A DFT study of Zr-S rotational barriers of (η5-C5H5)2Zr(Cl)(SR); the origin of an inverse steric effect

2001 ◽  
Vol 79 (5-6) ◽  
pp. 809-816 ◽  
Author(s):  
P K Das ◽  
Susan Stanislav Alguindigue ◽  
Michael T Ashby
Keyword(s):  
Low Temperature ◽  
Transition State ◽  
Ground State ◽  
Steric Effect ◽  
Rotational Transition ◽  
The Other ◽  
Steric Interaction ◽  
Rotational Barriers ◽  
State Orientation ◽  
Thiolate Ligand

We have previously reported the experimental rotational barriers about the M–S bonds of 16e– bent metallocene monothiolates (η5-C5H5)2Zr(Cl)(SR) (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3] (1a–d): 32, 33, 35, and 26 kJ mol–1, respectively. The ground-state orientation about the Zr–S bonds of 1 that maximizes S(pπ) [Formula: see text] M(dπ) bonding (Cl-Zr-S-R [Formula: see text] 90°) also maximizes Cp « R (Cp = η5-C5H5) steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R [Formula: see text] 0°) is one that minimizes S(pπ) [Formula: see text] M(dπ) bonding and maximizes Cl « R steric interaction. Deviation from a ground-state orientation that is ideal for S(pπ ) [Formula: see text] M(dπ ) bonding might be expected as the size of the R-group and Cp « R steric interaction increases. The present study employs hybrid density function computational methods to reproduce the experimental rotational barriers. The computational rotational barriers decrease in the order 1a (R = CH3) > 1b (R = CH2CH3) > 1c (R = CH(CH3)2) at low temperature. However, opposing enthalpic factors reverse this trend at higher temperatures. The aberrant trend (unexpectedly low rotational barrier) that is observed for the 1d (R = C(CH3)3) derivative is due to dominant ground-state steric (enthalpic) effects. We conclude the thiolate ligand of 1d is misdirected in the ground-state with respect to the other thiolate derivatives.Key words: misdirected ligands, metallocene, thiolate, structure, bonding, dynamics.

INTERMEDIATE VALENCE METALS

2008 ◽  
Vol 22 (13) ◽  
pp. 1273-1295 ◽  
Author(s):  
JON LAWRENCE
Keyword(s):  
Rare Earth ◽  
Low Temperature ◽  
Ground State ◽  
Fermi Liquid ◽  
Spin Dynamics ◽  
The Other ◽  
Spin Fluctuations ◽  
Intermediate Valence ◽  
Neutron Spectra ◽  
Kondo Impurities

The ground state of rare earth intermediate valence (IV) metals is that of a heavy mass Fermi liquid. The transport, optical conductivity, dHvA signals, and Q-resolved neutron spectra reflect the existence of a Fermi surface with strongly renormalized masses and a hybridization gap. On the other hand, properties such as the susceptibility, specific heat, valence and spin dynamics that are dominated by the spin fluctuations, which are highly localized, can be understood qualitatively (and sometimes quantitatively) as those of a collection of non-interacting Anderson/Kondo impurities. Anomalies exist in some compounds both in the low temperature behavior and in the rate of crossover from Fermi liquid to local moment behavior which may reflect non-universal behavior of the Anderson lattice.

scholarly journals Ground state and stability of the fractional plateau phase in metallic Shastry–Sutherland system TmB4

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Matúš Orendáč ◽  
Slavomír Gabáni ◽  
Pavol Farkašovský ◽  
Emil Gažo ◽  
Jozef Kačmarčík ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Phase Diagram ◽  
Ground State ◽  
Constant Magnetic Field ◽  
Paramagnetic Phase ◽  
The Other ◽  
Zero Field ◽  
Plateau Phase ◽  
Low Field ◽  
Zero Field Cooling

AbstractWe present a study of the ground state and stability of the fractional plateau phase (FPP) with M/Msat = 1/8 in the metallic Shastry–Sutherland system TmB4. Magnetization (M) measurements show that the FPP states are thermodynamically stable when the sample is cooled in constant magnetic field from the paramagnetic phase to the ordered one at 2 K. On the other hand, after zero-field cooling and subsequent magnetization these states appear to be of dynamic origin. In this case the FPP states are closely associated with the half plateau phase (HPP, M/Msat = ½), mediate the HPP to the low-field antiferromagnetic (AF) phase and depend on the thermodynamic history. Thus, in the same place of the phase diagram both, the stable and the metastable (dynamic) fractional plateau (FP) states, can be observed, depending on the way they are reached. In case of metastable FP states thermodynamic paths are identified that lead to very flat fractional plateaus in the FPP. Moreover, with a further decrease of magnetic field also the low-field AF phase becomes influenced and exhibits a plateau of the order of 1/1000 Msat.

