Event-Driven Power-Law Relaxation in Weak Turbulence
Date: January 5, 2009
Creator: Silvestri, Ludovico; Fronzoni, Leone; Grigolini, Paolo & Allegrini, Paolo
Description: This article discusses event-driven power-law relaxation in weak turbulence. Abstract: We characterize the spectral properties of weak turbulence in a liquid crystal sample driven by an external electric field, as a function of the applied voltage, and we find a 1/f noise spectrum S(f) ∝ 1/fn within the whole range 0< ɳ <2. We theoretically explore the hypothesis that the system complexity is driven by non-Poisson events resetting the system through creation and annihilation of coherent structures, retaining no memory of previous history (crucial events). The authors study the time asymptotic regime by means of the density ψ(τ) of the time distances between two crucial events, yielding ɳ = 3 - μ, where μ is defined through the long-time form ψ(τ) ∝ 1/τµ, with 1 < µ < 3. The system regression to equilibrium after an abrupt voltage change experimentally confirms the theory, proving violations of the ordinary linear response theory for both ɳ > 1 and ɳ < 1.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc40395/
Evidence for Strong Tantalum-to-Boron Dative Interactions in (silox)3Ta(BH3) and (silox)3Ta(ɳ2-B,CI-BCI2Ph) (silox = tBu3SiO)1
Date: January 24, 2007
Creator: Bonanno, Jeffrey B.; Henry, Thomas P.; Wolczanski, Peter T.; Pierpont, Aaron W. & Cundari, Thomas R., 1964-
Description: This article discusses strong tantalum-to-boron dative interactions. Abstract: Treatment of (silox)3 Ta (1, silox = tBu3SiO) with BH3·THF and BCI2Ph afforded (silox)3Ta(BH3) (2) and (silox)3Ta-(ɳ2-B,CI-BCI2Ph) (3), which are both remarkably stable Ta(III) compounds. NMe3 and ethylene failed to remove BH3 from 2, and no indication of BH3 exchange with BH3-THF-d8 was noted via variable-temperature 1H NMR studies. Addition of BH3-THF to (silox)3TaH2 provided the borohydride-hydride (silox)3HTa(ɳ3-BH4) (5), and its thermolysis released H2 to generate 2. Exposure of 2 to D2 enabled the preparation of isotopologues (silox)3Ta(BH3-nDn) (n = 0,2; 1,2-D; 2, 2-D₂; 3, 2-D₃) for isotopic perturbation of chemical shift studies, but these failed to distinguish between "inverse adduct" (i.e., (silox)3Ta→BH3) or (silox)3Ta(ɳ2-B,H-BH3) forms of 2. Computational models (RO)3Ta(BH3) (R = H, 2′-, SiH3, 2SiH SiMe3, 2SiMe, and SitBu3, 2SiBu) were investigated to assess the relative importance of steric and electronic effects on structure and bonding. With small R, ɳ2-B,H structures were favored, but for 2SiMe and 2SiBu, the dative structure proved to be similar in energy. The electronic and vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the ɳ2-B,H conformer of 2SiBu. In related computations pertaining to 3, small R ...
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc77129/
Evidence for the Net Addition of Arene C-H Bonds across a Ru(II)-Hydroxide Bond
Date: September 27, 2005
Creator: Feng, Yuee; Lail, Marty; Barakat, Khaldoon A.; Cundari, Thomas R., 1964-; Gunnoe, T. Brent & Petersen, Jeffrey L.
Description: This articles discusses evidence for the net addition of arene C-H bonds. TpRu(PMe3)2(OH) (1) reacts with C6D6 to initiate H/D exchange between the hydroxide ligand and the deuterated benzene. In addition, complex 1 catalyzes H/D exchange between H20 and C6D6. Mechanistic and computational studies suggest that a likely reaction pathway for the H/D exchange involves loss of PMe3 to produce {TpRu(PMe3)(OH)}, followed by the net addition of a benzene C-H(D) bond across the Ru-OH bond to form the putative complex TpRu(PMe3)(OH2)(Ph).
