Liquid CH3CN Research Articles

Raman spectroscopy study of acetonitrile at low temperature.

We report the low-temperature studies of liquid CH3CN by Raman spectral measurements at ambient pressure with decreasing the temperature from 20 to -196°C. Detailed internal modes especially the lattice modes analysis revealed that the structural phase transitions of acetonitrile from liquid to solid phase β and solid phase β to solid phase α were occurring at -50 and -60°C, respectively. Further, the Fermi resonance parameters between the fundamental ν2 and combination (ν3+ν4) of CH3CN at different temperatures were calculated based on the Bertran's equations. It is found that the Fermi resonance parameters as a function of temperature become discontinued at -50 and -60°C, which coincides with discontinuities observed in the Raman shifts of CH3CN at -50 and -60°C. The results suggest that the Fermi resonance parameters could be used as an indicator to assess the structural phase transition for CH3CN under low temperature.

Temperature-dependent study of Fermi resonance of CH3CN and CH3CN---Li+ complex in CH3CN-LiClO4 mixture by Raman spectroscopy.

The Raman spectra of acetonitrile-LiClO4 mixture solution have been measured in the temperature range 20 to -196 °C at ambient pressure. Detailed Raman spectroscopy analysis revealed that, in acetonitrile-LiClO4 mixture solution, the liquid CH3CN transformed into solid phase β at approximately -50 °C, and then into solid phase α at approximately -60 °C. Besides, the Fermi resonance parameters of CH3CN and CH3CN---Li+ complex at different temperatures were calculated by using the Bertran's equations, respectively. It was found that the Fermi resonance coefficient W of CH3CN---Li+ complex was not sensitive to the variation of temperature from 20 to -45 °C. In the case of CH3CN, however, the Fermi resonance coefficient W decreased from the temperature of 20 to -196 °C during which a sudden increase was observed at the temperature of -50 °C coinciding with the temperature of phase transition from liquid to solid phase β. Finally, the temperature induced precipitation behavior of LiClO4 and the structural evolution of CH3CN on the Fermi resonance have been analyzed.

Nature of Excess Electrons in Polar Fluids: Anion-Solvated Electron Equilibrium and Polarized Hole-Burning in Liquid Acetonitrile.

Unlike most polar liquids, excess electrons in liquid CH3CN take on two distinct forms – solvated electrons (esolv–) and solvated molecular anions – that are in equilibrium with each other. We find that excitation of esolv– leads to a short-lived excited state that has no effect on the equilibrium but that excitation of the molecular anion instantaneously leads to the production of new esolv–. We also find that esolv– produced by excitation of dimer anions relocalize to places far enough from their original location to alter their recombination dynamics. Finally, we show using polarized transient hole-burning that esolv– in liquid CH3CN have an inhomogeneously broadened spectrum, demonstrating that these electrons almost certainly reside in a cavity. Because there is no polarized hole-burning for esolv– in water or methanol, these results have important implications for the nature of excess electrons in all polar liquids.

Differences in the dynamic properties of liquid CH3CN and CD3CN above 40 °C revealed by Rayleigh–Brillouin scattering spectroscopy

Remarkable differences in some physical properties, such as number density, shear viscosity, sound velocity and adiabatic compressibility, were observed above 40 °C in the isotopomer pair CH3CN and CD3CN. These differences do not fall within the normal dynamic isotope effects; they were demonstrated by studying the Brillouin spectra of liquid CD3CN as a function of temperature and wavevector, correlating the scattering measurements with those of density and viscosity. An interpretation of these peculiar effects is attempted on the basis of the small differences in the intermolecular interactions in the two isotopomers. Copyright © 1999 John Wiley & Sons, Ltd.

Differences in the dynamic properties of liquid CH3CN and CD3CN above 40 °C revealed by Rayleigh-Brillouin scattering spectroscopy

Differences in the dynamic properties of liquid CH3CN and CD3CN above 40 °C revealed by Rayleigh-Brillouin scattering spectroscopy

Thermodynamic studies of chemical equilibrium in supercritical carbon dioxide-cosolvent solutions using UV-Vis spectroscopy

The keto-enol tautomeric equilibrium of 5,5-dimethyl-1,3-cyclohexanedione (dimedone) was investigated using UV-Vis spectroscopy in various solvents, including liquid acetonitrile (CH 3CN), supercritical carbon dioxide (SC CO 2) and SC CO 2–CH 3CN. The content of the enol tautomer in liquid CH 3CN is much higher than that in SC CO 2. With the addition of a small amount of cosolvent CH 3CN into SC CO 2, the enol content increases with respect to that in pure SC CO 2. The comparison of thermodynamic functions for the tautomerism was made in these solvents. It was found that the change of standard internal energy (Δ U 0) in SC CO 2–CH 3CN is close to that in liquid CH 3CN and remarkably different from that in pure CO 2. On the other hand, the maximum decrease in the change of the standard entropy (Δ S 0) was observed in SC CO 2–CH 3CN. It indicates that the cosolvent preferentially aggregates about the two tautomers. The Δ S 0 becomes the dominant thermodynamic driving force of the tautomerization in the cosolvent-modified SC CO 2.

