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D Atoms (d + atom)
Selected AbstractsAmino-phosphanes in RhI -Catalyzed Hydroformylation: New Mechanistic Insights Using D2O as Deuterium-Labeling AgentEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2006Jacques Andrieu Abstract In previous work, we have demonstrated that the dangling amino group in amino-phosphane ligands increases the rate of Rh-catalyzed styrene hydroformylation as a function of the amino group basicity and of the distance between the P and N functions. We now report additional stereochemical and mechanistic insights resulting from new catalytic experiments performed with Rh-,-P,N catalytic systems in the presence of D2O. In addition to the expected D0 product, the formation of the ,-D1 aldehyde, PhCH(CH2D)CHO was observed in all cases by 1H and 13C NMR spectroscopy, indicating that H/D exchange occurs for the rhodium-hydride complex. Minor amounts of a ,-D2 product, PhCH(CHD2)CHO, were also formed under certain conditions, demonstrating the reversibility of the olefin coordination step. The composition of the aldehyde mixture is slightly affected by the nature of the catalytic precursor or the P,N ligand used. In the specific case of the ,-P,N ligand [,-P,N = (SAr,SC)-Ph2PCH{o -C6H4Cl(Cr(CO)3)}NHPh], in combination with the [RhCl(COD)]2 precatalyst, products PhCD(CH3)CHO (,-D1) and PhCD(CH2D)CHO (,,,-D2) were also produced. This result suggests a reversible deprotonation assisted by an intramolecular H-bonding interaction between the dangling ammonium function and the carbonyl moiety. This isotopic exchange process decreases the asymmetric induction from 14 to 7,% ee when using the enantiopure version of this ligand. Aldehydes bearing a D atom on the formyl group, e.g. PhCH(CH3)CDO, were never observed. The latter observation excludes protonolysis of the rhodium-acyl intermediate as the aldehyde forming step. In addition, it also excludes a bimolecular reaction involving the rhodium-acyl and rhodium-hydride intermediates.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2001The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co,C*D(CH3)CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72,h and 27,d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*,D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co,C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co,C bond to form the inverted 1-cyanoethyl group. [source] Neutron powder diffraction study of orthorhombic and monoclinic defective silicaliteACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2000G. Artioli The crystal structure of silicalite (SiO2) with a substantial amount of structural hydroxyl groups [Si(1,x)O(2,4x)(OH)4x, with 0.08 < x < 0.10] has been refined from neutron powder diffraction data measured using the HRPD instrument at the ISIS pulsed neutron source. Powder data were collected on the as-synthesized orthorhombic sample at 298,K, and on the deuterated monoclinic sample at 100,K. Preferential location of Si-atom vacancies was found on four out of 12 independent T sites in the orthorhombic silicalite [Si(6), Si(7), Si(10) and Si(11)], although the H atoms of the substituting hydroxyl groups could not be located because of the low statistical site occupancy on multiple sites. No significant population of D atoms or of Si vacancies was found in the tetrahedral sites of the monoclinic sample. The detected long-range order of adjacent Si atoms in defective orthorhombic [MFI] structures is compatible with a mechanism of Si vacancy clustering and with the model of hydroxyl nests assumed in the literature on the basis of IR spectroscopic evidence. Crystal data: orthorhombic, Pnma, Z = 8, a = 20.0511,(1), b = 19.8757,(1), c = 13.36823,(9),Å, V = 5327.62,(5),Å3, Dx = 1.798,g,cm,3, Mr = 721.01; monoclinic, P21/n, Z = 4, a = 19.8352,(2), b = 20.0903,(2), c = 13.3588,(1),Å, , = 90.892,(1)°, V = 5322.78,(6),Å3, Dx = 1.799,g,cm,3, Mr = 1442.02. [source] Unambiguous determination of H-atom positions: comparing results from neutron and high-resolution X-ray crystallographyACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2010Anna S. Gardberg The locations of H atoms in biological structures can be difficult to determine using X-ray diffraction methods. Neutron diffraction offers a relatively greater scattering magnitude from H and D atoms. Here, 1.65,Å resolution neutron diffraction studies of fully perdeuterated and selectively CH3 -protonated perdeuterated crystals of Pyrococcus furiosus rubredoxin (D-rubredoxin and HD-rubredoxin, respectively) at room temperature (RT) are described, as well as 1.1,Å resolution X-ray diffraction studies of the same protein at both RT and 100,K. The two techniques are quantitatively compared in terms of their power to directly provide atomic positions for D atoms and analyze the role played by atomic thermal motion by computing the , level at the D-atom coordinate in simulated-annealing composite D-OMIT maps. It is shown that 1.65,Å resolution RT neutron data for perdeuterated rubredoxin are ,8 times more likely overall to provide high-confidence positions for D atoms than 1.1,Å resolution X-ray data at 100,K or RT. At or above the 1.0, level, the joint X-ray/neutron (XN) structures define 342/378 (90%) and 291/365 (80%) of the D-atom positions for D-rubredoxin and HD-rubredoxin, respectively. The X-ray-only 1.1,Å resolution 100,K structures determine only 19/388 (5%) and 8/388 (2%) of the D-atom positions above the 1.0, level for D-rubredoxin and HD-rubredoxin, respectively. Furthermore, the improved model obtained from joint XN refinement yielded improved electron-density maps, permitting the location of more D atoms than electron-density maps from models refined against X-ray data only. [source] Preliminary neutron and ultrahigh-resolution X-ray diffraction studies of the aspartic proteinase endothiapepsin cocrystallized with a gem -diol inhibitorACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 12 2007Han-Fang Tuan Endothiapepsin has been cocrystallized with the gem -diol inhibitor PD-135,040 in a low solvent-content (39%) unit cell, which is unprecedented for this enzyme,inhibitor complex and enables ultrahigh-resolution (1.0,Å) X-ray diffraction data to be collected. This atomic resolution X-ray data set will be used to deduce the protonation states of the catalytic aspartate residues. A room-temperature neutron data set has also been collected for joint refinement with a room-temperature X-ray data set in order to locate the H/D atoms at the active site. [source] | ||||||||||