Corrosion and weathering of uranium dioxide (UO2) is a serious concern in a broad range of technological and environmental systems. Oxidation of UO2 can compromise the integrity of nuclear fuel rods, as well as result in the bioavailability of uranium in contaminated ground water at mines, mills, and nuclear waste storage facilities. How the oxidation proceeds, however, is not well understood. In this work, density-functional theory and ab initio thermodynamics are used to delineate the initial stages of surface and subsurface oxidation of UO2 at the (111) surface as a function of temperature and oxygen pressure. Initially, chemisorption of oxygen on the clean stoichiometric surface results in formation of highly stable triple-bonded uranyl groups and oxidation of the topmost uranium atoms to U6+ at a minimal p(O2) near 0K. Once the surface is saturated with uranyl groups and the oxygen chemical potential increases above −1.0eV, subsurface oxidation becomes thermodynamically favored. The degree of oxidation of the subsurface uranium atoms is determined by quantifying the transfer of electrons from the localized U 5f bands to those dominated by the delocalized O 2p bands as oxygen atoms occupy octahedral interstitial sites in the UO2 lattice. Occupation of the octahedral site nearest the surface results in an expansion of the lattice, whereas movement of the oxygen interstitial to deeper sites results in a net contraction.
The six germyl cations FnGe(OH)(3-n)(+) and FnGe(NH2)(3-n)(+) (n = 0-2), recently observed in the gas phase from the cascade reactions between GeF3+ and H2O or NH3 (J. Mass Spectrom. 46 (2011) 465-477), were theoretically investigated by MP2 and CCSD(T) calculations. The bond distances and the electronic properties of these ions, and the stabilities of their complexes with H2O, NH3, and HF are regularly affected by the nature of the substituents. In particular, any Ge-X bond (X = F, OH, NH2) becomes progressively shorter by increasing the electronegativity of the two adjacent groups, and the most electronegative substituents produce also highest atomic charges at the Ge atom. In addition, replacing F with OH and NH2, the thermochemical stability of the cations substantially increases. This reflects in Lewis acidities of the F2Ge(X)(+) and FGe(X)(2)(+) (X = OH, NH2) which are invariably lower than GeF3+. The mechanisms of formation of the FnGe(OH)(3-n)(+) and FnGe(NH2)(3-n)(+) (n = 0-2) were also investigated and found to be describable by the initial formation of encounter complexes between the reactant ions GeF3+, F2Ge(X)(+) and FGe(X)(2)(+) (X = OH, NH2) and H2O or NH3, which eventually dissociate into FnGe(OH)(3-n)(+) and FnGe(NH2)(3-n)(+) (n = 0-2) and HF passing through two consecutive isomerizations. (C) 2012 Elsevier B.V. All rights reserved.
A review of our work on phase transitions, coexistence and crystal growth dynamics in ionic nanoclusters is presented. The foundations and limitations of the proposed models are discussed and perspectives for extended treatments are given. Additionally, supported on a compilation of the asymptotic behaviour of the properties towards bulk conditions, new results concerned with the operational meaning of the thermodynamic limit are also presented. Some topics are complemented with link references to on-line animations that provide a visualisation of the focused behaviours. The simulations were carried out by molecular dynamics on KCl, NaCl, LiCl and NaI clusters.
Segmented all-electron contracted double zeta valence plus polarization function (DZP) basis sets for the element Pt were constructed for use in conjunction with the non-relativistic and Douglas–Kroll–Hess (DKH) Hamiltonians. The DZP–DKH set is loosely contracted and thus offer computational advantages compared to the generally contracted relativistic basis sets, while their sufficiently small size allows it to be used in place of effective core potentials (ECP) for routine studies of molecules. Using the one-parameter hybrid functional mPW1PW, the performance of the basis sets is assessed for predicting the molecular structures and atomic charges of platinum(II) antitumor drugs, cisplatin and carboplatin. These results can be used as reference values to calibrate further ECP calculations. Despite their compact size, the DZP sets demonstrate consistent, efficient, and reliable performance and will be especially useful in calculations of molecular properties that require explicit treatment of the core electrons.
