Site Exchange Process Research Articles

A Dehydrogenated Cycloadduct of N-Phenylmaleimide and the Azomethine Ylide Derived from Ninhydrine and Phenylalanine: Structure and Conformation

The structure of the dehydrogenation product 1′,3a′-dihydro-3′-((1,3-dioxoindan-2-ylidene)-phenyl-methyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′-(5′H, 6a′H)-tetrone derived from the cycloadducts (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((R)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone and/or (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((S)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone, which were synthesized by 1,3-dipolar cycloaddition of N-phenylmaleimide to 2-((2-(1,3-dioxoindan-2-yl)-2-phenyl-ethenyl)-imino)-indan-1,3-dione, was determined by X-ray analysis. Crystal data (CCD, 180 K): rhombohedral, R&3macr;;, a = 34.0871(7), c = 13.9358(5) A, Z = 18; the structure was solved by direct methods and refined by full-matrix least-squares procedures to R(F, I ≥ 3σ(I)) = 0.053. The molecule contains a central folded ring system of two cis-fused 5-membered heterocyclic rings; each ring is nearly planar, and the angle between the rings amounts to 59.0°. Dynamic 1H NMR spectroscopy of the product revealed an exchange process caused by restricted rotation of the double bonded 1,3-indandione moiety and the phenyl group about the Csp2-Csp2 single-bonds. Molecular modeling and complete lineshape analysis indicated a four site exchange process for which free energies of activation and free energies could be established. ΔG‡ values for the barriers of rotation are in the range of 57–59 kJ · mol − 1 at 273 K, which is unusually high for an unsubstituted phenyl group.

Oxygen-driven rearrangement of Ag and Au monolayers on W( [formula omitted]) studied by photoemission spectroscopies

Exposure of Ag or Au monolayers deposited on W(1 1 0) to molecular oxygen leads to the formation of admetal double layers and open W patches covered with atomic oxygen. We have studied this rearrangement process in its dependence on admetal coverage, oxygen exposure and substrate temperature employing valence band and core-level photoemission. We present experimental evidence that the first reaction step, dioxygen dissociation, occurs through the closed Ag monolayer on W(1 1 0). Subsequent site-exchange processes lift Ag atoms into the second layer. We compare the restructuring of Ag layers with the one of Au layers. While the basic mechanism appears to be similar, we observe a different dependence on sample temperature during oxyen exposure.

High resolution photoemission core level spectroscopy study and TEM analysis of the Ge/As/Si(0 0 1) growth

The deposition of 1 ML of As on Si(001)-(2×1) surface and the heteroepitaxial Ge/As/Si(001)-(2×1) growth were studied by using high resolution core-level spectroscopy and transmission electron microscopy (TEM). From the spectral decomposition of the Si2p core levels, collected on the Ge/Si interfaces obtained by codepositing As and Ge atoms at different Ge thickness, we identify the contribution of different Si layers to the Si2p lineshape, in agreement with recent theoretical final-state pseudopotential calculations. The bulk-like Ge3d core levels reflected the occurrence of a complete Ge–As site-exchange process, while the pseudomorphic layer-by-layer Ge/Si growth was confirmed by TEM investigation.

Coordination sphere flexibility of active-site models for Fe-only hydrogenase: studies in intra- and intermolecular diatomic ligand exchange.

A series of dinuclear complexes, (mu-SRS)Fe(2)(CO)(6) (R = -CH(2)CH(2)-, -CH(2)CH(2)CH(2)-, -CH(2)-C(6)H(4)-CH(2)-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable-temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu-pdt)Fe(2)(CO)(6) while in the (mu-o-xyldt)Fe(2)(CO)(6) compound, the bridge is fixed. Temperature-dependent (13)C NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu-SRS unit. Kinetic studies of intermolecular CO/CN(-) ligand-exchange reactions establish associative or I(a) mechanisms in sequential steps to form the dicyano dianion, (mu-SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN(-)/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H(2) activation/production process in the enzyme.

