Isocyanide insertion reactions into the Ta—C bonds of cationic and zwitterionic tantalocenes

2003 ◽  
Vol 81 (11) ◽  
pp. 1137-1148 ◽  
Author(s):  
Kevin S Cook ◽  
Warren E Piers ◽  
Brian O Patrick ◽  
Robert McDonald
Keyword(s):  
Ground State ◽  
Room Temperature ◽  
Structural Changes ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Insertion Reactions ◽  
H Nmr ◽  
Agostic Interaction ◽  
Different Temperatures ◽  
The Difference

The insertion of isonitriles R' NC (R' = t-Bu, C6H11, CH2C6H5) occurs exclusively into the Ta—CH2 bonds of the zwitterionic compounds (C5H4R)2Ta[CH2B(C6F5)3]CH3 (R = H, 1a; R = CH3, 1b) at relatively slow rates at room temperature to form N-out isomers as the sole kinetic products. By comparison, insertion of the same isonitrile substrates into a Ta—CH3 bond of the non-zwitterionic analogs [(C5H4R)2Ta(CH3)2][A] (R = H, A = B(C6F5)4, 3a; R = CH3, A = BF4, 3b) occurs much more rapidly, again to form N-out isomers exclusively under kinetic conditions. The difference in rate is attributed to the presence of a ground state α-agostic interaction in the zwitterionic compounds, which is not featured in the dimethyl ion pairs. All of the N-out isomers formed undergo thermal and irreversible conversion to the corresponding N-in isomers at rates that are conveniently followed by 1H NMR spectroscopy. The rates were studied at different temperatures to obtain activation parameters for each transformation. The rate and activation trends for this isomerization were analyzed as a function of the structural changes in the compounds. It was found that the rate decreased as the steric bulk of the isonitrile substitutent R' increased and that the rates were faster for the series that incorporated the more electron-donating C5H4CH3 ancillary ligand. Furthermore, isomerization rates for the zwitterionic N-out compounds were faster than those found in the non-zwitterionic series. This study represents one of the more extensive kinetic analyses of the rate of N-out to N-in isomerization as a function of structural changes. The observations are consistent with the mechanistic picture that has been developed for this process, involving dissociation of the η2 iminoacyl ligand, rotation about the M—Ciminoacyl bond and recoordination to the inside site of the metallocene wedge. Key words: cationic metallocenes, isocyanide insertion, agostic interactions, tantalum.

5d and 4f electron configuration of CeB6 at 340 and 535 K

2008 ◽  
Vol 64 (5) ◽  
pp. 534-549 ◽  
Author(s):  
Ryoko Makita ◽  
Kiyoaki Tanaka ◽  
Yoshichika Ōnuki
Keyword(s):  
Electron Transfer ◽  
Crystal Field ◽  
Ground State ◽  
Room Temperature ◽  
Atomic Orbital ◽  
Energy Levels ◽  
Electron Configuration ◽  
Acta Cryst ◽  
X Ray ◽  
The Difference

X-ray atomic orbital (XAO) analysis revealed that at both temperatures the electrons are transferred from B 2px (= py ) to Ce 5d and 4f orbitals. At 340 K 5d(j = 5/2)Γ8 orbitals are occupied partially, but 4f(j = 5/2)Γ8 orbitals are more populated than 4f(j = 5/2)Γ7 orbitals, in contrast to our observation at 430 K [Makita et al. (2007). Acta Cryst. B63, 683–692]. At 535 K the XAO analysis revealed clearly that the order of the energy levels of 4f(j = 5/2)Γ8 and Γ7 states reversed again and is the same as that at room temperature. It also limited the possible 5d configurations to three models among the nine possible ones. However, the XAO analysis could not decide which of the three models was the best with the present accuracy of the measurement. Two of them have partially and fully occupied 5d(j = 5/2)Γ7 orbitals and the remaining one has a fully occupied 5d(j = 3/2)Γ8 orbital. Since the lobes of 5d(j = 3/2)Γ8 or 5d(j = 5/2)Γ7 orbitals do not overlap with the 4f(j = 5/2)Γ8 orbitals as well as the 5d(j = 5/2)Γ8 orbitals, the order of the energy levels of the 4f(j = 5/2) orbitals became the same as that at room temperature. These results indicate that the crystal field varies with temperature due to the electron transfer from B 2p to Ce 5d orbitals. The difference densities after the spherical-atom refinement at the three temperatures clearly revealed the different combinations of 4f and 5d orbitals which are occupied. In the present study positive peaks due to the 4f electrons appear near the Ce nucleus and those due to 5d orbitals are found in the area outside the 4f peaks. Between the two areas there is a negative area distributed spherically at 340 K. The negative area produced by the contraction of 4f(j = 5/2)Γ8 orbitals seems to reduce the electron repulsion of the 5d(j = 5/2)Γ8 orbitals and helps the 4f(j = 5/2)Γ8 orbitals to remain as the ground state.

