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The variational self-consistent field molecular-orbital method is used to compute both atomic and molecular full configuration interaction (FCI) energies of the hydrogen anion and the hydrogen molecule, for the Yukawa potential. The spin orbitals have been expanded as linear combinations of Gaussian basis functions, for which complete analytical formulas for all the required basic integrals are available. Both the ionization potential of the hydrogen anion and the behavior of the molecular properties of the hydrogen molecule have been analyzed in detail with extensive basis sets at the FCI level of theory. Finally, the growing importance of the electron correlation energy as the screening parameter increases has been demonstrated clearly by our calculations.

Alternant X2Y2 (X = N, P, As, and Sb and, Y = O, S, Se, and Te) cyclic planar and butterfly like isomers have been studied at the complete-active-space self-consistent-field (CASSCF) molecular orbital and second-order configuration-interaction (SOCI) levels of theory with optimized basis sets including f-functions. The salient geometrical features, and trends in the intraanular X−X bond are discussed along with the relative energies between the isomers. Rationalization of the trends found in the relative energy is based on examining the various forms and magnitudes of bond strain present in each isomer.

An improved Becke functional for the exchange energy is used in conjunction with various correlation functionals to calculate a number of properties of selected weakly bound complexes, for which experimental data are available. Comparison of the calculated properties with earlier calculations using both approximate density functional methods and second-order Moller-Plesset theory methods, with respect to experimental data, shows an overall improved prediction, at no extra computational cost. In particular, the performance of our method for the studied homomolecular hydrogen-bonded complexes is very encouraging.

In order to elucidate possible reaction paths to the formation of P−N bonds in interstellar space and to provide data for the interpretation of recent experiments, the reaction of P+ (3P) with methylamine has been investigated. In particular, the G2(MP2) methodology has been applied to the triplet potential energy surface of [H5, C, N, P]+. Numerous minima on this surface have been found, and transition state structures connecting those which lie along important reaction paths have been determined. The global minimum on the PES is the ion−molecule complex P+ NH2CH3, which lies 95.7 kcal/mol below the separated reactants. Barrier-free reaction paths leading to (PNH2+ + CH3), (PH + CH2NH2+), and atomic and molecular hydrogen abstraction have been found. In agreement with experimental results, these first two reactions appear to be favored.

The bπ.aσ charge-transfer complex formed by ethyne and chlorine monofluoride has been studied with various approximate pure and hybrid density functional methods and the second-order Moller−Plesset...

The transition energies and the oscillator strengths of both the ultraviolet and visible series of the helium atom have been calculated in a hot, weak coupling plasma environment, modeled in terms of the Yukawa potential. Our approach, based on the full configuration interaction method, is found to be superior to those based on the atomic many-body perturbation theory, for it converges smoothly to accurate vacuum values as the screening parameter approaches to zero and also behaves smoothly with respect to increasing screening. Thus, it could be useful for plasma diagnostic purposes.

A study of both the singlet and triplet potential energy surfaces of the [H3, P2]+ system has been carried out using the G2 procedure. Various stable structures along with a number of reaction mech...

G2 methodology has been used to characterize minima on both doublet and quartet potential energy surfaces of the PSi2 radical system. We found that for states with doublet spin multiplicity the most stable isomer is the cyclic 2A1. Linear isomers lie more than 24 kcal/mol above in energy. For the quartets the most stable state isomer is the cyclic 4A2, and the most stable linear isomer, i.e., Si-Si-P(4∑−), lies 10.68 kcal/mol higher in energy. The structural features of the various isomers characterized have been rationalized in terms of the bonding features of the molecular orbitals involved. Key words: ab initio, excited states, radical.

The dissociation energy of the Ti(OH2)+ ion–molecule complex was calculated by the multiconfigurational self-consistent field theory, coupled cluster theory, and two density functional theory based methods, using both all-electron basis sets and effective core potentials. The calculations show that approximate density functional theory gives results in better agreement with experiment than either the multiconfigurational self-consistent field theory or the coupled cluster theory, with both all-electron basis sets and effective core potentials. Nevertheless, the optimized geometries and harmonic vibration frequencies are very similar, irrespective of the level of theory used. The interconfigurational energy ordering of the two valence electronic configurations dn−1s and dn−2s2 of the 4F electronic state of the titanium cation were also calculated and are discussed. Key words: ab initio, dissociation energy, ion–molecule complex, effective core potentials, transition metals.