Host–guest chemistry of rotaxanes and catenanes: application of a polarizable all-atom force field to cyclobis(paraquat-p-phenylene) complexes with disubstituted benzenes and biphenyls †

Modeling of host–guest complexes of cyclobis(paraquat-p-phenylene) with benzidine, biphenol, 1,4-diaminobenzene, and benzohydroquinone in the gas-phase and in liquid CH3CN solution with molecular mechanics and Monte Carlo statistical mechanics has been performed. The complexes are important structural elements for a wide variety of self-assembling rotaxanes and catenanes with prospective use in nanoscale devices. However, their highly charged nature presents potential challenges for accurate modeling. In particular, the need for explicit polarization has been considered through computation of association energies using an all-atom force field with and without non-additive electrostatic polarization terms. The effect of including PF6– counterions has also been addressed. Polarization generally strengthens the gas-phase interactions, but has modest effects on the structures of the complexes and on the relative free energies of binding in solution, which are in reasonable agreement with experimental data.

Spatial distribution functions: liquid CH 3CN and CO 2

Monte Carlo simulations of CH 3CN and CO 2 fluid phases exhibit anisotropy in the spatial distribution functions (SDFs) that is readily interpreted by mere inspection of 2D topographical plots of the SDFs: The local structure in condensed CH 3CN and CO 2 is dominated by dimer-like arrangements. The fact that this local structure derives from (directional) electrostatic interactions is obvious from comparisons of SDF topographies obtained from simulations with and without the partial changes associated with CH 3CN and CO 2.

Raman Scattering: Fast Axial Rotation of Methylhalides II

AbstractIn extension of a previous study of liquid CH3CN, CH3I, CD3I, CH3Cl and CH3F, the density dependence of the degenerate methyl stretch Raman band of fluid CH3F at 50°C is investigated. The Raman bands are analyzed in the slow, intermediate and fast modulation limit of the Kubo model as well as in terms of the J‐diffusion model both with and without Coriolis interaction. A modification of the J‐diffusion model with decoupled collision frequency parameters for the reorientation of different spherical tensor components is also applied. Following theoretical arguments, the experimental correlation functions are decomposed into two partial correlation functions g1(2)(t) and g2(2)(t). The partial correlation functions g2(2)(t) are found to be systematically disturbed by non‐orientational, presumably collision‐induced effects. The behaviour of the partial correlation functions g1(2)(t) is in reasonable agreement with nuclear magnetic relaxation results. The J‐diffusion model in its simple form, characterized by only one collision frequency, is found to be incapable to simultaneously describe the spinning and tumbling motion of the molecules studied.

Light scattering studies of rotational and vibrational relaxations of acetonitrile in carbon tetrachloride

The rotational and vibrational relaxation times of acetonitrile–carbon tetrachloride solutions were investigated as a function of concentration, viscosity, and temperature using depolarized Rayleigh and Raman scattering. Using a Fabry-Perot interferometer and single frequency laser source, we have shown that reliable results for the single particle orientational correlation times (τs) for CH3CN can be obtained by carrying out a concentration dependent depolarized Rayleigh scattering study. Raman scattering was shown to yield inconsistent results for τs in CH3CN. At constant viscosity, it was found that the Rayleigh scattering relaxation time (τRay) of CH3CN in CCl4 does not change with CH3CN concentration, indicating that the orientational pair correlation factor of liquid CH3CN is close to unity. This result suggests that the dynamic pair correlation in CH3CN is just as important as the static pair correlation. The experimental data were also compared with the predictions of the hydrodynamic stick and slip models for a rotational diffusion. The CH3CN data were found to be close to the prediction of the slip model. The isotropic relaxation time (τiso) of the C≡N stretching mode was also studied as a function of concentration and viscosity using Raman spectroscopy. This viscosity dependence of τiso also decreases with decreasing number density of CH3CN, suggesting that pair correlations are also important in the Raman scattering of CH3CN.

Molecular reorientational motion in liquid acetonitrile: Raman band shapes, diffusion constants and activation energy of reorientation

Isotropic I α(ω) and anisotropic I β(ω) Raman band shape analyses of the ν1(a1) fundamentals in liquid CH3CN and CD3CN are reported. The ν2 and ν4 fundamentals are complicated by secondary structure and therefore were not studied in detail. Values of the perpendicular rotational diffusion constants D⊥, derived from the ν1(a1) fundamentals of both isotopic species are in good agreement with those obtained using NMR relaxation and dielectric methods and in addition agree closely with values calculated from microviscosity theory. The latter is based upon a model which involves a connection between the rotational and translational motions in liquids. In this model, reorientations perpendicular to the C3 axis proceed by small step Brownian diffusion and D⊥ (Raman) for CD3CN yields an activation energy for liquid state molecular reorientations perpendicular to the C3 symmetry axis of 2.0 ± 0.3 kcal/mole, in good agreement with existing NMR and microviscosity values. Comparison of Raman data and results reported here with available infrared results reveals that the D⊥(ir) values are too high by a factor of 2. The source of the infrared errors is discussed.

The Rotation-Vibration Spectrum of Methyl Cyanide in the Region 1.6μ—20μ

The infrared spectra of gaseous and liquid CH3CN have been investigated in the region 1.6μ—20μ with a Perkin-Elmer spectrometer using LiF, NaCl, and KBr prisms. The parallel type bands show, in general, a sharp Q branch and broad but more intense P and R branches. The perpendicular type bands are well resolved and show the expected intensity alternation ``strong, weak, weak, strong . . .'' of the subbands (Q branches). All the fundamentals, except ν8(e), which is beyond the region investigated, have been definitely established and vibrational assignments have been made for all the observed bands. Values for the rotational constants from microwave data have been used to obtain the Coriolis interaction constants,ζi, from the observed separations of the subbands of the ⊥ bands. The following values have been obtained: ζ5=0.10, ζ6=−0.44, and ζ7=0.41.