Search

We have analyzed the chemical bonding and reactivity in the cubic molecule octahydridosilsesquioxane, Si8H8O12, and its counterpart Ge8H8O12 by means of ab initio quantum chemical methods and group theory. Density functional theory and MP2 methods combined with the basis sets 6-311+G(d) and 6-311++G(2d,p) were used for geometry optimization and vibrational frequency analysis. The geometries of Si8H8O12 and Ge8H8O12 are unstable under Oh symmetry and distort to the rare Th molecular symmetry. The energy gained from this pseudo-Jahn–Teller distortion ranges from 0.78 to 6.14 kcal mol−1 depending on methodological treatment. The Fukui functions and the molecular electrostatic potential were both used as DFT-based reactivity descriptors. Our study shows that Si8H8O12 and Ge8H8O12 are both hard amphoteric molecules. The cavity within each cage is acidic and able to encapsulate hard small bases such as F−. The exterior of the cages is basic and can form stable exohedral complexes with hard acids, as in the case of H+. The insertion of F− in Si8H8O12 and Ge8H8O12 cages gives the most stable endohedral complexes of the series studied, characterized by formation energies of −3.50 and −3.45 eV at CAM-B3LYP/6-311+G(d) and −3.61 and −3.68 eV at the MP2/6-311++G(d,p) level, respectively. The calculated formation energies of the exohedral and endohedral complexes align with the DFT reactivity descriptor analysis. [ABSTRACT FROM AUTHOR]

The polarizable valence state potential energy curve (pVS-PEC) is derived through the systematic improvement of the valence state potential energy curve (VS-PEC) and is given as U(R) = -[(K1/R) + (K2/R (super 4)) + (K3/R (super 7)) + T/R exp (-Lambda R). The potential is tested on a broad variety of homonuclear diatoms and heteronuclear halides and hydrides.

We have explored the structural and energetic properties of a series of RMX3‐NH3 (M=Si, Ge; X=F, Cl; R=CH3, C6H5) complexes using density functional theory and low‐temperature infrared spectroscopy. In the minimum‐energy structures, the NH3 binds axially to the metal, opposite a halogen, while the organic group resides in an equatorial site. Remarkably, the primary mode of interaction in several of these systems seems to be hydrogen bonding (C‐H‐‐N) rather than a tetrel (N→M) interaction. This is particularly clear for the RMCl3‐NH3 complexes, and analyses of the charge distributions of the acid fragment corroborate this assessment. We also identified a set of metastable geometries in which the ammonia binds opposite the organic substituent in an axial orientation. Acid fragment charge analyses also provide a clear rationale as to why these configurations are less stable than the minimum‐energy structures. Matrix‐isolation infrared spectra provide clear evidence for the occurrence of the minimum‐energy form of CH3SiCl3–NH3, but analogous results for CH3GeCl3–NH3 are less conclusive. Computational scans of the M‐N distance potentials for CH3SiCl3–NH3 and CH3GeCl3–NH3, both in the gas phase and bulk dielectric media, reveal a great deal of anharmonicity and a propensity for condensed‐phase structural change. [ABSTRACT FROM AUTHOR]

Polarized-ion models—with and without dipole reaction fields and charge-quadrupole interaction—are evaluated in calculating atomization energies, bond angles, and bending force constants of groups 2 and 12 dihalides. A study of the sensitivity to changes in the input parameters reveals a strong dependence on dipole polarizabilities. The charge–quadrupole bending terms of a multipole expansion are important for quasi-linear molecules, where the preceding terms in the expansion tend to cancel each other. They normally stabilize the linear geometry, but enhance bent structures, if a certain criterion for the ratio of quadrupole polarizabilities of metal and halide ions is fulfilled. The models explain the bent alkaline earth halides, but fail spectacularly for the group 12 halides—HgBr[sub 2] and HgI[sub 2] are even calculated as unstable relative to the constituent atoms. The unexpected weakness of ionic bonds is related to the large increase of the ionization energies by relativistic effects and the lanthanide contraction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The existence and persistence of quintuple bonding in isomers of model RMMR molecules of different geometry are investigated, where R = H, F, Cl, Br, CN, and C[H.sub.3] and M = Cr, Mo, and W. A low-symmetry [C.sub.s] structure is identified and the MM quintuple bond is persisting, regardless of the preference of the isomer of the RMMR molecule.

The links between structural preferences in the monomers, dimers, and extended solid-state structures of the group 2 dihalides are examined theoretically. The rigidly linear monomers are shown to condense to form extended solids with low coordination numbers, 4 or 6. The reasons for these correlations are explored.

The relationship between the newly introduced (atomic and molecular) compressibility and polarizability is rationalized. Evidence of linear variation of a plot of the molecular compressibility against polarizability is found and analysis is made on assumptions that atomic polarizability is proportional to the atomic volume(super 11) and the molecular polarizability is roughly proportional to the sum of the atomic polarizabilities.