LOW-TEMPERATURE NORMAL-STATE HALL EFFECT IN HIGH-TcBi2Sr2-xLaxCuO6+δ REVEALED BY 60 T MAGNETIC FIELDS

2002 ◽  
Vol 16 (20n22) ◽  
pp. 3171-3174
Author(s):  
F. F. BALAKIREV ◽  
J. B. BETTS ◽  
G. S. BOEBINGER ◽  
S. ONO ◽  
Y. ANDO ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Low Temperature ◽  
Carrier Concentration ◽  
Hall Coefficient ◽  
Normal State ◽  
Ground State ◽  
Low Temperatures ◽  
Optimal Doping ◽  
Underdoped Sample ◽  
Additional Support

We report low-temperature Hall coefficient in the normal state of the high-Tc superconductor Bi 2 Sr 2-x La x CuO 6+δ. The Hall coefficient was measured down to 0.5 K by suppressing superconductivity with a 60 T pulsed magnetic field. The carrier concentration was varied from overdoped to underdoped regimes by partially substituting Sr with La in a set of five samples. The observed saturation of the Hall coefficient at low temperatures suggests the ability to extract the carrier concentration of each sample. The most underdoped sample exhibits a diverging Hall coefficient at low temperatures, consistent with a depletion of carriers in the insulating ground state. The Hall number exhibits a sharp peak providing additional support for the existence of a phase boundary at the optimal doping.

Approaches to Cyclopropa-Fused Quinones. The Synthesis and Photolysis of Some 4,9-Disubstituted 3,3-Dimethyl-3H-benz[f]indazoles

2003 ◽  
Vol 56 (9) ◽  
pp. 903 ◽  
Author(s):  
Gavin E. Collis ◽  
Dieter Wege
Keyword(s):  
Low Temperature ◽  
Functional Group ◽  
Alkoxy Group ◽  
The Other ◽  
Acetoxy Group

Addition of 2-diazopropane to 1,4-naphthoquinone at low temperature, followed by in situ enolization and acetylation or silylation gave 3,3-dimethyl-1H-benz[f]indazol-4,9-diyl diacetate and 3,3-dimethyl-9-(t-butyl-dimethylsilyloxy)-1H-benz[f]indazol-4-ol, respectively. Functional group manipulation of the latter compound provided a number of other 4,9-disubstituted 3,3-dimethyl-3H-benz[f]indazoles. Irradiation of the diacetate led to clean extrusion of nitrogen to give the naphtho[b]cycloproparene and an alkene. Attempts to elaborate the cycloproparene into the derived cyclopropanaphthoquinone were unsuccessful. Of the other 4,9-disubstituted 3,3-dimethyl-3H-benz[f]indazoles examined, only the compound possessing an acetoxy group at C9 was photoactive, and afforded the expected cycloproparene and alkene. Compounds bearing a hydroxy or alkoxy group at C9 were photochemically inert.

Electron drift caused by rf field gradient creates many plasma phenomena: An attempt to distinguish the cause and the effect

2011 ◽  
Vol 78 (2) ◽  
pp. 165-174 ◽  
Author(s):  
C. L. XAPLANTERIS ◽  
E. D. FILIPPAKI ◽  
I. S. MISTAKIDIS ◽  
L. C. XAPLANTERIS
Keyword(s):  
Experimental Data ◽  
Steady State ◽  
Low Temperature ◽  
Mathematical Models ◽  
Field Gradient ◽  
Collision Frequency ◽  
The Other ◽  
Frequency Effect ◽  
Electron Drift ◽  
Plasma Parameters

AbstractMany experimental data along with their theoretical interpretations on the rf low-temperature cylindrical plasma have been issued until today. Our Laboratory has contributed to that research by publishing results and interpretative mathematical models. With the present paper, two issues are being examined; firstly, the estimation of electron drift caused by the rf field gradient, which is the initial reason for the plasma behaviour, and secondly, many new experimental results, especially the electron-neutral collision frequency effect on the other plasma parameters and quantities. Up till now, only the plasma steady state was taken into consideration when a theoretical elaboration was carried out, regardless of the cause and the effect. This indicates the plasma's complicated and chaotic configuration and the need to simplify the problem. In the present work, a classification about the causality of the phenomena is attempted; the rf field gradient electron drift is proved to be the initial cause.

Abiotic Reactivity of De Novo Designed Artificial Copper Peptides (ArCuPs): The Case of C-H Activation

2022 ◽  
Author(s):  
Divyansh Prakash ◽  
Suchitra Mitra ◽  
Morgan Murphy ◽  
Saumen Chakraborty
Keyword(s):  
Steric Effect ◽  
De Novo ◽  
Outer Sphere ◽  
The Other ◽  
Easy Access ◽  
Oxygen Species ◽  
Redox Potentials ◽  
Spectroscopic Feature ◽  
Detailed Assessment ◽  
Active Variant

We report a series of de novo designed Artificial Cu Peptides (ArCuPs) that oxidize and peroxygenate C-H bonds of model abiotic substrates via electrochemically generated Cu-oxygen species using H2O2 as the terminal oxidant, akin to native Cu enzymes. Detailed assessment of kinetic parameters established the catalytic nature of the ArCuPs. Selective alteration of outer sphere steric at the d layers above and below the Cu site allows facilitated access of substrates, where a more pronounced effect on catalysis is observed when space is created at the d layer below the Cu site via Ile to Ala mutation producing a kcat of 6.2 s-1, TONmax of 14800 and catalytic proficiency (kcat/KM/kuncat) of 340 M-1 for the oxidation of benzyl alcohol. Independent spectroscopic studied revealed that the rate of formation of the Cu-oxygen species and the spectroscopic feature of the most active variant is distinct compared to the other ArCuPs. Systematic alteration of outer sphere hydrophobicity led to a correlated tuning of the T2 Cu site redox potentials by ~80 mV. The enhanced activity of the ArCuP variant is attributed to a combination of steric effect that allows easy access of substrates, the nature of Cu-oxygen species, and stability of this construct compared to others, where Ile to Ala mutation unexpectedly leads to a higher thermostability which is further augmented by Cu binding.

