Librational Peaks Research Articles

Evidence of exciton-libron coupling in chirally adsorbed single molecules

Interplay between motion of nuclei and excitations has an important role in molecular photophysics of natural and artificial structures. Here we provide a detailed analysis of coupling between quantized librational modes (librons) and charged excited states (trions) on single phthalocyanine dyes adsorbed on a surface. By means of tip-induced electroluminescence performed with a scanning probe microscope, we identify libronic signatures in spectra of chirally adsorbed phthalocyanines and find that these signatures are absent from spectra of symmetrically adsorbed species. We create a model of the libronic coupling based on the Franck-Condon principle to simulate the spectral features. Experimentally measured librational spectra match very well the theoretically calculated librational eigenenergies and peak intensities (Franck-Condon factors). Moreover, the comparison reveals an unexpected depopulation channel for the zero libron of the excited state that can be effectively controlled by tuning the size of the nanocavity. Our results showcase the possibility of characterizing the dynamics of molecules by their low-energy molecular modes using µeV-resolved tip-enhanced spectroscopy.

Structure and vibrational spectra of small water clusters from first principles simulations

The structure and vibrational spectra of (H(2)O)(n) (n=2-5) clusters have been studied based on first-principles molecular dynamics simulations. Trends of the cluster structures with the cluster size show that water molecules in cluster are bound more tightly. The vibrational spectra as a function of cluster size and temperature are obtained using Fourier transformation of the velocity autocorrelation function. Results of the clusters in ground state show that when the cluster size increases, the librational peaks shift to blue and the bonded intramolecular OH stretching bands shift to red due to the clusterization and hydrogen-bond strengthening. Meanwhile, there are no significant shifts in the intramolecular bending and free OH stretching modes, indicating that the free hydrogen atoms are insensitive to the local bonding environment. The temperature-dependent vibrational spectra, which exhibit similar behaviors from the dimer to pentamer, show that there are significant broadenings of the spectra with temperature caused by thermal motions. Moreover, different bands shift to different directions, where librational bands shift to red while bonded OH stretching bands shift to blue, although the blueshifts are quite small for the dimer and trimer.

Development of an Empirical Force Field for Silica. Application to the Quartz−Water Interface

Interactions of pulverized crystalline silica with biological systems, including the lungs, cause cell damage, inflammation, and apoptosis. To allow computational atomistic modeling of these pathogenic processes, including interactions between silica surfaces and biological molecules, new parameters for quartz, compatible with the CHARMM empirical force field were developed. Parameters were optimized to reproduce the experimental geometry of alpha-quartz, ab initio vibrational spectra, and interactions between model compounds and water. The newly developed force field was used to study interactions of water with two singular surfaces of alpha-quartz, (011) and (100). Properties monitored and analyzed include the variation of the density of water molecules in the plane perpendicular to the surface, disruption of the water H-bond network upon adsorption, and space-time correlations of water oxygen atoms in terms of Van Hove self-correlation functions. The vibrational density of states spectra of water in confined compartments were also computed and compared with experimental neutron-scattering results. Both the attenuation and shifting to higher frequencies of the hindered translational peaks upon confinement are clearly reproduced by the model. However, an upshift of librational peaks under the conditions of model confinement still remains underrepresented at the current empirical level.

Recent progress on our understanding of water around biomolecules

The aim of this research was to study the interaction of water with different biomolecules on the molecular level using inelastic neutron scattering (INS) techniques. These methods provide information on the so-called interfacial (or bound) water molecules that are within a few hydration shells of the macromolecules and show that the interfacial water molecules can be readily distinguished from those in the bulk (bulk water), especially using data for inelastic neutron scattering over the 20–130 meV range corresponding to the librational peak and the molecular optic peaks. Advances in molecular dynamics simulations of water, coupled with neutron scattering measurements of interfacial water could lead to a better understanding of the hydration properties of water around biomolecules such as DNA and proteins.

Multiple Time-Scale Behavior of the Hydrogen-Bond Network in Water

The temperature-dependent changes in the hydrogen-bond network of SPC/E water have been examined using power spectral analysis of fluctuations in tagged-molecule potential energies and local tetrahedral order parameters. The clear signatures of multiple time-scale or 1/fα behavior in the power spectra are shown to depend sensitively on the strength of hydrogen bonding. The analysis focuses on three specific power spectral features: the frequency of crossover to white noise behavior, the exponent in the 1/fα regime, and the librational peak. The exponent of the tagged-particle potential-energy fluctuations is shown to be strongly correlated with the diffusivity in the temperature range of 230 to 300 K. This correlation is strongest in the temperature−density regimes where the mechanism for diffusion is likely to be dominated by translational−rotational coupling, suggesting that the value of the exponent is a measure of the efficiency of the coupling of librational modes with network vibrations. The temperature dependence of all power spectral features was found to be strongest along the 0.9-g cm-3 isochore, which corresponds closely to the density of minimum diffusivity for the temperature range studied here. The static distributions of the tagged-particle quantities were examined to determine the degree of heterogeneity of the local molecular environment and its relationship with power spectral features.