The stereoelectronic interactions governing the conformational isomerism of 2-halocyclohexanones have been investigated by using an isodesmic reaction model. It has been found that 2-axial halogenation of cyclohexanone is thermodynamically favoured, whilst insertion of an equatorial bromine or iodine is not. Overall, inclusion of axial halogens in cyclohexanone is preferred to equatorial entrance and, according to NBO calculations, this behaviour is due to electron donation from nonbonding and C–X orbitals to πCO∗ antibonding orbital, in addition to steric and electrostatic effects.
The thermal Curtius rearrangement of benzoyl azide in the presence of Lewis acids has been studied by DFT (PBE/TZ2P) method. The complexation of Lewis acids (BF3, AlCl3, SbCl5) with benzoyl azide leads to the formation of 1:1 and 1:2 stable complexes with interaction of catalysts with O and N atoms of carbonyl azide group. The potential energy surfaces of the catalytic rearrangement have been calculated for each complex and the relation between the complexes and the transition states on potential energy surface have been established by IRC calculation. The energy barriers for catalytic reactions are significantly lower in the most cases in comparison with an uncatalyzed reaction. The activation energy is decreasing in the range of Lewis acids AlCl3>SbCl5>BF3 and it correlates with the decreasing of Lewis acids strength. The Mulliken bond population analysis has been done for three compounds RCON3 (R=H, Me, Ph) and for their complexes, and for all corresponding transition states using the B3LYP/6-311G∗ approximation. The interaction of Lewis acids with carbonyl azide group causes the decreasing of N1N2 bond strength and it helps the thermal Curtius rearrangement to proceed.
Mayer’s energy decomposition method was applied inthe study of the relative stability of cis and trans isomers of 1,2-disubstituted ethylenes, XHCCHX (X=F, Cl, Br) and 2-butene. The trans to cis isomerization energy for each system was determined at the Hartree–Fock level, with several basis sets, and then divided into monoatomic and diatomic energy contributions. The results point to a different energy distribution for the dihaloethylenes, known for exhibiting a cis isomer that is more stable than the trans one, a behavior that is known as the cis effect, when compared to 2-butene. The main stabilizing effects of the cis isomer in the dihaloethylenes, at this level of theory, are energy terms associated with the interaction of the X substituents with the C atoms.
A computational study predicts a number of unusual Be- and Mg-containing compounds with general formula X–M–N2–Li (X=F, Cl, Br; M=Be, Mg). Generally, the X–Be–N2–Li species were found to be energetically stable with respect to the LiX+Be+N2 fragments and with respect to the LiBeX+N2 fragments, whereas the Mg-containing species by comparison were found to be unstable. Harmonic vibrational frequencies and various bonding parameters were also computed and found useful in rationalizing the relative stabilities and trends (for varying X) of these unusual compounds. The high stability of X–Be–N2–Li is thought to be due mainly to strong electrostatic interactions between the constituent atoms and especially the Be atom in its +2 valence state.
DFT calculations show that 1-(pyridin-2-yl)-5-(quinolin-2-yl)pentane-2,4-dione tautomeric form is unstable. On the other hand, transfer of two methylene protons to the carbonyl oxygen (resulting in the formation of the enol) and to the quinoline nitrogen atoms (resulting in the formation of enaminone) seemed to proceed spontaneously. HOMA and NICS(1) parameters were used to follow the changes in aromaticity of the (quasi)rings in the tautomeric forms considered.
