Trends In Relative Stabilities Research Articles

BiSbWO6: Properties of a mixed 5s/6s lone-pair-electron system

• Stereochemical activity of lone-pair electrons as an alloy-design principle. • BiSbWO 6 alloys share narrow thermodynamic stability range. • Sb/Bi lone-pair effects on electronic structure identified and decoupled. • WO 3 used as a control model to isolate role of mixed lone-pair electrons. • Local distortions due to Sb 5s lone-pair electrons upshift conduction band edges. We investigate the behavior of lone-pair electrons in a mixed Sb(5s)/Bi(6s) crystal-environment. Density functional theory is used to calculate the electronic properties of Sb-alloyed-Bi 2 WO 6 and to study the effects of introducing Sb 5s orbitals to the band structure. The band edge positions, partial charge analyses, and band decomposed charge densities of BiSbWO 6 are used to explain the observed trends in relative stabilities and band edge shifts. To isolate the role of the mixed lone-pair, we considered WO 3 as a control model. We find that local distortions caused by Sb 5s lone-pair electrons lead to upshifts in both valence and conduction band edges.

Solvation of actinide salts in water using a polarizable continuum model.

In order to determine how actinide atoms are dressed when solvated in water, density functional theory calculations have been carried out to study the equilibrium structure of uranium plutonium and thorium salts (UO2(2+), PuO2(2+), Pu(4+), and Th(4+)) both in vacuum as well as in solution represented by a conductor-like polarizable continuum model. This information is of paramount importance for the development of sensitive nanosensors. Both UO2(2+) and PuO2(2+) ions show coordination number of 4-5 with counterions replacing one or two water molecules from the first coordination shell. On the other hand, Pu(4+), has a coordination number of 8 both when completely solvated and also in the presence of chloride and nitrate ions with counterions replacing water molecules in the first shell. Nitrates were found to bind more strongly to Pu(IV) than chloride anions. In the case of the Th(IV) ion, the coordination number was found to be 9 or 10 in the presence of chlorides. Moreover, the Pu(IV) ion shows greater affinity for chlorides than the Th(IV) ion. Adding dispersion and ZPE corrections to the binding energy does not alter the trends in relative stability of several conformers because of error cancelations. All structures and energetics of these complexes are reported.

An atoms-in-molecules study on selected tautomeric triads

A new method of investigating tautomeric equilibria is described. The method is based on the partition of the total molecular electronic energies among atoms, according to the quantum theory of the atoms in molecules. We employ our methodology to study several model tautomeric systems, so called tautomeric triads. The differences in the atomic energies between tautomers are exhaustively discussed and general trends in relative stabilities of C, S, N and O atoms calculated in different molecular environments are presented.

Comparative semiempirical and DFT study of styrylnaphthalenes and styrylquinolines and their photocyclization products

AbstractSemiempirical molecular orbital (PM3, PM6, and RM1) and density functional theory (DFT) (B3LYP/6‐31G*) studies are carried out for 1‐ and 2‐styrylnaphthalenes and their aza‐derivatives—2‐ and 4‐styrylquinolines. Relative stabilities of three isomeric forms: E‐ and Z‐isomers and the closed‐ring dihydrocyclophotoproduct (derivative of dihydrophenanthrene) are calculated. Compared to PM3, PM6 and especially RM1 understate heats of formation; in some cases, PM6 and RM1 even place Z‐isomer in energy below E‐isomer. PM3 rather close to DFT predicts heats of isomerization reaction, whereas PM6 and especially RM1 underestimate these values. Semiempirical methods in comparison with DFT markedly underestimate heats of cyclization reaction; however, reproduce trends in relative stabilities of different isomers in dependence on the structure of styrylnaphthalenes and styrylquinolines. Qualitative correlation is found between calculated relative stabilities of the closed‐ring forms (heats of cyclization reaction) and experimental data: cyclized products with low heats of cyclization are observed in steady‐state photolysis and those with high heats of cyclization are not. In the latter case, the closed‐ring compounds, if formed in the excited state, due to thermal instability decompose rapidly with ring opening in the ground state that prevents their observation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

CONFORMATIONAL EQUILIBRIUM AND SPECTROSCOPIC PROPERTIES OF CALIX[4]ARENE: THEORETICAL STUDY USING <i>AB INITIO</i> METHOD

