The singlet potential hypersurface of Si_{3}H_{2} has been examined from the viewpoint of intramolecular H-transfer actions. The likely candidate for the global minimum is trisilacyclopropenylidene. Several equilibrium ans transition structures contain H in bridging positions. The low rearragnement barriers, and the occurence and stability of a planar, formally tetravalent Si centre are among the particularly noteworthy findings presented here.
Standard enthalpies, entropies, and Gibbs functions of formation for a large number of silicon hydrides containing one to five Si atoms have been calculated. The choice of compounds includes cyclic and acyclic silanes, silyl-radicals, silylenes, disilenes and cyclic diradicals. The thermodynamic functions were calculated using an empirically corrected ab initio scheme. The electronic energies were obtained with multiconfiguration reference Averaged Coupled-Pair Functional (ACPF) wavefunctions. Two different basis sets were employed to demonstrate the validity of the correction scheme. The computed data are compared with experimental and theoretical data from other laboratories. Increment rules and substituent effects are presented, various reaction enthalpies are tabulated and discussed. The performance of the correction scheme and potential pitfalls in its general applicability are discussed.Because of the large number of systems, we did not include tables of reaction enthalpies, but left their calculation from standard enthalpies of formation to the reader. Alternatively, tables may be downloaded here (PostScript):
In the following abstract, the boldface atom symbols C and Si mean a divalent carbon or silicon atom, respectively.
Torsion potentials about the X-X-H bond in homosubstituted primary carbenes (X=C) and silylenes (X=Si) have been investigated at the multi-reference averaged coupled pair functional (MR-ACPF) level of theory. For the triplet species, the potentials are quite flat, but large barriers of torsion have been observed for the singlet states of all carbenes and silylenes whose carbon or silicon atom adjacent to the divalent atom formes small bond angles with two of its further substitutents; other geometry parameters, even the bond angle at the divalent atom, proved to be of little or no importance. The said kind of deformation encourages the formation of a weak dative π-like bond between the X and X atoms, which by its twofold symmetry with respect to torsion about the bond axis, is responsible for the observed two-minima torsion potential.
Thermodynamic state function (enthalpy, entropy, heat capacity) were calculated for several types of silicon hydrides taking into account the strongly anharmonic character of some of the molecular vibrations (internal rotation, inversion, pseudorotation). The anharmonic motions were treated as one-dimensional motions taking place along the harmonic normal coordinates, neglecting anharmonic coupling terms. Partition functions were calculated from the idealized numerical eigenvalue spectrum in the case of pseudorotation; for the other types of large amplitude motions, we used quantum-corrected classical partition functions. Following the work of Knyazev and Tsang, we derived a novel partition function for an asymmetric double well potential. We then used the data to calculate enthalpies, entropies and free energies of reaction for several types of chemical reactions among silicon hydrides, at both the harmonic and the anharmonic level. Differences arising from the inclusion of anharmonicity are discussed.
We describe a general numerical method for the calculation of pseudorotation constants at an accuracy suited for thermodynamic applications. The pseudorotation is treated using classical mechanics along a pseudorotational pathway, which is spanned by molecular structures obtained from conventional ab initio geometry optimization. We present numerical results for systems with vertical pseudorotation arising from ring puckering (cyclopentane, cyclopentasilane) and for systems with in-plane pseudorotation arising from Jahn–Teller distortion of planar rings (benzene cation, cyclopentadienyl radical, cyclopropenyl radical).
A novel partition function for one isolated internal rotation degree of freedom is presented. Our partition function is designed for torsion potentials with one antiperiplanar and two isoenergetic synclinal (gauche) minima, as in 1,2-dichloroethane or butane. Calculating thermodynamic functions (U, S, C_{v}) for the internal rotation in a number of small carbon and silicon compounds, we compare the results to those obtained with a symmetrical internal rotation partition function. The relative energy of the conformers affects the heat capacity most strongly, and gives an additive increment to the internal energy at high temperatures.
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