The structures of six different extraframework aluminum (EFAL) species, possibly present in zeolites, were studied by density functional theory methods. A T 6 cluster (T ) Si, Al), with different Si/Al ratios, was used to simulate the real zeolite Y structure and the coordination of the chosen EFAL species (Al 3+ , Al(OH) 2+ , AlO + , Al(OH) 2 + , AlO(OH), and Al(OH) 3 ). The monovalent cations prefer to attain bicoordination with the framework AlO 4moiety, while di-and trivalent cations usually achieve tetracoordination. One important result is that, in all cases, coordination occurs with the oxygen atoms nearest to the framework aluminum ones. A single water molecule addition to the optimized Al 3+ ‚T 6 cluster produces a strongly exothermic reaction, leading to formation of a hydroxyaluminum cation and an acidic site on the zeolite. The addition of a second water molecule produces only minor energetic and structural changes.

Long-term exposure to arsenic in drinking water has been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate in humans. It is therefore important to understand the structural aspects of arsenic in water, as hydrated arsenic is most likely the initial form of the metalloid absorbed by cells. We present a detailed experimental and theoretical characterization of the coordination environment of hydrated arsenite. XANES analysis confirms As(III) is a stable redox form of the metalloid in solution. EXAFS analysis indicate, at neutral pH, arsenite has a nearest-neighbor coordination geometry of approximately 3 As-O bonds at an average bond length of 1.77 A, while at basic pH the nearest-neighbor coordination geometry shifts to a single short As-O bond at 1.69 A and two longer As-O bonds at 1.82 A. Long-range ligand scattering is present in all EXAFS samples; however, these data could not be fit with any degree of certainty. There is no XAS detectable interaction between As and antimony, suggesting they are not imported into cells as a multinuclear complex. XAS results were compared to a structural database of arsenite compounds to confirm that a 3 coordinate As-O complex for hydrated arsenite is the predominate species in solution. Finally, quantum chemical studies indicate arsenite in solution is solvated by 3 water molecules. These results indicate As(OH)3 as the most stable structure existing in solution at neutral pH; thus, ionic As transport does not appear to be involved in the cellular uptake process.

We present a comparative study of the spatial distribution of the spin density of the ground state of CuCl2 using Density Functional Theory (DFT), quantum Monte Carlo (QMC), and post-Hartree-Fock wave function theory (WFT). A number of studies have shown that an accurate description of the electronic structure of the lowest-lying states of this molecule is particularly challenging due to the interplay between the strong dynamical correlation effects in the 3d shell and the delocalization of the 3d hole over the chlorine atoms. More generally, this problem is representative of the difficulties encountered when studying open-shell metal-containing molecular systems. Here, it is shown that qualitatively different results for the spin density distribution are obtained from the various quantum-mechanical approaches. At the DFT level, the spin density distribution is found to be very dependent on the functional employed. At the QMC level, Fixed-Node Diffusion Monte Carlo (FN-DMC) results are strongly dependent on the nodal structure of the trial wave function. Regarding wave function methods, most approaches not including a very high amount of dynamic correlation effects lead to a much too high localization of the spin density on the copper atom, in sharp contrast with DFT. To shed some light on these conflicting results Full CI-type (FCI) calculations using the 6-31G basis set and based on a selection process of the most important determinants, the so-called CIPSI approach (Configuration Interaction with Perturbative Selection done Iteratively) are performed. Quite remarkably, it is found that for this 63-electron molecule and a full CI space including about 10(18) determinants, the FCI limit can almost be reached. Putting all results together, a natural and coherent picture for the spin distribution is proposed.

The long-range correction (LC) for treating electron exchange in density functional theory, combined with the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional, was used to determine (hyper)polarizabilities of polydiacetylene/polybutatriene oligomers. In comparison with coupled-cluster calculations including single and double excitations as well as a perturbative treatment of triple excitations, our values indicate that the tendency of conventional functionals to result in a catastrophic overshoot for these properties is alleviated but not eliminated. No clear-cut preference for LC-BLYP over Hartree-Fock values is obtained. This analysis is consistent with the calculations of Sekino et al. [J. Chem. Phys. 126, 014107 (2007)] on polyacetylene and molecular hydrogen oligomers. Thus, the performance of LC-BLYP with regard to (hyper)polarizabilities of quasilinear conjugated systems is now well characterized.

