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Theoretical studies of the radical reaction H 2CSiH 2 + H
Abstract
Ab initio molecular orbital calculations on the transition states and
barrier heights for the addition of atomic hydrogen to silaethylene are
carried out. The activation energy for the addition to the silicon site
is lower than that to the carbon site, while the exothermicity is
smaller.
Results of ab initio calculations on the reaction pathways for the hydrogen addition to methanediazonium ion (CH3NN+) and methyl isocyanide (CH3NC) at both central and terminal atoms as well as for the stereomutation of α-adducts are reported. Structures of points on the energy surfaces were determined at the UHF/3-21 G level while their relative energies were estimated at UMP4SDQ/6-31 G**. Thermochemical properties were also computed. The α-addition of H˙ either to the terminal nitrogen of CH3NN+ or to the carbon atom of CH3NC is not stereospecific. Only a single transition structure can be located in each case. At the transition state, the substrate is only marginally bent. Soon after the transition state is passed, a bifurcation of the addition pathway occurs leading to the formation of both cis- and trans-isomers of the adduct. In both cases, the trans-adduct is more stable than the cis; the cis–trans stereomutation takes place via nitrogen inversion of the methyl group and requires only 5–6 kcal mol–1 therefore the trans-isomer should be the major adduct. The β-addition to central nitrogen exhibits, in both cases, an appreciably larger energy barrier which precludes the formation of the β-adduct.
Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.
The excitation of tetramethylsilane (Me4Si) into its lowest excited Rydberg state is followed by two main decomposition channels: a simple SiC bond breaking process with a quantum yield of Φ = 0.45 ± 0.05 and a methane elimination process with the concomitant formation of dimethylsilaethylene (Φ = 0.17 ± 0.04). Other very minor primary processes occur, with quantum yields of the order of Φ ⩽ 5 × 10−3, but their nature could not be identified with certainty. The reactions leading to the stable products are dominated by radical-radical processes and by radical addition reactions to Me2SiCH2. The addition reaction to the SiC double bond occurs preferentially at the Si site. Satisfactory material balance was obtained indicating that the products were mostly recovered. A number of relative rate constants were determined. Reactions in the presence of NO, MeOH, GeH4 and SF6 were also studied. An explanation of the photophysics by a three-state model was attempted. From the experiments, it was concluded that the two decomposition channels occur from different electronic states. The lack of dependence of the CH4 quantum yield on the experimental parameters (liquid or gaseous phase, etc.) suggests a decomposition from a strongly predissociating state, which is identified with the lowest excited state, while the SiC bond breaking process is thought to occur from the triplet state. Molecules which reach the ground state live sufficiently long so that deactivation competes successfully with decomposition.
High-level ab initio molecular orbital calculations are reported for the global minima on the SiCHn (n = 0-4), SiCHn+ (n = 0-5), SiC2Hn (n = 0-4, 6), and SiC2Hn+ (n = 0-5, 7) potential energy surfaces. The results have been used to calculate standard enthalpies of formation at 298 K for each compound, ionization energies at 0 K, and proton affinities at 298 K for the neutral species. The single-carbon compounds have been investigated at levels of theory up to PMP4SDTQ(fu11)/6-311++G(2df,p)//MP2(full)/6-311G(d,p) and QCISD(T)(full)/6-311++G(2df,p)//MP2(full)/6-311G(d,p); those with two carbons have been investigated at levels up to PMP4(fc)/6-311++G(2df,p)//MP2(full)/6-31G(d,p) and QCISD(T)(full)/6-311++G(2df,p)//MP2(full)/6-31G(d,p). Harmonic frequency calculations, performed on each optimized geometry, established all the structures to be at minima and also provided zero-point vibrational energies. Calculated thermodynamic values at the PMP4 and QCI levels of theory are in good agreement with each other (except where the removal of spin contamination by spin annihilation is inadequate) and are in good agreement with well-established experimental values where such comparisons are possible. Calculated ionization energies are consistently lower than the experimental values for the carbon analogues; proton affinities of the organosilicon species are higher than those of the carbon analogues for which experimental data are available.
