Paolo Tosi

Tautomerism of Uracil Probed via Infrared Spectroscopy of Singly Hydrated Protonated Uracil

Tautomerism of the nucleobase uracil is characterized in the gas phase through IR photodissociation spectroscopy of singly hydrated protonated uracil created with tandem mass spectrometric methods in a commercially available Fourier transform ion cyclotron resonance mass spectrometer. Protonated uracil ions generated by electrospray ionization are re-solvated in a low-pressure collision cell filled with a mixture of water vapor seeded in argon. Their structure is investigated by IR photodissociation spectroscopy in the NH and OH stretching region (2500-3800 cm(-1)) with a tabletop IR laser source and in the 1000-2000 cm(-1) range with a free-electron laser. In both regions the IR photodissociation spectrum exhibits well-resolved spectral signatures that point to the presence of two different types of structure for monohydrated protonated uracil, which result from the two lowest-energy tautomers of uracil. Ab initio calculations confirm that no water-catalyzed tautomerization occurs during the re-solvation process, indicating that the two protonated forms of uracil directly originate from the electrospray process.

CRITICAL REVIEW OF N, N+, N-2(+), N++, And N-2(++) MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

This paper is a detailed critical review of the production processes and reactions of N, N+, N+ 2, N++, and N++ 2 of relevance to Titans atmosphere. The review includes neutral, ion-molecule, and recombination reactions. The review covers all possible active nitrogen species under Titans atmospheric conditions, specifically N2 (A3Σ+ u), N (4 S), N (2 D), N (2 P), N+ 2, N+ (3 P), N+ (1 D), N++ 2, and N++ species, and includes a critical survey of the reactions of N, N+, N+ 2, N++, and N++ 2 with N2, H2, D2, CH4, C2H2, C2H4, C2H6, C3H8 and the deuterated hydrocarbon analogs, as well as the recombination reactions of N+ 2, N+, N++ 2, and N++. Production processes, lifetimes, and quenching by collisions with N2 of all reactant species are reviewed. The N (4 S) state is reactive with radicals and its reactions with CH2, CH3, C2H3, and C2H5 are reviewed. Metastable states N2(A3Σ+u), N (2 D), and N (2 P) are either reactive or quenched by collisions with the target molecules reviewed. The reactions of N+ (1 D) have similar rate constants as N+ (3 P), but the product branching ratios differ significantly. Temperature effects and the role of the kinetic energy content of reactants are investigated. In all cases, experimental uncertainties of laboratory data are reported or estimated. Recommended values with uncertainties, or estimated values when no data are available, are given for rate constants and product branching ratios at 300 K and at the atmospheric temperature range of Titan (150-200 K for neutral reactions and 150 K for ion reactions).

The reaction of argon ions with hydrogen and deuterium molecules by crossed beams: Low energy resonances and role of vibronic levels of the intermediate complex

In a crossed beam experiment, cross sections have been measured for the ion–molecule reactions Ar++H2→ArH++H and Ar++D2→ArD++D. Low collision energies (0.025≤E≤ 1 eV) and high resolution (ΔE∼10 meV, half‐width at half‐maximum) have been obtained using the method of guiding the ion beam by an octopole field and the technique of supersonic beams for H2 or D2. A structure in the energy dependence of cross sections has been found and attributed to a manifestation of vibronic resonances. Calculations are presented and compared to experimental findings to illustrate this effect, which arises because of the successive population of vibronic levels of the charge transfer complex Ar–H2+ or Ar–D2+, which are the intermediates for these reactions. Empirical potential energy surfaces for the entrance channels have been constructed accounting explicitly for the open shell nature and spin–orbit effects in Ar+(2PJ); symmetry considerations have also been used to establish the sequence of pertinent vibronic surfaces of t...

Infrared spectroscopy of copper-resveratrol complexes: A joint experimental and theoretical study

Infrared multiple-photon dissociation spectroscopy has been used to record vibrational spectra of charged copper-resveratrol complexes in the 3500-3700 cm(-1) and 1100-1900 cm(-1) regions. Minimum energy structures have been determined by density functional theory calculations using plane waves and pseudopotentials. In particular, the copper(I)-resveratrol complex presents a tetra-coordinated metal bound with two carbon atoms of the alkenyl moiety and two closest carbons of the adjoining resorcinol ring. For these geometries vibrational spectra have been calculated by using linear response theory. The good agreement between experimental and calculated IR spectra for the selected species confirms the overall reliability of the proposed geometries.

Bond-forming reactions of molecular dications with rare gas atoms: Production of ArC2+ in the reaction CO2++Ar

Integral cross sections for the bond-forming reaction CO2++Ar→ArC2++O have been measured as a function of collision energy in a guided-ion beam mass spectrometer. The energy dependence is consistent with an endoergic reaction. Since the title reaction is in competition with several charge-transfer processes, the cross section at the maximum is only 0.023 A2 at a collision energy of about 3 eV. Simple kinematics considerations suggest that the falloff of the cross section at higher energies might be due to the vibrational predissociation of ArC2+. State correlation diagrams are used for discussing the reaction mechanism.