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc77138/
Excess Molar Volumes of Binary Mixtures of Cyclohexane and y-Butyrolactone with Aromatic Hydrocarbons
Date: April 1985
Creator: Acree, William E. (William Eugene); Gholami, Kheirollah; McHan, Danny R. & Rytting, J. Howard
Description: This article discusses excess molar volumes of binary mixtures of cyclohexane and y-butyrolactone with aromatic hydrocarbons. Abstract: Excess molar volumes of cyclohexane + benzene, cyclohexane + toluene, cyclohexane + p-xylene, y-butyrolactone + benzene, y-butyrolactone + toluene, and y-butyrolactone + m-xylene have been measured at 298.15 K. For the first three systems, Vᴱ is positive throughout the entire concentration range. Vᴱ is negative for binary mixtures containing y-butyrolactone, which indicates significant interaction between y-butyrolactone and the aromatic hydrocarbon.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc152457/
Experimental and Computational Studies of Ruthenium(II)-Catalyzed Addition of Arene C-H Bonds to Olefins
Date: September 10, 2004
Creator: Lail, Marty; Bell, Christen M.; Conner, David; Cundari, Thomas R., 1964-; Gunnoe, T. Brent & Petersen, Jeffrey L.
Description: This article discusses experimental and computational studies of Ruthenium(II)-Catalyzed addition of arene C-H bonds to olefins. Abstract: Hydroarylation reactions of olefins are catalyzed by the octahedral Ru(II) complex TpRu-(CO)(NCMe)(Ph) (1) (Tp = hydridotris(pyrazolyl)borate). Experimental studies and density functional theory calculations support a reaction pathway that involves initial acetonitrile/olefin ligand exchange and subsequent olefin insertion into the ruthenium-phenyl bond. Metal-mediated C-H activation of arene to form a Ru-aryl bond with release of alkyl arene completes the proposed catalytic cycle. The cyclopentadienyl complex CpRu(PPh3)2(Ph) produces ethylbenzene and styrene from a benzene/ethylene solution at 90 ˚C; however, the transformation is not catalytic. A benzene solution of (PCP)Ru(CO)(Ph) (PCP = 2,6-)CH2Pt-Bu2)2C6H3) and ethylene at 90 ˚C produces styrene in 12% yield without observation of ethylbenzene. Computational studies (DFT) suggest that the C-H activation step does not proceed through the formation of a Ru(IV) oxidative addition intermediate but rather occurs by a concerted pathway.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc77180/
Experimental and Computational Studies of the Isomerization Reactions of Bidentate Phosphine Ligands in Triosmium Clusters: Kinetics of the Rearrangements from Bridged to Chelated Isomers and X-ray Structures of the Clusters Os3 (CO)10 (dppbz), 1,1-Os3 (CO)10 (dppbzF4), HOs3 (CO)9 [μ -1,2-PhP (C6H4-ɳ1) C6H4PPh2], and HOs3 (CO)9- [μ-1,2-PhP (C6H4-ɳ 1) C6F4PPh2]
Date: February 22, 2011
Creator: Zhang, Xue; Kandala, Srikanth; Yang, Li; Watson, William H.; Wang, Xiaoping; Hrovat, David A. et al
Description: This article discusses isomerization reactions of bidentate phosphine ligands in triosmium clusters. Abstract: The diphosphine ligand 1,2-bis(diphenylphosphino) benzene (dppbz) reacts with the activated cluster 1,2-Os3 (CO)10 (MeCN)2 (1) at room temperature to furnish a mixture of the triosmium clusters 1,2-Os3 (CO)10 (dppbz) (2) and 1,1-Os3 (CO)10 (dppbz) (3), along with a trace amount of the hydride cluster HOs3 (CO)9 [μ -1,2-PhP (C6H4-ɳ1) C6H4PPh2] (4). The dppbz-bridged cluster 2 forms as the kinetically controlled product and irreversibly transforms to the corresponding chelated isomer 3 at ambient temperature. The disposition of the dppbz ligand in 2 and 3 has been established by X-ray crystallography and 31P NMR spectroscopy, and the kinetics for the conversion 2 → 3 have been followed by UV-vis spectroscopy in toluene over the temperature range 318-343 K. The calculated activation parameters (ΔH‡ = 21.6(3)kcal/mol; ΔS‡ = -11(1)eu) and lack of CO inhibition support an intramolecular isomerization mechanism that involves the simultaneous migration of phosphine and CO groups about the cluster polyhedron. The reaction between 1 and the fluorinated diphosphine ligand 1,2-bis(diphenylphosphino) tetrafluorobenzene (dppbzF4) was examined under similar reaction conditions and was found to afford the chelated cluster 1,1-Os3 (CO)10 (dppbzF4) (6) as the sole observable product. The absence ...