Local structure change of luminescent Ag zeolite-A and -X studied by in situ XAFS and IR spectroscopy

2020 ◽  
Vol 27 (6) ◽  
pp. 1640-1647
Author(s):  
Takafumi Miyanaga ◽  
Yushi Suzuki ◽  
Sho Narita ◽  
Reki Nakamura
Keyword(s):  
Local Structure ◽  
Room Temperature ◽  
Structural Changes ◽  
Zeolite A ◽  
Zeolite X ◽  
Zeolite Cavity ◽  
Ag Clusters ◽  
The Difference ◽  
X Ray Absorption

The in situ X-ray absorption fine structure (XAFS) for the structural changes of Ag clusters produced in the cavity of luminescent zeolites by thermal treatment of Ag zeolite-A and Ag zeolite-X has been studied. The following procedures are compared: (i) samples are heated and cooled to room temperature under atmosphere (under air); (ii) samples are heated and cooled to room temperature in a vacuum and then exposed to air. It was confirmed that the Ag clusters were broken when the Ag zeolite was exposed to air for Ag zeolite-X, which complements our previous results for Ag12-A. It is suggested that the deformation of the Ag clusters plays an important role in the generation of a strong photoluminescence band, and Ag clusters may not be direct species producing the strong photoluminescence. The local structure of the Ag ions was found to be slightly different from that of the unheated species. The difference may originate from the formation and breakdown of Ag clusters in the zeolite cavity.

An acidity scale of phosphonium tetraphenylborate salts and ruthenium dihydrogen complexes in dichloromethane

2006 ◽  
Vol 84 (2) ◽  
pp. 164-175 ◽  
Author(s):  
Tianshu Li ◽  
Alan J Lough ◽  
Cristiano Zuccaccia ◽  
Alceo Macchioni ◽  
Robert H Morris
Keyword(s):  
Room Temperature ◽  
Equilibrium Constants ◽  
Ion Pairs ◽  
H Nmr ◽  
Protonation Reaction ◽  
Dihydrogen Complexes ◽  
Continuous Scale ◽  
Nmr Techniques ◽  
Acidic Compounds ◽  
Acidity Scale

Equilibrium constants (KDM) for reactions between acids and bases of the title compounds in CD2Cl2 (DM) have been determined by 31P and 1H NMR spectroscopy at room temperature. [HPCy3]BPh4 and [HPCy3]BF4, with pKDM assigned by literature convention to 9.7, have been used as the anchor compounds for the pKDM determinations. A continuous scale of pKDM values covering the range 9.7 to 5.7 is created with the acidic compounds [HPR3]BPh4. Those acids with pKDM greater than 6 are stable, while those with more acidic cations HPR3+ protonate BPh4– to produce R3PBPh3 and benzene. The literature pKTHF values reported for [HPBu2Ph]BPh4, [HPMePh2]BPh4, and [HPEtPh2]BPh4 are questionable because of this protonation reaction. NOE and PGSE 1H NMR techniques are used to show that [HPCy2Ph]BPh4 in DM exists as ion pairs and higher aggregates up to quadrupoles at the concentrations used in the acid–base studies. The new dihydrogen complexes [Ru(H2)Cl(PPh3)2(dach)]BF4 (dach = (1R,2R)-(–)-diaminocyclohexane) and [Ru(H2)Cl{tmeP2(NH)2}]BF4 (tmeP2(NH)2 = PPh2C6H4CH2NHCMe2CMe2NHCH2C6H4PPh2) were prepared by reaction of RuHCl(PPh3)2(dach) and RuHCl{tmeP2(NH)2} with HBF4. Their crystal structures are reported, and the pKDM values of their BPh4– salts were determined to be 8.6 and 6.9, respectively.Key words: acidity, dihydrogen complex, hydride, phosphonium, dichloromethane.

scholarly journals Jet-cooked papermaking starches studied using 1H NMR-relaxometry and viscometry

2020 ◽  
Vol 35 (3) ◽  
pp. 376-385
Author(s):  
Jenna Raunio ◽  
Ekaterina Nikolskaya ◽  
Yrjö Hiltunen
Keyword(s):  
Room Temperature ◽  
Relaxation Rates ◽  
H Nmr ◽  
Nmr Relaxometry ◽  
Surface Sizing ◽  
Cooking Temperature ◽  
Degree Of Oxidation ◽  
Different Temperatures ◽  
Retrograded Starch ◽  
Solids Content

