177 Matching Results

Search Results

Advanced search parameters have been applied.

Inter- and Intramolecular Experimental and Calculated Equilibrium Isotope Effects for (silox)₂(ᵗBu₃SiND)TiR + RH (silox = ᵗBu₃SiO): Inferred Kinetic Isotope Effects for RH/D Addition to Transient (silox)₂Ti=NSiᵗBu₃

Description: This article discusses inter- and intramolecular experimental and calculated equilibrium isotope effects.
Date: August 23, 2000
Creator: Slaughter, LeGrande M.; Wolczanski, Peter T.; Klinckman, Thomas R. & Cundari, Thomas R., 1964-
Partner: UNT College of Arts and Sciences

Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects

Description: Article on 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.
Partner: UNT College of Arts and Sciences

Determination of the Evaporation Coefficient of D2O

Description: The evaporation rate of D{sub 2}O has been determined by Raman thermometry of a droplet train (12-15 {micro}m diameter) injected into vacuum ({approx}10{sup -5} torr). The cooling rate measured as a function of time in vacuum was fit to a model that accounts for temperature gradients between the surface and the core of the droplets, yielding an evaporation coefficient ({gamma}{sub e}) of 0.57 {+-} 0.06. This is nearly identical to that found for H{sub 2}O (0.62 {+-} 0.09) using the same experimental method and model, and indicates the existence of a kinetic barrier to evaporation. The application of a recently developed transition state theory (TST) model suggests that the kinetic barrier is due to librational and hindered translational motions at the liquid surface, and that the lack of an isotope effect is due to competing energetic and entropic factors. The implications of these results for cloud and aerosol particles in the atmosphere are discussed.
Date: March 26, 2008
Creator: Drisdell, Walter S.; Cappa, Christopher D.; Smith, Jared D.; Saykally, Richard J. & Cohen, Ronald C.
Partner: UNT Libraries Government Documents Department

Variational transition state theory. Progress report, July 1, 1979-June 30, 1980

Description: The variational transition state theory (VTST) of chemical reaction rates has been further developed and two previously developed and one new version have been illustrated and tested by various applications to collinear and three-dimensional reactions of the type A + BC ..-->.. AB + C. The first two versions considered are canonical variational theory (CVT), which is based on curves of free energy of activation as functions of location of the VTST dividing surface, and microcanonical variational theory (..mu..VT), which is based on minimizing the reactive flux through the VTST dividing surface at each total energy. CVT is simpler but ..mu..VT is more accurate. The new theory, improved canonical variational theory (ICVT), is almost as simple as CVT but almost as accurate as ..mu..VT. This has been demonstrated by applications to H, O, F, Cl, and I reacting with H/sub 2/, H reacting with F/sub 2/ and Cl/sub 2/, and various isotopic analogs and model systems. It was also demonstrated that VTST leads to very good agreement with accurate quantal results for several collinear reactions. Another project used VTST to explore the systematics of kinetic isotope effects for three-dimensional reactions. The predictions sometimes differ considerably from those of the conventional theory.
Date: February 1, 1980
Creator: Truhlar, D.G.
Partner: UNT Libraries Government Documents Department

High Precision Measurement of Isotope Effects on Noncovalent Host-Guest Interactions

Description: Isotope effects (IEs) are a powerful tool for examining the reactivity of, and interactions between, molecules. Recently, secondary IEs have been used to probe the nature of noncovalent interactions between guest and host molecules in supramolecular systems. While these studies can provide valuable insight into the specific interactions governing guest recognition and binding properties, IEs on noncovalent interactions are often very small and difficult to measure precisely. The Perrin group has developed an NMR titration method capable of determining ratios of equilibrium constants with remarkable precision. They have used this technique to study small, secondary equilibrium isotope effects (EIEs) on the acidity of carboxylic acids and phenols and on the basicity of amines, measuring differences down to thousandths of a pK{sub a} unit. It occurred to us that this titration method can in principle measure relative equilibrium constants for any process which is fast on the NMR timescale and for which the species under comparison are distinguishable by NMR. Here we report the application of this method to measure very small EIEs on noncovalent host-guest interactions in a supramolecular system.
Date: June 23, 2009
Creator: Mugridge, Jeffrey S.; Bergman, Robert G. & Raymond, Kenneth N.
Partner: UNT Libraries Government Documents Department

An Isotope Effect in Photosynthesis

Description: In the course of some kinetic studies on photosynthesis of barley seedlings, it has been found that plants utilize C{sup 12}O{sub 2} faster than C{sup 14}O{sub 2}. The plants were placed in a closed system containing an infra-red absorption-cell for the analysis of total CO{sub 2} and an ionization chamber for the determination of C{sup 14}O{sub 2} in the gas phase, both instruments recording continuously. Carbon dioxide, containing about 2% C{sup 14}O{sub 2}, was introduced in the dark and the specific activity at this point taken as unity. After a short dark period, the lights were turned on and photosynthesis was allowed to take place. A figure shows the result of a typical experiment. During the initial dark period the specific activity fell because of dilution by inactive respired CO{sub 2}. However, as photosynthesis proceeded, the specific activity of the residual CO{sub 2} rose until, when only 1/6 of it remained, the specific activity reached a peak some 20% higher than it had been at the start of photosynthesis. At this point the steady respiratory dilution became an appreciable fraction of the total remaining CO{sub 2}, and the specific activity dropped rapidly.
Date: November 23, 1948
Creator: Weigl, J.W. & Calvin, M.
Partner: UNT Libraries Government Documents Department