PLn+ clusters (n = 1, 2 and L = NH3, OH2, FH, PH3, SH2, CIH) in both their triplet and singlet states have been characterized by common approximate density functional methods, SVWN, BVWN, BLYP, and B3LYP. The phosphorus–ligand distances (R), dissociation energies (D0), triplet–singlet gaps (Δt−s), and several bond properties, such as the electron density (ρ(rc)), the Laplacian [Formula: see text] and the local energy density H(rc) at the bond critical point, were compared with those obtained by accurate ab initio molecular orbital theory, namely, second-order Møller–Plesset (MP2) and G2 theory. In general, it is observed that the local spin density approximation (SVWN) yields stronger bonds than ab initio molecular orbital theory. However, addition of gradient corrections to the exchange functional (BVWN) yields ion–molecule bonds that are too weak. Finally, taking account also of gradient corrections to the correlation functional (BLYP) leads to very close agreement with ab initio results. Among these functional, Becke's hybrid functional, B3LYP, best fit the second-order Møller–Plesset and G2 data, reproducing the qualitative trends observed for the above-mentioned properties of phosphorus clusters, except for [Formula: see text] This fit is particularly good for distances, dissociation energies, and electron densities at the bond critical point, and both methods show similar deviations of the values of binding energies and triplet–singlet gap with respect to the G2 data. Compared with our most accurate ab initio molecular orbital data, namely G2, significant overbinding for the singlets, larger for one-ligand than for two-ligand complexes, and significant overestimation of the triplet–singlet gap for one-ligand complexes is observed for both methods, namely, B3LYP and MP2. The deviations at the second-order Møller–Plesset level of theory are mainly due to the lack of quadratic configuration interaction (QCI) corrections, and this deficiency is also present to some extent in B3LYP. However, for larger clusters these corrections are smaller, therefore the B3LYP functional is expected to lead to accurate descriptions. Key words: DFT, Bader analysis, G2, ion–molecule complexes, phosphorus.

Equilibrium geometries, dipole moments, harmonic vibrational frequencies and atomization energies for a set of 32 small neutral molecules were calculated with the 6-31G ∗ standard basis set and approximate density functional theory methods. Comparison with experimental results confirmed that selecting properly the β parameter in the Becke's gradient corrected exchange functional results in a better overall fit for all the molecular properties studied. In particular the atomization energies are noticeably improved, and for other properties slightly better results are always obtained.

Accurate ab initio molecular orbital and density functional theories are used to study the structure of silyl isocyanate in the gas phase, in solution, and in the crystalline state. Calculations predict a linear backbone with a C3v symmetry for both the gas phase and in solution. Model calculations on the dimer and the trimer yield markedly bent structures because of acid−base interactions between different silyl isocyanate units. This result explains the Cs bent geometry found for this compound in the crystalline state. Lewis acids are also found to yield bent structures upon complexation on both the nitrogen and the oxygen, the former type of interaction being more stabilizing than the latter.

Ab initio calculations of phosphorus ionic clusters of the type PL+·L, PL2+·L, PL3+, and PL4+, with L = NH3, OH2, and FH, in both triplet and singlet states have been carried out, in order to determine their geometries and binding energies. The nature of the binding has also been extensively studied by means of both the topological analysis of the total electron charge density and the natural bond orbital analysis. When the ligand is bound as a second ligation shell through a formal hydrogen bond, significant cooperative effects between the two ligand shells have been detected. Substantial covalency is found for these hydrogen bonds, along with the weakening of the hydride bond with the inner ligand and the reinforcing of the bond between the inner ligand and the phosphorus. These processes are greatly favored for the singlet state of the phosphorus, and for the case of ammonia and water ligands, conversion reactions leading to PXHn-1·XHn+1+ complexes are observed. It has been found that the maximum numbe...

