. The most reasonable physical explanation of the more anharmonicity at lower volume concentration of CS2, where the effect of weak hydrogen bonds on CS2are the strongest, is that this bonding puts the C=S in a less symmetrical environment and, of course, the Fermi coupling coefficient, W, to be correspondingly greater. Therefore, the vl and2v2FR of CS2in CH2C12, CHC13and C6H6become stronger as the strength of the weak hydrogen bond effect on CS2is increased when the CS2volume concentration is decreasing. Besides, the electron density redistributed, which caused by the weak hydrogen bond formed between CS2and CH2Cl2, CHCl3or C6H6, between C=S and C-H bonds is too small to have a significant influence on the force constant for the C=S symmetric stretching vibration, thus we hardly observed any significant changes of wavenumber. Therefore, we concluded that the weak hydrogen bond formed between CS2and CH2Cl2, CHCl3or C6H6shows a marked effect on the overtone,2v2, of the bending mode while the v1symmetric stretching vibration mode is not sensitive to the weak hydrogen bond formation.(3) We used Raman spectroscopy to study the effect of anti-hydrogen bond on the v1-v12FR of pyridine at different volume concentrations in three systems, pyridine/water, pyridine/formamide, pyridine/carbonate tetrachloride, which can provide varying degrees of strength for the diluent-pyridine anti-hydrogen bond complex. The values of FR coupling coefficient W of the ring breathing mode v1and the triangle mode v12of pyridine in carbonate tetrachloride, water and formamide at different volume concentrations are calculated based on the FR theory. The result shows that the solution with the strongest anti-hydrogen bond, water, shows the fastest change in the Fermi coupling coefficient W of pyridine as the volume concentration varies, followed by the formamide and carbonate tetrachloride solutions. In addition, we also found that no matter whether carbonate tetrachloride, formamide or water is used to dilute pyridine, all the C-H stretching mode v2of pyridine will shift toward higher frequency as the volume concentration decreases, and the only difference is that the stronger the anti-hydrogen bond is, the faster the C-H stretching mode shifts. Finally, we give a qualitative explanation for the anti-hydrogen bond effect in reducing the strength of the v1-v12FR and increasing the frequency of the C-H stretching mode of pyridine.