Crystal structures and coordination-site exchange reactions of palladium(II) and platinum(II) complexes containing tris[2-( tert-butylthio)ethyl]amine

Palladium(II) and platinum(II) complexes, [PdX(NS 3 1 Bu)]BPh 4 (X=Cl, Br, I; NS 3 1 Bu=tris[2-( tert-butylthio)ethyl]amine) and [PtCl(NS 3 1 Bu)]BPh 4, were prepared, and their structures were determined by X-ray analyses. The geometry around the palladium and platinum atoms is square planar. The NS 3 1 Bu ligand functions as a tridentate ligand and one sulfur atom is not coordinated to the metal. The 1H NMR spectrum of [PdCl(NS 3 1 Bu)]BPh 4 in acetone-d 6 exhibited a dynamic behavior. At 20°C the spectrum showed a singlet signal at 1.60 ppm that can be assigned to tert-butyl protons, whereas at −70°C three singlet signals were observed at 1.36, 1.61, and 1.70 ppm with an intensity ratio of 1: 0.25: 2. The signals at 1.36 and 1.70 ppm are assigned to the tert-butyl protons in the square-planar structure, and these signals are consistent with the X-ray structure. The signal at 1.61 ppm can be assigned to the tert-butyl protons in a trigonal-bipyramidal structure where the three tert-butyl groups are magnetically equivalent. Thus, we concluded that the coordination-site exchange occurred via a trigonal-bipyramidal intermediate. The square-planar and trigonal-bipyramidal species of [PdCl(NS 3 1 Bu)]BPh 4 are in equilibrium in acetone-d 6. The equilibrium was shifted toward the square-planar species on decreasing the temperature. The 1H NMR spectra for [PdX(NS 3 1 Bu)]BPh 4 (X=Cl, Br, and I) were similar to one another at the same temperature, suggesting that the site-exchange process is insensitive to the kind of coexisting halogen ligand. The site exchange reaction of [PtCl(NS 3 1 Bu)]BPh 4 seems to occur more slowly than that of the palladium(II) analogue.

Surface Segregation in Ge/Si System Studied by Medium Energy Ion Scattering.

The Ge segregation mechanism in the Si/Ge system has been studied by medium-energy ion scattering (MEIS). One monolayer of Ge was grown on Si(001) at 450oC and annealed at 780oC. We find that the Ge redistributes during the very early stages of the annealing process and remains constant even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time (less than 1 min). In addition, we find that the activation energy for the site exchange process occurring at the subsurface region is lower than the previously reported value when Si overlayer is grown on the Ge thin film. The exchange between the second and third layers occurs readily during the growth, even though these layers are buried, and are not expected to reduce the surface free energy. The contribution of the second layer to Ge segregation is similar to that of the topmost layer.

Analysis of1H and13C NMR spectra ofcis- andtrans-10-substituted decal-2-ones by 2D NMR techniques

The 1H and 13C NMR chemical shift assignments of 10-substituted decal-2-ones, cis (H, CH3 and CO2CH3) and trans (H, CH3, OH and CO2C2H5) were based on 2D COSY and HETCOR experiments, supported by selective spin decoupling experiments. The discrimination between the cis and the trans conformations was ruled out on the basis of the J(1a,9) magnitudes related to dihedral angles (Karplus rule) and also on the 13C shielding values of C-4 of the cis compounds, which are much more shielded than those of the trans counterparts. The best and fastest criterion of differentiation between cis and trans conformers is the constant appearance in the 13C PND spectra of the cis series of some signals of broadened linewidth and low intensity, essentially C-4, C-5 and C-7, a clue to site exchange processes; the trans counterparts showed only sharp signals because of their existence as a single conformation. Copyright © 2000 John Wiley & Sons, Ltd.