AbstractTwo wet-end starches (potato and barley), one surface sizing starch (barley) and one coating binder starch (barley) were jet-cooked. Samples were collected and stored at 90, 60 and 40 °C. 1H NMR-relaxometry and viscometry were used to monitor the jet-cooked solutions as they cooled to room temperature. Samples stored at different temperatures were also monitored using 1H NMR-relaxometry and viscometry. A sediment formed into the surface sizing and coating binder starches stored at 90 °C. The sediment and supernatant were separated and collected, and measured using 1H NMR-relaxometry. The {T_{2}} relaxation rates of jet-cooked starches showed significant differences between potato and barley starches, as had also been examined in previous studies. The NMR method was also sensitive to differences in solids content and chemical modification (degree of cationization, degree of oxidation and molecular weight). The cooking temperature, cooking speed and viscosity did not influence {T_{2}} relaxation rates. The sediment separated from the surface sizing and coating binder starches held at 90 °C had a significantly higher relaxation rate than the supernatant, indicating that the sediment contained a high amount of retrograded starch.

scholarly journals Improvement of Mechanical, Tribological, and Fricative Reduction Properties of Pure Titanium by Boriding

2021 ◽  
Vol 11 (11) ◽  
pp. 4862
Author(s):  
Tong Chen ◽  
Shinji Koyama ◽  
Lihua Yu
Keyword(s):  
Room Temperature ◽  
Present Report ◽  
High Surface ◽  
Pure Titanium ◽  
Surface Hardness ◽  
Hardened Layer ◽  
Layer Growth ◽  
Different Temperatures ◽  
The Difference ◽  
Hardened Layer Thickness

Surface boriding of pure titanium was performed using dissolved salt impregnation to modify the surface hardness and improve wear performance. The effect of boriding temperature (950–1150 °C) on the microstructure, composition, and room-temperature tribological properties of the borided samples was investigated by X-ray diffraction, scanning electron microscopy, and ball-on-disc tribometry, respectively. Gibbs free energy was also calculated to evaluate the compounds generated during the boriding at different temperatures. After a detailed analysis of the crystal structures and the growth morphologies of TiB, the diffusion mechanisms for B atoms in TiB and TiB2 were discussed in the present report. The boriding temperature had a large effect on the microstructure, mechanical properties, and room-temperature tribological behavior of the borided samples, attributed to the changes in the composition and the increased hardened layer under elevated boriding temperature. The modeling of layer growth kinetics was also discussed in this paper. The actual value of hardened layer thickness was compared to the calculated value, and the difference was analyzed. The fricative value of the borided samples showed a minimum value of 88.9 dB for a boriding temperature of 1050 °C. The depth and width of the wear tracks decreased gradually with increasing boriding temperature. The worn surface of the samples borided at higher temperatures showed very good wear resistance. A boriding condition of 1050 °C was considered optimal, as it provided sufficiently high surface hardness and a low fricative value to reduce vibrations during practical use.

STRUCTURAL CHANGES OF Ni-DOPED YBCO AT DIFFERENT TEMPERATURE

2005 ◽  
Vol 19 (01n03) ◽  
pp. 319-322
Author(s):  
L. ZHANG ◽  
H. L. DU ◽  
X. F. RUI ◽  
X. F. SUN ◽  
H. ZHANG
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Tetragonal Phase ◽  
Room Temperature ◽  
Structural Changes ◽  
Close Correlation ◽  
Bond Lengths ◽  
Ni Content ◽  
The Difference ◽  
Tetragonal Phase Transition

In YBa 2 Cu 3-x Ni x O y at about 480°C all the samples show a tetragonal phase. Comparing the structural changes at 480°C with those at room temperature, the bond lengths such as Cu (1)- O (4) and Cu (2)- O (4) vary regularly with Ni -content increasing at room temperature, but not at high temperature. It suggests that there is a close correlation between structural changes and superconductivity in this system. The difference of orthorhombic- tetragonal phase -transition is also discussed between undoped YBCO and Ni -doped YBCO at high temperature.

X-ray Study of M II(ClO4)2.6H2O (M II = Zn, Ni): Twinning, Disorder and Phase Transitions

1997 ◽  
Vol 53 (4) ◽  
pp. 639-644 ◽  
Author(s):  
S. Ghosh ◽  
M. Mukherjee ◽  
A. Seal ◽  
S. Ray
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Structural Changes ◽  
Probability Of Error ◽  
X Ray ◽  
Heat Treated ◽  
Different Temperatures ◽  
Orthorhombic Cell ◽  
Perchlorate Hexahydrate ◽  
Nickel Perchlorate

X-ray studies of zinc and nickel perchlorate hexahydrate, Zn(ClO4)2.6H2O and Ni(ClO4)2.6H2O, respectively, at different temperatures have been carried out to correlate the structural changes with phase transitions in the compounds. The crystals are pseudohexagonal (P63 mc), exhibiting a three-component orthorhombic twinning (Pmn21). At high temperatures a slight deviation of the b/a ratio of the three twinned orthorhombic cells from 31/2 results in a three-component splitting of each spot in the X-ray photograph, which on cooling to room temperature coalesce into single ones, thus restoring the original b/a ratio. The diffuse streaks disappear in the high-temperature photographs due to a decrease in the probability of error in repetition along the b axes of the three orthorhombic cells with temperature. A successful refinement of the heat-treated ordered Zn(ClO4)2.6H2O crystal verifies the continuous perchlorate–water arrangement and three-component twinning of the orthorhombic cell. Low-temperature X-ray photographs indicate no structural change.