A theoretical study of the reaction of P{sup +} with ammonia, as a possible source of interstellar phosphorus-nitrogen compounds, has been carried out. The geometries of the possible products of this reaction, (HNP){sup +} and (H{sub 2}NP){sup +}, as well as of the relevant species on the triplet (H{sub 3}NP){sup +} potential energy surface have been optimized at the Hartree-Fock level. Relative energies were obtained through correlated calculations at the fourth-order Moller-Plesset theory. Calculations demonstrate that PNH{sup +} is the lowest lying (HNP){sup +} triplet state, with HPN{sup +} lying 50 kcal/mol higher at the correlated level. In the case of the (H{sub 2}NP){sup +} system, PNH{sub 2}{sup +} is found to be the ground state with trans-HPNH{sup +} and H{sub 2}PN{sup +} lying 44 and 93 kcal/mol, respectively, higher in energy at the correlated level. The PNH{sub 3}{sup +} ion-molecule complex is the lowest lying triplet (H{sub 3}NP){sup +} structure by a relatively high value, i.e. 40 kcal/mol. The stability order of this system is PNH{sub 3}{sup +} > HPNH{sub 2}{sup +} > H{sub 2}PNH{sup +} > H{sub 3}PN{sup +}. The reaction of P{sup +} with ammonia has been found to be endothermic in all cases except for themore » production of PNH{sub 2}{sup +}. In addition, two different mechanisms for producing PNH{sub 2}{sup +} that are barrier free have been found. Therefore, the reaction of P{sup +} with NH{sub 3}, leading to PNH{sub 2}{sup +}, seems to be a plausible interstellar process, since it is exothermic and also proceeds with no activation barrier.« less

Accurate ab initio G1 and G2 protocols have been used to characterize 14 (HnP C)+, with n = 1−4, species. We have predicted that the C-site protonation of the earlier characterized P C (2B1) species is preferred over the P-site by 1.73 eV at the G2 level. For the (H2P C)+ system, five stable molecular structures have been found for each of the singlet and triplet spin multiplicities. In particular, the (HP CH)+ singlet isomers do not present either biradicaloid character or P−C cross bonding, while both the (H2P C)+ and (P CH2)+ singlet molecules are almost pure biradical species with no P−C cross bonding. For every case the singlet state has been found to be more stable than the triplet. Finally, vibrational spectra of these species, which may help in their detection, are discussed.

The reaction of the iron oxide cation with the hydrogen molecule is studied in this work using the diffusion quantum Monte Carlo (DMC) method. Previous attempts achieved a qualitative agreement between theory and experiments, but not a satisfactory quantitative explanation which concurs with the experimental facts. The DMC method provides a consistent explanation of the experimental results all along the reaction path. This is due to the fact that DMC accounts for almost all of the correlation energy, which is of vital importance for the correct description of this reaction.

The natural orbital functional theory provides two complementary representations of the one-electron picture in molecules, namely, the natural orbital (NO) representation and the canonical orbital (CO) representation. The former arises directly from the optimization process solving the corresponding Euler equations, whereas the latter is attained from the diagonalization of the matrix of Lagrange multipliers obtained in the NO representation. In general, the one-particle reduced-density matrix (1-RDM) and the Lagrangian cannot be simultaneously brought to the diagonal form, except for the special Hartree-Fock case. The 1-RDM is diagonal in the NO representation, but not the Lagrangian, which is only a Hermitian matrix. Conversely, in the CO representation, the Lagrangian is diagonal, but not the 1-RDM. Combining both representations we have the whole picture concerning the occupation numbers and the orbital energies. The Piris natural orbital functional 5 leads generally to the localization of the molecular orbitals in the NO representation. Accordingly, it provides an orbital picture that agrees closely with the empirical valence shell electron pair repulsion theory and the Bent’s rule, along with the theoretical valence bond method. On the other hand, the equivalent CO representation can afford delocalized molecular orbitals adapted to the symmetry of the molecule. We show by means of the extended Koopmans’ theorem that the one-particle energies associated with the COs can yield reasonable principal ionization potentials when the 1-RDM remains close to the diagonal form. The relationship between NOs and COs is illustrated by several examples, showing that both orbital representations complement each other.