Analysis of 1H and 13C NMR spectra of cis‐ and trans‐10‐substituted decal‐2‐ones by 2D NMR techniques

The 1H and 13C NMR chemical shift assignments of 10-substituted decal-2-ones, cis (H, CH3 and CO2CH3) and trans (H, CH3, OH and CO2C2H5) were based on 2D COSY and HETCOR experiments, supported by selective spin decoupling experiments. The discrimination between the cis and the trans conformations was ruled out on the basis of the J(1a,9) magnitudes related to dihedral angles (Karplus rule) and also on the 13C shielding values of C-4 of the cis compounds, which are much more shielded than those of the trans counterparts. The best and fastest criterion of differentiation between cis and trans conformers is the constant appearance in the 13C PND spectra of the cis series of some signals of broadened linewidth and low intensity, essentially C-4, C-5 and C-7, a clue to site exchange processes; the trans counterparts showed only sharp signals because of their existence as a single conformation. Copyright © 2000 John Wiley & Sons, Ltd.

Ge segregation mechanism during Si/Ge multilayer growth

The Ge segregation mechanism during Si/Ge multilayer growth has been investigated using medium-energy ion scattering (MEIS) and computer simulation with the stochastic Monte Carlo technique. We found that Ge atoms segregated not only in the topmost layer but also in the second and third layers. The segregation probability is found to be quadratic in the Ge concentration at the surface. The exchange between the second and the third layer also occurs readily during growth, even although these layers are buried and therefore are not expected to reduce the surface free energy. We found that the activation energy for the site exchange process at the subsurface is lower than the previously believed value, and flip-flop exchange at the subsurface plays a more important role in the Ge segregation process.

Oxidative Addition of HX (X = H, SiR3, GeR3, SnR3, Cl) Molecules to the Complex Os(η5-C5H5)Cl(PiPr3)2

The cyclopentadienyl complex Os(η5-C5H5)Cl(PiPr3)2 (1) reacts with molecular hydrogen to give a mixture of the isomers transoid-dihydride OsH2(η5-C5H5)Cl(PiPr3) (2a) and cisoid-dihydride OsH2(η5-C5H5)Cl(PiPr3) (2b). Isomer 2a has a rigid structure in solution. However, the hydride ligands of 2b undergo a thermally activated site exchange process and show quantum exchange coupling. The reaction of the isomeric mixture of 2a and 2b with NaBH4 and methanol leads to the trihydride OsH3(η5-C5H5)(PiPr3) (3). In solution, the cisoid hydride ligands of this complex also undergo a thermally activated site exchange process. The activation parameters are ΔH⧧ = 15.1 ± 0.3 kcal·mol-1 and ΔS⧧ = 0.5 ± 0.7 cal·mol-1·K-1. Complex 1 also reacts with group 14 element hydride compounds to afford OsH(η5-C5H5)Cl(ER3)(PiPr3) [ER3 = SiEt3 (4), Si(CH2−CHCH2)Me2 (5), SiPh3 (6), SiHPh2 (7), SiH2Ph (8), GeEt3 (9), GePh3 (10), GeHPh2 (11), SnnBu3 (12), SnPh3 (13)]. The structure of 7 has been determined by an X-ray investigation. The distribution of ligands around the metallic center can be described as a piano stool geometry with the triisopropylphosphine and diphenylsilyl ligands disposed mutually transoid. Complex 12 reacts with HSnnBu3 to give the dihydride−stannyl derivative OsH2(η5-C5H5)(SnnBu3)(PiPr3) (14). The reaction of 1 with HCl leads to a mixture of the monohydrides OsH(η5-C5H5)Cl2(PiPr3) (15) and [OsH(η5-C5H5)Cl(PiPr3)2]+ (16). Complexes 15 and 16 can also be obtained by reaction of 6 with HCl and by protonation of 1 with HBF4, respectively.