The 6‐31G* and 6‐31G** basis sets previously introduced for first‐row atoms have been extended through the second‐row of the periodic table. Equilibrium geometries for one‐heavy‐atom hydrides calculated for the two‐basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6‐31G* structures, obtained for two‐heavy‐atom hydrides and for a variety of hypervalent second‐row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first‐row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6‐31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split‐valance 3‐21G basis set for both normal‐ and hypervalent compounds. Normal‐mode vibrational frequencies derived from 6‐31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3‐21G basis set. Hydrogenation energies calculated for normal‐ and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.
One development in silaolefin chemistry has been the preparation of the first adamantyl-substituted silaethylene which is stable at room temperature. It has been characterized by ir, NMR, and mass spectroscopy. Another key discovery was the first convincing demonstration of the laboratory preparation of the unsubstituted parent silaethylene. A brief outline of these developments is presented. Other aspects of silaolefin chemistry are described. Theory is expected to play a significant role. 3 figures, 2 tables.
A simple level of ab initio molecular orbital theory with a split-valence shell basis with d-type polarization functions (6-31G*) is used to predict equilibrium geometries for the ground and some low-lying excited states of AHn molecules and cations where A is carbon, nitrogen, oxygen or fluorine. The results are shown to be close to the limit for single determinant wave functions in cases where corresponding computations with more extensive bases are available. Comparison with experimental results also shows good agreement although a systematic underestimation of bond lengths up to 3 per cent is evident. For systems where no experimental data are available, the results provide predictions of equilibrium geometry.
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The addition of CF3 radicals to substrates possessing substituted reaction centres was investigated in the gas phase over the temperature range 65-180°C. The following reaction centres were studied : (Chemical Equation Presented) A methyl or Cl substituent located on an olefinic, dienic, or acetylenic hydrocarbon reduces the A factor of addition by ∼5, this result being accounted for in terms of restricted internal rotation of a CF3 group around an incipient C ⋯ CF3 bond. A further slight reduction in the A factor is observed when the addition involves dimethylated centres. The A factors for the CF3 addition to acetylenes are higher by a factor of 3.5 than that for the respective olefins-a result explained again in terms of the transition state theory. Methylation of a substrate lowers the activation energy of the addition, a linear relation being found between E and the ionization potential for each family of substrates (olefins, dienes, acetylenes). The close similarity of CF3 radical and O atom addition indicates that both reagents are electrophilic and that the transition state of both reactions involves an incipient C ⋯ X bond, i.e., the activated complex is described more correctly as a σ- rather than a π-complex.
The importance of the overlapping between the highest occupied (HO) molecular orbital (MO) of an electron donor and the lowest unoccupied (LU) MO of an electron acceptor in determining the favorable position and spatial direction of a chemical reaction is emphasized by setting up the following auxiliary principles: the principle of a positional parallelism between the chargetransfer and the bond-interchange, the principle of a narrowing of the inter-frontier energy-level separation, and the principle of a growing frontier density along the reaction path. These subprinciples work in a cooperative manner to enable us to arrive at this general governing principle: most of the chemical interactions are liable to occur at the position and in the direction where the overlapping of the HO and the LU of the respective reactants is at its maximum; in an electron-donating reactant, HO predominates in the overlapping interaction with the MO’s of the other reactant, whereas LU does so in an electron-accepting reactan...
The 6-31G* and 6-31G** basis sets previously introduced for first-row atoms have been extended through the second-row of the periodic table. Equilibrium geometries for one-heavy-atom hydrides calculated for the two-basis sets and using Hartree--Fock wave functions are in good agreement both with each other and with the experimental data. HF/6-31G* structures, obtained for two-heavy-atom hydrides and for a variety of hypervalent second-row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first-row compounds, where limiting Hartree--Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6-31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split-valance 3-21G basis set for both normal- and hypervalent compounds. Normal-mode vibrational frequencies derived from 6-31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%--15%; they are of much more uniform quality than those obtained from the 3-21G basis set. Hydrogenation energies calculated for normal- and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.
Ab initio molecular orbital calculations on the transition state and barrier height for the addition of atomic hydrogen to ethylene are carried out. The higher reactivity of olefinic compared to acetylenic bonds toward free radicals is examined in terms of chemically interpretable contributions.
