Chemically activated methylcyclobutane exothermicity of singlet methylene reactions and the heat of formation of singlet methylene

The specific decomposition rates of chemically activated methylcyclobutane produced from CH₂(¹A₁) reaction with cyclobutane have been determined. CH₂(¹A₁)was produced from ketene photolyses at 3340 and 3130 Å and from diazomethane photolyses at 4358 and 3660 Å. Comparisons of the excitation energies of the methylcyclobutane, determined by RRKM theory calculations, and the experimental results for the ketene systems, with thermochemically predicted maximum excitation energies, favor an Arrhenius A factor in the range of 5 × 10¹⁵ to 1 × 10¹⁶ sec^?1 for methylcyclobutane. This result is consistent with (1) the comparison of RRKM theory calculations and the experimental unimolecular falloff for methylcyclobutane, (2) the comparison of experimental A factors for cyclobutane and other alkylcyclobutane decompositions, and (3) two out of three reported experimental A factors for methylcyclobutane. An analysis of these and previous results leads to a value of the CH₂(¹A₁) ? CH₂(³B₁) energy splitting of 9±3 kcal/mole.     

Calculated photoelectron spectra of singlet and triplet methylene

Vertical ionization potentials for singlet and triplet methylene are calculated by a CI perturbation method based on ab initio SCF molecular orbitals (6–31 G^** basis). The shape and vibrational fine structure of the first photoelectron band are investigated using the MINDO/3 method. The computed singlet-triplet splitting for methylene is 16.4 kcal/mol, in reasonable agreement with the experimental value of 19.5 kcal/mol.     

DFT study on mechanism of carbonyl hydrosilylation catalyzed by high-valent molybdenum (IV) hydrides

Density functional theory (DFT) calculations have been employed to investigate hydrosilylation of carbonyl compounds catalyzed by three high-valent molybdenum (VI) hydrides: Mo(NAr)H(Cp)(PMe₃) (1A), Mo(NAr)H(PMe₃)₃ (1B), and Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate) (1C). Three independent mechanisms have been explored. The first mechanism is “carbonyl insertion pathway”, in which the carbonyls insert into Mo?H bond to give a metal alkoxide complex. The second mechanism is the “ionic hydrosilylation pathway”, in which the carbonyls nucleophilically attacks η¹-silane molybdenum adduct. The third mechanism is [2 + 2] addition mechanism which was proposed to be favorable for the high-valent di-oxo molybdenum complex MoO₂Cl₂ catalyzing the hydrosilylation. Our studies have identified the “carbonyl insertion pathway” to be the preferable pathway for three molybdenum hydrides catalyzing hydrosilylation of carbonyls. For Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate), the proposed nonhydride mechanism experimentally is calculated to be more than 32.6?kcal/mol higher than the “carbonyl insertion pathway”. Our calculation results have derived meaningful mechanistic insights for the high-valent transition metal complexes catalyzing the reduction reaction.     

Asymmetric reduction of the carbonyl group by chiral hydrides. 1. Stereochemical considerations

Conclusions A quadrant rule is proposed for the enantioselective reduction of acetophenone by chiral hydrides based on LiAlH₄, NaBH₄, LiBH₄, and BH₃. This enables the configuration of the product to be explained and predicted, depending on the preferred conformations of the ligands in these hydrides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 843–848, April, 1987.     

Quantum chemical study of mechanism for insertion of singlet methylene into C-C1 bond

Conclusions Quantum chemical calculation by the expanded Huckel method revealed that the mechanism for the insertion of singlet methylene into the C-C1 bond of methyl chloride consists in the electrophilic attack by methylene on the C-Cl bond, with a subsequent synchronous transfer of the chlorine atom to the methyl and the formation of ethyl chloride; here the vacant p-orbital of methylene does not react with the unshared pairs of the chlorine atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2622–2625, November, 1973.     

Relativistic effects on inversion barriers of pyramidal group 15 hydrides

Quantum chemistry is an important tool for determining general molecular properties, although relativistic corrections are usually required for systems containing heavy and super heavy elements. Non‐relativistic along with relativistic two‐ and four‐component electronic structure calculations done with the CCSD‐T method and the new RPF‐4Z basis set have therefore been applied for determining inversion barriers, corresponding to the change from a pyramidal (C_3v) ground‐state structure to the trigonal planar (D_3h) transition state, TS, of group 15 hydrides, XH₃ (X= N, P, As, Sb, and Bi). The ground‐state structure of the McH₃ molecule, which contains the super heavy element Moscovium, is also predicted as pyramidal (C_3v), with an atomization energy of 90.8 kcal mol⁻¹. However, although non‐relativistic calculations still provided a D_3h planar TS for McH₃, four‐component relativistic calculations based on single‐reference wave functions are unable to elucidate the definitive TS geometry in this case. Hence, the results show that relativistic effects are crucial for this barrier determination in those hydrides containing Bi and Mc. Moreover, while the scalar relativistic effects predominate, increasing barrier heights by as much as 17.6 kcal mol⁻¹ (32%) in BiH₃, the spin‐orbit coupling cannot be disregarded in those hydrides containing the heaviest group 15 elements, decreasing the barrier by 2.5 kcal mol⁻¹ (4.5%) in this same molecule.     