TM@ZniSi nanoclusters have been characterized by means of the Density Functional Theory, in which Transition Metal (TM) stands from Y to Cd, and i = 12 and 16. These two nanoclusters have been chosen owing to their highly spheroidal shape which allow for favored endohedral structures as compared to other nanoclusters. Doping with TM is chosen due to their magnetic properties. In similar cluster-assembled materials, these magnetic properties are related to the Transition Metal-Transition Metal (TM-TM) distances. At this point, endohedral doping presents a clear advantage over substitutional or exohedral doping, since in the cluster-assembled materials, these TM would occupy the well-fixed center of the cluster, providing in this way a better TM-TM distance control to experimentalists. In addition to endohedral compounds, surface structures and the TS’s connecting both isomers have been characterized. In this way the kinetic and thermal stability of endohedral nanoclusters is predicted. We anticipate that silver and cadmium endohedrally doped nanoclusters have the longest life-times. This is due to the weak interaction of these metals with the cage, in contrast to the remaining cases where the TM covalently bond to a region of the cage. The open-shell electronic structure of Ag provides magnetic properties to Ag@ZniSi clusters. Therefore, we have further characterized (Ag@Zn12S12)2 and (Ag@Zn16S16)2 dimers both in the ferromagnetic and antiferromagnetic state, in order to calculate the corresponding magnetic exchange coupling constant, J. This research was funded by Eusko Jaurlaritza (the Basque Government), and the Spanish Office for Scientific Research. The SGI/IZO-SGIker UPV/EHU (supported by Fondo Social Europeo and MCyT) is gratefully acknowledged for generous allocation of computational resources. JMM would like to thank Spanish Ministry of Science and Innovation for funding through a Ramon y Cajal fellow position (RYC 2008-03216). We thanks Elixabete Rezabal for cheerful discussion.

The electronic spin filtering capability of a single chiral helical peptide is measured. A ferromagnetic electrode source is employed to inject spin-polarized electrons in an asymmetric single-molecule junction bridging an α-helical peptide sequence of known chirality. The conductance comparison between both isomers allows the direct determination of the polarization power of an individual chiral molecule.

We report a joint photoelectron spectroscopy and theoretical study on AuC4−, AuC6−, and AuCnH− (n = 2, 4, and 6) using high-resolution photoelectron imaging and ab initio calculations. The ground state of AuC2H−, AuC4H−, and AuC6H− is found to be linear, while that of AuC4− and AuC6− is bent. All the species are found to be linear in their neutral ground states. The electron affinities (EAs) are measured to be 3.366(1) and 3.593(1) eV for AuC4 and AuC6, respectively. Both bending and stretching frequencies are resolved in the spectra of AuC4− and AuC6−. High-resolution data of AuCnH− reveal major vibrational progressions in the Au—C stretching and bending modes. AuC2H− has a ground state stretching frequency of 445(10) cm−1 and a bending frequency of 260(10) cm−1; AuC4H− has a ground state stretching frequency of 340(10) cm−1; AuC6H− has a ground state stretching frequency of 260(10) cm−1 and a bending frequency of 55(10) cm−1. The EAs are measured to be 1.475(1), 1.778(1), and 1.962(1) eV for AuC2H, AuC4...

In this paper, we present for the first time a description based on the natural orbital functional theory (NOFT) of the group VI dimers, namely, Cr(2), Mo(2) and W(2). The PNOF5, Piris Natural Orbital Functional, has been used throughout this work, and the results are compared to multireferential perturbation theory (CASPT2) results. Both methods have been combined with effective core potentials to take into account the scalar relativistic effects. In addition, for Cr(2), an all-electron TZVP quality basis set has also been used to recover the core-valence dynamical correlation. In all cases, PNOF5 shows better behavior than CASPT2, which needs a larger basis set to recover comparable amounts of dynamical correlation. PNOF5 is able to account for the non-dynamical electron correlation, which is responsible for the multireferential nature of these dimers. However, it does not fully recover the dynamical correlation, which is crucial for the accurate description of these challenging potential energy curves. Consequently, PNOF5 predicts longer equilibrium distances and lower dissociation energies than the experimental values. Unlike CASPT2, the PNOF5 results do not significantly improve by using larger basis sets. These new findings represent a major step in the NOFT development, since PNOF5 is the first functional of the natural orbitals reported to yield a chemically balanced and accurate description of these challenging transition metal dimers.

