The Molecular Conformations Of Liquid Normal Monohydric Alcohols And Solvent Effect On Photoinduced Reactions

In this dissertation, we employ spontaneous Raman spectroscopy and laser flash photolysis to study the molecular conformation of normal monohydric alcohols in liquid、the self-assembly process of surfactant in aqueous solution solvent effect on the photoinduced reaction of thioxanthen-9-one. The dissertation mainly includes the following three aspects:①we identify the gauche-conformer in the isotropic Raman spectra of ethanol、n-propanol、n-butanol and n-pentanol, and we estimate the population ratio of gauche-conformer and trans-conformer;②in the vertically polarized Raman spectra of sodium dodecyl sulfate (SDS), we study the process of SDS micelle formation by measuring the change of band position at different concentrations in aqueous solution, and we obtain the first critical micelle concentration and the second critical micelle concentration of SDS.③we study solvent effect on the electron-proton transfer reaction between thioxanthen-9-one and diphenylamine by laser flash photolysis. The results are listed as follow:The gauche-conformer in liquid ethanolIn the isotropic Raman spectrum of CD3CH2OH, there are two bands, the band at2886cm-1is contributed from trans-conformer and gauche-conformer, and the band at2970cm-1is only contributed from gauche-conformer. Thus, the band at2970cm-1can indicate gauche-conformer. To eliminate the contribution of-CH2Fermi resonance of gauche-conformer to the band at2970cm-1, we measure the Raman spectra of liquid/gaseous CD3CHDOH and the crystal CD3CH2OH/CD3CD2OH. Apart from a very similar doublet, the Fermi resonance of the-CαH bending overtones are observed in all these spectra. For CD3CHDOH, the overtone bands locate in a range of2550～2750cm-1, and for CD3CH2OH/CD3CD2OH, two overtone bands appear at2915cm-1and2933cm-1. Therefore, in the isotropic spectrum of CD3CH2OH, the band at2970cm-1is contributed from the-CαH symmetric stretching; according to the calculated results, the band at2970cm-1is assigned to the-CαH stretching vibration in the Cα-O-H plane of gauche-conformer (-CαHip), and the band at2886cm-1is contributed from both the-CαH2symmetrical stretching vibration of trans-conformer and the-CαH stretching vibration out of the Cα-O-H plane of gauche-conformer (-CαHop). Furthermore, using the integral intensity from the spectral fitting with Lorentz function to the experimental isotropic spectrum of CD3CH2OH, we estimate the population of gauche-conformers is54%in liquid ethanol, which agrees well with that from our molecular dynamics simulation. Notably, in the isotropic spectra of CH3CH2OH and CD3CH2OH, the intensities of the band at～2970cm-1are almost equal, which indicates that the-CαHip stretching vibration of gauche-conformer contributes to this band rather than-CH3asymmetric stretching vibration in the isotropic spectrum of CH3CH2OH.The gauche-conformer in liquid n-propanol、n-butanol and n-pentanolIn the isotropic Raman spectra of CD3CD2CH2OH、CD3CD2CD2CH2OH and CD3CD2CD2CD2CH2OH, a doublet is find for all alcohols at below2900cm-1and above2950cm-1, respectively, and the band above2950cm-1is only contributed from gauche-conformer. Similar to ethanol, the band above2950cm-1can indicate gauche-conformer. According to the calculated results, the band below2900cm-1is contributed from both the-CαH2symmetrical stretching vibration of trans-conformer and the-CaHop stretching vibration of gauche-conformer (-CαHOp), and the band above2950cm-1is assigned to the-CαHip stretching vibration of gauche-conformer. For each deuterated alcohol and its normal alcohol, for example, CD3CD2CH2OH and CH3CH2CH2OH, the intensities of the band at-2970cm-1are almost equal in their isotropic spectra, which indicates that the-CαHip stretching vibration of gauche-conformer contributes to this band in the isotropic spectra of CH3CH2CH2OH、 CH3CH2CH2CH2OH and CH3CH2CH2CH2CH2OH. Furthermore, using the integral intensity from