Molecular structures were optimized for the calix[4]arene by ab initio method at the Hartree-Fock level of theory using LANL2DZ and 6-311G basis sets. Conformational equilibrium of four calix[4]arene conformers are reported. The results are compared with experiment, force field, and semiempirical molecular orbital calculations. General trends in relative stabilities of calix[4]arene decrease in following order: cone > partial-cone > 1,2-alternate > 1,3-alternate. The most stable conformer is the cone conformer that is stabilized by an array of four hydrogen bonds and these results agree with the reported experimental observations. All structures were analyzed using theoretical IR, UV-Vis, and 1H NMR spectra attributed to the conformational equilibrium at the Hartree-Fock level of theory using LANL2DZ basis set. Keywords: ab initio calculation, calix[4]arene, conformations, cone

STUDY ON THE CONFORMATIONS OF p-(NITRO)METHOXYCALIX[4]ARENE AND p-(tert-BUTYL)METHOXYCALIX[4]ARENE USING HIGH LEVEL AB INITIO METHOD

Ab initio calculations have been performed in order to investigate the conformational characteristics of p-(nitro)methoxycalix[4]arene and p-(tert-butyl)methoxycalix[4]arene. The structures of four types (cone, partial cone, 1,2-alternate and 1,3-alternate) of conformers for each compound have been optimized by ab initio method at the restricted Møller-Plesset fourth-order perturbation (RMP4) level of theory using 6-311G and 6-311G++(d,p) basis sets. General trends in relative stabilities of p-(nitro)methoxycalix[4]arene and p-(tert-butyl)methoxycalix[4]arene are similar and decrease in following order: partial cone > cone > 1,3-alternate > 1,2-alternate. The calculated results of the most stable conformation of partial cone structure agree with the reported NMR experimental observations. Keywords: ab initio, RMP4, conformational characteristics, p-(nitro)methoxycalix[4]arene, p-(tert-butyl)methoxycalix[4]arene

Exohedral silicon fullerenes: SiNPtN∕2 (20⩽N⩽60)

Using density functional theory method we show that hollow silicon fullerene cages, SiN (20⩽N⩽60), can be fully stabilized by exohedrally coated platinum atoms (PtN∕2), denoted as SiNPtN∕2. The exohedral coating PtN∕2 passivates the dangling bonds of the silicon cages, thereby making the silicon cages SiN to retain the symmetry and structure of homologous carbon fullerenes CN. In particular, the Ih symmetrical, 60-atom silicon buckminsterfullerene cage (Si60) can be fully stabilized by exohedrally coated 30 Pt atoms. Properties of SiNPtN∕2, such as the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap and relative stability of cage isomers, are calculated and compared with their carbon counterparts. It is found that the HOMO-LUMO gaps of SiNPtN∕2 are close to their carbon fullerene counterparts (CN). The trend in relative stability for exohedral fullerene isomers SiNPtN∕2 is similar to that for the homologous carbon fullerenes (CN). The exohedral Pt coating offers a possible molecular design towards stabilizing the silicon fullerene cages.

Ab Initio Study of the Conformations of Tetramethoxycalix[4]arenes

In this study we have performed ab initio computer simulations to investigate the conformational characteristics of the tetramethoxycalix[4]arenes (1 and 2). The structures of four types (cone, partial cone, 1,2-alternate, and 1,3-alternate) of conformers for each compound have been optimized using ab initio RHF/6-31G and 6-31<TEX>$G^{**}$</TEX> methods. General trends in relative stabilities of tetramethyl ether derivatives of calix[4]arene 1 and p-tert-butylcalix[4]arene 2 are similar and decrease in following order: partial cone (most stable) > cone > 1,3-alternate > 1,2-alternate. The calculated results of the most stable conformation of partial cone structure agree with the reported NMR experimental observations.

Hartree−Fock Dispersion Probe of the Equilibrium Structures of Small Microclusters of Benzene and Naphthalene: Comparison with Second-Order MØeller−Plesset Geometries

The equilibrium structures of small microclusters of benzene and naphthalene were computed by a simple Hartree−Fock dispersion (HFD) model, in which a self-consistent field calculation is supplemented by an empirical dispersion term. The minimum energy conformers so obtained with the 6-31G basis set are essentially identical to those obtained from a second-order MØeller−Plesset calculation with the same basis set. The trends in relative stabilities are also in general accord with those from the correlated ab initio calculation. These results demonstrate the utility of the HFD models in the conformational search of aromatic clusters.

