Angel Gonzalez Urena

Chemiluminescent reactions of excited calcium atoms with HCl and HBr: Selective charge-transfer “harpooning” and synchronized intermediate complex rearrangement

A “harpooning” mechanism is investigated for the chemiluminescent reactions of Ca*(1D2) atoms with HCl (leading to CaCl* in the A state) and with HBr (leading to CaBr* in both the A and B states). A model of the interactions in the entrance channels, which involves an electron jump and leads to a charge-transfer intermediate complex, accounts for the selective dependence of reactivity on the relative orientation between the molecule and the outer electronic orbital of the atom. The dynamical treatment incorporates in the Landau–Zener approach the synchronization between times for nonadiabatic transitions and for triggering the rearrangement of the intermediate collision complex. The treatment accounts for the observed features of the translational energy dependence of the cross sections: The steep initial increase with a pronounced maximum, the sharp decline and also fine details, such as a stairlike behavior.

Collision energy effects in the Cs+ICH3→CsI+CH3 reaction

Differential and total reaction cross sections for the Cs+ICH3→CsI+CH3 system have been measured as a function of the collision energy by using the crossed beam technique. The analysis of the center‐of‐mass angular and recoil velocity distribution of the products indicated: (a) a backward peak character corresponding to a direct rebound mechanism at low collision energy; (b) over the collision energy range from 0.15 up to 0.56 eV, the backward character shifts to near sideways scattering. A direct interaction with product repulsion (DIPR) model analysis of this collision energy evolution showed an increasing participation of bent transition state configurations, which might be in competition with the purely collinear one at lower collision energy; (c) that the average products’ translational energy E’T increases approximately linearly with increasing collision energy ET as follows: E’T/kJ mol−1 =0.62 ET/kJ mol−1+58.5. Complete laboratory differential reaction cross‐section measurements were carried ou...

Spectroscopy and kinetics of tyrosinase catalyzed trans-resveratrol oxidation

The spectroscopy and kinetics of the tyrosinase catalyzed trans-resveratrol oxidation were investigated by measuring both UV-vis absorption spectra over the 200-500 nm range and Raman spectra over the 600-1800 cm(-1) region. Room temperature UV-vis absorption spectra, as a function of time, showed the presence of two isosbestic points located at λ(1) = 270 nm and λ(2) = 345.5 nm delimiting two different regions: the reactant region around 300 nm, where the absorption decreased with time, and the product region over the low wavelength (λ < 260 nm) and high wavelength (λ > 390 nm) wavelength zone in which the absorption increased with time until, in both cases, constant values were achieved. A first-order kinetics was deduced with a rate coefficient of k(1) = (0.10 ± 0.001) min(-1), which turned out to be independent of substrate concentration over the 50-5 μM range; a feature that was rationalized by invoking the limiting case of the Michaelis-Menten scheme appropriate for substrate concentration much lower than the respective Michaelis constant. The observation of the distinct resonance enhanced Raman lines, specifically those peaking at 830 cm(-1), 753 cm(-1), and 642 cm(-1) together with their time evolution, permitted us to gain insight into some crucial features and steps of the catalytic reaction. Namely, that the formation of the so-called trans-resveratrol and tyrosinase (S)P complex with its O-O bridge plays a crucial role in the first steps of this enzymatic reaction and that the hydroxylation of the ortho C-H bond of the trans-resveratrol OH group occurs after O-O bond cleavage in the tyrosinase active site. The present study makes clear that a class of potential inhibitors of tyrosinase can be found in compounds able to bind the two Cu (II) ions of the enzyme bidentate form.

An optical–optical double resonance study of the perturbed O2 d3sσg(1Πg) Rydberg state excited via single rotational levels of the b(1Σg+) valence state

The perturbed v=3 level of the d3sσg(1Πg) Rydberg state of O2 has been excited in an optical–optical double resonance (OODR) experiment via J=0–16 of v=0 of the b(1Σg+) state. The d(1Πg) state resonances were detected by ionization with one further probe photon near 340 nm. The range of J levels of the d(1Πg) state now accessed reveals a lack of variation in line widths that is not predicted by previous models of state-dependent predissociation. Instead, intensities of rotational lines in the ionization spectrum appear to be controlled by a J-dependent mixing of the d(1Πg) state with a nearby valence state that has a much lower ionization cross section at the probe wavelengths used.

Angular momenta correlation in kinematically constrained reactions. II. Application to the B + OH → BO + H system

Extensive quasiclassical trajectory calculations have been carried out to study the disposal of both rotational and orbital angular momenta in the Ba + HI → BaI + H system. As expected for this kinematically constrained reaction, a complete transfer of the reactant orbital (rotational) momentum L(J) into the product rotational (orbital) angular momentum J′(L′) was found. However, significant deviations from the kinematic limit are noticeable for the J → L′ vector correlation, particularly at low reagent rotational excitation. The results are analysed in the light of the stereodynamic picture associated with these kinematically restricted reactions. In addition, the possibility of controlling the spatial distribution of the reaction products by laser polarization of the reagent rotational angular momentum is also discussed.

