The Silicon Hydride Clusters Si3Hn (n ≤ 8) and Their Anions: Structures, Thermochemistry, and Electron Affinities

Gas Phase Synthesis of the Elusive Trisilacyclopropyl Radical (Si3H5) via Unimolecular Decomposition of Chemically Activated Doublet Trisilapropyl Radicals (Si3H7)

The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; X²Π) with disilane (Si₂H₆; X¹A_1g) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (Si₃H₅)-the isovalent counterpart of the cyclopropyl radical (C₃H₅)-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-Si₃H₇) collision complex(es). Possible hydrogen-atom roaming on the doublet surface proceeds to molecular hydrogen loss accompanied by ring closure. The chemical dynamics are quite distinct from the isovalent methylidyne (CH)-ethane (C₂H₆) reaction, which leads to propylene (C₃H₆) radical plus atomic hydrogen but not to cyclopropyl (C₃H₅) radical plus molecular hydrogen. The identification of the trisilacyclopropyl radical (Si₃H₅) opens up preparative pathways for an unusual gas phase chemistry of previously inaccessible ring-strained (inorgano)silicon molecules as a result of single-collision events.

Large entropic effects on the thermochemistry of silicon nanodusty plasma constituents

Determination of the thermodynamic properties of reactor constituents is the first step in designing control strategies for plasma-mediated deposition processes and is also a key fundamental issue in physical chemistry. In this work, a recently proposed multistructural statistical thermodynamic method is used to show the importance of multiple structures and torsional anharmonicity in determining the thermodynamic properties of silicon hydride clusters, which are important both in plasmas and in thermally driven systems. It includes five different categories of silicon hydride clusters and radicals, including silanes, silyl radicals, and silenes. We employed a statistical mechanical approach, namely the recently developed multistructural (MS) anharmonicity method, in combination with density functional theory to calculate the partition functions, which in turn are used to estimate thermodynamic quantities, namely Gibbs free energy, enthalpy, entropy, and heat capacity, for all of the systems considered. The calculations are performed using all of the conformational structures of each molecule or radical by employing the multistructural quasiharmonic approximation (MS-QH) and also by including torsional potential anharmonicity (MS-T). For those cases where group additivity (GA) results are available, the thermodynamic quantities obtained from our MS-T calculations differ considerably due to the fact that the GA method is based on single-structure data for isomers of each stoichiometry, and hence lack multistructural effects; whereas we find that multistructural effects are very important in silicon hydride systems. Our results also indicate that the entropic effect on the thermochemistry is huge and is dominated by multistructural effects. The entropic effect of multiple structures is also expected to be important for other kinds of chain molecules, and its effect on nucleation kinetics is expected to be large.

A coordination compound of Ge(0) stabilized by a diiminopyridine ligand

Reduction of the cationic Ge(II) complex [dimpyrGeCl][GeCl3] (dimpyr=2,6-(ArN=CMe)2NC5H3, Ar=2,6-iPr2C6H3) with potassium graphite in benzene affords an air sensitive, dark green compound of Ge(0), [dimpyrGe], which is stabilized by a bis(imino)pyridine platform. This compound is the first example of a complex of a zero-valent Group 14 element that does not contain a carbene or carbenoid ligand. This species has a singlet ground state. DFT studies revealed partial delocalization of one of the Ge lone pairs over the π*(C=N) orbitals of the imines. This delocalization results in a partial multiple-bond character between the Ge atom and imine nitrogen atoms, a fact supported by the X-ray crystallography and IR spectroscopy data.

Silicon Hydride Clusters Si5Hn (n = 3−12) and Their Anions: Structures, Thermochemistry, and Electron Affinities

The molecular structures, electron affinities, and dissociation energies of the Si(5)H(n)/Si(5)H(n)(-) (n = 3-12) species have been calculated by means of three density functional theory (DFT) methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The first Si-H dissociation energies for neutral Si(5)H(n) and its anion have also been reported.

Ge3H(n)- anions (n = 0-5) and their neutral analogues: a theoretical investigation on the structure, stability, and thermochemistry