Density functional MO calculation for stacked DNA base-pairs with backbones

In order to elucidate the effect of the sugar and phosphate backbones on the stable structure and electronic properties of stacked DNA base-pairs, we performed ab initio molecular orbital (MO) calculations based on the density functional theory and Slater-type basis set. As a model cluster for stacked base-pairs, we employed three isomers for the dimer unit of stacked guanine-cytosine pairs composed with backbones as well as base-pairs. These structures were fully optimized and their electronic properties were self-consistently investigated. By including the backbones, the difference in total energy among the isomers was largely enhanced, while the trend in relative stability was not changed. The effect of backbones on the electronic properties is remarkable: the MOs with the character of the PO 4 parts of backbones appear just below the highest-occupied MO. This result indicates that the PO 4 parts might play a rule as a reaction site in chemical processes concerning DNA. Therefore, we conclude that the DNA backbones are indispensable for investigating the stability and electronic properties of the stacked DNA base-pairs.

Lanthanide and alkaline-earth complexes of EDTA in water: a molecular dynamics study of structures and binding selectivities †

We report a molecular dynamics (MD) study on M3+ lanthanide and M2+ alkaline-earth complexes of ethylenediaminetetraacetate tetraanion (EDTA4−) in aqueous solution. First, a consistent set of Lennard-Jones parameters for La3+, Eu3+ and Lu3+ cations has been derived from MD and free energy calculations. It reproduces the experimental differences in their hydration free energies, lanthanide–water distances and coordination numbers in water. Next, the uncomplexed EDTA4− ligand has been simulated in the presence of Na+, Ca2+ or Eu3+ neutralizing counterions, leading to the spontaneous complexation of Na+, but not of Ca2+ or Eu3+ whose complexes are more stable. The endo 1∶1 complexes of M2+ and M3+ cations, simulated for up to 1 ns, remain of inclusive type during the whole simulation. The calculated binding selectivities nicely reproduce experimental trends in relative stabilities with EDTA4− in each cation series: Ca2+ > Sr2+ > Ba2+ and Lu3+ > Eu3+ > La3+. Thus, despite the simplicity of the cation models (1–6–12 pairwise additive interactions without explicit polarization or charge transfer effects), structural and energy features of lanthanide and alkaline-earth complexation by polyaminocarboxylate ligands are, at least, qualitatively, accounted for.

Ab initio MO study of structure and stability of heteronuclear trihalide anions X 2Y − ( X, Y = Cl, Br, or I) in the gas phase and in solution

The structure and dissociation of heteronuclear trihalide anions X 2Y − ( X, Y = Cl, Br, or I) in the gas phase and in solutions were investigated by ab initio MO calculations. The solvent effect was treated by our generalized Born (GB) model. The calculations including solvent effects were carried out for ϵ = 5 and 79 where ϵ is the dielectric constant of the solvent; the latter value corresponds to aqueous solution. When Y is heavier than X ( Y ⪢ X), the (XYX) − and (YYX) − isomers were calculated to be more stable than the (YXX) − and (YXY) − isomers, respectively, both in the gas phase and in solutions; the relative stability of (XYX) − is considerably reduced in solutions, while the relative stability of (YXY) − tends to be somewhat enhanced in solutions. The dissociation energies of the trihalide ions are largely reduced by solvent effects and, hence, the potential energy curves for dissociations become considerably flatter in solutions. As a result, various processes are expected to proceed easily in solutions in agreement with experimental observations. For example, the reaction between X 2 and Y − ( Y ⪢ X) will yield (XYX) − via (YXX) − as an intermediate [ Y − + X 2 → ( Y X X) − → Y X + X − → ( X Y X) −]. Though the dissociation energies in aqueous solution were overestimated, experimental trends in relative stability of trihalide ions in aqueous solution were by and large reproduced. The reactions YX + Y − → Y 2 + X − ( Y ⪢ X) in aqueous solution were calculated to be exothermic in agreement with experimental equilibrium constants.

Theoretical study of I2O

We present high-level density functional calculations (DFT) on the unknown I2O molecule. The results suggest that the compound may be sufficiently stable for detection and synthesis. Our results also suggest that the DFT method is a reliable and computationally cheap alternative to G2, for estimating thermodynamic properties. The trends in relative stabilities within the HOX and X2O series are discussed (X=halogen). © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:383–385, 1998

Effect of three-body forces on the statics and dynamics of SF6–(Rg)n and (Rg)13 clusters

