A new approach to local hardness

The applicability of the local hardness as defined by the derivative of the chemical potential with respect to the electron density is undermined by an essential ambiguity arising from this definition. Further, the local quantity defined in this way does not integrate to the (global) hardness - in contrast with the local softness, which integrates to the softness. It has also been shown recently that with the conventional formulae, the largest values of local hardness do not necessarily correspond to the hardest regions of a molecule. Here, in an attempt to fix these drawbacks, we propose a new approach to define and evaluate the local hardness. We define a local chemical potential, utilizing the fact that the chemical potential emerges as the additive constant term in the number-conserving functional derivative of the energy density functional. Then, differentiation of this local chemical potential with respect to the number of electrons leads to a local hardness that integrates to the hardness, and possesses a favourable property; namely, within any given electron system, it is in a local inverse relation with the Fukui function, which is known to be a proper indicator of local softness in the case of soft systems. Numerical tests for a few selected molecules and a detailed analysis, comparing the new definition of local hardness with the previous ones, show promising results.Comment: 30 pages (including 6 figures, 1 table

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.This work was supported by the German Research Foundation (DFG) within the Collaborative Research Center (SFB) 613 (F.S., P.B., W.P., and U.H.), the Priority Programs SPP 1931 (C.S., M.H., and W.P.), and SPP 1840 (St.F., S.N., and W.P.); the Basque Government (grant IT-756-13 UPV/EHU) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.); and the Spanish Ministerio de Economía y Competitividad (grants FIS2016-76617-P and FIS2016-76471-P) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.) and Fondo Europeo de Desarrollo Regional (FEDER) (CTQ2016- 80375-P) (M.T.-S.). N.M.K. acknowledges hospitality and financial support from the theory group in cooperation with the small quantum systems (SQS) research group of European XFEL.Peer Reviewe

Role of vibrational anharmonicity in atmospheric radical hydrogen-bonded complexes

12 pages, 2 figures, 8 tables.-- PMID: 19809669 [PubMed].-- Available online Jun 19, 2009.Harmonic and anharmonic vibrational frequency calculations are reported for the most stable hydrogen bonded complexes formed between the hydroperoxyl radical and formic, acetic, nitric, and sulfuric acids which are of atmospheric interest. A comparison between the calculated IR spectra of the hydrogen bonded complexes with the corresponding separate monomers is also reported with the aim to facilitate a possible experimental identification of these complexes. The calculations have been carried out using the second-order vibrational perturbative treatment implemented by Barone applied to the PES obtained with the B3LYP functional using the 6-31+G(d,p) and 6-311+G(2d,2p) basis sets. Our calculations for the separate monomers predict vibrational frequencies with quite a good agreement with the experimental values. The anharmonic contribution results in differences of around 40 cm-1 with respect to the harmonic values; although in some cases involving highly anharmonic modes, these differences can rise up to 300 and 450 cm-1.M.T. acknowledges the European Community for financial help through postdoctoral grant MEIF-CT-2006-025362. The financial support of the Spanish MEC and the Catalan Departament d’Universitats, Recerca i Societat de la Informació (DURSI), respectively, is acknowledged according to the following programmes: CTQ2008-06696/BQU and 2005SGR- 00238 (J.M.L.); and CTQ2005-07790 and 2005SGR00111 (J.M.A.).Peer reviewe

The reactions of SO3 with HO2 radical and H2O...HO2 radical complex. Theoretical study on the atmospheric formation of HSO5 and H2SO4

