Crossed Beam Experiments and Computational Studies of Pathways to the Preparation of Singlet Ethynylsilylene (HCCSiH; X1A'): The Silacarbene Counterpart of Triplet Propargylene (HCCCH; X3B)

Ethynylsilylene (HCCSiH; X¹A') has been prepared in the gas phase through the elementary reaction of singlet dicarbon (C₂) with silane (SiH₄) under single-collision conditions. Electronic structure calculations reveal a barrierless reaction pathway involving 1,1-insertion of dicarbon into one of the silicon-hydrogen bonds followed by hydrogen migration to form the 3-sila-methylacetylene (HCCSiH₃) intermediate. The intermediate undergoes unimolecular decomposition through molecular hydrogen loss to ethynylsilylene (HCCSiH; C_s; X¹A'). The dicarbon-silane system defines a benchmark to explore the consequence of a single collision between the simplest "only carbon" molecule (dicarbon) with the prototype of a closed-shell silicon hydride (silane) yielding a nonclassical silacarbene, whose molecular geometry and electronic structure are quite distinct from the isovalent triplet propargylene (HCCCH; C₂; ³B) carbon-counterpart. These organosilicon transients cannot be prepared through traditional organic, synthetic methods, thus opening up a versatile path to access the previously largely elusive class of silacarbenes.

Theoretical study on the reaction between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane): An alternative approach to the formation of heterocyclic silylene

The reaction mechanism between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane) has been systematically investigated at the B3LYP/6-311+G* level of theory in order to better understand the reactivity of unsaturated cyclic silylene. Geometry optimizations and vibrational analyses have been conducted for the stationary points on the potential energy surface of the system. Calculations show that the Si-spiroheterocyclic intermediate and four-membered heterocyclic silylene compound could be produced through the insertion process and subsequent dissociation process between silacyclopropenylidene and three-membered heterocyclic compounds. For the insertion process, it is easier for silacyclopropenylidene to insert into C-N bond of azirane than into C-O bond of oxirane. This study is helpful to understand the reactivity of silacyclopropenylidene, the evolution of silicon-bearing molecules in space, and to offer an alternative approach to the formation of enlarged heterocyclic silylene compound.

Are Nonadiabatic Reaction Dynamics the Key to Novel Organosilicon Molecules? The Silicon (Si(3P))-Dimethylacetylene (C4H6(X1A1g)) System as a Case Study

The bimolecular gas phase reaction of ground-state silicon (Si; ³P) with dimethylacetylene (C₄H₆; X¹A_1g) was investigated under single collision conditions in a crossed molecular beams machine. Merged with electronic structure calculations, the data propose nonadiabatic reaction dynamics leading to the formation of singlet SiC₄H₄ isomer(s) and molecular hydrogen (H₂) via indirect scattering dynamics along with intersystem crossing (ISC) from the triplet to the singlet surface. The reaction may lead to distinct energetically accessible singlet SiC₄H₄ isomers (¹p8-¹p24) in overall exoergic reaction(s) (-107_-20⁺¹² kJ mol^-1). All feasible reaction products are either cyclic, carry carbene analogous silylene moieties, or carry C-Si-H or C-Si-C bonds that would require extensive isomerization from the initial collision complex(es) to the fragmenting singlet intermediate(s). The present study demonstrates the first successful crossed beams study of an exoergic reaction channel arising from bimolecular collisions of silicon, Si(³P), with a hydrocarbon molecule.

Flash Vacuum Pyrolysis: Techniques and Reactions

Flash vacuum pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser pyrolysis, pulsed pyrolysis with supersonic jet expansion, very low pressure pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.

Flash (Vacuum) Pyrolysis Apparatus and Methods

The history of pyrolysis equipment, methods, and reactions is narrated in the Introduction. Detailed descriptions of flash vacuum pyrolysis (FVP) (or thermolysis, FVT) apparatus for preparative and spectroscopic (UV, IR, electron spin resonance) purposes with product isolation at 77 K or in Ar matrices at ~10 K are presented. Very low pressure pyrolysis (VLPP), laser pyrolysis, and pulsed pyrolysis (jet flash pyrolysis) are also described together with illustrations of apparatus. The solvent spray flash vacuum pyrolysis (SS-FVP) of liquids or solutions of compounds of low volatility is described together with methods for the addition of solids to a pyrolysis tube, in particular details of pipto-pyrolysis (‘falling solid pyrolysis’). Methods used for catalytic vacuum gas–solid reactions (VGSR) are also summarised.

Structures and transition states of Ge2CH2

In this study a systematic theoretical investigation of Ge2CH2 is carried out. The singlet potential energy surface (PES) was explored using state-of-the-art theoretical methods including self-consistent field (SCF), coupled cluster theory incorporating single and double excitation (CCSD), perturbative triple [CCSD(T)] and full triples [CCSDT] with perturbative quadruple (Q), together with a variety of correlation-consistent polarized valence basis sets cc-pVXZ (where X = D, T, and Q). A total of eleven stationary points have been located on the Ge2CH2 singlet ground state PES. Among them, seven structures are minima (1S-7S), two are transition states (TS1 and TS2), and two are second-order saddle points (SSP1 and SSP2). The global minimum is predicted to be an exotic hydrogen-bridged structure 1S. The energy ordering of the seven minima (in kcal mol(-1)) obtained from focal point analysis using the extrapolation to complete basis set (CBS) limit with zero point vibrational energy (ZPVE), core correlation, diagonal Born-Oppenheimer (DBOC) and relativistic correction is 1S [0.0] < 2S [17.2] < 3S [18.3] < 4S [31.7] < 5S [39.9] < 6S [58.1] < 7S [82.1].