Ab initio, POL-CI calculations on the barriers to hydrogen migration in the title compounds are reported. For CâHâ, CâHâ, and CHâCH the predicted barriers are 57, 46, and 53 kcal/mol, respectively. For the first two molecules barriers to C-H bond cleavage are also calculated and found to be lower than the migration barriers. A qualitative analysis of the wave functions indicates that the high migration barriers are due to a geometrical constraint placed on the electronic structure of the transition state. A comparison to hydrogen migration in a closed-shell molecule (vinylidene-acetylene) is also presented.
For the reactions H + C2H4 ⇄ C2H5 and F + C2H4 → C2H4F → H + C2H3F, the equilibrium geometries and transition structures were fully optimized at the HF/3-21G, HF/6-31G*, and MP2/3-21G levels. Vibrational frequencies were computed for each structure at the HF/3-21G level. While basis set and correlation effects cause large changes in the energy differences, only small and predictable variations are found in the geometries. The β-fluoroethyl radical adopts a gauche conformation in agreement with ESR measurements and matrix isolation IR spectra. The three transition structures are quite similar with the attacking (or leaving) atom ca. 2 Å from the carbon and making an angle of ca. 105° with the CC double bond. The ethylenic moiety is only slightly distorted in the transition state. The bending frequencies for the attacking (or leaving) atom are somewhat higher than previous estimates: 242 and 405 cm-1 for F + C2H4, 415 and 448 cm-1 for H + C2H4, 461 and 552 cm-1 for H + C2H3F.
Mø–Plesset theory, in which electron correlation energy is calculated by perturbation techniques, is used in second order to calculate energies of the ground states of atoms up to neon. The unrestricted Hartree–Fock (UHF) Hamiltonian is used as the unperturbed system and the technique is then described as unrestricted Mø–Plesset to second order (UMP2). Use of large Gaussian basis sets suggests that the limiting UMP2 energies with a complete basis of s, p, and d functions account for 75–84% of the correlation energy. Preliminary estimates of the contributions of basis functions with higher angular quantum numbers indicate that full UMP2 limits give even more accurate total energies.
An approximate fourth-order expression for the electron correlation energy in the Møller–Plesset perturbation scheme is proposed. It takes into account all the contributions to the fourthorder energy neglecting only those of the triple-substituted determinants. It is size consistent and correct to fourth order for an assembly of isolated two-electron systems. Illustrative calculations are reported for a series of small molecules.
Two new split-valence basis sets, termed 6-21G and 3-21G, are proposed for use in molecular orbital calculations on molecules containing first-row elements. The valence functions for the smaller representation (3-21G) have been taken directly from the larger (6-21G), preventing their collapse inwards to make up for deficiencies in the inner-shell region. This is necessary to ensure a good description of bonding interactions which necessarily involve overlap of valence functions. Equilibrium geometries, vibrational frequencies, relative energies, and electric dipole moments calculated using the 3-21G basis set are nearly identical with those obtained from the larger 6-21G representation. Compared to experiment they are consistently superior to properties derived from the STO-3G minimal basis set, and of comparable quality to those obtained from the larger 4-21G and 4-31G representations. One notable exception is that the 4-31G basis set yields hydrogenation energies in significantly better agreement with experiment than those obtained from 3-21G. The 3-21G basis set comprises the same number of primitive Gaussian functions as STO-3G (although nearly twice the number of basis functions) and should be nearly as efficient computationally as that representation for applications which require evaluation of energy derivatives as well as the energy itself (e.g., determination of equilibrium geometry and calculation of vibrational frequencies). It is less costly to apply than either the 4-21G or 4-31G split-valence basis sets, and in those areas where the performance of the two is comparable it would appear to be the method of choice.
A perturbation theory is developed for treating a system of n electrons in which the Hartree-Fock solution appears as the zero-order approximation. It is shown by this development that the first order correction for the energy and the charge density of the system is zero. The expression for the second-order correction for the energy greatly simplifies because of the special property of the zero-order solution. It is pointed out that the development of the higher approximation involves only calculations based on a definite one-body problem.