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The structures of carbatranes XC(SiH2CH2CH2)3N (8) and XC(SCH2CH2)3N (9), where X = H, F, and their in and out protonated forms were studied at the MP2 level of theory using the 6-31G*, 6-311G**, a...

Titanium dichloride was investigated as a potential catalyst for the bis-silylation reaction of ethylene with hexachlorodisilane. Ab initio electronic structure calculations at the restricted Hartree−Fock (RHF), density functional (DFT), second-order perturbation (MP2), and couple cluster (CCSD) levels of theory were used to find optimized structures, saddle points, and minimum-energy paths that connect them. The reaction was found to have a net zero barrier at the DFT, MP2, and CCSD levels of theory. Dynamic correlation is found to be important for this reaction.

The nature of the multiple bonding of H2TiEH2 (E = C, Si) and H2TiEH (E = N, P) molecules has been investigated with the multiconfiguration SCF (MCSCF) method. It is shown that the Ti−C and Ti−P bond lengths decrease with bond rotation, whereas the Ti−Si bond length increases. MCSCF geometry optimization shows that Ti−N has triple-bond character with a linear Ti−N−H bond angle that results from strong back-bonding from the N lone pair into the empty Ti d orbitals. The rotation barrier and bond dissociation energy for TiE are estimated with the MCSCF + multireference second-order perturbation theory (MRMP2) method. The singlet rotation barriers in TiC, TiSi, and TiP are estimated to be 15.9, 8.6, and 9.3 kcal/mol, respectively. The TiC and TiSi bond dissociation energies are estimated to be 83.4 and 56.9 kcal/mol, respectively. The Ti⋮N triple-bond energy is about 30 kcal/mol larger than that of the carbon species, whereas the energy of the TiP bond is about 8 kcal/mol smaller than that of the silicon species.

Ab initio calculations with full geometry optimization have been used to investigate the O and N atom proton affinities and the molecular structures of silatranes (RSi(OCH2CH2)3N), as well as the r...

Among the reactions of hydrogen peroxide that are catalyzed by methyltrioxorhenium, the oxidation of alkylsilanes is unique. It is not a reaction in which an oxygen atom is added to a substrate, but one featuring a net insertion, R{sub 3}Si{single{underscore}bond}H + H{sub 2}O{sub 2}{r{underscore}arrow} R{sub 3}Si{single{underscore}bond}OH + H{sub 2}O. Kinetic studies were carried out on 10 compounds. Rate constants were determined for the bimolecular reaction of the silane with the peroxo compound CH{sub 3}Re(O)({eta}{sup 2}-O{sub 2}){sub 2}(H{sub 2}O). The variation of rate constant with the alkyl groups R follows two trends: the values of log(k) are linear functions of (a) the stretching frequency of the Si{single{underscore}bond}H group and (b) the total Taft constant for these substituents. The reactions of (n-Bu){sub 3}Si{single{underscore}bond}H and (n-Bu){sub 3}Si{single{underscore}bond}D exhibit a kinetic isotope effect of 2.1 at 0 C. From these data, a model for the transition state was formulated in which O{single{underscore}bond}H and Si{single{underscore}bond}O bond making accompany Si{single{underscore}bond}H bond breaking. Quantum mechanical calculations have been carried out on the gas-phase reaction between Et{sub 3}SiH and CH{sub 3}Re(O){sub 2}({eta}{sup 2}-O{sub 2}). These results support this structure, calculating a structure and energy that are in agreement. The theoretical activation energy is 28.5 kcal mol{sup {minus}1}, twicemore » the experimental value in aqueous acetonitrile, 12.4 kcal mol{sup {minus}1}. The difference can be attributed to the solvation of the polar transition state in this medium.« less

Reactions involving two [H,C,Si]- anions with the protonated neutral analogues (HSiCH, H2CSi, and H2SiC) have been studied. Calculations through fourth-order perturbation theory (MP4) suggest possible routes for single-proton transfer between the silaacetylide [HCSi]- anion and neutral species. Various important reaction paths for proton-transfer reactions and internal rearrangements of ion−molecule complexes are discussed. Accurate estimates of the acidity of HSiCH and H2SiC are presented and discussed in light of earlier experimental studies.

Methyltrioxorhenium (MTO) catalyzes the 1,3-transposition of allylic alcohols to generate the more stable isomer at equilibrium. The direction of the equilibrium is largely decided by the nature of the OH group, i.e., whether it is primary, secondary, or tertiary. In the case of aliphatic allylic alcohols, tertiary is preferred to secondary which is preferred to primary. For aromatic allyl alcohols, the more conjugated isomer predominates largely at equilibrium. Oxygen-18 labeling showed that the OH groups of the parent and product are the same. The reaction is first order with respect to both allyl alcohol and MTO but strongly inhibited by traces of water. Theoretical calculations suggest the same results in the case of aliphatic allyl alcohols, although aromatic allyl alcohols do not follow the predictions. Studies of deuterium-labeled substrates show a large equilibrium isotope effect (K = 1.20 ± 0.02). For isomeric allyl alcohols differing in the position of deuterium only, the isomer with the deuteri...