Effect of Medium on Reactivity for Alkaline Hydrolysis of p-Nitrophenyl Acetate and S-p-Nitrophenyl Thioacetate in DMSO-H2O Mixtures of Varying Compositions: Ground State and Transition State Contributions

2021 ◽  
Author(s):  
Ik-Hwan Um ◽  
Seungjae Kim
Keyword(s):  
Transition State ◽  
Alkaline Hydrolysis ◽  
Ground State ◽  
Rate Constants ◽  
Kinetic Data ◽  
Reaction Medium ◽  
Stepwise Mechanism ◽  
Rate Determining Step ◽  
Leaving Group ◽  
Hydrolysis Of

Second-order rate constants (kN) for reactions of p-nitrophenyl acetate (1) and S-p-nitrophenyl thioacetate (2) with OH‒ have been measured spectrophotometrically in DMSO-H2O mixtures of varying compositions at 25.0 ± 0.1 oC. The kN value increases from 11.6 to 32,800 M‒1s‒1 for the reactions of 1 and from 5.90 to 190,000 M‒1s‒1 for those of 2 as the reaction medium changes from H2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H2O. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state (TS), i.e., the reactions of 1 and 2 with OH‒ proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step (RDS). Addition of DMSO to H2O would destabilize OH‒ through electronic repulsion between the anion and the negative-dipole end in DMSO. However, destabilization of OH‒ in the ground state (GS) is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. The effect of medium on reactivity has been dissected into the GS and TS contributions through combination of the kinetic data with the transfer enthalpies (ΔΔHtr) from H2O to DMSO-H2O mixtures for OH‒ ion.

Comparative Spectroscopic Studies on Luminescence Performance of Er3+ Doped Tellurite Glass Embedded with Different Nanoparticles (Ag Co and Fe) at 0.55 μm Emission

2021 ◽  
Vol 317 ◽  
pp. 81-86
Author(s):  
Syariffah Nurathirah Syed Yaacob ◽  
Md. Rahim Sahar ◽  
Faizani Mohd Noor ◽  
Nur Liyana Amiar Rodin ◽  
Siti Khadijah Mohd Zain ◽  
...  
Keyword(s):  
Energy Transfer ◽  
Ground State ◽  
Tellurite Glass ◽  
Spectroscopic Studies ◽  
The Other ◽  
X Ray Diffraction ◽  
Melt Quenching ◽  
X Ray ◽  
Doped Glass ◽  
Luminescence Performance

The spectroscopic performance of Er3+ doped glass at 0.55 mm emission contain different nanoparticles NPs have been comparatively evaluated. Glass containing 1.0 mol % of Er3+ doped with different NPs (Ag, Co and Fe ) have been prepared using melt quenching technique. X-ray diffraction analysis reveals the all the prepared samples are amorphous. The UV-Vis absorption spectra of all glasses show several prominent peaks at 525 nm, 660 nm, 801nm, 982 nm and 959 nm due to transition from ground state 4I15/2 to different excited of 2H11/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2. The emission of Er3+ at 0.55 mm for glass contain Ag NP shows significant enhancement about 3 folds up to 0.6 mol%. On the other hand, the emission of Er3+ at 0.55 mm for glass containing Fe NPs and Co NPs intensely quench probably due to the energy-transfer from Er3+ ion to NPs and magnetic contributions.

scholarly journals XX.—On Dimethyl-Thetine and its Derivatives

1878 ◽  
Vol 28 (2) ◽  
pp. 571-582 ◽  
Author(s):  
Crum Brown ◽  
E. A. Letts
Keyword(s):  
Low Temperature ◽  
The Other ◽  
Similar Relation ◽  
The One ◽  
Elementary Textbooks

The analogies existing between elements belonging to one “family,” such, for instance, as the nitrogen family or the sulphur family, have long been recognised, and are pointed out and insisted upon even in elementary textbooks; but the very important analogies existing between substances of different quantivalence are apt to be forgotten or overlooked. For illustrations of such analogies we may point to boron and silicon, elements closely resembling one another in themselves and also in their compounds,—differing, indeed, in little else but that the one is triad and the other tetrad. A similar relation exists between gold and platinum.The elementary substances, sulphur and phosphorus, have many points of similarity: both fuse at a comparatively low temperature, both are transformed by heat into amorphous insoluble modifications, and both have anomalous vapour densities.

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