Intramolecular quenching of the excited singlet state of a naphthyl group by halogeno substituents

The halogeno group in a number of naphthyl alkyl halides has been shown to quench the fluorescence of the naphthyl group. The efficiency of quenching is dependent on the length of the chain and the nature of the hologeno group. By comparison of triplet yields in fluid solution and a rigid matrix it is shown that the magnitude of triplet production is also dependent upon the orientation of the halogeno group with respect to the aromatic nucleus.     

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH₄, GeH₄, SnH₄, and PbH₄. The well‐established spherical complex optical potential and complex scattering potential‐ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets.     

Synthesis of C-phosphorylated acetamidines containing CH-acidic methylene group

C-Phosphorylated acetamidines with the methylene group linked to dialkoxyphosphoryl and amidine groups possessing CH-acidic properties were obtained and characterized. At the reaction of these amidines with sodium one hydrogen atom is substituted to form sodium derivatives interesting as intermediates for the syntheses of the organophosphorus compounds of versatile nature. Based on the CH-acidity of C-phosphorylated acetamidines a convenient method is developed for the synthesis of C-phosphorylated bromoacetamidines.     

Synthesis and reactivity of dimethyl platinum(IV) hydrides in water

New hydrophilic ligands of the di(2-pyridyl)methanesulfonate family, L = dpms and Me-dpms, enable the synthesis of methyl platinum(IV) hydrides, LPtMe2H, the study of very fast CH reductive coupling, and reductive elimination of these complexes in water. In dichloromethane solutions, 13CH4 reacts with (Me-dpms)PtMe2H to produce isotopomeric complexes.     

9-Triptycenyl complexes of group 13 and 15 halides and hydrides

The reactions of 9-lithiotriptycene with AlCl₃, GaCl₃ and InBr₃ have been carried out and have yielded the complexes, [(tript)AlCl₂(OEt₂)], [(tript)GaCl₂(THF)] and [(tript)InBr(μ-Br)₂Li(OEt₂)₂], tript=9-triptycenyl. The latter two complexes have been structurally characterised. The corresponding reactions of 9-lithiotriptycene with ECl₃ (E=P, As, Sb or Bi) afforded the complexes, [(tript)ECl₂], of which all but the phosphorus compound have been crystallographically authenticated. In addition, the high yield syntheses of the thermally stable primary pnictanes [(tript)EH₂] (E=As, Sb) have been achieved by reaction of the relevant halide complex with LiAlH₄. The X-ray crystal structure of the antimony hydride complex is reported.     

Studies of benzylisoquinolines. Comparison of the reactivity of the methylene group in 1-benzyl and 4-benzylisoquinolines

While examining the oxidation of 4-benzylisoquinolines to the corresponding 4-benzoylisoquinolines as well as the reactivity of these ketones towards hydroxylamine, reduction reagents and Grignard's reagents, there was found a definite difference in the behavior of the methylene group for these compounds as compared to that of papaverine (1-benzylisoquinolines).     

Poisoning of heterogeneous, late transition metal dehydrocoupling catalysts by boranes and other group 13 hydrides

Borane reagents are widely used as reductants for the generation of colloidal metals. When treated with a variety of heterogeneous catalysts such as colloidal Rh, Rh/Al2O3, and Rh(0) black, BH3.THF (THF = tetrahydrofuran) was found to generate H2 gas with the concomitant formation of a passivating boron layer on the surface of the Rh metal, thereby acting as a poison and rendering the catalyst inactive toward the dehydrocoupling of Me2NH.BH3. Analogous poisoning effects were also detected for (i) colloidal Rh treated with other species containing B-H bonds such as [HB-NH]3, or Ga-H bonds such as those present in GaH3.OEt2, (ii) colloidal Rh that was generated from Rh(I) and Rh(III) salts using borane or borohydrides as reductants, and (iii) for other metals such as Ru and Pd. In contrast, analogous poisoning effects were not detected for the catalytic hydrogenation of cyclohexene using Rh/Al2O3 or the Pd-catalyzed Suzuki cross-coupling of PhB(OH)2 and PhI. These results suggest that although this poisoning behavior is not a universal phenomenon, the observation that such boron layers are formed and surface passivation may exist needs to be carefully considered when borane reagents are used for the generation of metal colloids for catalytic or materials science applications.