Hollow CdS nanoclusters were predicted to trap alkali metals and halogen atoms inside their cavity. Furthermore, electron affinities (EA) of endohedrally halogen doped clusters and ionization potentials (IE) of endohedrally alkali doped clusters were predicted to be very similar. This makes them suitable to build cluster-assembled materials, in the same vein as do related ZnO, ZnS and MgO nanoclusters, which yield porous solid materials. With this aim in mind, we have focused on the assembly of bare Cd(i)S(i) and endohedral K@Cd(i)S(i)-X@Cd(i)S(i) (i = 12, 16, X = Cl, Br) clusters in order to obtain solids with tailored semiconducting and structural properties. Since these hollow nanoclusters possess square and hexagonal faces, three different orientations have to be considered, namely, edge-to-edge (E-E), square-to-square (S-S) and hexagon-to-hexagon (H-H). These three orientations lead to distinct zeolite-like nanoporous bulk CdS solid phases denoted as SOD, LTA and FAU. These solids are low-density crystalline nanoporous materials that might be useful in a wide range of applications ranging from molecular sieves for heterogeneous catalysis to gas storage templates.

The excited states of the gold dimer have been investigated using modern theoretical tools including the multiconfigurational exact molecular mean-field intermediate Hamiltonian Fock-space Coupled Cluster, X2Cmmf-IHFSCC, and the complete active space self-consistent field followed by second order perturbation theory, CASSCF/CASPT2. The computed optically active transitions have been benchmarked against the available experimental data and compared with time-dependent density functional theory, TDDFT, results, both in the two- and four-component schemes. We explored in great detail several spectroscopic properties such as bond lengths, potential energy surfaces (PES), vibrational frequencies and vibrational progressions of the ground and low-lying excited states. Our data show excellent agreement with the experimental measurements and present a significant improvement compared to previous ab initio calculations. They also permit a detailed investigation of the intriguing a ← X and A′ ← X experimental bands that, according to our calculations, show an avoided energy level crossing. The location of this crossing is critical for a correct estimation of the vibrational progression and oscillator strengths of these two states. Moreover, among the exchange-correlation (xc) potentials, the SAOP gives the best excitation energies, followed by the hybrid B3LYP functional. Pure functionals like BLYP give by far the worst results. This journal is © the Owner Societies 2012., I.I. would like to thank the Minister of Science and Innovation for granting a Juan de la Cierva Fellowship. I.I., F.R. and J.M.U would like to thank the Eusko Jaurlaritza (GIC 07/85 IT-330-07) for financial support and the Spanish Office for Scientific Research (CTQ2011-27374). The work of M.D.M. on the gold dimer is supported by the National Science Foundation under grant No. CHE-0808984. K.R.G. acknowledges the support by CSIC under the JAE-DOC program Junta para la Ampliacion de Estudios co-financed by FSE, and in part, by the University of the Basque Country UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02-02).

Citrate is the main low mass molecule chelator of aluminum in serum, and knowledge of the interaction mode of this organic molecule with this cation is necessary to understand aluminum speciation in biosystems. However, the 1 ∶ 1 complexation of citric acid to Al(III) is a complex process due to the myriad of coordination sites and protonation states of this molecule. Moreover, due to the acidic character of the complex, its entire experimental characterization is elusive. The system is also challenging from a computational point of view, due to the difficulties in getting a balanced estimation of the large range of solvation free energies encountered for the different protonation states of a multiprotic acid in both situations, complexed and uncomplexed with a trivalent cation. Herein, the deprotonation process of the free citric acid in solution and that interacting with Al(III) have been investigated considering all possible coordination modes and protonation states of the citric acid. All the structures were optimized in solution combining the B3LYP density function method with the polarizable continuum IEFPCM model. In addition, different schemes have been employed to obtain reliable solvation energies. Taking into account the most stable isomer of each protonation state, the pKa values were computationally estimated for the free citric acid and that interacting with Al(III), showing a good agreement with the experimental data. All these results shed light on how the deprotonation process of the citric acid takes place, and show that Al(III) not only increases the acidity of the molecule, but also changes qualitatively the deprotonation pattern of the citric acid. This information is highly relevant to understand aluminum speciation in biological environments, for which citrate is the main low molecular weight chelator, and responsible for its cellular in-take.