Calorimetric and neutron scattering studies on the boson peak of lithium chloride aqueous solution glasses

The heat capacities of the glassy and liquid states of three lithium chloride aqueous solutions (LiCl⋅4H2O,LiCl⋅6H2O,LiCl⋅7.5H2O) were measured with an adiabatic calorimeter in the temperature range 5–340 K; the crystalline state was also measured for LiCl⋅4H2O. For all of the glassy samples, a large non-Debye excess heat capacity appeared below 30 K. The neutron scattering spectra of glassy LiCl⋅4H2O and LiCl⋅6H2O were measured with two spectrometers operating with different energy windows (E<20 meV and E<100 meV). A boson peak, corresponding to the excess heat capacity, appeared at around 4 meV in the S(Q,E) spectra of both glassy samples. From the analysis combining the heat capacity and neutron scattering data, the excess density of states due to the boson peak (compared with hexagonal ice) was found to be 0.10 degree of freedom per water molecule for both glassy samples. A librational peak of water molecules appeared at around 60 meV in glassy samples. This energy is about 10 meV lower than that of hexagonal and amorphous ices, indicating that the hydrogen bonds connecting water molecules are broken due to the strong coordination of water molecules to lithium ions.

Methyl Group Dynamics in Poly(methyl methacrylate): From Quantum Tunneling to Classical Hopping

We present an extensive neutron scattering study on the ester methyl dynamics of poly(methyl methacrylate) (PMMA) in the temperature range from 1 to 200 K. The crossover from quantum tunneling to classical hopping has been investigated in an energy window from 0.5 μeV to 2 meV. Inelastic measurements of the librational levels have been performed in the millielectronvolt range. The results have been analyzed in terms of a model that considers a Gaussian distribution of potential barriers for methyl group rotation. A 6-fold correction to the main 3-fold term of the potential has been added to get a good description of the librational peak. This distribution of mixed-symmetry barriers gives an excellent description of the spectra in the whole temperature range.

Low-temperature dynamics of confined methyl iodide

The rotational tunneling and librational excitations of the one-dimensional quantum rotor, methyl iodide, confined in porous glass monoliths (mean pore diameter of 58 \AA{}) have been investigated using incoherent inelastic neutron scattering. When the pores are completely filled with the solid at low temperatures $(5\mathrm{K}lTl35\mathrm{K}),$ the system exhibits a tunneling spectrum composed of two sets of inelastic peaks: one set at \ifmmode\pm\else\textpm\fi{}2.5 \ensuremath{\mu}eV and the other at \ifmmode\pm\else\textpm\fi{}4 \ensuremath{\mu}eV. Tunneling spectra measured with partially filled pores display peaks at \ifmmode\pm\else\textpm\fi{}4.0 \ensuremath{\mu}eV corresponding to tunneling of molecules near the pore walls and show that the peaks at \ifmmode\pm\else\textpm\fi{}2.5 \ensuremath{\mu}eV in the filled pores, which are similar to bulk solid peaks, can be attributed to molecules in the center of the pore. The lowest-lying librational excitation of the methyl group exhibits behavior consistent with a two-component system with a bulk solidlike librational excitation and a lower-energy librational peak due to the surface molecules. The methyl dynamics are well-described as one-dimensional rotationally hindered rotors, with the core molecules under the influence of a single hindering barrier while the surface molecules experience a distribution of barrier heights. Quantitative estimates of this distribution are presented. Finally, the temperature dependence of the tunneling spectra for both the completely filled and partially filled pores is discussed.

Single-particle dynamics in dimethyl–sulfoxide/water eutectic mixture by neutron scattering

We study water and dimethyl–sulfoxide (DMSO) dynamics in the eutectic DMSO/water mixture, 2H2O:1DMSO, by incoherent quasielastic and inelastic neutron scattering. A temperature range from room down to −27 °C is investigated. Both water and DMSO translational dynamics are significantly slowed in the mixture compared to pure liquids. They exhibit different diffusion dynamics, pointing to the absence of stable hydrogen bonded complexes. Further, the presence of a solute suppresses the dynamic anomalies of water: self-diffusion coefficients for water in the mixture, as well as residence times for jump diffusion reveal an Arrhenius temperature dependence, in contrast to the strong non-Arrhenius behavior in pure water. The density of vibrational states shows a shift of the characteristic librational peak in pure water towards higher energies in the mixture, reflecting hindrance of large amplitude proton motions. These results are compatible with computer simulations which predict longer hydrogen-bond lifetimes in the mixture, compared to pure water. However, rotational relaxation times for water protons in the mixture are estimated to be ∼1 ps, as for pure water. This experimental finding suggests that unlike in pure, low temperature water, proton librations should no longer be the main mechanism responsible for hydrogen-bond breaking in the mixture. Thus, the correspondence between the proton hindered rotational relaxation time and the “hydrogen-bond lifetime” should not apply in DMSO–water mixtures.

