Pressure Effects On Molecular Fermi Resonance In Pyridine

Pyridine is a kind of usual solvents existing widely in natural world. It can beused as denaturant, catalyst, Dyeing auxiliary, etc. Also, it can be used as the startingmaterials of the combination of many kinds of materials such as medicine, dye,seasoner, adhesion agent, explosive substance, etc. Therefore, to investigate pyridineis very important. Pyridine is a kind of heterocyclic compound with a N-atom in it.The overlapping together with their sp2hybrid orbits of C-atom and N-atom,σ-bond formed and therefore formed a plane with6hexagons. Meanwhile, a largeclosed π-bond formed too at the side of p-orbit. Electronegativity of N-atom in thering of pyridine is relative strong. Pyridine behaves quite distinct in optical properties,either.Fermi resonance（FR） is a phenomenon about molecular vibrational coupling andenergy-transfer occurring widely in intramolecular and intermolecular. It can acceleratethe transfer speed of the particles and the energy within the two vibrational modesinvolved in FR, and change the behavior of molecular dynamics, and the chemicalreaction speed, etc. Study of FR is theoretically meaningful in investigatingmolecular vibrational states, intercoupling of electronic states, molecular structure andproperties, etc i Physics. Moreover, it is extensively useful in the identification and theassignment of spectral lines in Materials, Biology and Chemistry, etc such as theinvestigation of biomolecules, the textual research on the efficacy of anti-cancer drugs,the analysis on internal pressure inside inclusion in Geology, the impurity detection incrystals, as well as the investigations of acoustical and optical instruments, etc.Nevertheless, a lot of theory and actual application about FR are still unknown andneed to be investigated further. Moreover, the investigation of FR in binary solution aswell as on the comparison between the FR behaviors occurred respectively in binarysolution and in neat liquid under high pressure, especially the investigation of twoFRs occurred simultaneously in one molecule under high pressure has not yet beenreported before.The followings are the main contents investigated in this work using high pressure technique and Raman spectrum technique, etc.（1） Investigates firstly here the variation law of bi-FR doublets occurredsimultaneously in one pyridine molecule under high pressure. The variation law ofthe bi-FR doublets of the one between ν1and ν12and the one between ν1+ν6and ν8occurred simultaneously in one pyridine molecule was analyzed with FR’s theoryaccording to the Raman spectra of C₅H₅N in solutions and in neat liquid under highpressure measured here, which demonstrates that the Raman activity of thefundamental ν1decreased gradually with pressure increasing, and eventuallydisappeared, which induced the weakening and even disappearance of the FR betweenν1and ν12.While, the variation of the intensity of ν1has no effect on the presence ofthe FR between ν1+ν6and ν8as well as its variation law with changing pressure. Thefundamental ν1itself still plays an important role in the FR between ν1+ν6and ν8. Weinterpret those phenomena above by group theory.（2） Raman spectra of C₅H₅N in H2O, in CH3OH and in neat liquid with changingpressure were measured respectively, and the parameters of FR between ν1and ν12such as the unperturbed wavenumber separation Δ0, the ratio of the two spectralintensities R, coupling coefficient W were calculated, which indicates that theseparameters in binary solution decreased with increasing pressure and decreased morequickly than those in neat liquid. When two kinds of molecules mixed together, theextending space of the molecules enlarged, the compressibility of the bond increased,the distance between two molecules lengthened, as a result, the interaction potentialsincreased, which meant that there is a relatively high pressure in solutions. Besides,there are many kinds of interactions such as electrostatic force, hydrogen bond,dispersion forces, etc which are much closed with the distance between twoneighboring molecules between solute molecule and solvent molecule. On the otherhand, these molecules-pyridine, methanol, water are all polar molecules, therefore, thesymmetry of these molecular vibrational modes weakened under high pressure, whichinduced splitting of the spectral line, even presence of phase transition, then the FRparameters such as Δ0, R and W decreased with increasing pressure.Similarly, the Raman spectra of C₆H₆in CCl₄and in neat liquid with changing pressure were measured respectively, which demonstrates that the two Raman bandsfundamental υ8and the combinationv₁+v₆involved in the FR betweenv₁+v₆and υ8shifted more quickly than their counterparts in neat liquid, moreover the combinationshifted more quickly than the fundamental, which induced the variation of thewavenumber separation Δ0between the unperturbed harmonic levels involved in FRmuch markedly than their counterparts in pure liquid. Therefore, the couplingcoefficient W of FR between υ1+υ6and υ8decreased markedly, and FR disappeared atvery low pressure. The spectral line split with the increasing pressure which meansthe symmetry of the vibrational modes worsened. These two factors lead theaccelerating the variation of FR by pressure effects in binary solution.（3） Raman spectra of CS₂in C₆H₆at different concentrations were measured here,the influence of solvent effects and FR on the RCSs of the FR doubletsv₁²v2wasinvestigated by the two RCSs calculated according to the related theory. We found thatwith decreasing concentration of CS₂the RCS ofν1decreased, while the one of2ν2increased. We thought that it was just the results of the joint action of both solventeffects and FR. Solvent effects lead them decreased monotonously concerned only,while together with the influence of FR, the phenomena mentioned above can beeasily interpreted.（4） Weak hydrogen bond was found between CS₂and C-H bond according to theRaman spectra of CS₂in CH2Cl2, CHCl3and C₆H₆at different concentrations measuredhere respectively. We also found that the bandv₁kept almost unchanged, while2v₂shifted towards higher wavenumbers with decreasing concentration of CS₂. Moreover,band C-H of CH2Cl2, CHCl3blue-shifted linearly at same speed with concentration.Intensity of weak hydrogen bond in binary solution varied with concentration of CS₂. Wof CS₂in different binary solution increased linearly at same slope with decreasingconcentration of CS₂. Weak hydrogen bond changed exactly the electron clouds of C=Sin CS₂, which induced the variation of anharmonic potential of CS₂. Therefore, FRbetweenv₁and2v2increased. The variation of the electron clouds lead no change in v₁, while lead great change in2v2, which means the two bands ofv₁and2v2shifted asymmetrically.