Structure Dynamics and Isomerism of Bis[μ-(2-methylphenolato)]bis[(η2:η2-cycloocta-1,5-diene)rhodium(I)] Complex

2006 ◽  
Vol 71 (3) ◽  
pp. 423-433 ◽  
Author(s):  
Jiří Zedník ◽  
Jan Sejbal ◽  
Jan Svoboda ◽  
Jan Sedláček ◽  
Jiří Vohlídal
Keyword(s):  
Room Temperature ◽  
Activation Parameters ◽  
Dynamic Processes ◽  
Methyl Groups ◽  
H Nmr ◽  
Structure Dynamics

Dinuclear rhodium(I) complex [{RhI(cod)}2(μ-OC6H4-2-Me)2] (1), exhibits the syn-anti isomerism consisting in the orientation of methyl groups of the bridge ligands with respect to the plane or distorted plane involving Rh and O atoms. The syn isomer predominates in CDCl3 solution (above 90%) at room temperature. EXSY 1H NMR measurements showed that, in CDCl3 solution, complex 1 undergoes at least two independent dynamic processes differing substantially in values of activation parameters: (i) rotation of 2-methylphenyl rings in bridge ligands along the O-C axis, and (ii) formal rotation of cod ligands along the Rh-cod axes, which proves fluxional behavior of cod ligand in RhI(cod) complexes.

Reciprocating Wear Behaviour of Two Dimensionally Reinforced Carbon-Phenolic and Carbon-Epoxy Composites

2011 ◽  
Vol 1 (4) ◽  
pp. 46-57
Author(s):  
Sourav Sarkar ◽  
V.G. Sekaran ◽  
E. Badisch ◽  
Manish Roy ◽  
R. Mitra
Keyword(s):  
Friction Surface ◽  
Room Temperature ◽  
Epoxy Composites ◽  
Wear Behaviour ◽  
Reciprocating Wear ◽  
Different Temperatures ◽  
The Difference ◽  
Surface Morphologies ◽  
Low Amplitude ◽  
Scanning Electron

A comparative study has been carried out on performance of two-dimensionally reinforced carbon/phenolic (C/P) and carbon/epoxy (C/E) composites, subjected to low amplitude reciprocating wear at different temperatures. The C/P composite has shown greater wear rate than the C/E composite, with the difference being modest at room temperature, and larger at 250 °C. The values of coefficient of friction, surface roughness, and depths of craters on worn surfaces have been measured, which along with surface morphologies examined by scanning electron microscope have been correlated to both amount of weight loss and mechanisms of damage by wear.

FERROMAGNETISM IN SEMICONDUCTOR C–Ni FILMS AT DIFFERENT ANNEALING TEMPERATURE

2016 ◽  
Vol 23 (03) ◽  
pp. 1650002 ◽  
Author(s):  
VALI DALOUJI ◽  
SMOHAMMAD ELAHI
Keyword(s):  
Coercive Field ◽  
Room Temperature ◽  
Annealing Temperature ◽  
Composite Films ◽  
Nickel Concentration ◽  
Maximum Value ◽  
Different Temperatures ◽  
The Difference ◽  
Quartz Substrates ◽  
Microstructure And Magnetic Properties

In this work, the microstructure and magnetic properties of carbon–nickel (C–Ni) composite films annealed at different temperatures (300–1000[Formula: see text]C) were investigated. The films were grown by radio frequency magnetron sputtering on quartz substrates at room temperature. The nickel concentration in the films are affected by changing of the value of evaporation nickel atoms and measured by Rutherford backscattering spectroscopy (RBS). Values of coercive field were measured under both increasing and decreasing applied magnetic field. It is shown that the coercive field of films strongly dependent on the annealing temperature and at 500[Formula: see text]C films has maximum value of 93.67[Formula: see text]Oe. The difference in the coercive fields increased for films annealed from 300 to 500[Formula: see text]C and then decreased from 500 to 1000[Formula: see text]C. The ID/IG ratio of Raman spectra would indicate the presence of higher sp2 bonded carbon in the films annealed at 800[Formula: see text]C.

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