10 pages, 6 figures, 2 tables.-- PMID: 20165760 [PubMed].-- Available online Jan 14, 2010.-- Supporting information available at: http://dx.doi.org/10.1039/b916659aThe influence of a single water molecule on the gas-phase reactivity of the HO2 radical has been investigated by studying the reactions of SO3 with the HO2 radical and with the H2OHO2 radical complex. The naked reaction leads to the formation of the HSO5 radical, with a computed binding energy of 13.81 kcal mol−1. The reaction with the H2OHO2 radical complex can give two different products, namely (a) HSO5 + H2O, which has a binding energy that is computed to be 4.76 kcal mol−1 more stable than the SO3 + H2OHO2 reactants ((E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol−1 below the energy of the reactants ((E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H2SO4 may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO2, HSO5, the HSO5H2O hydrogen bonded complex, H2SO4, and two H2SO4H2O complexes, in order to help with the possible experimental identification of some of these species.This research has been supported by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, grant CTQ2008-06536/BQU) and by the Generalitat de Catalunya (Grant 2009SGR01472).Peer reviewe

Theoretical study of the switching between Hückel and Möbius topologies for expanded porphyrins

The expanded porphyrins have become a useful tool to synthesize new Hückel-to-Möbius topological switches. Only applying small changes in the external conditions (temperature, solvent, redox potential) or in the structure of the ring allows the expanded porphyrins to achieve Hückel and Möbius topologies with distinct aromaticities and magnetic and electric properties. In this work, we report a theoretical investigation of the reaction mechanism of the conformational switch between the Hückel planar and the singly twisted Möbius structures for two different meso-substituted [28]hexaphyrins(1.1.1.1.1.1); that is, the meso-substituents are the hydrogen atoms and pentafluorophenyls. Calculations are performed with the B3LYP, BH&HLYP, CAM-B3LYP, M05-2X, and MP2 methodologies along with the 6-31G and 6-311G(d,p) basis sets. For benchmarking purposes, single-point energies are calculated at the CCSD(T)/6-31G level. Our results conclude that, in overall, CAM-B3LYP and M05-2X are the methods that provide the most equilibrated results for the different steps of the topological switch. In addition, we point out that the ring strain and the steric effect of the meso-substituents play a relevant role in the thermochemistry and kinetics of the Hückel-to-Möbius topological switches.This research has been supported by the Research Executive Agency (Grant Agreement no. PERG05-GA-2009-249310) and by the Generalitat de Catalunya (Grant 2009SGR01472). The calculations described in this work were carried out at the Centre de Supercomputació de Catalunya (CESCA). M.T-S. acknowledges the CSIC for the JAE-DOC contract, and E.M. acknowledges a contract under the PERG05-GA-2009-249310 grant. We are grateful to Ramon Crehuet for his help in the NEB calculationsPeer Reviewe

Sulfuric acid as autocatalyst in the formation of sulfuric acid

Sulfuric acid can act as a catalyst of its own formation. We have carried out a computational investigation on the gas-phase formation of H 2SO4 by hydrolysis of SO3 involving one and two water molecules, and also in the presence of sulfuric acid and its complexes with one and two water molecules. The hydrolysis of SO3 requires the concurrence of two water molecules, one of them acting as a catalyzer, and our results predict an important catalytic effect, ranging between 3 and 11 kcal·mol-1 when the catalytic water molecule is substituted by a sulfuric acid molecule or one of its hydrates. In these cases, the reaction products are either bare sulfuric acid dimer or sulfuric acid dimer complexed with a water molecule. There are broad implications from these new findings. The results of the present investigation show that the catalytic effect of sulfuric acid in the SO3 hydrolysis can be important in the Earth's stratosphere, in the heterogeneous formation of sulfuric acid and in the formation of aerosols, in H2SO4 formation by aircraft engines, and also in understanding the formation of sulfuric acid in the atmosphere of Venus. © 2012 American Chemical Society.This work was supported by the Spanish DGYCIT (CTQ2011-27812) and the Generalitat de Catalunya (Grant 2009SGR01472). The calculations described in this work were carried out at the Centre de Supercomputació de Catalunya (CESCA) and at CTI-CSIC. M.T.-S. acknowledges the CSIC for a JAE-Doc contract. We also thank the reviewers for their comments and suggestions regarding this work.Peer Reviewe