Manifestations of Slow Site Exchange Processes in Solution NMR: A Continuous Gaussian Exchange Model

The effects of site exchange due to slow conformational changes in rapidly rotating molecules in solution are examined in detail. Significant gaps in the currently available theory are filled. The effects of site exchange on the lineshape, decay of a simple spin-echo, decay of the even echoes in a Carr–Purcell–Meiboom–Gill (CPMG) pulse-sequence, and decay of the transverse magnetization in a resonant spin-locking field are investigated. Both trajectory and stochastic operator approaches are formulated and shown to be completely equivalent whenever the dynamics of population transfers among the inequivalent sites is governed by either a stationary or a nonstationary Markov process. A nonstationary Markov process may result from Brownian dynamics (a stationary Markov process) in a larger conformational space that contains the subspace of inequivalent sites. A continuous Gaussian exchange model is formulated in which a nucleus undergoes continuous one-dimensional motion in a harmonic potential well that is located in a linear chemical shift gradient. The effects of this Gaussian exchange model on the lineshape, simple spin-echo decay, and decay of the even echoes of a CPMG pulse train are treated rigorously via the trajectory approach. Compact analytical expressions are obtained for the relevant correlation functions in each case. The relevant decays are found to be exponential in the very short time and long time limits, which are not necessarily experimentally significant in any given case. In the fast exchange limit the relevant decays are exponential at all times, and explicit formulas are given for their decay rates. In the long time limit, all discrete multisite models with the same intrinsic Ro2 at every site are shown to be completely equivalent to a continuous Gaussian model with appropriate relaxation time and variance of the Larmor frequency. The effects of this Gaussian exchange model on the decay of the transverse magnetization in a resonant spin-locking field are treated heuristically by a trajectory approach. The intrinsic contribution (Ro1ρ) of rapid rotations and dipole–dipole interactions to relax the transverse magnetizations of two nuclei of the same kind in the presence of a (nearly) resonant spin-locking field is also derived and found to be practically the same as the intrinsic contribution, Ro2, of those same rotations to the simple and CPMG spin-echo decay rates and linewidth. Literature data for the linewidth, decay rate of the CPMG even spin-echoes, and R1ρ decay rate for the A9-H2 protons of adenines at the central TpA step in the sequence, 5′-GCAGGTTTAAACCTCG-3′, are analyzed using the Gaussian exchange model to assess the time-scale and variance of the site exchange process as well as the intrinsic Ro2 rate. Although a single Gaussian exchange process with appropriate parameters can fit these three A9-H2 data rather well, this particular “solution” cannot be reconciled with NMR relaxation data on other protons in the same DNA molecule. Rather good agreement with all of the observations is obtained by using a model of two concurrent Gaussian exchange processes, whose relaxation times, τ = 7 and 460 μs, differ in time-scale by a factor of 65. The insensitivity of R1ρ in the presence of a fast site exchange process to much slower concurrent site exchange processes is explicitly demonstrated. Protocols for detecting and characterizing a second slow site exchange process are suggested.

Synthesis and Characterization of OsH2Cl[κN,κO-(ONCR2)](PiPr3)2 (CR2 = C(CH2)4CH2, R = CH3): Influence of the L2 Ligand on the Nature of the H2 Unit in OsH2ClL2(PiPr3)2 (L2 = ONCR2, NHC(Ph)C6H4) Complexes

The dihydride−dichloro complex OsH2Cl2(PiPr3)2 (1) reacts with cyclohexanone oxime and acetone oxime in the presence of Et3N to give the dihydride derivatives (2) and OsH2Cl{κN,κO-[ONC(CH3)2]}(PiPr3)2 (3), respectively. The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid, with the triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the hydride ligands, the chlorine, and the oximate group, which acts with a bite angle of 36.6(1)°. In solution the hydride ligands of 2 and 3 undergo an intramolecular thermally activated site exchange process. The activation parameters of this process are ΔH⧧ = 11.9(±0.7) kcal mol-1 and ΔS⧧ = −0.5(±1.4) cal mol-1 K-1 for 2 and ΔH⧧ = 11.7(±0.8) kcal mol-1 and ΔS⧧ = −0.8(±2.0) cal mol-1 K-1 for 3. To understand why complexes 2 and 3 are dihyride derivatives, while the previously reported complex is an elongated dihydr...