Dynamics of hydrogen-bonded water networks under high pressure: Neutron scattering and computer simulation

Molecular dynamics simulation of proton-ordered high pressure ice modifications II and IX was performed. Dynamics of both isotope varieties, H2O and D2O, was simulated. Rectangular simulation box of ice II contained 576 and that of ice IX 768 molecules. The average kinetic energy corresponded to 82 and 201 K for ice II and to 87 and 203 K for ice IX. One-phonon densities of states were calculated via Fourier transformation of velocity autocorrelation functions and compared with those found experimentally from inelastic incoherent neutron scattering. This characteristic was calculated for all the molecules, as well as for the molecules of a particular crystallographic type. Both simulated ice modifications contain molecules of two different structural types. Dynamic characteristics of molecules of different types are slightly different. Splitting of the librational peak at about 60–70meV observed in the ice II experimental spectrum is mainly due to such difference. In the case of ice II simulated spectra reproduce experimental ones quite reasonably in the whole range of energies, while in the case of ice IX agreement with the experiment is worse.

Rotational dynamics ofC60in(ND3)K3C60

The orientational dynamics of ${\mathrm{C}}_{60}$ in the ammoniated fulleride $({\mathrm{ND}}_{3}{)\mathrm{K}}_{3}{\mathrm{C}}_{60}$ are studied by the neutron inelastic-scattering technique. Well-defined excitations are observed in the low-energy spectra with energies varying between 3.11(7) and 2.38(5) meV, as the temperature increases from 30 to 310 K. The dependence of their intensities on the scattering vector is consistent with their assignment to small-amplitude librational motion of the ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ ions. The librations in $({\mathrm{ND}}_{3}{)\mathrm{K}}_{3}{\mathrm{C}}_{60}$ are both softer and broader than those in the parent fulleride ${\mathrm{K}}_{3}{\mathrm{C}}_{60},$ reflecting the existence of a weaker and more anisotropic orientational potential upon ammoniation. The estimated barrier of the hindrance potential, ${E}_{a}\ensuremath{\sim}250 \mathrm{meV}$ is approximately half the size of that in ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ and comparable to that found in the primitive cubic fulleride ${\mathrm{Na}}_{2}{\mathrm{RbC}}_{60}.$ The temperature dependence of the energies and widths of the librational peaks in $({\mathrm{ND}}_{3}{)\mathrm{K}}_{3}{\mathrm{C}}_{60}$ is also more reminiscent of the behavior of ${\mathrm{Na}}_{2}{\mathrm{RbC}}_{60}$ rather than ${\mathrm{K}}_{3}{\mathrm{C}}_{60}.$ The result has been rationalized by involving crystal-packing arguments, related to the increase in size of the tetrahedral interstices upon ammoniation.

Polarized librational spectra of proton-ordered ice XI by molecular dynamics simulations

A complete set of vibrational spectra of ice XI, proton-ordered phase of ice Ih, has been obtained by molecular dynamics simulations using the Kumagai, Kawamura, and Yokokawa (KKY) potential model which allows unconstrained atomic motions. A remarkable difference between the power spectra of ice Ih and of ice XI was found in the librational band. In ice XI four major librational peaks were observed and assigned to three librational modes: wag, twist, and rock, whereas ice Ih is well known to have a broad librational band. Moreover, characteristic spectral properties, due to the ferroelectric structure of ice XI, were observed in the polarized power spectra of the atom velocities along the crystal axes.

Molecular dynamics of C 60 in superconducting Na 2CsC 60

We have measured the low-energy inelastic neutron scattering spectra of superconducting Na 2CsC 60 in the temperature range 100–350 K. Well-defined librational peaks are observed at 2.7(1) meV; their position and width are temperature independent. They survive well above the order-disorder transition at 299 K. A quasielastic feature with a width 1.03(6) meV at an average scattering vector of 1.09 Å −1 is observed at 350 K, arising from diffusive motion of the C 60 3− ions. Evidence is found for influence of the Na + ions on the rotational motion of the C 60 3− ions.

Observation of Fano interference in high-pressure ice VII.

