Agostic interaction in the methylidene metal dihydride complexes H2MCH2 (M=Y, Zr, Nb, Mo, Ru, Th, or U)

Multiconfigurational quantum chemical methods (complete active space self-consistent field (CASSCF)/second-order perturbation theory (CASPT2)) have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U. The geometry of some of these complexes is highly irregular due to the formation of a three center bond CH...M, where the electrons in the CH bond are delocalized onto empty or half empty orbitals of d- or f-type on the metal. No agostic interaction is expected when M=Y, where only a single bond with methylene can be formed, or when M=Ru, because of the lack of empty electron accepting metal valence orbitals. The largest agostic interaction is found in the Zr and U complexes.     

Variational calculations for the hydrogen-antihydrogen system with a mass-scaled Born-Oppenheimer potential

The problem of proton-antiproton motion in the H- $\bar H$ system is investigated by means of the variational method. We introduce a modified nuclear interaction through mass-scaling of the Born-Oppenheimer potential. This improved treatment of the interaction includes the nondivergent part of the otherwise divergent adiabatic correction and shows the correct threshold behaviour. Using this potential we calculate the vibrational energy levels with angular momentum 0 and 1 and the corresponding nuclear wave functions, as well as the S-wave scattering length. We obtain a full set of all bound states together with a large number of discretized continuum states that might be utilized in variational four-body calculations. The results of our calculations gives an indication of resonance states in the hydrogen-antihydrogen system.     

De novo sequencing of proteolytic peptides by a combination of C-terminal derivatization and nano-electrospray/ collision-induced dissociation mass spectrometry

A series of synthetic peptides (3-15 residues), C-terminally derivatized with 4-aminonaphthalenesulfonic acid (ansa), have been analyzed on a hybrid magnetic sector-orthogonal acceleration time-of-flight tandem mass spectrometer, fitted with a nano-electrospray (nano-ES) interface. Deprotonated molecules generated by negative-ion ES were subjected to collision-induced dissociation (CID) using either methane or xenon as the collision gas, at a collision energy of 400 eV (laboratory frame of reference). As a consequence of charge localization on the sulfonate group, only C-terminal fragment ions were formed, presumably by charge-remote fragmentation mechanisms. Interpretable CID spectra were obtained from fmol amounts of the small peptides (up to 6 residues), whereas low pmol amounts were required for the larger peptides. CID spectra were also recorded of derivatized, previously noncharacterised peptides obtained by proteolysis of cytosolic hamster liver aldehyde dehydrogenase. Interpretation of these CID spectra was based on rules established for the fragmentation of the synthetic peptides. This study shows that derivatization with ansa may be useful in the de novo sequencing of peptides.     

Synthesis of Styrenes by Palladium(II)‐Catalyzed Vinylation of Arylboronic Acids and Aryltrifluoroborates by Using Vinyl Acetate

One Heck of a reaction : Treatment of arylboronic acids or aryltrifluoroborates with vinyl acetate by using a palladium(II) catalyst gives the corresponding styrenes (see scheme). No palladium reoxidant is needed and the vinylation is performed under non‐inert conditions

     

Microwave‐Promoted Palladium(II)‐Catalyzed C?P Bond Formation by Using Arylboronic Acids or Aryltrifluoroborates

The first Pd^II‐catalyzed P arylation has been performed by using palladium acetate, the rigid bidentate ligand dmphen (dmphen=2,9‐dimethyl‐1,10‐phenanthroline), and without the addition of base or acid. Couplings of arylboronic acids or aryl trifluoroborates with H‐phosphonate dialkyl esters were conducted in 30 min with controlled microwave (MW) heating under non‐inert conditions. Aryl phosphites were also synthesized at room temperature with atmospheric air as the sole reoxidant. The arylated phosphonates were isolated in 44–90 % yields. The excellent chemoselectivity of the method was illustrated in the synthesis of a Mycobacterium tuberculosis glutamine synthetase (MTB‐GS) inhibitor. Online ESIMS was used to detect cationic palladium species in ongoing reactions directly, and a catalytic cycle has been proposed based on these results.     

An ab initio study of the molecular structure and vibration-rotation spectrum of the triplet radical HCCN

The bent triplet cyanocarbene H-C-CN and the linear triplet allene H-C=C=N have been studied by the CASSCF and CI methods, using a DZP basis. Relaxation of all geometrical parameters for the CASSCF energy results in a bent molecule with CCH angle 133° and a barrier to linearity of 6.4 kcal/mol, which was lowered to 2.3 kcal/mol in a subsequent CI calculation. The Davidson correction lowered it further to 1.8 kcal/mol.A 26-term analytical potential energy surface (PES) was fitted to CASSCF, CI, and Davidson corrected CI energies in 94 different geometries. Using these three potentials, the semi-rigid bender model predicts a CCH bending frequency of 782, 505, and 503 cm^–1, resp., which compares favourably with an experimentally observed IR transition line at 458 cm^–1. For the deuterated species, the corresponding frequencies are 610, 407, and 402 cm^–1, to be compared with two possible absorption lines at 405 and 317.5 cm^–1.The PES was then parametrized by adding a variable CCH angle dependence, and a comprehensive vibration-rotation spectrum was calculated variationally, using the exact 4-atom vibration-rotation kinetic Hamiltonian, for a range of barrier heights. Comparison with experiment indicates a barrier in the range 1±0.5 kcal/mol.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

A theoretical study of the diffuseness of the V(1B1u) state of planar ethylene

The ground (N) state and the ¹B_1u(V) excited state of planar ethylene have been studied at the CPF and MR -SDCI levels of theory, using an extended CGTO basis set of the ANO type. The investigation especially addresses the near-degeneracy problem in the ground state and the coupling between the diffuse character of the π* orbital and the amount of correlation included in the wave function of the V state. The MR -SDCI results yield a vertical excitation energy in the range 7.8–8.0 eV, whereas the CPF result is 7.9 eV. The best MR -SDCI result for 〈1π*‖z²‖1π*〉 is 7.8, whereas CPF calculations based on MR -SDCI INOS give the value 6.7. It is clear from the results that these numbers have not converged and that more extended calculations than was possible in the present work would yield an even more compact wave function.     

Ab initio DFT study of Z–E isomerization pathways of <Emphasis Type="Italic">N</Emphasis>–benzylideneaniline

The ground state properties and absorption spectra of N-benzylideneaniline (NBA) have been studied at the density functional (DFT) and at the time-dependent density functional (TD-DFT) level of the theory. The equilibrium geometries of the E and Z isomers in the ground state and their vibrational frequencies have been computed. Furthermore, the excitation energies of the lowest excited singlet and triplet states and the potential energy curves along the torsion and the inversion isomerization coordinates were evaluated. The results are discussed in light of the available experimental data.