A new glance at ruthenium sorption mechanism on hydroxy, carbonate, and fluor apatites: Analytical and structural studies

The sorption mechanism of Ru3+ ions on hydroxy (HAp), carbonate (CO3HAp), and fluor apatites (FAp) has been studied in detail. Ru apatites were obtained by reaction of the apatites with RuCl3 in aqueous solution. The structure and composition of the ruthenium-modified apatites were studied by several techniques: elemental analysis, XRD, EXAFS, IR, NMR, SEM-EDS, TEM, and thermal analysis. The amount of Ru in the modified apatite varies from 7.8 to 10.5 wt% and is not related to the initial composition or the specific surface area of the apatite. The different characterization techniques show that in the Ru-modified apatites Ru is surrounded by six oxygen atoms and do not contain any chlorine. For Ru-HAp and Ru-CO3HAp the new phase is amorphous whereas it is crystalline for FAp. The catalytic oxidation ability is higher for Ru-HAp and Ru-CO3HAp compared to Ru-FAp apatite in the oxidation of benzylic alcohol.     

Vibrational dynamics of 9-fluorenemethanol using infrared-ultraviolet double-resonance spectroscopy

Vibrational spectroscopy of jet-cooled 9-fluorenemethanol and its clusters 9-fluorenemethanol-H2O, 9-fluorenemethanol-CH3OH, 9-fluorenemethanol-C2H5OH, and 9-fluorenemethanol-C3H7OH has been carried out using an IR-UV double-resonance method. The spectrum of the OH stretching vibration, v(OH), has been measured for the 9-fluorenemethanol monomer and for each of the clusters. Two conformers of 9-fluorenemethanol, symmetric (sym) and unsymmetric (unsym), have been identified using a combination of spectroscopy and quantum chemical calculations with B3LYP and HF methods using the 6-31G(d) basis set. Vibrational dynamics resulting from IR excitation has also been studied using the S0-S1 transition probed by a nanosecond-time-delayed UV laser. The data suggest that isomerization occurs as a result of the IR excitation, but the breadth of the probe spectra makes an unequivocal conclusion difficult. The effect of hydrogen bonding on the v(OH) of 9-fluorenemethanol has also been studied in clusters with water, methanol, ethanol, and propanol by measuring the IR spectra. Cluster dissociation dynamics have also been studied following IR excitation. It is observed that upon excitation of the cluster of a particular conformation the monomer product is generally produced in both conformer forms. Energetic considerations indicate that isomerization occurs before dissociation.     

Femtosecond time-resolved electronic sum-frequency generation spectroscopy: a new method to investigate ultrafast dynamics at liquid interfaces

We developed a new surface-selective time-resolved nonlinear spectroscopy, femtosecond time-resolved electronic sum-frequency generation (TR-ESFG) spectroscopy, to investigate ultrafast dynamics of molecules at liquid interfaces. Its advantage over conventional time-resolved second harmonic generation spectroscopy is that it can provide spectral information, which is realized by the multiplex detection of the transient electronic sum-frequency signal using a broadband white light continuum and a multichannel detector. We studied the photochemical dynamics of rhodamine 800 (R800) at the air/water interface with the TR-ESFG spectroscopy, and discussed the ultrafast dynamics of the molecule as thoroughly as we do for the bulk molecules with conventional transient absorption spectroscopy. We found that the relaxation dynamics of photoexcited R800 at the air/water interface exhibited three characteristic time constants of 0.32 ps, 6.4 ps, and 0.85 ns. The 0.32 ps time constant was ascribed to the lifetime of dimeric R800 in the lowest excited singlet (S(1)) state (S(1) dimer) that is directly generated by photoexcitation. The S(1) dimer dissociates to a monomer in the S(1) state (S(1) monomer) and a monomer in the ground state with this time constant. This lifetime of the S(1) dimer was ten times shorter than the corresponding lifetime in a bulk aqueous solution. The 6.4 ps and 0.85 ns components were ascribed to the decay of the S(1) monomer (as well as the recovery of the dimer in the ground state). For the 6.4 ps time constant, there is no corresponding component in the dynamics in bulk water, and it is ascribed to an interface-specific deactivation process. The 0.85 ns time constant was ascribed to the intrinsic lifetime of the S(1) monomer at the air/water interface, which is almost the same as the lifetime in bulk water. The present study clearly shows the feasibility and high potential of the TR-ESFG spectroscopy to investigate ultrafast dynamics at the interface.     