Infrared absorption spectra of ice VII have been measured up to 100 GPa at room temperature. The OH stretching peaks, which showed a rapid shift to low frequency with an enormous line broadening with increasing pressure, were found to interfere with the OH bending and librational peaks at very high pressures above 40 GPa. Appearance of such Fano-type interference in absorption line shape indicates that the OH stretching modes form a continuous (or quasicontinuous) energy band in their excitation state. The formation of the energy continuum is closely related to the conversion from double- to single-minimum shape in the hydrogen-bond potential, and hence considered to be a signature of the onset of hydrogen-bond symmetrization.

Rotational dynamics of solid C70: A neutron-scattering study.

We report the results of neutron-diffraction and low-energy neutron-inelastic-scattering experiments on high-purity solid ${\mathrm{C}}_{70}$ between 10 and 640 K. Thermal hysteresis effects are found to accompany structural changes both on cooling and on heating. The observed diffuse scattering intensity does not change with temperature. At 10 K broad librational peaks are observed at 1.82(16) meV [full width at half maximum=1.8(5) meV]. The peaks soften and broaden further with increasing temperature. At and above room temperature, they collapse into a single quasielastic line. At 300 K, the diffusive reorientational motion appears to be somewhat anisotropic, becoming less so with increasing temperature. An isotropic rotational diffusion model, in which the motions of adjacent molecules are uncorrelated, describes well the results at 525 K. The temperature dependence of the rotational diffusion constants is consistent with a thermally activated process having an activation energy of 32(7) meV.

Simulation of the intermolecular vibrational spectra of liquid water and water clusters

We report simulated Raman and infrared spectra of liquid water and water clusters in the frequency range 0–1000 cm−1. The librational peak in the Raman spectrum of the liquid, which has a strong dependence on the anisotropy of the assumed gas-phase polarizability tensor, allows us to choose between various models for that tensor. Most of the spectroscopically probed dynamics of the liquid are present in the small clusters, with N as low as 5. The librational peaks in the pentamer spectra are shown to redshift with increasing temperature.

Neutron-scattering study of C60n- (n=3,6) librations in alkali-metal fullerides.

We have measured the low-energy inelastic neutron scattering spectra of superconducting ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and insulating ${\mathrm{Rb}}_{6}$${\mathrm{C}}_{60}$. Well-defined peaks are observed at room temperature at 3.59(4) meV [full width at half maximum (FWHM) =1.11(15) meV] and 5.49(38) meV [FWHM = 3.41(44) meV], respectively. They harden with decreasing temperature and the dependence of their intensities on the scattering vector shows that they are due to small-amplitude librational motion. No anomalous behavior of the librational peak was observed on cooling through the superconducting transition temperature, indicating that electron-librational coupling is weak. The energy barrier for reorientation is estimated to be at least twice as large in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ as in ${\mathrm{C}}_{60}$. Substantial disorder persists for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ in the superconducting state.

Methyl tunnelling in trihalogeno-trimethyl-benzenes

Results of inelastic neutron scattering (INS) from 1,3,5-trihalogeno-2,4,6-trimethyl-benzenes are reported for the chloro-, the bromo- and the iodo-case. For the whole family the spectra show numerous similarities, in the μeV as well as in the meV range. We observe three tunnelling lines of equal intensity corresponding to three crystallographically inequivalent methyl rotors. The observed librational peaks are at much lower energies than expected from a pure threefold potential. Within the halogen family a systematic reduction of the barrier height is found with larger halogen sizes. The temperature dependence of the tunnelling energies shows the most pronounced difference between TBM and TIM, where the corresponding tunnelling lines even shift in opposite directions.

Weak infrared bands in solid CD4

The spectrum of solid CD4 has been studied between 900 and 5500 cm−1 with improved sensitivity. The sideband associated to the normal mode ν3 has been resolved: two librational peaks are observed in CD4 II, and six in CD4 III. The latter result indicates the existence of three inequivalent sites in phase III of methane, as predicted by the model of Maki et al. In CD4 II, the A–T tunneling splitting of the librational ground state has been determined by scaling the observed librational frequency. The librations associated to the mode ν4, and to its overtones 2ν4 and 3ν4 have also been observed: in CD4 II, the dependence of the librational energy on the vibrational final state is much stronger than in CH4 II.

Librational spectra of water molecules in Ni, Co and Fe dichloride hydrates

The infrared absorption spectra of FeCl 2·4H 2O, NiCl 2·6H 2O, CoCl 2·6H 2O and of their respective deuterates were recorded at liquid nitrogen temperature in the 1000-300 cm −1 wavenumber region. In all the crystals studied, we separated the librational bands from those of the metal-oxygen stretching modes. A comparison between tetra- and hexahydrates was used to distinguish coordinated and lattice water librations. Considerations of the isotopic frequency ratios, of site symmetries, and of the molecular distortions produced by the different hydrogen bond strengths, were used to reach more specific proposals concerning the character of the librational peaks. Our conclusions are in general agreement with the calculations based on a recently proposed theoretical model.