Strained Ge/Si(001) interface in the presence of Bi and Sb surfactants

Abstract The adsorption of Ge on the Bi/Si(001)2×5 surface (Bi saturation coverage of 0.7 ML) and on Sb/Si(001)2×1 (Sb saturation coverage of 0.9 ML), was investigated by core level photoemission spectroscopy. For the Bi/Si(001)2×5 interface, at submonolayer Ge coverage, the presence of a bulk component largely dominating the Ge 3d core level, demonstrates that the favorite adsorption sites are the Bi-terminated terraces, where the Ge–Bi site exchange takes place. At larger Ge coverage, in addition to the previous process, the occurrence of a pure Ge epitaxy on the bare fraction of the substrate is indicated by the presence of the 2×1 surface components in the Ge 3d core level. In the case of Sb/Si(001)2×1 interface, the Ge 3d shows a complete lack of surface components, which indicates the absence of dimerized Ge atoms at the top layer, and demonstrates that the Ge–Sb site exchange process has taken place on the whole Si(001) surface.

Thermally Activated Site Exchange and Quantum Exchange Coupling Processes in Unsymmetrical Trihydride Osmium Compounds.

Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the trihydride derivative OsH(3){kappa-N,kappa-S-(2-Spy)}(P(i)Pr(3))(2) (2). The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. The equatorial plane contains the pyridine-2-thiolato group, attached through a bite angle of 65.7(1) degrees, and the three hydride ligands. The theoretical structure determination of the model complex OsH(3){kappa-N,kappa-S-(2-Spy)}(PH(3))(2) (2a) reveals that the hydride ligands form a triangle with sides of 1.623, 1.714, and 2.873 Å, respectively. A topological analysis of the electron density of 2a indicates that there is no significant electron density connecting the hydrogen atoms of the OsH(3) unit. In solution, the hydride ligands of 2 undergo two different thermally activated site exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The activation barriers of both processes are similar. Theoretical calculations suggest that the transition states have a cis-hydride-dihydrogen nature. In addition to the thermally activated exchange processes, complex 2 shows quantum exchange coupling between the central hydride and the one situated close to the sulfur atom of the pyridine-2-thiolato group. The reactions of 1 with L-valine and 2-hydroxypyridine afford OsH(3){kappa-N,kappa-O-OC(O)CH[CH(CH(3))(2)]NH(2)}(P(i)Pr(3))(2) (3) and OsH(3){kappa-N,kappa-O-(2-Opy)}(P(i)Pr(3))(2) (4) respectively, which according to their spectroscopic data have a similar structure to that of 2. In solution, the hydride ligands of 3 and 4 also undergo two different thermally activated site exchange processes. However, they do not show quantum exchange coupling. The tetranuclear complexes [(P(i)Pr(3))(2)H(3)Os(&mgr;-biim)M(TFB)](2) [M = Rh (5), Ir (6); H(2)biim = 2, 2'-biimidazole; TFB = tetrafluorobenzobarrelene] have been prepared by reaction of OsH(3)(Hbiim)(P(i)Pr(3))(2) with the dimers [M(&mgr;-OMe)(TFB)](2) (M = Rh, Ir). In solution the hydride ligands of these complexes, which form two chemically equivalent unsymmetrical OsH(3) units, undergo two thermally activated site exchanges and show two different quantum exchange coupling processes.

Relationship of Hydride Fluxionality in Ir(H)2L(phosphine)2n+ to Properties of L: An Exceptional Range of Barrier Heights

The site exchange process illustrated here shows ΔH⧧ values from 4.8 to 11.6 kcal/mol, depending on the identity of L (halide, OSO2CF3, HN3, CO, H2, NHC(O)CH3, PiPr2Ph, PCy2Ph).