The structure, stability, and thermochemistry of various Ge3H(n)- isomers (n = 0-5) and of their neutral analogues have been investigated at the B3LYP/6-311+G(d), MP2(full)/6-31G(d), and Gaussian-2 (G2) level of theory. For Ge3H(-), both the B3LYP and the G2/MP2 methods predict the cyclic, H-bridged structure 1a- as the global minimum, more stable than another cyclic isomer and an open-chain isomer by ca. 10 and 25 kcal mol(-1), respectively. For Ge3H2(-), the B3LYP and the G2/MP2 methods provide a somewhat different description of the potential energy surface. At the G2/MP2 level of theory, the global minimum is the cyclic, H2Ge-bridged structure 2a-, separated by other three nearly degenerate isomers by ca. 10 kcal mol(-1). On the other hand, at the B3LYP level of theory, the cyclic, H-bridged structure 2e-, not located at the MP2 level of theory, is more stable than 2a- by ca. 1 kcal mol(-1). For Ge3H3(-), both the B3LYP and the G2/MP2 methods predict the cyclic, H3Ge-bridged isomer 3a- as the global minimum, but the energy differences with the other five located isomeric structures predicted by the two methods are quantitatively different. Similar to Ge3H2(-), the B3LYP and the G2/MP2 theoretical levels provide a somewhat different description of the Ge3H4(-) potential energy surface. At the G2/MP2 level of theory, the global minimum is the cyclic structure 4b- of C(2v) symmetry, featuring a Ge2H4 moiety and a Ge-bridged atom, which is more stable than other three located isomers by 3, 9, and 17 kcal mol(-1). On the other hand, at the B3LYP level of theory, the open-chain isomer 4a- of H3Ge-Ge-GeH(-) connectivity is more stable than 4b- by ca. 1 kcal mol(-1) and nearly degenerate with the alternative open-chain isomer H3Ge-GeH-Ge(-). For Ge3H5(-), both the B3LYP and the G2/MP2 methods predict the 2-propenyl-like isomer H3Ge-Ge-GeH2(-) as the global minimum, with energy differences with other four isomeric structures which range from ca. 1-2 to 13-17 kcal mol(-1). At the G2 level of theory and 298.15 K, the electron affinities of Ge3H(n) are computed as 2.17 (n = 0), 2.57 (n = 1), 1.70 (n = 2), 2.41 (n = 3), 2.07/1.80 (n = 4), and 2.71/2.46 eV (n = 5). The two alternative values reported for Ge3H4 and Ge3H5 reflect the alternative conceivable choice for the structure of the involved neutrals and ions. The G2 enthalpies of formation of Ge3H(n) and Ge3H(n)- (n = 0-5) have also been calculated using the atomization procedure. Finally, we have briefly discussed the implications of our calculations for previously performed mass spectrometric experiments on the negative ion chemistry of GeH4.

Negative gas-phase ion chemistry of silane: A quadrupole ion trap study

Silicon clusters are of considerable interest for their importance in astrophysics and chemical vapour deposition processes, as well as from a fundamental point of view. Here, we present a quadrupole ion trap study of the self-condensation ion/molecule reactions of anions of silane. In the high-pressure regime, several ion clusters are formed with increasing size: the largest ions detected are Si5Hn- (n = 0-3). Selective ion isolation and storage allowed detection of the main reaction sequences occurring in the reacting system. The most frequent condensation step is followed by single or multiple dehydrogenation, this latter being particularly observed for the high-mass reactant ions. As a consequence, the most abundant ions in the mass spectra are those with a low content of hydrogen, namely Si2H-, Si3H-, and Si4H-. These results are discussed with reference to literature data on silicon cluster anions and related systems.

Silicon Monohydride Clusters SinH (n = 4−10) and Their Anions: Structures, Thermochemistry, and Electron Affinities

The molecular structures, electron affinities, and dissociation energies of the Si(n)H/Si(n)H- (n = 4-10) species have been examined via five hybrid and pure density functional theory (DFT) methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. The three different types of neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The first Si-H dissociation energies, D(e)(Si(n)H --> Si(n) + H) for neutral Si(n)H and D(e)(Si(n)H- --> Si(n)- + H) for anionic Si(n)H- species, have also been reported. The structures of the ground states of these clusters are traditional H-Si single-bond forms. The ground-state geometries of Si5H, Si6H, Si8H, and Si9H predicted by the DFT methods are different from previous calculations, such as those obtained by Car-Parrinello molecular dynamics and nonorthogonal tight-binding molecular dynamics schemes. The most reliable EA(ad) values obtained at the B3LYP level of theory are 2.59 (Si4H), 2.84 (Si5H), 2.86 (Si6H), 3.19 (Si7H), 3.14 (Si8H), 3.36 (Si9H), and 3.56 (Si10H) eV. The first dissociation energies (Si(n)H --> Si(n) + H) predicted by all of these methods are 2.20-2.29 (Si4H), 2.30-2.83 (Si5H), 2.12-2.41 (Si6H), 1.75-2.03 (Si7H), 2.41-2.72 (Si8H), 1.86-2.11 (Si9H), and 1.92-2.27 (Si10H) eV. For the negatively charged ion clusters (Si(n)H- --> Si(n)- + H), the dissociation energies predicted are 2.56-2.69 (Si4H-), 2.80-3.01 (Si5H-), 2.86-3.06 (Si6H-), 2.80-3.03 (Si7H-), 2.69-2.92 (Si8H-), 2.92-3.18 (Si9H-), and 2.89-3.25 (Si10H-) eV.