the spectral fitting to the experimental spectra, we estimate the population of gauche-conformers is67%、70%and68%in liquid n-propanol、 n-butanol and n-pentanol, which agree well with those from our molecular dynamics simulation.The process of SDS micelle formation in aqueous solutionSodium dodecyl sulfate (SDS) is anionic surfactant, and features amphiphilic property. Due to the hydrophobic interaction of long hydrocarbon chain, SDS molecules can associate to form micelle in aqueous solution. Combined with the experimental results of vertically polarized Raman spectra and surface tension, we find that the process of SDS micelle formation could be classified into several concentration regions. In the0.45mM/L≤[SDS/H2O]<8.52mM/L region, only SDS monomer exists at the air-solution interface and in bulk solution, and the monomer concentration increases with the increase of the SDS concentration. When [SDS/H2O]=8.52mM/L, the SDS adsorption is saturated at the air-solution interface, and in bulk solution, the SDS monomer begin to associate into spherical micelle. In the8.52mM/L<[SDS/H2O]<69.68mM/L region, the absorbed SDS molecule at the interface remains constant, and the spherical micelle concentration increases with the increase of the SDS concentration in bulk solution. When [SDS/H2O]=69.68mM/L, the absorbed SDS molecule at the interface remains constant, and the spherical micelle begin to associate into rodlike micelle. In the69.68mM/L<[SDS/H20]^294.79mM/L region, with the increase of the SDS concentration, the rodlike micelle concentration increases in bulk solution, however, the surface tension slightly decrease, which may be influenced by the change of micelle structure in bulk solution. According to the micelle formation process, in aqueous solution, the first critical micelle concentration (FCMC) and second critical micelle concentration (SCMC) of SDS is8.52mM/L and69.68mM/L, respectively. Furthermore, in the presence of100mM/L sodium halide in aqueous solution, the FCMC and SCMC of SDS respectively is1.76mM/L and34.28mM/L. The decrease in the FCMC and SCMC of SDS, may be due to the electrostatic attraction between Na+and SO4-, screening the charge repulsion that exists between SDS molecules.Solvent effect on the mechanism between TX and DPAPhotoinduced chemical reaction between thioxanthen-9-one (TX) and diphenylamine (DPA) are investigated by the nanosecond laser flash photolysis. With irradiation at355nm, the triplet TX (3TX*) is produced via a Franck-Condon excitation of So�'Si and intersystem crossing of Si�'Ti. A typical two-step mechanism is proposed for the reaction between3TX*and DPA:an electron transfer from DPA to3TX*primarily occurs to produce TX-anion and DPA+cation, followed by a proton transfer between TX and DPA+to form TXH radical. In the transition absorption spectra of the title reaction system in pure CH3CN, the major band at627nm is assigned to absorption of3TX*, and the bands at420nm,683nm and780nm are contributed to TXH, TX-and DPA+, respectively. With the polarity of solvent increasing, the blue-shift was observed for all absorption bands of the intermediates. The quenching rate constants of3TX*by DPA in various polar solvents are determined as1.09×1010l·mol-1·s-1(in CH3CN),1.02×1010l·mol-1·s-1(in CH3CN/CH3OH) and0.96×1010l·mol-1·s-1(in CH3CN/H2O), respectively. No obvious medium effect is observed for the electron transfer between3TX*and DPA, although more37ππ*characteristics are believed for the triplet TX in a polar solvent. Otherwise, a solvent effect is find in the dynamic decay of a primary product of TX-anion. In strongly acidic solvent,3TX*can combine with one proton to generate protonated triplet3TXH+*, and the absorption band of3TXH+*locates at520nm. By comparing the change of3TX*quenching rate, we find that the protonation process largely accelerates the3TX*quenching. However, no obvious pH dependence is find for the quenching rate of3TX*by DPA, whatever in acidic and alkaline aqueous acetonitrile solutions.