Ab initio calculations were performed to study the stability of various pyrophosphate species in the gas phase: H 4 P 2 O 7 , H 3 P 2 O 7 -, H 2 P 2 O 7 2-, HP 2 O 7 3-, P 2 O 7 4-, and their complexes with Mg 2+ . It is found that the metal cation allows the existence of highly charged anions in the gas phase. We also study the isomerization reactions Mg‚H 2 P 2 O 7 f (H 2 PO 4 ‚Mg‚PO 3 ), (Mg‚HP 2 O 7 ) -f (HPO 4 ‚Mg‚PO 3 ) -, and (Mg‚P 2 O 7 ) 2f (PO 4 ‚Mg‚PO 3 ) 2-, at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G** basis set with diffuse and polarization functions. Other basis sets, including one of valence triple ζ plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is found that the same mechanism occurs for the isomerizations of the three species: one of the P-O bridging bonds of the reactant is longer than the other, and the route to the products proceeds through its elongation. This asymmetry is induced by the metal cation in the case of the evenly charged anions. In all cases the metal cation coordinates the transition states and the leaving groups. The structures found for the complexes (H 2 PO 4 ‚Mg‚PO 3 ), (HPO 4 ‚Mg‚PO 3 ) -, and (PO 4 ‚Mg‚PO 3 ) 2are different from those reported previously, the metal cation being enclosed by the two phosphates. The activation barrier increases with the charge of the anion, from ∆G°q ) 5.6 kcal/mol for the neutral complex Mg‚H 2 P 2 O 7 , to ∆G°q ) 10.4 kcal/mol for the monoanion (Mg‚HP 2 O 7 ) -, to ∆G°q ) 13.5 kcal/mol for the dianion (Mg‚P 2 O 7 ) 2-. The positive value found for the energy of the isomerization (Mg‚P 2 O 7 ) 2f (PO 4 ‚Mg‚PO 3 ) 2-, ∆G°q ) 1.8 kcal/mol, predicts the synthesis to be spontaneous in the gas phase, opposite of what occurs in the aqueous solution. This result supports the view that the hydration energy makes a large contribution to the energy of hydrolysis. The gas-phase hydrolysis reaction H 2 O + Mg 2+ + H 2 P 2 O 7 2f Mg 2+ + H 2 PO 4-+ H 2 PO 4is also studied as a multistep reaction, involving the isomerization of H 2 O + (Mg‚H 2 P 2 O 7 ) f H 2 O + (PO 3 ‚Mg‚H 2 PO 4 ) as an intermediate step. It is found that the equilibrium in the gas phase yields H 2 PO 4 ‚Mg‚H 2 PO 4 as the final species; an energy input is required for separating the metal cation from the phosphate anions.

We report periodic B3LYP6-31G(**) density functional theory calculations on Li-doped polythiophene at various dopant concentrations using (SC(4)H(2))(m)Li(2) unit cells for m=2, 6, and 10. Uniform doping by Li atoms and by pairs of Li atoms on adjacent thiophene rings are considered with the primary aim of comparing polaron versus bipolaron properties. Properties examined include geometries, charge distributions, polaron/bipolaron formation energies, dopant binding energies, band structures, and densities of states.

We report the results of Born-Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI molecule at room temperature using the cluster microsolvation approach including 32 water molecules. The electronic structure calculations were done using the M062X hybrid exchange-correlation functional in conjunction with the 6-31G** basis sets for oxygen and hydrogen. For the iodine and samarium atoms the Stuttgart-Köln relativistic effective-core potentials were utilized with their associated valence basis sets. Starting from the optimized geometry of SmI embeded in the microsolvation environment, we find a swift substitution of the iodine ions by eight tightly bound water molecules around Sm(II). Through the Sm-O radial distribution function and the evolution of the Sm-O distances, the present study predicts a first rigid Sm(II) solvation shell from 2.6 to 3.4 Å, whose integration leads to a coordination number of 8.4 water molecules, and a second softer solvation sphere from 3.5 to ca. 6 Å. The Sm(II)-O radial distribution function is in excellent agreement with that reported for Sr from EXAFS studies, a fact that can be explained because Sr and Sm have almost identical ionic radii (ca. 1.26 Å) and coordination numbers: 8 for Sr and 8.4 for Sm. The theoretical EXAFS spectrum was obtained from the BOMD trajectory and is discussed in the light of the experimental spectra for Sm(III). Once microsolvation is achieved, no water exchange events were found to occur around Sm, in agreement with the experimental data for Eu (which has a nearly identical charge-to-ionic radius relation as Sm), where the mean residence time of a water molecule in [Eu(HO)] is known to be ca. 230 ps.

The structure and energy of the isobutonium cations, protonated isobutane, were studied by ab initio methods. At MP2(full)/6-31G** level, besides the C-isobutonium cation (5), the 2-H-isobutonium cation (6), and the 1-H-isobutonium cation (7), two additional structures, representing the van der Waals complex between methane and isopropyl cation (8) and hydrogen plus tert-butyl cation (9), could also be characterized. The energy increases in the order 9 < 8 < 5 < 6 < 7, indicating the lower energy of the van der Waals complexes. The experimental proton affinity of isobutane is in good agreement with the calculated values for the van der Waals complexes 8 and 9, indicating the facility of rupture of the three center bond in 5 and 6. On the other hand, the relative order of stability of the isobutonium cations can explain the experimental gas phase protonation of isobutane by small electrophiles, such as H 3 + and H 3 O + , as well as the H-D exchange in liquid superacid.