The reaction between Fe+ in the ground 6D and excited 4F states and silaethylene have been studied using multiconfigurational (CASSCF) wave functions and multiconfigurational wave functions augment...

Ab initio geometry optimizations for a series of azatitanatranes, ZTi[−NR(CH2)2−]3N, where Z = CH3, NH2, OH, F, and N(CH3)2 and R = H and CH3, have been performed using a triple-ζ basis set for Ti and a 6-31G(d) basis set for all other atoms. An analysis of the transannular Ti−N interaction indicates that it is significantly stronger than that found in the analogous azasilatranes. In addition there is reasonable agreement between the calculated structure and the available X-ray data for Z = N(CH3)2 and R = CH3. Of special significance in this calculation is the fact that theory correctly predicts that the axial Ti−N bond is shorter than the equatorial Ti−N bonds.

The Si2CH4+ potential energy surface has been studied theoretically using ab initio unrestricted Hartree−Fock, restricted open shell Hartree−Fock, and fourth-order perturbation theories. In additio...

An ab initio analysis of the electronic structure of high-valent, transition-metal alkylidenes as models for olefin metathesis catalysts is presented. The catalyst models studied fall into three categories: {open_quotes}new{close_quotes} metathesis catalyst models-tetrahedral M(OH){sup 2}(XH)(CH{sub 2}) complexes; {open_quotes}old{close_quotes} metathesis catalyst models-tetrahedral MCl{sub 2}(Y)(CH{sub 2}) complexes and alkylidene-substituted Mo metathesis catalysts, Mo(OH){sub 2}(NH)(=C(H)Z). The effect on the bonding caused by modification of either the metal, ligands, or alkylidene substitutents is considered. 21 refs., 2 figs., 5 tabs.

The addition reaction of SiX2 (X = H, CH3, t-Bu, F, and Cl) to acetylene is considered using ab initio calculations. Second-order perturbation theory (MP2) and coupled cluster CCSD(T) calculations confirm there is no barrier for SiH2, Si(CH3)2, and Si(t-Bu)2 additions, while electronegative substituents such as F and Cl have nonzero barriers. It is concluded that electronegativity is more important than steric effects in determining barrier heights for these reactions. The CCSD(T)/6-31+G* barriers for SiF2 and SiCl2 additions are computed to be about 24 and 8 kcal/mol, respectively.

Calculs ab initio (MP3/6-31G * /3-21G) sur les 13 isomeres cycliques: le silylsilacyclopropenylidene est le plus stable; le disilatetraedrane est situe tres haut sur la surface d'energie; aucun minimum n'existe pour les 2 disila-1,2cyclobutadienes plans; le disila-1,3cyclobutadiene a une structure stable a caractere de diradical

group, (E)- and (Z)-3-d 1 [(E),(Z)-3-dd were labeled >95% at C3 of the 1-propenyl group, and 4-d 1 was deuterated at a silyl methyl. Cleavage of a silacyclopropylmethylene intermediate to 1,1-dimethyl-1-silaethene accounts for the labeling of 4-d 1 and the deuteration of 2-d 1 and (E),(Z)-3-d 1 suggests the intermediacy of 1,1-dimethyl-1-sila-1,3-butadiene, while the 13% contribution by a photoprotodesilylation pathway explains the partial C3 labeling of 2. Additional evidence for silicon-carbon doubly bonded intermediates is the formation of 31% 2-methoxy-2,4,4-trimethyl-3-vinyl-2,4-disilapentane and 4.8% 2-methoxy-2,4,4-tri­ methyl-2,4-disilapentane upon 214 nm photolysis of 1 with 0.25 M methoxytrimethylsilane in pentane. Product yields versus tert-butyl alcohol concentration were determined to probe whether equilibrating, ground-states-cis (gauche) and s-trans conformers of silabutadiene were involved in the formation of the 2 and (E),(Z)-3. Optimized geometries using the 6-31G(d) basis set were calculated for the cis, gauche, and s-trans structures of 1-sila-1,3-butadiene and 1,3-butadiene, as well as relative energies and barriers for interconversion of the gauche and trans species at the MP2/6-31G(d) level.

Des calculs ab initio ont ete realises afin d'analyser l'aptitude d'un, deux ou trois groupes vinyles a stabiliser les ions CH 3 + et SiH 3 +