Description: The unclassified literature covering 2498 reports from 1907 through 1957 has been searched for isotopic exchange and isotepic separation reactions involving U and the lighter elements of the periodic chart through atomic number 30. From 1953 to 1957, all elements were included Numerous references to isotope properties, isotopic ratios, and kinetic isotope effects were included. This is a complete revision of TID-3036 (Revised) issued June 4, 1954. An author index is included. (auth)
Date: October 28, 1959
Creator: Begun, G.M.
Partner: UNT Libraries Government Documents Department

The Acid Hydrolysis Mechanism of Acetals Catalyzed by a Supramolecular Assembly in Basic Solution

Description: A self-assembled supramolecular host catalyzes the hydrolysis of acetals in basic aqueous solution. The mechanism of hydrolysis is consistent with the Michaelis-Menten kinetic model. Further investigation of the rate limiting step of the reaction revealed a negative entropy of activation ({Delta}S{double_dagger} = -9 cal mol{sup -1}K{sup -1}) and an inverse solvent isotope effect (k(H{sub 2}O)/k(D{sub 2}O) = 0.62). These data suggest that the mechanism of hydrolysis that takes place inside the assembly proceeds through an A-2 mechanism, in contrast to the A-1 mechanism operating in the uncatalyzed reaction. Comparison of the rates of acetal hydrolysis in the assembly with the rate of the reaction of unencapsulated substrates reveals rate accelerations of up to 980 over the background reaction for the substrate diethoxymethane.
Date: September 24, 2008
Creator: Pluth, Michael D.; Bergman, Robert G. & Raymond, Kenneth N.
Partner: UNT Libraries Government Documents Department


Description: A compilation of available data on the spectra of thorium is presented, including energy levels and classified lines with Zeeman effects and isotope shifts, as well as certain data on unclassified lines. The present status of knowledge of these spectra is discussed. (auth)
Date: May 22, 1958
Creator: Charles, G.W.
Partner: UNT Libraries Government Documents Department

On the ``inverse'' isotope effect in organic superconductors: New findings and implications

Description: The inverse deuterium isotope effect, previously found in K-(BEDT-TTF){sub 2}CU(NCS){sub 2}, is also found in two other BEDT-TTF-based superconductors with different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three superconductors, ca. +0.26 {+-} 0.06 K. These results, when taken together with the recent results of Lang et al. on the uniaxial pressure derivatives of T{sub c}, suggest that the inverse isotope effect may not have a direct relationship to the pairing mechanism but instead is a reflection of the change in the internal lattice pressure.
Date: July 24, 2000
Creator: Kini, A. M.; Schlueter, J. A.; Ward, B. H.; Geiser, U. W. & Wang, H. H.
Partner: UNT Libraries Government Documents Department

Reactions of atomic hydrogen in water : solvent and isotope effects.

Description: It has been known for many years that hydrogen atoms can be easily created and studied in water using radiolytic techniques [1]. The use of CW EPR detection coupled with electron radiolysis proved extremely useful in estimating many reaction rates, and revealed the interesting phenomenon of chemically induced dynamic electron polarization (CIDEP) [2]. In recent years, we have made use of pulsed EPR detection to make precision reaction rate measurements which avoid the complications of CIDEP [3]. Activation energies and H/D isotope effects measured in these studies [4-14] will be described below. An interesting aspect of the hydrogen atom reactions is the effect of hydrophobic solvation. EPR evidence--an almost gas-phase hyperfine coupling and extremely narrow linewidth--is quite convincing to show that the H atom is just a minimally perturbed gas phase atom inside a small ''bubble''. In several systems we have found that the hydrophobic free energy of solvation dominates the solvent effect on reaction rates.
Date: June 10, 1999
Creator: Bartels, D. M.
Partner: UNT Libraries Government Documents Department