The electron pair density of a core-valence separable system can be decomposed into three parts: core-core, core-valence, and valence-valence. The core-core part has a Hartree-Fock like structure. The core-valence part can be written as Γcv (1,2) = γc (1,1)γv (2,2) − γc (1,2)γv (2,1) + γc (2,2)γv (1,1) − γc (2,1)γv (1,2), where only the 1-matrices from the core and valence orbitals contribute. The valence-valence part is left to be determined from the reduced frozen-core type wave function, which often contains the essential information on the electron correlation and the chemical bond. We demonstrate the analysis to the ground state of negative ion Li− and 21Σu+ excited state of the Li2 molecule. © 2012 Wiley Periodicals, Inc.

The possibility for an Al-superoxide complex to reduce Fe(III) to Fe(II), promoting oxidative damage through the Fenton reaction, is investigated using highly accurate ab initio methods and density functional theory in conjunction with solvation continuum methods to simulate bulk solvent effects. It is found that the redox reaction between Al-superoxide and Fe(III) to produce Fe(II) is exothermic. Moreover, the loss of an electron from the superoxide radical ion in the Al-superoxide complex leads to a spontaneous dissociation of molecular oxygen from aluminum, recovering therefore an Al(3+) hexahydrated complex. As demonstrated in previous studies, this complex is again prone to stabilize another superoxide molecule, suggesting a catalytic cycle that augments the concentration of Fe(II) in the presence of Al(III). Similar results are found for Al(OH)(2+) and Al(OH)(2)(+) hydrolytic species. Our work reinforces the idea that the presence of aluminum in biological systems could lead to an important pro-oxidant activity through a superoxide formation mechanism.

Natural orbital functional theory (NOFT) is used for the first time in the analysis of different types of chemical bonds. Concretely, the Piris natural orbital functional PNOF5 is used. It provides a localization scheme that yields an orbital picture which agrees very well with the empirical valence shell electron pair repulsion theory (VSEPR) and Bent's rule, as well as with other theoretical pictures provided by valence bond (VB) or linear combination of atomic orbitals-molecular orbital (LCAO-MO) methods. In this context, PNOF5 provides a novel tool for chemical bond analysis. In this work, PNOF5 is applied to selected molecules that have ionic, polar covalent, covalent, multiple (σ and π), 3c-2e, and 3c-4e bonds.

We report on the interaction between ZnS quantum dots and several surface ligands by means of pure Quantum Mechanical (QM) and hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) methods. To shed light on the nature of the interaction, we focus our discussion on the structural and energetic aspects. The Zn6S6 cluster has been chosen to model the quantum dot core, while trimethylamine (NMe3), trimethylphosphine (PMe3), trymethylphosphine oxide (OPMe3), methanol (MeOH), methanethiol (MeSH), and methaneselenol (MeSeH) have been employed to model the passivating ligands. Our results concerning the interaction between the cluster and one ligand of each type reveal that NMe3, PMe3, and OPMe3 show a significantly greater affinity to Zn6S6 than MeOH, MeSH, and MeSeH. We noticed that the softer the heteroatom of the ligand bonded to the cluster, the greater the interaction energy. A comparative study of different amines shows that the interaction is strengthened with the number and the length of the alkyl substituents in the ligand. We demonstrated that the interaction is mainly electrostatic, even if an important polarization of the charge density is observed. Fully passivated complexes have also been investigated, and our calculations point out that the bond is weaker than in the complexes with a single bonded ligand, suggesting that the repulsive interactions between the ligands and the diminished charge acceptor capacity of the cluster come into play.

Serum transferrin (sTf) carries iron in blood serum and delivers it into cells by receptor-mediated endocytosis. The protein can also bind other metals, including aluminum. The crystal structures of the metal-free and metal-loaded protein indicate that the metal release process involves an opening of the protein. In this process, Lys206 and Lys296 lying in the proximity of each other form the dilysine pair or, so-called, dilysine trigger. It was suggested that the conformational change takes place due to variations of the protonation state of the dilysine trigger at the acidic endosomal pH. In 2003, Rinaldo and Field (Biophys. J. 85, 3485-3501) proposed that the dilysine trigger alone can not explain the opening and that the protonation of Tyr188 is required to prompt the conformational change. However, no evidence was supplied to support this hypothesis. Here, we present several 60 ns molecular dynamics simulations considering various protonation states to investigate the complexes formed by sTf with Fe(III) and Al(III). The calculations demonstrate that only in those systems where Tyr188 has been protonated does the protein undergo the conformational change and that the dilysine trigger alone does not lead to the opening. The simulations also indicate that the metal release process is a stepwise mechanism, where the hinge-bending motion is followed by the hinge-twisting step. Therefore, the study demonstrates for the first time that the protonation of Tyr188 is required for the release of metal from the metal loaded sTf and provides valuable information about the whole process.