The accurate quantum-chemical computations based on DFT, Hartree-Fock and second-order Moller-Plesset (MP2) methods have been performed for the first time to study the interaction of the Cu2+ ion and [Cu(H2O)(1-4)](2+) complexes with trans-resveratrol, its cis-stereoisomer, phospholipid and deoxythymidine 5'-monophosphate in vacuum and water medium. On the basis of the interaction energies we have demonstrated that O3 and O5 oxygen atoms of the stereoisomers of resveratrol form the most stable chelate complexes with Cu2+. It has been shown that the capacity of trans-resveratrol to chelate Cu2+ is higher than that of cis-resveratrol. The trans-resveratrol-Cu2+-phospholipid [trans-resveratrol-Cu(H2O)(4)-phospholipid](2+) trans-resveratrol-Cu2+-deoxythymidine 5'-monophosphate and [trans-resveratrol-Cu(H2O)(4)-deoxythymidine 5'-monophosphate](2+) systems in which Cu2+ and [Cu(H2O)(4)](2+) aqua complex coordinate to the negatively charged oxygen atoms of 5'-monophosphate group of deoxythymidine 5'-monophosphate and phospholipid are characterised by the lowest interaction energy. We have proved that the hydration of the Cu(II) ion by four water molecules contributes to the dramatic reduction of the energetical stability of all the complexes studied. It was pointed out that in aqueous medium the stability of all complexes is significantly higher than in vacuum. The result suggests that trans-resveratrol-Cu(II) and [trans-resveratrol-Cu(H2O)(4)](2+) complexes can bind with negatively charged oxygen atoms of 5'-monophosphate anion of phospholipids which form a polar shell surrounding LDL particles. Furthermore, it has been found that trans-resveratrol-3-O-Cu(II) and trans-resveratrol-5-O-Cu(II) complexes are capable of forming the most stable complexes with deoxythymidine 5'-monophosphate and phospholipid. The findings obtained satisfactorily explain the experimental data and give insight into understanding of therapeutical and biological activity of trans-resveratrol-Cu2+ complexes. (C) 2010 Elsevier B.V. All rights reserved.
The merits and limitations of a new formula developed for the standard enthalpy of formation of perfect gas molecules, Delta H(f)degrees = Sigma(K)F(K.) + ZPE + (H(T) - H(0)) - Sigma(k>l)epsilon(kl) - (CNE - E(nb)(KL)), are illustrated by numerous examples and comparisons with experimental results. F(K.),F(L.)... are invariable parameters derived from the energies of the chemical bonds found in the hypothetical electroneutral precursors K degrees, L degrees... of the free radicals K., L degrees.. In no way do these F parameters depend on one another. ZPE + H(T) - H(0) is the familiar zero-point plus heat-content energy of the molecule: 41 is the intrinsic bond energy connecting K and L in the ground-state molecule, to be calculated from the NMR chemical shifts of atoms k is an element of K degrees and l is an element of L degrees,... etc. CNE-the so-called 'Charge Neutralization Energy'-takes care of the fact that K and K. (as well as L and L, etc.) are usually not isoelectronic. Finally, E(nb)(KL). stands for the sum of nonbonded interactions over all pairs of groups K and L, not counting similar interactions within the fragments K and L themselves. Considerable simplifications stem from the fact that in many situations CNE - E(nb)(KL) similar or equal to 0 is an acceptable approximation, greatly facilitating the calculation of Delta H(f)degrees, near experimental accuracy, even though detailed information regarding the electronic structure of large pluriatomic groups may be lacking. (C) 2009 Elsevier B.V. All rights reserved.
[1,3]-Sigmatropic rearrangement of 2-vinylsilirane (2-vinylsilacyclopropane) to silacyclopent-3-ene has been studied for the first time with quantum chemical methods. The PBE/TZ2P and B3LYP/6-31G(d) levels of theory have been used to explore the potential energy surface of the title reaction. The energies of the stationary points have also been evaluated with the G3(MP2)//B3LYP method. Several transition states corresponding to suprafacial and antarafacial pathways of the rearrangement have been found. The lowest energy pathway is a symmetry-allowed suprafacial sigmatropic shift with G3(MP2)//B3LYP activation energy of 97kJ/mol.
A simple coarse grained description of the electron density changes in molecular systems due to change in external potential, which may include the effect of external electric fields in addition to the potential due to the nuclei, has been proposed in terms of the induced atom–atom charges and atomic dipoles. The density functional perturbation theory has been used for deriving the expressions for the interaction energy and the effective chemical potentials in terms of these coarse grained variables. A route to the calculation of these quantities and hence the dipole polarizability of the molecular system is provided. The proposed approach would also be useful for obtaining polarizable charge based force field for intermolecular interaction in computer simulation.
The dimerization and trimerization of thioformaldehyde as well as the dimerization of thioketene has been studied using G3(MP2) calculations. The investigations have elucidated the reaction mechanisms. The activation Gibbs energy of the trimerization of thioformaldehyde has been determined as 118.1kJ/mol and that of the dimerization of thioketene as 139.2kJ/mol. The trimerization of thioformaldehyde is shown to proceed through an open chain dimer with the activation Gibbs energy 74.0kJ/mol. The results explain that the direct dimerization of thioformaldehyde to 1,3-dithietane is not experimentally observed.
