Evidence for an Intermediate in the Methylation of CB11H12−with Methyl Triflate: Comparison of Electrophilic Substitution in Cage Boranes and in Arenes
The trideuteriomethylation of BH vertices in CB11H12− and its derivatives with CD3OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of BCD3 and BCHD2 substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD3OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially …
Measured and Calculated Oxidation Potentials of 1-X-12-Y-CB11Me10– Anions
Cyclic voltammetry of 31 icosahedral carborane anions 1-X-12-Y-CB(11)Me(10)(-) at a Pt electrode in liquid SO(2) revealed a completely reversible one-electron oxidation even at low scan rates, except for the anions with Y = I, which are oxidized irreversibly up to a scan rate of 5.0 V/s, and the anion with X = COOH and Y = H, whose oxidation is irreversible at scan rates below 1.0 V/s. Relative reversible oxidation potentials agree well with RI-B3LYP/TZVPP,COSMO and significantly less well with RI-BP86/TZVPP,COSMO or RI-HF/TZVPP,COSMO calculated adiabatic electron detachment energies. Correlations with HOMO energies of the anions are nearly as good, even though the oxidized forms are subjec…
Ab initio study of the internal rotation potential of p-disilanylbenzene and p-(1,1′,2,2′-tetramethyldisilanyl)benzene
Abstract We present a detailed ab initio study of the geometrical structure and torsional potential of p -disilanylbenzene and its methyl derivative p -(1,1′,2,2′-tetramethyldisilanyl)benzene. The geometries of the different conformers have been fully optimized at the Hartree-Fock 3-21G ∗ and 6-31G ∗ levels. In all cases, the most stable conformation corresponds to the conformer in which the phenylene group lies in a plane perpendicular to the four silicon atoms. The torsional potential curves calculated using the 3-21G ∗ and 6-31G ∗ basis sets show similar features. In the case of p -(1,1′,2,2′-tetramethyldisilanyl)benzene the interactions between the hydrogen atoms of phenylene and the me…
Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes
Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findi…
Photoelectron spectrum of poly(methyl-n-propylsilane): a valence band theoretical interpretation
Abstract The results of a theoretical investigation of the photoelectron spectrum of poly(methyl-n-propylsilane) are reported. The simulation of the UPS spectrum has been obtained from band structure calculations performed using the valence effective Hamiltonian method (VEH). The theoretical spectrum is fully consistent with the experimental UPS spectrum, and an excellent quantitative agreement is observed when the energies of the main peaks of both spectra are compared. A complete analysis of the contributions to the main peaks, together with a detailed interpretation of all the photoemission bands observed in the experimental spectrum have also been performed.