Collision dynamics of Cs + ICH3→ Csl + CH3: backward vs. sideways scattering as a function of collision energy

Differential reaction cross-sections for the Cs + ICH3→ CsI + CH3 system have been measured as a function of the collision energy using a simple molecular-beam apparatus. A new oven design suitable for producting stable beams of highly reactive metals was used to produce a Cs beam via the (oven) chemical reaction Ba(s)+ CsCl(s)→ BaCl(s)+ Cs(v). The Csl was detected over the whole angular laboratory range as a function of collision energy, ET, from 0.15 to 0.56 eV. The analysis of the centre-of-mass angular and recoil velocity distributions of the product indicated (a) a backward peak corresponding to a direct, rebound mechanism, (b) increasing forward scattering as the collision energy increases, and (c) that the average translational energy of the products, text-decoration:overlineE′T, increases approximately linearly with increasing collision energy, text-decoration:overlineET, as follows: text-decoration:overlineE′T/kJ mol–1= 0.62text-decoration:overlineET/kJ mol–1+ 64.6. The backward to near-sideways scattering evolution with increasing ET is discussed in the light of a possible insertion mechanism in addition to the (low collision energy) abstraction mechanism.

Transmission Resonance Raman Spectroscopy: Experimental Results Versus Theoretical Model Calculations

A laser spectroscopic technique is described that combines transmission and resonance-enhanced Raman inelastic scattering together with low laser power (< 30 mW) and good spatial resolution (< 200 μm) as operational features. The monitoring of the transmitted inelastic scattering provides an increased signal-to-noise ratio because the low fluorescence background and, on the other hand, the resonant character of the laser excitation, leads to enhanced analytical sensitivity. The spectroscopic technique was applied to investigate the carotenoid content (specifically the β-carotene concentration) of distinct samples that included fruits, reaching a detection limit of the order of hundreds of picograms in solid samples, which is below the level needed for typical food control analysis. Additional features of the present development are direct sampling, noninvasive character, and fast analysis that is not time consuming. From a theoretical point of view, a model for the Raman signal dependence on the sample thickness is also presented. Essentially, the model considers the sample to be homogeneous and describes the underlying physics using only three parameters: the Raman cross-section, the laser-radiation attenuation cross-section, and the Raman signal attenuation cross-section. The model was applied successfully to describe the sample-size dependence of the Raman signal in both β-carotene standards and carrot roots. The present technique could be useful for direct, fast, and nondestructive investigations in food quality control and analytical or physiological studies of animal and human tissues.

Polarization of the CaI* chemiluminescence from the Ca*+ CH3I → CaI*+ CH3 reaction: evidence for Hund's case (c)coupling

The polarization emission of the excited CaI* produced in the Ca(3Pj, 1D2)+ CH3I → CaI*+ CH3 reaction has been measured showing evidence for Hunds case (c) in the excited CaI diatom.

Chemiluminescence and kinetic studies in O(3P)+ thiophene, pyrrole and furan mixtures

Using fast-flow-discharge apparatus the reactions between oxygen atoms and thiophene, pyrrole and furan have been studied. Chemiluminescence has been observed from the electronically excited states of SO2 and CH in thiophene + O(3P); NO, CH, CN and NO2 in pyrrole + O(3P) and OH in furan + O(3P), and the evolution of its intensity has been monitored as a function of reactant concentration. Kinetic determinations have been done for the reaction of O(3P) and thiophene by following the SO*2 emission intensity and the O(3P) decay, using the titration of residual oxygen atoms with NO, at several temperatures, and for the reaction of O(3P) and pyrrole at 318 K by following the decay of O(3P) with NO. For the thiophene reaction, rate constants in the range T= 379–525 K can be fitted to the Arrhenius expression k=(1.6 ± 0.2)× 1012 exp (–240 ±/RT) cm3 mol–1 s–1 and for the pyrrole reaction the value of the rate constant at 318 K is k=(2.8 ± 0.3)× 1012 cm3 mol–1 s–1. Different reaction mechanisms are proposed in the light of the observed emissions and the empirical correlation existing between the kinetic parameters and the ionization potential of the aromatic hydrocarbons. A comparison is made with the reactions of O(3P) and benzene and O(3P) and pyridine.

Translational energy threshold for the reaction of oxygen atoms with benzene molecules

Reactive scattering of O atoms with C6H6 molecules has been studied over the range of initial translational energy E= 15–35 kJ mol–1 using a supersonic beam of O atoms seeded in an He–Ne buffer gas mixture. The total reaction cross-section was measured as a function of initial translational energy, yielding an estimated threshold energy E0= 11 ± 3 kJ mol–1 corresponding to a barrier in the entrance valley of the potential-energy surface. The barrier is higher than those estimated for O + C2H2, C2H4 despite these molecules having higher ionisation potentials. This accords with the spin uncoupling model of Bader et al. but not the charge transfer model of Cvetanovic.