Radical formation through hydrogen abstraction and C-C and O-O homolytic bond cleavage from selected molecules is investigated by use of natural orbital functional theory in its PNOF5 natural orbital functional implementation, and the results are compared to high-level ab initio complete active space self-consistent field (CASSCF) and complete active space with second-order perturbation theory (CASPT2) methods and experimental data. It is observed that PNOF5 is able to treat the strong electron correlation effects along the homolysis of X-H (X = C, N, O) and X-X (X = C, O) bonds, leading, in general, to the correct trends in the corresponding bond strengths and a good description of the resultant electronic structure for these radicals. In general, PNOF5 bond energies are lower than the experimental ones, because of partial lack of dynamical electron correlation. However, the part of dynamical electron correlation recovered by PNOF5 allows it to give more accurate results than CASSCF methods with a minimum window required to treat near-degeneracy effects. In addition, inspection of the natural orbital occupancies with respect to the CASSCF ones shows an outstanding performance of PNOF5 in treating degenerate and quasidegenerate states, giving a correct description of diradicals and diradicaloids formed upon C-C cleavage in cyclopropane and derivatives.

Theoretical and experimental studies on the cycloaddition between methoxyketene and a model conjugated imine predict the preferential formation of the [2+2] cycloadduct. A configuration interaction level of theory is required to reproduce correctly the observed periselectivity.

Theoretical studies on the Staudinger reaction between methoxyketene and a simple imidate provide an explanation for the trans stereoselectivity observed. The key step responsible for this unusual outcome is the facile interconversion between two zwitterionic intermediates through a biradical transition state.

An ab initio molecular orbital study has been carried out for the C 2 P radical, a molecule which could be of astrophysical interest, on the doublet and quartet surfaces. Geometries have been obtained at the MP2/6-31G * level, whereas MP4 theory has been employed to compute electronic energies. We have found that the CPC isomer lies quite high in energy, but there are two states quite close in energy: linear CCP ( 2 Π state) and cyclic C 2 P ( 2 B 2 state). We predict linear CCP to be the ground state, but with cyclic C 2 P lying just about 6 kcal/mol above

The different modes of interaction between achiral ketenes and homochiral aldimines derived from chiral aldehydes to give the two possible diastereoisomers of cis-β-lactams have been examined using the AM 1 method. It has been found that the step which determines the final stereochemistry is the conrotatory ring closure between C 3 and C 4 atoms. The high diastereoselectivity predicted by our calculations is in excellent agreement with available experimental data. The origin of this high stereocontrol is rationalized on the basis of the stabilizing interaction between the C-X a orbital (X being an electronegative atom) and the p atomic orbital of the C 3 atom

Calculations of the Hartree-Fock and local-density approximation exchange energies for all atoms with Z

Serum transferrin (sTf) transports iron in serum and internalizes in cells via receptor mediated endocytosis. Additionally, sTf has been identified as the predominant aluminum carrier in serum. Some questions remain unclear about the exact mechanism for the metal release or whether the aluminum and iron show the same binding mode during the entire process. In the present work, simulation techniques at quantum and atomic levels have been employed in order to gain access into a molecular level understanding of the metal-bound sTf complex, and to describe the binding of Al(III) and Fe(III) ions to sTf. First, hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations were carried out in order to analyze the dynamics of the aluminum-loaded complex, taking into account the different pH conditions in blood and into the cell. Moreover, the complexes formed by transferrin with Al(III) and Fe(III) were optimized with high level density functional theory (DFT)/MM methods. All these results indicate that the interaction mode of Al(III) and Fe(III) with sTf change upon different pH conditions, and that the coordination of Al(III) and Fe(III) is not equivalent during the metal intake, transport and release processes. Our results emphasize the importance of the pH on the metal binding and release mechanism and suggest that Al(III) can follow the iron pathway to get access into cells, although once there, it may show a different binding mode, leading to a different mechanism for its release.