Trajectory on-the-fly molecular dynamics approach to tunneling splitting in the electronic excited state: A case of tropolone

The semiclassical tunneling method is applied to evaluate the tunneling splitting of tropolone due to the intramolecular proton transfer in the electronic excited state, first time, in a framework of the trajectory on-the-fly molecular dynamics (TOF-MD) approach. To prevent unphysical zero-point vibrational energy transfer among the normal modes of vibration, quantum zero-point vibrational energies are assigned only to the vibrational modes related to intramolecular proton transfer, whereas the remaining modes are treated as bath modes. Practical ways to determine the tunnel-initiating points and tunneling path are introduced. It is shown that the tunneling splitting decreases as the bath-mode energy increases. The experimental tunneling splitting value is well reproduced by the present TOF-MD approach based on the Wentzel-Kramers-Brillouin (WKB) approximation.     

Probing state-to-state reaction dynamics using H-atom Rydberg tagging time-of-flight spectroscopy

In the last decade or so, the H-atom Rydberg tagging time-of-flight (HRTOF) technique has made a significant impact in the study of state-to-state reaction dynamics, and especially in the study of transition state dynamics of elementary chemical reactions and quantum state resolved dynamics of molecular photodissociation of important molecules. In this perspective, we will discuss mainly the state-to-state dynamics of three important elementary reactions: H + H(2), O((1)D) + H(2) and F + H(2) that have been studied in our laboratory in recent years using the HRTOF method. In addition, we will also mention briefly the experimental results of other reactive systems. In the end, we will also present a brief research outlook in the study of molecular reaction dynamics using this powerful experimental method.     

Conformational dynamics of poly(acrylic acid). A study using surface plasmon resonance spectroscopy

The conformational dynamics of poly(acrylic acid) induced by pH change is reported here. Poly(acrylic acid) immobilized on gold surface was exposed to pH changes, and the conformational changes thus induced were followed in real time using surface plasmon resonance spectroscopy. The temporal profile of the stretching-coiling phenomenon showed a minimum point, which was proposed to be arising due to the contradictory behavior of two different property changes in the polymeric system. Normally surface plasmon resonance (SPR) response would be a convoluted effect of the thickness and refractive index changes, but the behavior observed here, where the SPR response is predominantly governed by either one of the two, is unique and to the author's knowledge is a feature that is observed for the first time. Analysis of the kinetics of the angle change revealed that it takes longer for the polymer to stretch than it takes for it to collapse, with the kinetic rate constants varying by at least an order of magnitude. The SPR angle change as well as the kinetic constants increased linearly with molecular weight. Effect of Ca2+ was studied, and it was found that the polymer was locked in its conformation due to the binding of the multivalent cations.     

ONIOM approach for non-adiabatic on-the-fly molecular dynamics demonstrated for the backbone controlled Dewar valence isomerization

Non-adiabatic on-the-fly molecular dynamics (NA-O-MD) simulations require the electronic wavefunction, energy gradients, and derivative coupling vectors in every timestep. Thus, they are commonly restricted to the excited state dynamics of molecules with up to ≈20 atoms. We discuss an approximation that combines the ONIOM(QM:QM) method with NA-O-MD simulations to allow calculations for larger molecules. As a proof of principle we present the excited state dynamics of a (6-4)-lesion containing dinucleotide (63 atoms), and especially the importance to include the confinement effects of the DNA backbone. The method is able to include electron correlation on a high level of theory and offers an attractive alternative to QM:MM approaches for moderate sized systems with unknown force fields.     

