1,720,989 research outputs found
Group Theory and Crystal Field Theory: a simple and rigorous derivation of the spectroscopic terms generated by the t2g2 electronic configuration in a strong octahedral field
No full textA DFT study on Cu(I) coordination in Cu-ZSM-5: effects of the functional choice and tuning of the ONIOM approach
No full textThe coordination of Cu+ at the T1 and T7 positions of the M7 ring of Cu-ZSM-5, and the interaction of NO with coordinated Cu+ were investigated by means of DFT/ONIOM calculations. The B3LYP, BLYP, PBE1PBE, PBE, M06 and M062X functionals with the
def2-TZVP (def2-QZVP for Cu) basis set were employed in the high-level part of ONIOM calculations, with the HF/3-21G, B3LYP/LANL2DZ, M06/LANL2DZ and M062X/LANL2DZ methods in the low-level part. The ability of suitable combinations of the above methods to reproduce (i) the crystallographic structure of purely siliceous ZSM-5, (ii) the tendency of Cu+ to be two- or fourfold-coordinated by framework oxygen atoms of Cu-ZSM-5 and (iii) the interaction energy and the N–O stretching frequency of adsorbed nitrogen oxide are discussed, showing that different results are obtained depending on the adopted computational approach. With reference to above properties, some considerations about the employment of the ONIOM approximations are also include
A DFT study on the mechanism of NO decomposition catalyzed by short-distance Cu(I) pairs in Cu-ZSM-5
No full textThe complete NO decomposition catalyzed by short-distance Cu+ pairs in Cu-ZSM-5 was studied by means of DFT calculations. After adsorption of two NO molecules, an hyponitrite species is formed. Further decomposition of hyponitrite occurs with activation energies ranging from about 4 to 24 kcal mol−1, depending on the initial geometry of the substrate-catalyst complex. An oxidized form of the catalyst, [Cu[sbnd]O[sbnd]Cu]2+ and a copper-coordinating N2O molecule are obtained. Further N2O decomposition may occur with oxygen transfer from N2O to [Cu[sbnd]O[sbnd]Cu]2+ and formation of N2 and O2, both adsorbed on the catalyst. Three different kinds of transition states were identified for the latter step, which appears to be rate-determining due to activation energies ranging from 39–40, to 44–45, and to 50–52 kcal mol−1, respectively. After this, N2 desorption occurs easily, whereas O2 desorption is endothermic (from 28.8 to 36.5 kcal mol−1), the highest value being associated to reductive O2 desorption from a peroxide-like complex. It turned out that the best way for N2O elimination is the direct, spin-forbidden decomposition on a reduced Cu+⋯Cu+ pair, with formation of [Cu[sbnd]O[sbnd]Cu]2+ and N2, as already suggested in the literature. The problem of how the reduced catalyst may be regenerated is left open. © 2017 Elsevier B.V
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
How to get rid of imaginary frequencies within ONIOM geometry optimizations: A DFT study on the effect of basis set and link atom distances in Cu-ZSM-5
No full textTwo extended clusters representing different portions of Cu-ZSM-5 were treated within a two-layer ONIOM approximation, employing DFT calculations for both the real and the model system. Despite a two-step optimization procedure successfully employed in previous work, a consistent number of imaginary and anomalous frequencies appeared after the vibrational analysis. These artefacts depend both on the basis set assigned to link atoms and on an improper setting of the O–H distances, where H are the link atoms at the boundaries of the model system. The latter problem, revealed for the first time in the present study, originates from the default scale factor employed by the ONIOM routine within Gaussian-09. Once basis set and g scale factor are properly set, all imaginary and anomalous frequencies disappear. The present findings may represent an interesting and practical solution to an annoying computational problem, whenever it occurs in the framework of ONIOM calculations
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 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
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