Direct determination of the dissociation probability in highly focused IR multiple photon dissociation

Laser-induced fluorescence (LIF) has been used to directly determine the dissociation probability per pulse in highly focused infrared multiple-photon dissociation (IRMPD). The fluence dependence of CDCl₃ IRMPD has been determined by LIF and FTIR spectrometry. The particular LIF irradiation and detection system’s geometry implemented allowed us to monitor the local CCl₂ radicals concentration in the intersection zone of the observation and the dissociation volumes. The fluence dependence of the LIF intensity was modeled with the cumulative log-normal distribution. The dependence of the global values of the fraction of molecules dissociated per pulse on fluence was obtained from FTIR spectrometry. The dissociation probabilities per pulse were derived from the deconvolution of these values using the cumulative log-normal distribution. A very good agreement between the values of the parameters σ and Φ _sat obtained from the deconvolution technique with those resulting from the fluorescence intensity fit was found, showing the validity of the method proposed.     

Dopaminergic modulation of the spectral characteristics in the rat brain oscillatory activity

Oscillatory activity can be widely recorded in the brain. It has been demonstrated to play an important role not only in the physiology of movement, perception and cognition, but also in the pathophysiology of a variety of diseases. In frequency domain, neurophysiological recordings show a power spectrum (PSD) following a log (PSD) ∝ log (f)^?β, that reveals an intrinsic feature of many complex systems in nature: the presence of a scale-free dynamics characterized by a power-law component (PLC).Here we analyzed the influence of dopaminergic drugs over the PLC of the oscillatory activity recorded from different locations of the rat brain. Dopamine (DA) is a neurotransmitter that is required for a number of physiological functions like normal feeding, locomotion, posturing, grooming and reaction time. Alterations in the dopaminergic system cause vast effects in the dynamics of the brain activity, that may be crucial in the pathophysiology of neurological (like Parkinson’s disease) or psychiatric (like schizophrenia) diseases.Our results show that drugs with opposite effects over the dopaminergic system, induce opposite changes in the characteristics of the PLC: DA agonists/antagonists cause the PLC to swing around a fulcrum point in the range of 20 Hz. Changes in the harmonic component of the spectrum were also detected. However, differences between recordings are better explained by the modulation of the PLC than by narrow peak activities in particular frequency ranges.Our findings suggest that the brain operates in a state of self-organized criticality (SOC) that is sensitive to dopaminergic stimulation. Nevertheless, understanding of the interactions between the rhythmic (harmonic component) and arhythmic component (fractal component) of brain activity remains as a challenge, which should motivate future studies to explore these phenomena.     

Crystal structure of M(II) bis-μ-diacetoamidopropionate,diaqua·4H2O (M=Cu,Zn, Co): An uncommon carbonyl-O-of-peptide-coordinated two-dimensional layered polymer

Crystal and molecular structures by X-ray diffraction analysis of Co, Cu and Zn complexes of 2,2-diacetoamidopropionic acid are reported. The results show that an uncommon bond from metal ion to a carbonyl-O-of-peptide atom is formed. The structures are isomorphic (Monoclinic,P2₁/c, with two formula units in the cell). The metal ion lies on a center of symmetry and it is six-oxygen coordinated in an octahedral-type configuration by pairs of water molecules, carboxylic (monodentate) groups and two carbonyl-O-of-peptide atoms. Ligand molecules bridge metal ions, so that the structure consists of a two-dimensional (sheet-type) polymer. Sheets are held together by a hydrogen-bond network making efficient use of the solvent water molecules.     

Infrared multiple-photon dissociation of CDCl3 induced by a TEA CO2 laser

Infrared multiple-photon dissociation (IRMPD) of CDCl₃ was studied using a tunable TEA CO₂ laser. Effects of number of irradiation pulses, wavelength and energy fluence as well as of sample pressure on the reaction yield are reported.     

Nonequilibrium fluctuations in the Rayleigh-Bénard problem for binary fluid mixtures

We have employed a simple Galerkin-approximation scheme to calculate nonequilibrium temperature and concentration fluctuations in a binary fluid subjected to a temperature gradient with realistic boundary conditions. When a fluid mixture is driven outside thermal equilibrium, there are two instability mechanisms, namely a Rayleigh (stationary) and a Hopf (oscillatory) instability, causing long-ranged fluctuations. The competition of these two mechanisms causes the structure factor associated with the temperature fluctuations to exhibit two maxima as a function of the wave number q of the fluctuations, in particular, close to the convective instability. In the presence of thermally conducting but impermeable walls the intensity of the temperature fluctuations vanishes as q goes to zero, while the intensity of the concentration fluctuations remains finite in the limit of vanishing q. Finally, we propose a simpler small-Lewis-number approximation scheme, which is useful to represent nonequilibrium concentration fluctuations for mixtures with positive separation ratio, even close to (but below) the convective instability.     

Low-frequency velocity correlation spectrum of fluid in a rectangular microcapillary

In addition to the fast correlation for local stochastic motion, the velocity correlation function in a fluid enclosed within the pore boundaries features a slow long time-tail decay. At late times, the flow approaches that of an incompressible fluid. Here, we consider the motion of a viscous fluid, at constant temperature, in a rectangular semipermeable channel. The fluid is driven through the rectangular capillary by a uniform main pressure gradient. Tiny pressure gradients are allowed perpendicular to the main flux. We solve numerically the three-dimensional Navier-Stokes equations for the velocity field to obtain the steady solution. We then set and solve the Langevin equation for the fluid velocity. We report hydrodynamic fluctuations for the center-line velocity together with the corresponding relaxation times as a function of the size of the observing region and the Reynolds number. The effective diffusion coefficient for the fluid in the microchannel is also estimated (Deff = 1.43 x 10(-10) m2.s-1 for Re = 2), which is in accordance with measurements reported for a similar system (Stepisnik, J.; Callaghan, P. T. Physica B 2000, 292, 296-301; Stepisnik, J.; Callaghan, P. T. Magn. Reson. Imaging 2001, 19, 469-472).     

Nanoparticulated silicas with bimodal porosity: chemical control of the pore sizes

Nanoparticulated bimodal porous silicas (NBSs) with pore systems structured at two length scales (meso- and large-meso-/macropores) have been prepared through a one-pot surfactant-assisted procedure by using a simple template agent and starting from silicon atrane complexes as hydrolytic inorganic precursors. The final bulk materials are constructed by an aggregation of pseudospherical mesoporous primary nanoparticles process, over the course of which the interparticle (textural) large pore system is generated. A fine-tuning of the procedural variables allows not only an adjustment of the processes of nucleation and growth of the primary nanoparticles but also a modulation of their subsequent aggregation. In this way, we achieve good control of the porosity of both the intra- and interparticle pore systems by managing independent variables. We analyze in particular the regulating role played by two physicochemical variables: the critical micelar concentration (cmc) of the surfactant and the dielectric constant of the reaction medium.