1,721,069 research outputs found
Vibrational Spectroscopy of Polyconjugated Systems and Molecular Nonlinear Optical Responses
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Vibrational spectra as a probe of structural order/disorder in chain molecules and polymers (Cap.3)
No full textOverall mobility of polymerhylene chains in the solid state: a spectroscopic study
No full textVibrational spectroscopy is used to characterize the molecular phenomena involved in pretransitional processes in polymethylene systems. — It is shown that the “soliton-like” model proposed to justify the mechanism of molecular mobility is consistent with different experimental data from vibrational spectroscopy and other techniques, namely, diffusion, transport of matter, and surface disordering. — By use of spectroscopic “probes” such as Raman band width, frequency shifts, and intensity changes, it is predicted that, if solitons exist, we should observe in the IR/Raman spectra narrow band width and a possible upward shift of a few specific vibrational modes. Moreover, an intensity behavior different for distinct modes is predicted as a consequence of their different coupling with the torsional modes which, in turn are activated by the conformational soliton-like distortion of the chain. Vibrational spectra agree with what is predicted. — A test case (n-alkanes included in urea clathrate) is provided to support the predictions
Vibrational spectra as a probe of local and collective flexibility in chain molecules: recent aspects.
No full textCONFORMATIONAL SOLITONS IN CHAIN MOLECULES - INFRARED-SPECTRA AND ABINITIO CALCULATIONS
No full textIn the search for experimental evidence for conformational solitons or twistons in polymethylene chains, results from 'ab initio' calculations are presented which suggest that just slight distortions from trans planarity of the carbon skeleton of these systems introduce sizeable perturbations in the electronic and dynamical properties of the CH2 groups. The vibrations of these occur at frequencies slightly higher than those of the transplanar structure and overlap with the absorption of the pinned, classical conformational defects such as GG, GTG', end-TG, etc. The experimental observations on selectively deuterated stearic acid are presented and discussed
TWISTONS AND PHASE-TRANSITIONS IN POLYMETHYLENE SYSTEMS - A SPECTROSCOPIC STUDY
No full textThe transport and diffusion properties observed for solid n-alkanes and the premelting phenomena in polymethylene systems imply that molecular chains possess conformational flexibility coupled to longitudinal mobility. Most of the authors have proposed models based on the creation and mobility of conformational kinks whose existence is not proven by spectroscopy. The existence of 'twistons' has been postulated. In this work vibrational i.r. and/or Raman spectroscopy is used as a probe for twistonic excitations. The conditions for the spectroscopic observation of twistons are discussed. The temperature dependence of the spectra of n-alkanes from the orthorhombic to 'alpha' (or 'pseudohexagonal') and to melt phases is studied and the spectra are interpreted in the attempt to find evidence of twistons. As a test case of the theoretical predictions the spectra of n-alkanes in urea clathrates are discussed. Indications are found that twistons start being produced while still in the orthorhombic phase when the lattice starts expanding. The 'alpha' phase (or premelting state) may be considered as a 'soft' phase consisting of a collection of twistonic chains out of register because of the propagation of twistons which also generate the spectroscopically observed surface disordering and diffusion
Enhancement of second order hyperpolarizabilities via SERS techniques in organic materials
No full textThe usual strategy to improve molecular hyperpolarizabilities beta and gamma is the chemical functionalization of polyconjugated molecular chromophores (e.g. by increasing the number of delocalized electrons or adding suitable electron-donating or electron-withdrawing functional groups) with the result of an increased molecular polarizability. In this work, we show that it is possible to increase the molecular polarizability in the presence of metal nanoparticles. The vibrational contribution to molecular polarizabilities has been evaluated by using experimental Raman intensities. SERS (surface enhanced Raman spectroscopy) techniques can enhance such signals and therefore improve the second-order hyperpolarizability by orders of magnitude. In particular we have measured the mean value over the entire scattering volume of the second-order hyperpolarizability for some organic molecules in a silver colloidal solution. We have observed an increase of second-order hyperpolarizability of about six orders of magnitude reaching values of about 10^-28 esu. This large value of gamma_vib has been reached in spite of the fact that molecules used in the experiments were not optimized for third order nonlinear optics as shown by the low value of the second order hyperpolarizability of the isolated molecule (10^-34 esu)
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