A unified view of coherent and incoherent dihydrogen exchange in transition metal hydrides by nuclear resonance and inelastic neutron scattering

In this paper a unified view of coherent and incoherent dihydrogen exchange in transition metal hydrides by nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) is presented. It is shown that both exchange processes coexist i.e. do not transform into each other although they may dominate the spectra in different temperature ranges. This superposition is the consequence of the incorporation of the tunnel frequency J of the coherent process into the nuclear two-spin hamiltonian of hydrogen pairs which allows to treat the problem using the well known density matrix theory of NMR line-shapes developed by Alexander and Binsch. It is shown that this theory can also be used to predict the line-shapes of the rotational tunneling transitions observed in the INS spectra of transition metal dihydrogen complexes and that both NMR and INS spectra depend on similar parameters

Selective C-H Activation at a Molecular Rhodium Sigma-Alkane Complex by Solid/Gas Single-Crystal to Single-Crystal H/D Exchange

The controlled catalytic functionalization of alkanes via the activation of C-H bonds is a significant challenge. Although C-H activation by transition metal catalysts is often suggested to operate via intermediate σ-alkane complexes, such transient species are difficult to observe due to their instability in solution. This instability may be controlled by use of solid/gas synthetic techniques that enable the isolation of single-crystals of well-defined σ-alkane complexes. Here we show that, using this unique platform, selective alkane C-H activation occurs, as probed by H/D exchange using D2, and that five different isotopomers/isotopologues of the σ-alkane complex result, as characterized by single-crystal neutron diffraction studies for three examples. Low-energy fluxional processes associated with the σ-alkane ligand are identified using variable-temperature X-ray diffraction, solid-state NMR spectroscopy, and periodic DFT calculations. These observations connect σ-alkane complexes with their C-H activated products, and demonstrate that alkane-ligand mobility, and selective C-H activation, are possible when these processes occur in the constrained environment of the solid-state

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.Comment: To appear in Space Science Reviews. The final publication is available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents

Untersuchungen an organischen Festkoerpern mit optischer Kernspinpolarisation und Multiquanten-NMR

Available from TIB Hannover: MA 6829 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

H-solid state NMR studies of tunneling phenomena and isotope effects in transition metal dihydrides

In many transition metal dihydrides and dihydrogen complexes the hydrogens are relatively weakly bound and exhibit a fairly high mobility, in particular with respect to their mutual exchange. Part of this high mobility is due to the exchange symmetry of the two hydrogens, which causes an energy splitting into even and odd spatial energy eigenfunctions, resulting in the typical coherent tunneling of a two-level system. Owing to the quantum mechanical symmetry selection principles the eigenfunctions are connected to the possible nuclear spin states of the system. If the tunneling frequency is in the proper frequency window it is thus possible to observe these tunneling transitions by NMR at very low temperatures, where no thermally induced exchange reactions overshadow the tunneling. The first part of this review gives an introduction into the interplay of chemical kinetics and tunneling phenomena in general, rotational tunneling of dihydrogen in a two-fold potential in particular and the Bell tunnel model, followed by a summary of solid state NMR techniques for the observation of these tunnel processes. Then a discussion of the effects of these processes on the (2)H NMR line shape is given. The second part of the review reports results of a (2)H-solid state NMR spectroscopy and T(1) relaxatiometry study of trans-Ru(D(2))Cl(PPh(2)CH(2)CH(2)PPh(2))(2)PF(6), in the temperature regime from 5.4 to 320 K. In the Ru-D(2) sample coherent tunneling and incoherent exchange processes on the time scale of the quadrupolar interaction are observed. From the spectra and T(1)-data the height of the tunneling barrier is determined. Next results of (2)H-spin-lattice relaxation measurements for a selectively eta(2)-D(2) labeled isotopomer of the complex W(PCy(3))(2)(CO)(3))(eta(2)-D(2)) are presented and discussed. The relaxation measurements are analyzed in terms of a simple one dimensional Bell tunnel model and comparison to incoherent neutron scattering (INS) data from the H(2) complex. The comparison reveals a strong isotope effect of 2 x 10(3) for the exchange rates of the deuterons versus hydrons

Revealing CSA tensors and hydrogen bonding in methoxycarbonyl urea: A combined C-13, N-15 and (CN2)-C-13-N-14 dipolar chemical shift NMR and DFT study

Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by C-13 and N-15 dipolar chemical shift NMR spectroscopy and ab initio calculations. Employing a combination of dipolar chemical shift NMR, selective isotope labeling and ab initio gas phase calculations, possible molecular structures and chemical shielding tensors of all N-15 nuclei and of two out of the three C-13 nuclei were revealed. Four possible stable configurations of the molecule with different energies were found in the calculations. The CSA tensors were calculated for these configurations. While the calculated C-13(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tenser orientation, there are pronounced differences between calculated and experimental tensor eigenvalues. These differences are a clear indication of the presence of intermolecular hydrogen bonds in the experimental sample, which are neglected in the gas phase calculations. Four different possible orientations of the experimental C-13(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the C-13 CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where delta(11) points approximately in direction of N(imide), delta(22) approximately in direction of the C=O bond, and delta(33) is oriented perpendicular to the molecular frame, is adopted in the molecule