Nucleophilic epoxide ring opening reactions can be effectively carried out with BF3 but not with BH3. This has been traced to the formation of an intermediate with an intramolecular dihydrogen bond which prefers to yield a species with frustrated Lewis acid–base pair after liberating H2 molecule in the BH3 catalyzed system. Quantum chemical analysis indicates that the H2 liberation is more favorable by 37.8kcal/mol in comparison to the desired aminoalcohol formation.
The conformations of cycloheptanone and cycloheptanethione were studied at the B3LYP and CCSD(T) levels of theory using the 6–311+G(d,p) basis set. Both molecules are remarkably similar in their conformational properties and are characterized by the presence of a broad potential well centred around the symmetrical twist-chair conformation. Pseudorotational barriers are found to be higher than in cycloheptane, whereas the transition from the chair family to the twist-boat conformation requires less energy.
Density functional theory (B3LYP) calculation is used to determine the gas-phase proton affinities of four series of nano-size diamines (1–2.3 nm) including –(CH2)n–C60–(CH2)n–, –(C6H4)n–, –(CHCH)n–, and –(CH2)n– spacers, where the maximum value of n is 5, 5, 9 and 15, respectively. The results showed that, in the case of diamines with two former spacers the first proton affinity, PA1, decreases by increasing the value of n, while in the case of two latter diamines it increases by increasing the value of n. However, the second proton affinity, PA2, and proton overallaffinity, PAov, of all above molecules, as expected, increases with increasing the value of n. Among all above compounds the greatest amounts of the PA1 and PA2 were calculated for H2NCH2–C60–CH2NH2 and H2N–(CH2)15–NH2 molecules, respectively. It seems that the C60 molecule has an increasing effect on PA1 of diamines, while a saturated long aliphatic chain which separates well the positively charged nitrogen atoms has an increasing effect on PA2 of diamines. The above results has led us to design new nano-size superbases with –N(CH3)2 or –NC{N(CH3)2}2 basic groups on fullerene molecule. The amounts of PA1, PA2 and PAov, for one of these superbases are 1126.3, 786.6 and 1912.9 kJ mol−1, respectively, indicating that it is one of the strongest superbases known so far.
The adsorption of Agn (n=2,4,8) clusters on the partially reduced rutile (110) surface have been studied using DFT slab calculations. The geometry and electronic structure of silver clusters adsorbed near the oxygen vacancy as well as binding mechanism have been investigated. It has been found that the binding occurs through two types of interaction. Firstly, it is the interaction between cluster’s HOMO and surface eigenstates located on hollow-Ti atoms and, secondly, the overlap of MOs of the cluster with bridging oxygen atoms. The first type leads to charge transfer from the adsorbed cluster to the surface, therefore silver particles become positively charged upon adsorption. The second type leads to the deformation of adsorbed silver cluster, thereby increasing the overlap. The interaction between molecular orbitals of the cluster and orbitals of atoms lying at the vacancy is unlikely. It has been shown that the adsorption energy of Agn clusters on stoichiometric surface are higher than those for reduced surface. Thus, the adsorption of silver clusters occurs preferably on stoichiometric surface far from oxygen vacancies.
We show that our procedure of constructing excited-state energy functionals by splitting k-space, employed so far to obtain exchange energies of excited-states, is quite general. We do so by applying the same method to construct modified Thomas-Fermi kinetic energy functional and its gradient expansion up to the second order for the excited-states. We show that the resulting kinetic energy functional has the same accuracy for the excited-states as the ground-state functionals do for the ground-states.
Spin-polarized Conceptual DFT descriptors, such as the sum of the spin potentials and the spin-philicity index, are used to characterize the vertical and adiabatic singlet–triplet energy separation of silylenes and p-benzynes. In addition, an (approximately) intrinsic stability scale is constructed for both systems, using a model linking bond dissociation enthalpies to chemical concepts such as the electrophilicity index, Pauling’s electronegativity and the stability of the individual radical fragments, used with success in a recent contribution for radical systems. The new stability scales are compared to other scales in the literature and are correlated with the singlet–triplet gaps and the electrophilicity index values for the examined compounds, investigating the relationship between reactivity and stability.