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc71815/
Experimental and Predicted Solubilities of 3,4-Dimethoxybenzoic Acid in Select Organic Solvents of Varying Polarity and Hydrogen-bonding Character
Date: 2013
Creator: Bowen, Kaci R.; Stephens, Timothy W.; Lu, Helen; Satish, Kalpana; Shan, Danyang; Acree, William E. (William Eugene) et al
Description: This article discusses the experimental and predicted solubilities of 3,4-dimethoxybenzoic acid in select organic solvents of varying polarity and hydrogen-bonding character. Abstract: Experimental solubilities are reported for 3,4-dimethoxybenzoic acid dissolved in 16 alcohol, 5 alkyl alkanoate, 5 alkoxyalcohol and 6 ether solvents. The measured solubility data were correlated with the Abraham solvation parameter model. Mathematical expressions based on the Abraham model predicted the observed molar solubilities to within 0.083 log units.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc157298/
Experimental Artifacts and Determination of Accurate Py Values
Date: 1986
Creator: Street, Kenneth W. & Acree, William E. (William Eugene)
Description: This article discusses experimental artifacts and determination of accurate Py values. Abstract: Recently the Py solvent scale has been introduced and a large body of data has been generated using fluorescence measurements. Numerous problems exist in the correct determination of these values, including instrumental and chemical artifacts. Among the instrumental problems associated with correct Py assignments are slit width effects, inner filtering and efficiencies associated with double-pass vs. single-pass cell compartment designs. These instrumental problems, together with chemical artifacts related to adequate blank correction and temperature control, were investigated in the work reported in this paper.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc152446/
Experimental evidence for a discrete transition to channeling for 1.0-MeV protons in Si〈100〉
Date: April 1998
Creator: Zhao, Z. Y.; Arrale, A. M.; Li, S. L.; Marble, D. K.; Weathers, Duncan L.; Matteson, Samuel E. et al
Description: This article discusses experimental evidence for a discrete transition to channeling for 1.0-MeV protons in Si〈100〉. Abstract: The present work reports the experimental evidence of anomalies exhibited by the energy loss and energy straggling of channeled protons in silicon in transmission measurements versus the incident angle. Results are presented for 1.0-MeV protons channeled along the 〈100〉 axis for a silicon foil of 3.8 μm thickness. It is shown that the transition from random to a channeling condition is discrete. The energy spectra of transmitted ions show a random peak (lower energy) and a channeled peak (higher energy). The random peak has a fixed energy, while the energy of the channeled peak increases as the target crystal's axis approaches alignment with the direction of the incident ion beam. The results support a model suggesting that the channeled ions lose energy only to valence electrons and are concentrated in a narrow cone about the direction of incidence when they emerge from the crystal. The energy straggling of channeled particles reaches a minimum in the hyper-channeled condition. Both the energy loss and the energy straggling of channeled protons show a dependence on the local electron density.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc139489/
Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects
Date: August 19, 2010
Creator: Gonzalez-James, Ollie M.; Zhang, Xue; Datta, Ayan; Hrovat, David A.; Singleton, Daniel A. & Borden, Weston T., 1943-
Description: This article discusses experimental evidence for heavy-atom tunneling. Abstract: The intramolecular 13C kinetic isotope effects for the ring-opening of cyclopropylcarbinyl radical were determined over a broad temperature range. The observed isotope effects are unprecedentedly large, ranging from 1.062 at 80 °C to 1.163 at -100 °C. Semiclassical calculations employing canonical variational transition-state theory drastically underpredict the observed isotope effects, but the predicted isotope effects including tunneling by a small-curvature tunneling model match well with experiment. These results and a curvature in the Arrhenius plot of the isotope effects support the recently predicted importance of heavy-atom tunneling in cyclopropylcarbinyl ring-opening.
Contributing Partner: UNT College of Arts and Sciences
Permallink:digital.library.unt.edu/ark:/67531/metadc71805/