From acetylene complexes to vinylidene structures: The GeC(2) H(2) system

The expansion of germanium chemistry in recent years has been rapid. In anticipation of new experiments, a systematic theoretical investigation of the eight low lying electronic singlet GeC(2) H(2) stationary points is carried out. This research used ab initio self-consistent-field (SCF), coupled cluster (CC) with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory and a variety of correlation-consistent polarized valence cc-pVXZ and cc-pVXZ-DK (Douglas-Kroll) (where X = D, T, and Q) basis sets. At all levels of theory used in this study, the global minimum of the GeC(2) H(2) potential energy surface (PES) is confirmed to be 1-germacyclopropenylidene (Ge-1S). Among the eight singlet stationary points, seven structures are found to be local minima and one structure (Ge-6S) to be a second-order saddle point. For the seven singlet minima, the energy ordering and energy differences (in kcal mol(-1) , with the zero-point vibrational energy corrected values in parentheses) at the cc-pVQZ-DK (Douglas-Kroll) CCSD(T) level of theory are predicted to be 1-germacyclopropenylidene (Ge-1S) [0.0 (0.0)] < vinylidenegermylene (Ge-3S) [13.9 (13.5)] < ethynylgermylene (Ge-2S) [17.9 (14.8)] < Ge-7S [37.4 (33.9)] < syn-3-germapropenediylidene (Ge-8S) [41.2 (37.9)] < germavinylidenecarbene (Ge-5S) [66.6 (61.6)] < nonplanar germacyclopropyne (Ge-4S) [67.8 (63.3)]. These seven isomers are all well below the dissociation limit to Ge ((3) P) + C(2) H(2) (X (1) Σ g+). This system seems particularly well poised for matrix isolation infrared (IR) experiments.

Unusual isomers of disilacyclopropenylidene (Si2CH2)

Nine electronic singlet state structures of Si(2)CH(2) have been systematically investigated by high level theoretical methods. This research employed coupled cluster (CC) methods with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] using the correlation-consistent polarized valence cc-pVXZ/cc-pV(X+d)Z (X = D, T, and Q) basis sets. Full valence complete active space self-consistent-field (CASSCF) wave functions were used for the interpretation of geometries and physical properties. Among the nine singlet stationary points, six structures (1S-6S) are found to be minima, two structures (7S and 8S) are transition states, and one structure (9S) is a second-order saddle point. The existence of the two peculiar hydrogen bridged isomers, 1S (Si...H...Si) and 4S (agostic CH...Si) is established. Extensive focal point analyses are used to obtain complete basis set (CBS) limit energies. For the six lowest-lying singlet minima, after focal point analyses, the energy ordering and energy differences (in kcal mol(-1), with the zero-point vibrational energy corrected values in parentheses) are predicted to be 1S [0.0 (0.0)] < 3S [14.7 (14.5)] < 4S [25.1 (25.3)] < 5S [28.2 (26.0)] < 6S [45.0 (45.4)] < 2S [73.8 (72.0)]. Their relative energies are strikingly different from those for the isovalent parent C(3)H(2) molecule. Geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities are reported for all equilibrium structures.

Theoretical study on isomeric stabilities of C2H2Si and its ionization potentials and electron affinities

The geometric structures and isomeric stabilities of various stationary points in C(2)H(2)Si neutral and its cation and anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For the geometrical survey, the basis sets used are of the Dunning's correlation consistent basis sets of triple-zeta quality (cc-pVTZ) for the neutral and cation. For the anions, the cc-pVTZ basis sets with diffuse functions (aug-cc-pVTZ) are used. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To lower lying neutrals and cations, the Dunning's correlation consistent basis sets of quadruple-zeta quality (cc-pVQZ) are also applied. Both the global minima of the C(2)H(2)Si neutral and cation, N-1 (C(2v):(1)A(1)) and C-1 (C(2v):(2)B(2)), respectively, are silacyclopropenylidene conformers, having a CCSi ring with a C[Double Bond]C double bond. No competitive stable isomers exist in the present C(2)H(2)Si neutral. In the cation, however, the second lowest lying isomer C-2 lies 10.8 kJ/mol above the most stable C-1. The vertical and adiabatic ionization potentials from the lowest lying neutral N-1 are 9.83 and 8.97 eV, respectively, at the CCSD(T)/cc-pVQZ level of theory. The electron addition to the N-1 does not result in the anion with positive (real) electron affinities. On the other hand, the electron addition to the N-2 isomer produces the global minimum anion A-1 (C(2v):(2)B(1)) with the positive electron affinities of 1.13 eV. The second lowest lying anion isomer A-2 with silylenylacetylene conformer, produced from an electron addition to the N-3 neutral, very well competes with the A-1 after the zero-point vibrational energy corrections. The energy difference between the two lowest lying isomers of the neutral and its anion, N-1 and A-1, is only 0.39 eV.