Alkaline‐Earth‐Catalysed Cross‐Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends,Scope and Mechanism

Alkaline‐earth (Ae=Ca, Sr, Ba) complexes are shown to catalyse the chemoselective cross‐dehydrocoupling (CDC) of amines and hydrosilanes. Key trends were delineated in the benchmark couplings of Ph₃SiH with pyrrolidine or tBuNH₂. Ae{E(SiMe₃)₂}₂ ? (THF)_x (E=N, CH; x=2–3) are more efficient than {N^N}Ae{E(SiMe₃)₂} ? (THF)_n (E=N, CH; n=1–2) complexes (where {N^N}^?={ArN(o‐C₆H₄)C(H)=NAr}^? with Ar=2,6‐iPr₂‐C₆H₃) bearing an iminoanilide ligand, and alkyl precatalysts are better than amido analogues. Turnover frequencies (TOFs) increase in the order Ca<Sr<Ba. Ba{CH(SiMe₃)₂}₂ ? (THF)₃ displays the best performance (TOF up to 3600 h^?1). The substrate scope (>30 products) includes diamines and di(hydrosilane)s. Kinetic analysis of the Ba‐promoted CDC of pyrrolidine and Ph₃SiH shows that 1) the kinetic law is rate=k[Ba]¹[amine]⁰[hydrosilane]¹, 2) electron‐withdrawing p‐substituents on the arylhydrosilane improve the reaction rate and 3) a maximal kinetic isotopic effect (k_SiH/k_SiD=4.7) is seen for Ph₃SiX (X=H, D). DFT calculations identified the prevailing mechanism; instead of an inaccessible σ‐bond‐breaking metathesis pathway, the CDC appears to follow a stepwise reaction path with N?Si bond‐forming nucleophilic attack of the catalytically competent Ba pyrrolide onto the incoming silane, followed by rate limiting hydrogen‐atom transfer to barium. The participation of a Ba silyl species is prevented energetically. The reactivity trend Ca<Sr<Ba results from greater accessibility of the metal centre and decreasing Ae?N_amide bond strength upon descending Group 2.     

Semicoherent elastic pion-carbon scattering at 25 and 40 GeV/c

In the frame of a systematic study of hadron-nucleus interactions performed at the Serpukhov accelerator (CERN-IHEP 5th experiment), semicoherent elastic scattering on carbon $\text{π}{}^{\mathrm{?12}}\text{C}\text{a}\text{?}\text{π}{}^{\mathrm{?12}}\text{C}{}^1\text{(4.44}\text{MeV}\text{)}$ was investigated with 25 and 40 GeV/c incident pions.The experimental data were obtained with a counter technique, looking at the coincidence between the scattered pion and the 4.44 MeV photon from the J^P = 2⁺ carbon excited state detected by a NaI counter.The value obtained for the integrated cross section at 40 GeV/c in the 0.0032 ? |t| ? 0.27 (GeV/c)² four-momentum transfer range isσ = 1.16 ± 0.11 mb.According to the theoretical models, this t-range at 40 GeV/c covers more than 99% of the angular distribution, so the above value almost corresponds to the total semicoherent elastic cross section.     

Evidence that absorption in the π+ π− π− system in nuclear matter is stronger in the JP = 0− than in the JP = 1+ state

In the reaction π^? + A → π⁺π^?π^? + A at 15.1 GeV/c the coherently produced 3π mass spectrum has a broad peak around 1.1 GeV which consists mainly of a J^P = 1⁺ state and of a smaller fraction of 0^-. Comparing the observed dependence of the production of these diffractive states on the atomic weight A of the target nuclei with the optical model, we have studied the absorption of the produced states in nuclear matter. We have found the following values of the absorption cross-sections: σ₂(0^-) = 49_?7⁺⁹ mb and σ₂(1⁺) = 15.8_?1.3^+1.5 mb.     

Nematics with quenched disorder: pinning out the origin of memory

Memory effects and glassy behavior have been repeatedly observed in disordered nematic liquid crystals but the connection between these effects and the system topology remained unrevealed. We present an analysis of the local and global topology of the nematic ordering in the presence of quenched disorder and we show that nematics with quenched disorder can be mapped into a system of pinned defect lines and that the memory of the system stems from the pinning of these strings.     

Phase-locked, time-delayed harmonic pulses for high spectral resolution in the extreme ultraviolet

We present experiments in the time and frequency domains aimed at confirming the measured mutual phase coherence of time-delayed, collinear harmonic pulses. We show that pairs of phase-locked harmonic pulses of medium order can be generated for peak intensities up to ~1.5 10(14)W/cm(2) in xenon, demonstrating the possibility of performing high-resolution spectroscopy in the extreme ultraviolet with Ramsey-like techniques.     

New experimental and simulated results on nuclear media effects in meson photoproduction off nuclei

New experimental approach to study meson photoproduction in nuclei is discussed. It is based on the measurement of the recoil nucleon as a tagger for identification of the initial step of reaction. Propagation of mesons and nucleons within a nucleus is described by the Intra-Nuclear Cascade (INC) model. Simulations and preliminary experimental data for the deuteron target obtained at GRAAL are presented.     

Fierz–Pauli equation for massive gravitons from Induced Matter theory of gravity

Starting with a 5D physical vacuum described by a 5D Ricci-flat background metric, we study the emergence of gravitational waves (GW) from the Induce Matter (IM) theory of gravity. We obtain the equation of motion for GW on a 4D curved spacetime which has the form of a Fierz–Pauli one. In our model the mass of gravitons mg$m_g$ is induced by a static foliation on the noncompact space-like extra dimension and the source-term is originated in the interaction of the GW with the induced connections of the background 5D metric. Here, relies the main difference of this formalism with the original Fierz–Pauli one.     

Barium‐Mediated Cross‐Dehydrocoupling of Hydrosilanes with Amines: A Theoretical and Experimental Approach

Alkaline‐earth (most prominently barium) complexes of the type [Ae{N(SiMe₃)₂}₂?(THF)_x] and [{N^N}Ae{N(SiMe₃)₂}?(THF)_x] are very active and productive precatalysts (TON=396, TOF up to 3600 h^?1; Ca相似文献