Depth profiling of water molecules at the liquid-liquid interface using a combined surface vibrational spectroscopy and molecular dynamics approach

The studies presented here combine experimental and computational approaches to provide new insights into how water structures and penetrates into the organic phase at two different liquid-liquid systems: the interfaces of carbon tetrachloride-water (CCl4-H2O) and 1,2-dichloroethane-water (DCE-H2O). In particular, molecular dynamics simulations are performed to generate computational spectral intensities of the CCl4-H2O and DCE-H2O interfaces that are directly comparable with experimental measurements. These simulations are then applied toward the generation of spectral profiles, responses that vary as functions of both frequency and interfacial depth. These studies emphasize the similarities and differences in the structure, orientation, and bonding of interfacial water as a function of interfacial depth for these two liquid-liquid systems and demonstrate the differing behavior of water monomers that penetrate into the organic phase.     

A new look at fluorescence depolarization and the dynamics of anisotropic rotational diffusion

Existing derivations of the time-dependent fluorescence anisotropy of an asymmetric molecule constitute a straightforward application of Favro's work on the rotational diffusion equation (RDE), and make no contribution to the elucidation of rotational dynamics as such. A new approach is developed, and the problem formulated in the parlance of conventional reaction kinetics by demonstrating, with the aid of the method of moments, that the RDE is completely equivalent to a set of ordinary linear differential equations, formally identical with those used to describe a first-order series-parallel reaction scheme.     

A new eletronic spectrum of the SiH3 radical observed using multiphoton ionization spectroscopy

The silyl radical, SiH₃, was observed between 365 and 410 nm using resonance-enhanced multiphoton ionization spectroscopy. The spectrum arises from two-photon resonance states which lie between 49229 and 54014 cm⁻¹. A vibrational progression interval of ≈ 800 cm⁻¹ was assigned to the resonant intermediate state symmetric deformation mode, ν₂.     

Investigation of the excimer dynamics in 9,10-dichloroanthracene crystals using picosecond spectroscopy

The dynamics of excimer formation, excimer migration, and excimer dissociation in β-9,10-dichloroanthracene crystals were investigated by means of picosecond time-resolved fluorescence spectroscopy. In the temperature range from T = 20 K to T = 40 K we were able to temporally resolve the relaxation into the excimer state. The excimer formation rates were determined to be k ≈ 1.8 × 10¹¹ s^?1 at T = 40 K and k ≈ 2.0 × 10¹⁰ s^?1 at T<30 K. The excimer migration was investigated by measuring the excimer annihilation rate as a function of temperature. At room temperature 20% of the excimers are dissociated. The excimer binding energy is estimated to be B = 1360 cm^?1. The experimental results are explained in terms of a kinetic scheme comprising the population and depopulation of exciton, trap, and excimer states. The nature of the trap state is identified and it is shown that thermal activation of a 25 cm^?1 librational mode induces the relaxation of the trap into the excimer state.     

A new magneto-optical effect in Raman spectroscopy

It is well known that the application of a static magnetic field can result in a change in both Raman spectral intensities and line positions. this effect is generally associated with a removal of spin degeneracy, and leads to a manifestation of chiral discrimination in both optically active and inactive samples. However, other magnetic field effects can be conferred upon the Raman process by virtue of direct magneto-optical interactions. It is here shown that for molecules belonging to one of the cubic or icosahedral point groups, any polarised Raman transition associated with a totally symmetric vibration becomes depolarised on application of a magnetic field directed along the usual right-angled direction of observation. Hence with a suitable polarisation filter, the corresponding Raman line can be “switched” into the spectrum by application of the field.     

A molecular dynamics investigation of rare-gas solvated cation-benzene clusters using a new model potential

The main static and dynamic properties of some ionic heteroclusters, involving K+, C6H6, and Ar, have been investigated. A new representation of the intermolecular potential energy, which takes into account both electrostatic and non-electrostatic contributions to the overall noncovalent interaction, was used. Dynamical calculations were performed for a microcanonical ensemble. Particular attention was paid to the opening of the isomerization and dissociation processes for K+-C6H6-Ar(n) and to the formation of some of its fragments at increasing temperatures of the cluster considered.     

