1,720,968 research outputs found
Nuova Secondaria
No full textPuò la scienza diventare sapienza
Dalla tavola periodica degli elementi alla crisi della ricerca scientifica
Nuova Secondaria
No full textCreazione ed evoluzione Alcune riflessioni sul rapporto tra scienza e fed
The epimerisation of 2-tetrahydropyranol catalysed by the tautomeric couples 2-pyridone/2-hydroxypyridine and formamide/formamidic acid as a model for the sugar's mutarotation: a theoretical study
No full textIn the present computational study a complete reaction mechanism for the sugar's mutarotation promoted by tautomeric catalysts in vacuum is outlined. The rate determining step of the process consists in the breaking of the C-O bond of the sugar ring which occurs by a concerted double proton transfer between the substrate and the catalyst. After ring opening, the -CHO group of the substrate rotates around the C2-C3 bond without dissociation of the substrate catalyst adduct. Theactivation energy of this step is generally less than 8-9 kcal mol(-1). Alternative reaction paths which require the dissociation of the substrate catalyst complex are less favourable for both probability and energetic reasons. When the mutarotation is catalysed by the 2-pyridone/2-hydroxypyridine (PD/HP) tautomeric couple, the process may be promoted by either PD or HP derived from the dissociation of the (PD)(2), (HP)(2) or PD-HP dimers. According to B3LYP and MP4 (SDQ) calculations the HP-promoted reaction path should be faster than the PD-promoted one. When the process is catalysed by the formamide/formamidic acid (F/FA) tautomeric couple, the reaction path promoted by FA, which is derived from the dissociation of the F-FA dimer, should be the most favourable, according to B3LYP, MP2 and MP4(SDQ) calculations. Solvent effects in benzene, calculated by the PCM method, did not affect to a relevant extent the mechanism outlined in vacuum. The present study suggests, differing from the hypothesis formulated in many experimental studies, that the sugar's mutarotation process is more efficiently catalysed by the less stable tautomer or catalyst dimer rather than by the more stable one of both the PD/HP and F/FA couples
A theoretical study of hydrogen bonding, proton transfer and kinetic isotope effects in the dimers of 2-tetrahydropyranol and in the 2-tetrahydropyranol-H2O adducts
No full textThe epimerization process of the model sugar 2-tetrahydropyranol was studied by means of ab initio calculations. The results suggest that the rate limiting step of sugar ring opening involves a high-energy
intramolecular proton transfer reaction or a low-energy process in which the proton transfer is mediated by a catalyst molecule, formic acid in the case investigated. The catalyzed process is an asynchronous concerted
double proton transfer reaction, where both protons are transferred within the same elementary step but one of them is transferred much earlier than the other one along the reaction coordinate. The motion of both
protons in the transition state of the catalyzed process is strongly coupled with the breaking of the C-O bond of the sugar ring. Geometry optimization at the B3LYP/6-31G* level, with additional p polarization functions located on the hydrogen atoms involved in proton transfer, appears to be suitable for further MP2/6-31G** single point energy calculations, as it provides hydrogen bond and activation energies in good agreement with those obtained from geometry optimization at the full MP2 level of theor
The use of ab initio and DFT calculations in the interpretation of ultraviolet photoelectron spectra: the rotational isomerism of anisole and thioanisole as a case study
No full textThe rotational isomerism of anisole and thioanisole was investigated by means of several computational methods, in the framework of the Møller–Plesset and Density Functional theories. Total energy curves as a function of the C–O(S)–C–C dihedral angle were calculated. For anisole, all methods employed, with the exception of HF/6-31G*, predicted the planar conformer to be the only stable species, in agreement with experiment. For thioanisole, HF and Møller–Plesset calculations
predicted the perpendicular conformer to be the only stable species. Nearly all DFT methods predicted, instead, the existence of two minima, in agreement with experimental findings. Ionisation energies calculated according to the Koopmans’ theorem were compared with the values obtained from UP spectra. DFT ionisation energies, especially those obtained by the B3LYP, B1LYP, mPW1PW and B3PW functionals, were found to be in fair agreement with experimental values. HF ionisation energies showed only a rough agreement with experiment
A computational study on the mechanism of NO decomposition catalyzed by Cu-ZSM-5: A comparison between single and dimeric Cu+ active sites
No full textThe present computational study investigates the mechanism of NO decomposition catalyzed by Cu-ZSM-5. It was considered that the active site could be of two kinds: (i) pairs of Cu+ ions located at opposite sides of the ten-membered rings of the zeolite, in the region at the intersection of the linear and sinusoidal channels; (ii) isolated Cu+ ions. DFT calculations were performed by using the B3LYP functional on cluster models suited to reproduce the geometrical constraints of the ZSM-5 structure. It was shown that both the Cu+-pair mechanism and that catalyzed by a single-Cu+ site proceed through the formation of N2O as a reaction intermediate, which further reacts with the oxygen atom of a ZCu-O-CuZ or ZCuO unit (Z = zeolite) to give O-2 + N-2. The latter step displayed a larger activation energy in the Cu+-pair mechanism, which is therefore unfavored although the pathway associated to Cu+-pairs occurs at lower energy. than that associated to a single-Cu+ site. Such a large activation energy is associated to the stability of the ZCu-O-CuZ fragment. A quantitative analysis, performed by means of the energetic span model of Kozuch and Shaik, reinforced the just outlined observations. (C) 2012 Elsevier B.V. All rights reserved
Basis set effects on Cu(I) coordination in Cu-ZSM-5: a computational study
No full textDFT calculations on the coordination of Cu+ to the framework oxygen atoms of Al-substituted ZSM-5 were performed by using combinations of different basis sets in order to investigate the dependence of the results on the adopted computational level. With low-end basis sets, a large basis set superposition error (BSSE) favors the coordination of Cu+ to three to four oxygen atoms of the framework, only two of which belong to the AlO4 tetrahedron corresponding to the investigated T-site. More extended basis sets considerably lower the BSSE and favor the coordination of Cu+ to only two oxygen atoms of the AlO4 tetrahedron. Upon interaction with NO, the Cu+ ion is always coordinated by two oxygen atoms of the AlO4 tetrahedron, independently of the basis set adopted and of the coordination number before NO adsorption. The shift from three-to twofold coordination caused by the Cu+-NO interaction requires a deformation energy that lowers the final adsorption energy. Such an effect is relevant with low-end basis sets, whereas it is substantially absent with more extended basis sets, which favor the twofold coordination of Cu+ even before NO adsorption. As a result, high-end basis sets increase the NO interaction energy with respect to that calculated by low-end basis sets, in agreement with experiments and suggesting a possible re-interpretation of the catalytic properties of the investigated sites. Provided suitable scale factors are employed, the N-O stretching frequencies of adsorbed nitrogen oxide calculated by sufficiently extended basis sets turned out in fair agreement with experimental findings
Critical Test of PM3-calculated Proton-Transfer Activation Energies: a Comparison with Ab initio and AM1 Calculations
No full text- …
