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M Taravillo

Publications and source records attributed to M Taravillo.

8 recordsLinked to original sources
PubMed2006

n-pentanol at high pressures: rotational isomerism in the liquid phase and the liquid-solid phase transition.

The vibrational spectrum of liquids constituted of chain molecules is difficult to analyze because it may have contributions of different rotational isomers. In turn, with a proper vibrational assignment, this feature allows us to extract information about the effect of temperature or pressure on the molecular conformations in the liquid state. In this regard, the information on the vibrational spectrum in the solid phase greatly simplifies the vibrational analysis of the different rotational conformers existing in the liquid, as the molecules usually present all-trans conformations in the crystalline state. Here we report room-temperature Raman experiments on n-pentanol performed in a sapphire-anvil cell up to 3 GPa. A detailed analysis of the liquid-solid phase transition occurring at 1.3 GPa is provided. The analysis of the Raman spectrum in the solid phase allows the identification of the bands due to the different rotational isomers present in the liquid. The analysis of the spectral region corresponding to skeletal vibrations of the carbon chain (800-1200 cm(-1)) indicates that gauche conformers are promoted by the application of pressure. The analysis of the intensity ratio of those bands assigned to trans and gauge conformations is used to calculate the change in molecular volume ascribed to the trans-gauge isomerization process. We find a value similar to that found in n-alkanes, i.e., -0.88 cm(3) mol(-1). In addition, we find indication that pressure varies the proportions of the different gauge conformers. Thus, it appears that the GTTt to TGTt transition in the carbon chain is favored at high pressures. As expected, a smaller change in the molecular volume accompanies this conformation change.

Journal Article↗
PubMed2006

Light-scattering study of vibrational relaxation in liquid xylenes.

Brillouin spectra obtained in dynamic light-scattering experiments are reported for the three isomeric xylenes (ortho-, meta-, and paradimethylbenzenes) between 288 and 363 K. Limiting sound velocities and relaxation times, as obtained from the polarized spectra using the theory developed by Mountain [J. Res. Natl. Bur. Stand. 70A, 207 (1966)], reveal the existence of a relaxation process. Our results suggest that the relaxation process in liquid xylenes has a purely vibrational nature. Vibrational-translational energy exchanges in xylenes are analyzed in terms of available molecular models and compared to those previously obtained for toluene and benzene. The results presented here confirm the important role played by the molecular geometry in the vibrational relaxation process, as the relative arrangement of the methyl groups has significant effect in determining the relaxing vibrational modes.

Journal Article↗
PubMed2005

Pressure tuning of the Fermi resonance in liquid methanol: implications for the analysis of high-pressure vibrational spectroscopy experiments.

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.

Methanol↗
PubMed2004

Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy.

We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.

Journal Article↗
PubMed2004

Dynamic light scattering in liquid and supercooled diphenylmethane.

We use a dynamic light scattering technique to measure both polarized (VV) and depolarized (VH) spectra of liquid diphenylmethane (DPM) between 288 and 362 K, covering both normal and supercooled liquid ranges. Our results allow extracting information on structural relaxation processes, rotational motions, rotation-translation couplings, and molecular reorientation phenomena in liquid DPM. The VV spectra are modeled according to the microscopic theory of Wang, which assumes that a structural relaxation process dominates the spectrum. We find that the relaxation time of the structural relaxation in DPM follows an Arrhenius behavior. The Rayleigh dip was observed in the VH spectra, which are described using the Andersen-Pecora theory. Our results are discussed in terms of the rotation-translation coupling parameter, which we find independent of temperature over the experimental range. The collective reorientation time also follows an Arrhenius behavior with temperature. Finally, we calculate the hydrodynamic volumes for the reorientation process from geometric molecular models in two hydrodynamic limits: slip and stick boundary conditions. Our results suggest that the DMP molecule reorientates in quasi-slipping conditions in the bulk liquid.

Journal Article↗
PubMed2002

Effect of pressure on hydrogen bonding in liquid methanol.

We have investigated the effect of pressure on the hydrogen bonding in liquid methanol using Raman spectroscopy. Specifically, we have measured the OH and CO stretching modes and assigned the bands, in agreement with recent IR and crossed molecular beam experiments on methanol clusters. At about 7 to 8 kbar, we note indications that the intrinsic nature of the methanol clusters in our samples has changed. Our results provide support for and extend conclusions derived from Monte Carlo simulations, explain anomalies observed by previous researchers, and provide new insights into general hydrogen-bonding phenomena.

Hydrogen Bonding↗