Bulk-likeSi(001) atomic rearrangement artificially created at theGe/Sb/Si(001) interface

We report here on the electronic properties of the Ge/Sb/Si(001) interface formation studied by very high-energy-resolution photoemission spectro-scopy using synchrotron radiation. Surface- and bulk-sensitive measurements of the core levels of the different atomic species give a clear indication of the occurrence of a site exchange process between germanium and antimony, when Ge is deposited on the already prepared Sb/Si(001) interface. We demonstrate that the use of surfactant atoms like Sb at the strained Ge/Si(001) interface produces a perfect epitaxial interface, where the Si atoms are in register with the bulk geometrical positions.

Hydrogen site exchange in CpIr(PR 3)H 3+ complexes

Ab initio molecular orbital calculations on CpIr(PH 3)H 3 + yield an optimized structure for the ground state which is very close to the neutron structure of a related complex. The hydrogen site exchange process is computed to occur with an activation energy of 17.0 kcal mol −1 via an η 2-H 2 species which constitutes a pairwise exchange mechanism in agreement with the NMR lineshape analysis of [(η-C 5R 5)Ir(L)H 3]BF 4 (RH, Me; L = various phosphines). No evidence was found for an ‘open’ or ‘closed’ η 3-H 3 isomer as a stationary point on the potential energy surface, which would correspond to a random exchange mechanism. The computed energy differences between hypothetical η 3-H 3 geometries and the ground state were found to be prohibitively high. Kinetic isotope effect calculations performed on partially deuterated analogs are in agreement with experimental data and confirm the viability of an η 2-H 2 species as the transition state.

A Density Functional Study of the Rotational Barrier of Tricarbonyl(eta(4)-norbornadiene)iron. Effect of the Torsional Angle on the Carbonyl Stretching Spectra.

According to density functional calculations, the rotational barrier of the Fe(CO)3 unit in tricarbonyl(η4-norbornadiene)iron (1) is 0.5 kcal/mol, consistent with the value deduced from dynamic IR spectroscopy and, thus, with a very fast CO site exchange process. An alternative explanation of the observed coalescence of the carbonyl stretching bands through an intrawell mechanism is not supported since the computed ν(CO) frequencies are almost independent of the relevant torsional angle.

Fluxional Behaviour of Group 9 Metal Carbonyl Clusters

The site exchange processes of carbonyl ligands in homo- and hetero-metallic dodecacarbonyls of Ir and Rh and their derivatives take place by bridge opening and closing of CO's involving three or four metal atoms, and by rotation of terminal CO's about the apical metal centre. Substitution of Ir by Rh in the basal face of the metal framework slows down the first two processes and accelerates the third one, reflecting a significantly more electropositive character of Ir relative to Rh.

STM studies of island formation and surface ordering of Si on GaAs (001), (2�4) and c(4�4): Implications for ?-doping

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si below 400‡C onto GaAs (001) surfaces grown in situ by molecular beam epitaxy (MBE). The emphasis is on the island formation and growth, as well as surface ordering, for submonolayer quantities of Si (up to 0.2 ML) deposited on two different As-rich reconstructions of GaAs (0 0 1) (2 × 4) and c(4 × 4). For deposition on the c(4 × 4) surface, an asymmetric (3 × 1) RHEED pattern is formed, a consequence of anisotropic “needle-like” islands, which grow adjacent to each other along the [1 1 0] direction and produce a three-fold periodic superstructure. Individual islands grow by a site exchange process in which the additional As layer of the c(4 × 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contrast, Si deposition on the (2 × 4) surface does not lead to any new surface periodicities as monitored by RHEED. The Si atoms form poorly ordered clusters distributed randomly across the surface. The site exchange process cannot occur in this case as the (2 × 4) surface is terminated with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.