Lipid profiles of oil from trout (Oncorhynchus mykiss) heads, spines and viscera: Trout by-products as a possible source of omega-3 lipids?

Lipid profiles of fish oil extracted from trout heads, spines and viscera using supercritical carbon dioxide and Randall extraction with hexane were measured. The amount of unsaturated fatty acids (as a percentage of total fatty acids) was within the range of 72.6-75.3% in all the substrates. A significant presence of the most important omega-3 fatty acids was detected. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) content in oil from spines, heads and viscera resulted to be 8.7% and 7.3%, 7.9% and 6.3%, and 6.4% and 6.0%, respectively. A low (≈3%), but worth noting, presence of lipids with omega-1 polyunsaturated fatty chains was observed in all the oils. Finally, significant differences were noticed in the relative amounts of triacylglycerides (TAG), diacylglycerides (DAG) and free fatty acids (FFA). Whereas oil from heads and spines was essentially composed of TAG (≈98%), in viscera oil the molar distribution ratio became TAG:DAG:FFA=87:8:5.

Transport of an ion beam through an octopole guide operating in the R.F.-only mode

Abstract Transport of an ion beam through an octopole guide operating in the r.f.-only mode has been investigated both experimentally and by ion trajectory calculations. The octopole transmission band has been characterized by two limit values of the r.f.-potential. The dependence of these limits on the characteristic energy e (e = 1 8 π2mf2R20, where m is ion mass, f is frequency and R0 is octopole free radius) has been studied considering different configurations of the ion guide.

Structure and charge transfer dynamics of the (Ar–N2)+ molecular cluster

In this paper we have investigated the interaction potential and the charge transfer processes at low collision energies in the (Ar–N2)+ system. The angular dependence of the lowest doublet potential energy surfaces (PES), correlating with Ar+(2Pj)–N2 and Ar–N2+(2Σ,2Π), has been given in terms of spherical harmonics, while the dependence on the intermolecular distance has been represented by proper radial coefficients. Such coefficients, which account for van der Waals, induction, charge transfer, and electrostatic contributions, have been predicted by empirical correlation formulas. The PES so obtained have been employed to calculate cross sections for the charge transfer process Ar++N2→Ar+N2+ at low collision energy (E⩽2 eV). A good agreement between calculated and experimental cross sections is obtained by assuming that the duration of the nonadiabatic transition has to match the time required for the molecular rearrangement into the final vibrational state. As a consequence the efficient formation of ...

Ar-dilution effects on the elastic and structural properties of hydrogenated hard carbon films deposited by plasma-enhanced chemical vapor deposition

Abstract Results of systematic investigations of the elastic and structural properties of hydrogenated amorphous carbon (a-C:H) films as a function of deposition conditions are reported. A multi-technique approach has been adopted, combining Brillouin light scattering (BLS), Raman spectroscopy and X-ray reflectivity (XRR). The mass density of the films has been determined by XRR, while BLS has been adopted to test the elastic properties of the films. As the fraction of methane in the feed gas is increased, the deposited film shows both a higher phase velocity of the surface Rayleigh wave, and an increasing density, thus suggesting the synthesis of a harder material. This hardening effect has been attributed to the occurrence of a higher degree of structural order in the sp2 lattice, as suggested by Raman scattering measurements. These experimental findings are related to the different chemical composition of plasmas with different CH4/Ar ratios.

Argon–hydrogen rf plasma study for carbon film deposition

In this work the effect of hydrogen addition on the physical properties and the sputtering efficiency of an radio-frequency (rf) (13.56 MHz) Ar plasma was investigated. The discharges in Ar–H2 were used to sputter-deposit carbon films from a graphite cathode, with a hydrogen concentration in the feed gas ranging from 0 to 100% (the useful range for film growth was however limited to 0–85%). The physical plasma parameters were determined using a Langmuir probe, which, coupled with a chemical modelling of the ion–molecule and electron–molecule reactions in gas phase, enabled us to define the energy flux conditions at the cathode. The results show that hydrogen exerts a positive effect on the film deposition rate at the lowest end of the hydrogen concentration range, an enhancing deposition effect correlated with a high density of ArH+ ions in the plasma and a high energy flux carried by the ions to the cathode. Nonetheless, an analysis of the processes at the cathode indicates that the sputtering mechanism was essentially physical in the low [H2] range (3–20%) but that a chemical assistance of the process should be considered too for the remaining [H2] range. Besides, even in the physical sputtering regime, the target material removal occurred with a reactive sputtering mechanism, which implies a chemical modification of the target surface layers and surface binding energy.