Molecular dynamics and Monte Carlo simulations are used to examine the effect on the structural properties of heterogeneous SF6–(Ar)n and SF6–(Kr)n clusters, and on the melting behavior of heterogeneous SF6–(Ar)n and homogeneous (Ar)13 and (Kr)13 clusters, of including the three-body Axilrod–Teller–Muto triple–dipole dispersion energies in the total potential energy surface governing the dynamics of the system. The behavior of these systems is governed by potentials constructed from the best available two-body interactions, and from accurate constrained dipole oscillator strength values for the triple–dipole dispersion energy coefficients reported here for the first time. The structural studies show that (virtually) all isomers are destablized by inclusion of the three-body terms, with the ‘‘stacked’’ or ‘‘nonwetting’’ structures being destablized relatively more than isomeric ‘‘monolayer’’ or ‘‘wetting’’ structures. However, the qualitative trends in relative stability are unchanged; in particular, the preference for the SF6 to be fully solvated in larger clusters formed with Ar, but to lie on the surface of larger clusters formed with Kr, remains unchanged. In contrast, the melting temperatures of the stacked and monolayer isomers of the heterogeneous SF6–(Ar)12 cluster undergo substantial change on inclusion of the three-body terms, the former dropping from ca. 30 to 22 K and the latter from 10 to 6 K. The melting temperatures of the homogeneous (Ar)13 and (Kr)13 icosahedral isomers also decreased on inclusion of the three-body interactions, and the resulting values are quite different than those obtained using the commonly accepted ‘‘effective’’ two-body LJ(12,6) pair potentials for these systems.

Vertical Electronic Spectra of the Isovalent Molecules H2CNH, H2SiNH, H2CPH and H2SiPH on the Basis of MRD-CI Calculations

Abstract Large-scale multi-reference single and double-excitation configuration interaction (MRD-CI) calculations are employed for the study of the isovalent compounds H2CNH, HLSiNH, hLCPH and H2SiPH in their ground state equilibrium geometry. The dipole moments and charge distributions are given. The vertical excitation energies to the intravalence states 3,1 (n, π*) and 3,1(π,π*) and to the first members of the Rydberg series originating from n and - MO's respectively are predicted; the first two ionization potentials and the Rydberg term values are also calculated. In H2CNH, mixing of Rydberg and valence-shell states with CN stretching is analyzed. The trends in relative stability of electronic and ionized states can be directly related to increased orbital stability of n relative to π as soon as a first-row constituent is replaced by a second-row atom. The calculations explain the diffuse character of the uv spectrum of imines; they treat the molecules H2SiNH and H2SiPH for the first time and present a large number of data for all four molecules which can serve as a basis for future experimental investigations on these and related compounds.

Vertical electronic spectra of the isovalent molecules H2CNH, H2SiNH, H2CPH, and H2SiPH on the basis of MRD-CI calculations

Large-scale multi-reference single and double-excitation configuration interaction (MRD-CI) calculations are employed for the study of the isovalent compounds H2CNH, H2SiNH, H2CPH, and H2SiPH in their ground state equilibrium geometry. The dipole moments and charge distributions are given. The vertical excitation energies to the intra-valence states 3.1(n, π*) and 3.1(π, π*) and to the first members of the Rydberg series originating from n and π MO's respectively are predicted; the first two ionization potentials and the Rydberg term values are also calculated. In H2CNH, mixing of Rydberg and valence-shell states with CN stretching is analyzed. The trends in relative stability of electronic and ionized states can be directly related to increased orbital stability of n relative to π as soon as a first-row constituent is replaced by a second-row atom. The calculations explain the diffuse character of the uv spectrum of imines; they treat the molecules H2SiNH and H2SiPH for the first time and present a large number of data for all four molecules which can serve as a basis for future experimental investigations on these and related compounds.

Effect of periodate and pyrophosphate on the seasonal changes in aggregate stabilization

Aggregates taken from sequential samplings of a range of red-brown earths showed a definite seasonal trend in relative stability. Maximum stability of the aggregates occurred in the summer, followed by a gradual decline during the autumn and early winter and an increase in the spring and early summer. In all phases of the crop-pasture rotation on all the red-brown earths, this trend was minimized by a combination of chemical treatments, designed to oxidize polysaccharides and polyuronides and to break the organic matter-polyvalent cation-clay bridge. The residual aggregate stability after the combined chemical treatment increased with the proportion of pasture phase in the rotation. Individually, the chemical treatments reduced the aggregate stability by a constant amount throughout all the seasons. The aggregate stabilizing mechanisms broken by the individual treatments make a constant contribution to stability throughout the year. This contribution increases with increasing cultivation in the rotation. In contrast, the aggregates taken from the solonized brown soil were stabilized by calcium carbonate. No seasonal trend or rotational effect was evident in these aggregates.