The geometries of ZrX2−6 (X=F,Cl,Br,I) were optimized at the ab initio Hartree–Fock SCF level using all-electron MIDI and effective core potential valence basis sets of double-zeta quality, supplemented with diffuse functions. In addition, geometries and energies of X2, as well as energies of gaseous Zr were determined in order to predict energies of formation of ZrX2−6. Effects of electron correlation were taken into account at the second order Mo/ller–Plesset level of theory. Vibrational frequencies were determined in the harmonic approximation and compared with available experimental data. Standard routines were employed to evaluate entropies, heat capacities, heats of formation and free enthalpies of formation of gaseous ZrX2−6. Electrostatic cohesive energies in hexahalogenozirconates were evaluated by the extended Ewald method. It was assumed that each ion gathers a formal charge +1, +2, or −2. Net atomic charges in complex ions were determined either from various population analyses or fitted to the ab initio quantum mechanical electrostatic potential. The Coulombic energies decrease gradually with the increase in volume of the simplest structural unit. A similar tendency is noted as regards distances between interacting centers. Theoretically determined properties are in a good agreement with available data, mostly of experimental origin.
Photoelectron spectra of $(\mathrm{HF}{)}_{3}^{\ensuremath{-}}$ reveal coexistence of two anionic isomers with vertical electron detachment energies (VDE) of 0.24 and 0.43 eV. The results of electronic-structure calculations, performed at the coupled cluster level of theory with single, double, and noniterative triple excitations, suggest that the two isomers observed experimentally are an open, zigzag, dipole-bound anion and an asymmetric solvated electron, in which the dipole-bound anion of $(\mathrm{HF}{)}_{2}$ is solvated by one HF monomer at the side of the excess electron. The theoretical VDE of 0.21 and 0.44 eV, respectively, are in excellent agreement with the experimental data.
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The method of analytic continuation of real stabilization graphs was applied to calculate positions and widths of electronic resonances of the FH−2 double-Rydberg anion at the experimental geometry of the parent FH+2 cation. In correlated calculations on FH−2, a full configuration interaction calculation was performed on the two outermost electrons; the remaining electrons occupied orbitals taken from the SCF-level treatment of the FH+2 core. All spatial symmetries and both singlet and triplet spin multiplicities were considered. Many Feshbach and core-excited shape resonances were found with lifetimes in the range (1 to 80) ×10−14 s. Different methods of fitting the coefficients of the characteristic polynomial used in the stabilization calculations were considered. Techniques to suppress incomplete basis set artifacts in the stabilization calculations were examined.
The H(NH(2)BH(2))(n)H oligomers are possible products from dehydrogenation of ammonia borane (NH(3)BH(3)) and ammonium borohydride (NH(4)BH(4)), which belong to a class of boron-nitrogen-hydrogen (BNH(x)) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNH(x)-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH(2)BH(2))(n)H (n = 1-4) oligomers, polyaminoborane (PAB), from NH(3)BH(3) monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH(3)BH(3) molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH(3)BH(3) molecules. We find that the formation of H(NH(2)BH(2))(n+1)H oligomers occurs first through reactions of the H(NH(2)BH(2))(n)H oligomers with BH(3) followed by reactions with NH(3) and the release of H(2), where the BH(3) and NH(3) intermediates are formed through dissociation of NH(3)BH(3). We also find that the dimerization of the NH(3)BH(3) molecules to form cyclic c-(NH(2)BH(2))(2) is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H(2) molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H(2)), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (H(N)) hydrogens interact with hydridic (H(B)) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH(2)(eta(2)-H(2)) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane.
Abstract Usefulness of different Gaussian basis sets for reproducing the “tail” region of the SCF wavefunctions employed in calculations of the exchange‐repulsion effect is investigated for the model He‐He interaction. It has been shown that extension of the monomer‐centered basis set in the scheme of regularized even‐tempered basis sets [M. W. Schmidt and K. Ruedenberg, J. Chem. Phys. 71 , 3951 (1979)] can be more efficient than augmentation of the fully energy‐optimized basis set with diffuse basis functions. It has been also found that Landshoff term vanishes and the “tail” region is well reproduced if monomer wavefunctions are calculated with the basis set of the dimer.
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