Heteronuclear dimers like CrMo, CrW, MoW, VCr, VMo, VW, and their anions have been investigated by means of multiconfigurational quantum chemistry methods, using the complete active space self-consistent field followed by second-order perturbation theory, CASSCF/CASPT2. We explored in great detail several spectroscopic properties such as bond length, potential energy surfaces, dissociation energies, ionization potentials, electron affinities, low-lying excited states, vibrational frequencies, and dipole moments. All proposed dimers show ground states with a pronounced multireference character. The group VI heterodimers have a (1)Σ(+) ground state, while the mixed group V-group VI heterodimers show a (2)Δ ground state. Among all dimers, only VCr presents a potential energy profile with a deep minimum in the d-d region and a shelf-like potential in the s-s region. All the remaining dimers show only the short-range minimum. The largest effective bond order is obtained for the MoW, with a value of 5.2, that is, a weak sextuple bond. Most of the obtained results are valuable tools to drive future experimental investigations.

The excitation energies of small ZnS nanoclusters characterized in previous studies have been calculated using TDDFT. The relativistic pseudopotentials of Stevens et al have been used, including Zn 4s(2) electrons and S 3s(2) and 3p(4) electrons as valence electrons. Results obtained with these pseudopotentials are compared to those obtained considering also Zn 3s(2)3p(6)3d(10) electrons in the valence part, and demonstrated to be consistent. The results show that spheroid-like bubble structures have absorption energies in the range of 5-5.3 eV for small sizes, which decreases to 5 eV with increasing particle size.

An explicit formulation of the Piris cumulant λΔ,Π matrix is described herein, and used to reconstruct the two-particle reduced density matrix (2-RDM). Then, we have derived a natural orbital functional, the Piris Natural Orbital Functional 5, PNOF5, constrained to fulfill the D, Q, and G positivity necessary conditions of the N-representable 2-RDM. This functional yields a remarkable accurate description of systems bearing substantial (near)degeneracy of one-particle states. The theory is applied to the homolitic dissociation of selected diatomic molecules and to the rotation barrier of ethylene, both paradigmatic cases of near-degeneracy effects. It is found that the method describes correctly the dissociation limit yielding an integer number of electrons on the dissociated atoms. PNOF5 predicts a barrier of 65.6 kcal/mol for the ethylene torsion in an outstanding agreement with Complete Active Space Second-order Perturbation Theory (CASPT2). The obtained occupation numbers and pseudo one-particle energies at the ethylene transition state account for fully degenerate π orbitals. The calculated equilibrium distances, dipole moments, and binding energies of the considered molecules are presented. The values obtained are accurate comparing those obtained by the complete active space self-consistent field method and the experimental data.

Natural orbital functional theory (NOFT) in its PNOF3 and PNOF4 implementations is used to investigate the potential energy surface (PES) of ethylene torsion. This rearrangement is taken as a case study to illustrate the satisfactory performance of the Piris Natural Orbital Functional (PNOF) toward rearrangements sensible to non-dynamical electron correlation or near-degeneracy effects. We show that the fulfillment of the G-positivity condition, as implemented in PNOF4, is key to yield a correct description of structures with significant diradical character. We have found that PNOF4 yields an accurate description of these delicate PES and predicts the correct trends of the occupation numbers as compared with multiconfigurational wavefunction methods.

We consider the problem of constructing kinetic energy functionals in density functional theory. We first discuss the functional generated through the application of local-scaling transformations to the exact analytic wavefunctions for the Hookean model of 4He (a finite mass three-particle system), and contrast this result with a previous one for ∞He (infinite mass system). It is shown that an exact non-Born-Oppenheimer treatment not only leads to mass-correction terms in the kinetic energy, but to a basically different functional expression. In addition, we report and comment on some recently advanced approximate kinetic energy functionals generated in the context of the constraint-based approach to orbital-free molecular dynamics. The positivity and non-singularity of a new family of kinetic energy functionals specifically designed for orbital-free molecular dynamics is discussed. Finally, we present some conclusions.