A new action photoelectron spectroscopy for anions

We present a new experimental approach, in which anion photodetachment spectroscopy is recorded with electrons of fixed kinetic energy. This approach circumvents some shortcomings of the zero electron kinetic energy method. Our method is based on a modified magnetic bottle photoelectron spectrometer (MBPES). A tunable laser is used to detach electrons from mass selected anions, drifting collinearly with the 40 cm MBPES drift tube. To avoid Doppler broadening, a low voltage pulse removes the velocity component of anions from the detached electrons. Spectra are recorded by collecting the wavelength dependence of electron-signal at a predetermined TOF window, corresponding to a specific electron-kinetic energy. We call this approach PEACE, denoting photoelectron action spectroscopy at constant kinetic energy. Our best resolution is 0.65 meV for 1.5 meV electrons. We present a PEACE spectrum of HgCl(-) together with the corresponding simulated theoretical spectrum. The method is similar in resolution and data collection rates to the slow electron velocity map imaging technique recently introduced by Neumark and co-workers.     

Photodissociation dynamics of the tert-butyl radical via photofragment translational spectroscopy at 248 nm

The photodissociation dynamics of the tert-butyl radical (t-C(4)H(9)) were investigated using photofragment translational spectroscopy. The tert-butyl radical was produced from flash pyrolysis of azo-tert-butane and dissociated at 248 nm. Two distinct channels of approximately equal importance were identified: dissociation to H + 2-methylpropene, and CH(3) + dimethylcarbene. Neither the translational energy distributions that describe these two channels nor the product branching ratio are consistent with statistical dissociation on the ground state, and instead favor a mechanism taking place on excited state surfaces.     

Native and unfolded cytochrome c--comparison of dynamics using 2D-IR vibrational echo spectroscopy

Unfolded vs native CO-coordinated horse heart cytochrome c (h-cyt c) and a heme axial methionine mutant cyt c552 from Hydrogenobacter thermophilus ( Ht-M61A) are studied by IR absorption spectroscopy and ultrafast 2D-IR vibrational echo spectroscopy of the CO stretching mode. The unfolding is induced by guanidinium hydrochloride (GuHCl). The CO IR absorption spectra for both h-cyt c and Ht-M61A shift to the red as the GuHCl concentration is increased through the concentration region over which unfolding occurs. The spectra for the unfolded state are substantially broader than the spectra for the native proteins. A plot of the CO peak position vs GuHCl concentration produces a sigmoidal curve that overlays the concentration-dependent circular dichroism (CD) data of the CO-coordinated forms of both Ht-M61A and h-cyt c within experimental error. The coincidence of the CO peak shift curve with the CD curves demonstrates that the CO vibrational frequency is sensitive to the structural changes induced by the denaturant. 2D-IR vibrational echo experiments are performed on native Ht-M61A and on the protein in low- and high-concentration GuHCl solutions. The 2D-IR vibrational echo is sensitive to the global protein structural dynamics on time scales from subpicosecond to greater than 100 ps through the change in the shape of the 2D spectrum with time (spectral diffusion). At the high GuHCl concentration (5.1 M), at which Ht-M61A is essentially fully denatured as judged by CD, a very large reduction in dynamics is observed compared to the native protein within the approximately 100 ps time window of the experiment. The results suggest the denatured protein may be in a glassy-like state involving hydrophobic collapse around the heme.     

Transient dynamics of cold-rolled and subsequently thermally rejuvenated atactic-polystyrene using broadband dielectric spectroscopy

The effect of plastic deformation on the molecular dynamics of atactic polystyrene (a-PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier-transform infrared spectroscopy (FTIR) and polarized-light microscopy. Sheets of a-PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass-transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α- and γ(I)-relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation-enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G-ratio); and (ii) processes III and IV were identified as helix-inversion processes of T₂G₂ helices as reported earlier for syndiotactic-rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized-light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.     

Dynamics of dewetting at the nanoscale using molecular dynamics

Large-scale molecular dynamics simulations are used to model the dewetting of solid surfaces by partially wetting thin liquid films. Two levels of solid-liquid interaction are considered that give rise to large equilibrium contact angles. The initial length and thickness of the films are varied over a wide range at the nanoscale. Spontaneous dewetting is initiated by removing a band of molecules either from each end of the film or from its center. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Consistent with the current understanding of wetting dynamics, film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. In addition, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These new results imply not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the effective viscosity of the film decreases with thickness.