Bjorn Pedersen

Characterisation of agostic interactions by a topological analysis of experimental and theoretical charge densities in [EtTiCl3(dmpe)] [dmpe = 1,2-bis(dimethylphosphino)ethane]

Topological analysis of the experimental and theoretical electron densities in [EtTiCl3(dmpe)] (dmpe = Me2PCH2CH2PMe2) suggests the presence of a (3, –1) bond critical point (CP) between titanium and the β-agostic hydrogen atom; the characteristic curvature in the Ti–Cα bond is proposed as a more general criterion for identifying a β-agostic interaction.

Structure of dehydroisoascorbic acid isomers in solution

Abstract N.m.r. spectroscopy ( 1 H and 13 C) shows that dehydroisoascorbic acid ( d - erythro -2,3-hexodiulosono-1,4-lactone) in solution is only partially similar to dehydroascorbic acid ( l - threo -2,3-hexodiulosono-1,4-lactone). In N,N -dimethylformamide, the preponderant species is an asymmetric dimer, only 7% of the symmetric dimer is present, and other species are also detected. In water, significant differences between dehydroascorbic acid and dehydroisoascorbic acid are observed. In fresh, aqueous solutions, both acids are present as bicyclic lactones, but, with time, dehydroisoascorbic acid is transformed irreversibly into approximately equal amounts of two pyranose anomers. In contrast, dehydroascorbic acid mainly changes into a hydrated 1,4-lactone having a free side-chain. This difference is probably caused by strain in the lactone ring of dehydroisoascorbic acid, because of the proximity of O-4 and O-5 after formation of the furanoid ring. In water, this leads to opening of the lactone ring prior to the furanoid ring, and a simple equilibrium between iso-ascorbic acid and its primary oxidation product is lost.

The IR, Raman and NMR spectra and conformations of cyclohexylallene

Abstract Three conformers of cyclohexylallene, designated I, II and III, were observed in the IR and Raman spectra. Metastable crystals formed by annealing an amorphous solid from 80 K to 140 K, and crystals formed under a pressure of 30 kbar at ambient temperature, contained conformer II. The stable crystals obtained after heating the metastable crystals to 195 K contained conformer I. The conformer III was observed as a minor component in a nitrogen matrix heated to 900 K before deposition. Negligible intensity variations between the bands of conformer I and II were observed with nozzle temperatures 300–900 K in the argon and nitrogen matrices, indicating a ΔHo(I-II) below 0.4 kJ mol−1 while ΔHo(III-II) was equal to 6.5 kJ mol−1 and a barrier higher than ca. 11 kJ mol−1 was observed between the latter. Variable temperature measurements in Raman indicate ΔHo(I-II) ≈ 0.9 kJ mol−1 in the liquid. The 13C NMR spectra show that in deuteromethanol solution the equatorial conformer dominates (≈ 2% a at 193 K, ΔGo(a–e) ≈ 6.4 kJ mol−1). Analysing the temperature dependence of the vicinal H-H coupling across the cyclohexane - allene carbon bond on the basis of a guache ↔ anti equilibrium it is found 40% anti and 60% gauche in methanol solution at ambient temperature. We conclude that the abundant conformers II and I being respectively the anti and the gauche conformer, while III is the anti axial conformer.

The crystal and molecular structures of trans-1,4-dichlorocyclohexane

Abstract A new phase transition in the title compound is observed at 260 K by DSC and NMR. Furthermore, the phase stable between 260 K and 280 K is found to be triclinic and not monoclinic as believed earlier. The structure is determined from single crystal X-ray data, and the molecules are found to be in the ee chair conformation. In the phase stable below 260 K the 13C CPMAS NMR spectrum shows that the molecules are in the aa chair conformation.

Magnetic couplings in Cs2IrCl6 and other iridium hexachlorides from NMR studies

NMR line shift and relaxation of 133Cs in FCC Cs2IrCl6 show that this salt is antiferromagnetic with TN < 1.6 K and θ = - 4 ± 2 K. The contact couplings AIi · Sj of 133Cs to Ir4+ are much larger than the corresponding dipolar couplings, A1/h = - 5.9 MHz to nearest neighbours and A2/h = 0.9 MHz to the next nearest neighbours. The corresponding couplings to 39K in K2IrCl6 are A1/h = - 0.3 MHz and A2/h = 0.05 MHz. The Ir4+-Ir4+ superexchange J1S j·Sk vary very rapidly with alkali ion A+ substitution in the iridium hexachlorides. We suggest that this is due to an increasing ferromagnetic coupling IrCl2-6 (↑)A+(↓)-Cl6Ir2-(↑) partly counteracting the decreasing antiferromagnetic coupling IrCl2-6(↑)-Cl6Ir 2-(↓) when we go from A+ = K+, NH+4 and Rb+ to A+ = Cs+. We give some very crude estimates for the strengths of the superexchanges.

NUCLEAR MAGNETIC RESONANCE STUDIES OF METAL HYDRIDES

ABSTRACT Applications of NMR spectroscopy to the study of structural and dynamic aspects of metal hydrides are reviewed. The low temperature proton spectra give information on the crystal structure. The method can not compete with neutron diffraction in the determination of more complex hydride structures. But as the proton spectrum is determined by the local order and neutron diffraction by the long range order unique information from NMR can be obtained in disordered structures. At higher temperatures the proton spectrum is narrowed by hydrogen selfdiffusion in all hydrides studied (both metallic and salt like). Both the mean life time of a hydrogen atom in a site and the mean distance a hydrogen atom jumps can be determined by NMR spectroscopy giving detailed microscopic information on the diffusion process. The deuteron spectra and the spectra of the different metal nuclei give information of the local symmetry of the occupied sites. Observed shifts of the proton resonance frequency and the metal resonance frequency give information about the electronic structure. The observed paramagnetic shifts in the metal resonance frequency is similar to shifts observed in alloys. Recently fairly large diamagnetic shifts in the proton resonance frequency have been reported, but the origin of these shifts are not known.