Alternate Substrates and Isotope Effects as a Probe of the Malic Enzyme Reaction

Description: Dissociation constants for alternate dirmcleotide substrates and competitive inhibitors suggest that the dinucleotide binding site of the Ascaris suum NAD-malic enzyme is hydrophobic in the vicinity of the nicotinamide ring. Changes in the divalent metal ion activator from Mg^2+ to Mn^2+ or Cd^2+ results in a decrease in the dinucleotide affinity and an increase in the affinity for malate. Primary deuterium and 13-C isotope effects obtained with the different metal ions suggest either a change in the transition state structure for the hydride transfer or decarboxylation steps or both. Deuterium isotope effects are finite whether reactants are maintained at saturating or limiting concentrations with all the metal ions and dinucleotide substrates used. With Cd^2+ as the divalent metal ion, inactivation of the enzyme occurs whether enzyme alone is present or is turning over. Upon inactivation only Cd^2+ ions are bound to the enzyme which becomes denatured. Modification of the enzyme to give an SCN-enzyme decreases the ability of Cd^2+ to cause inactivation. The modified enzyme generally exhibits increases in K_NAD and K_i_metai and decreases in V_max as the metal size increases from Mg^2+ to Mn^2+ or Cd^2+, indicative of crowding in the site. In all cases, affinity for malate greatly decreases, suggesting that malate does not bind optimally to the modified enzyme. For the native enzyme, primary deuterium isotope effects increase with a concomitant decrease in the 13-C effects when NAD is replaced by an alternate dinucleotide substrate different in redox potential. This suggests that when the alternate dinucleotides are used, a switch in the rate limitation of the chemical steps occurs with hydride transfer more rate limiting than decarboxylation. Deuteration of malate decreases the 13-C effect with NAD for the native enzyme, but an increase in 13-C effect is obtained with alternate dinucleotides. These suggest the presence of a ...
Date: August 1988
Creator: Gavva, Sandhya Reddy
Partner: UNT Libraries

Review of D-T Experiments Relevant to Burning Plasma Issues

Description: Progress in the performance of tokamak devices has enabled not only the production of significant bursts of fusion energy from deuterium-tritium (D-T) plasmas in the Tokamak Fusion Test Reactor (TFTR) and the Joint European Torus (JET) but, more importantly, the initial study of the physics of burning magnetically confined plasmas. The TFTR and JET, in conjunction with the worldwide fusion effort, have studied a broad range of topics including magnetohydrodynamic stability, transport, wave-particle interactions, the confinement of energetic particles, and plasma boundary interactions. The D-T experiments differ in three principal ways from previous experiments: isotope effects associated with the use of deuterium-tritium fuel, the presence of fusion-generated alpha particles, and technology issues associated with tritium handling and increased activation. The effect of deuterium-tritium fuel and the presence of alpha particles is reviewed and placed in the perspective of the much large r worldwide database using deuterium fuel and theoretical understanding. Both devices have contributed substantially to addressing the scientific and technical issues associated with burning plasmas. However, future burning plasma experiments will operate with larger ratios of alpha heating power to auxiliary power and will be able to access additional alpha-particle physics issues. The scientific opportunities for extending our understanding of burning plasmas beyond that provided by current experiments is described.
Date: December 21, 2001
Creator: Hawryluk, R.J.
Partner: UNT Libraries Government Documents Department

Correlation of isotope chemistry with molecular structure by second moment perturbation theory

Description: Expansion of the isotopic reduced partition function ratio in terms of a number of arguments has led to greater insight into the relationship between isotopic fractionation factors and molecular structure than can be obtained even from a critical analysis of each of the terms that contribute to the isotopic reduced partition function ratio. The most significant of these expansion methods are: (1) expansion of ln(s/s')f, logarithm of the isotopic reduced partition function ratio, in terms of the even moments of the vibrational frequencies, and (2) expansion of either (s/s')f or ln(s/s')f around a reference molecule which has no off-diagonal elements in the G (kinetic energy) and F (force constant) matrices whose solution yield the molecular vibrations. The diagonal element approximation, ln(s/s')f/sub 0/(ln(s/s')f/sub 0/ is the logarithm of the isotopic reduced partition function for the reference molecules which has eigenvalues defined by lambda/sub i0/ = f/sub ii/g/sub ii/.), gives very good results for hydrogen isotope substitution. The latter is a consequence of the fact that the off-diagonal G matrix elements involving the mass of hydrogen are either zero, or negligible, and the relatively light mass of hydrogen compared with other elements. The off-diagonal element corrections to ln(s/s')f/sub 0/ are of the order of 10 to 20% of the latter for carbon and oxygen isotope substitutions. These are easily evaluated by the WINIMAX polynomials. The correction terms, through second order (h/kT)/sup 4/, added to ln(s/s')f/sub 0/ provide an analytical formulation for the direct evaluation of ln(s/s')f for any molecule in terms of the F and G matrix elements. Through this method it is now possible to analyze ln(s/s')f in terms of the stretching, bending, and interaction force constants in a molecule.
Date: January 1, 1978
Creator: Bigeleisen, J.; Lee, M.W. & Ishida, T.
Partner: UNT Libraries Government Documents Department


Description: From American Chemical Society 145th National Meeting, New York, Sept. 1963. The pyrolysis of diborane was examined using a chemical shock tube as a reactor. Additional evidence for the existence of hexaborane-12 and heptaborane- 11 and -13 was obtained. The presence of a large net normal isotope effect in the formation of tetraborane and hexaborane and a net inverse isotope effect in the formation of pentaborane-9 are observed. A mechanism, consisting of a series of competitive reactions and eqailibria, in which tetraboraue is the precursor of pentaborane-11 and hexaborane but not of pentaborane-9 is shown to be compatible with the observed isotope effect. Data obtained by examining the pyrolysis of various mixtures of boron hydrides are also consistent with the mechanism. (auth)
Date: January 1, 1963
Creator: Fehlher, T P & Koski, W S
Partner: UNT Libraries Government Documents Department

An Isotope Effect in a Simple Chemical Reaction

Description: It has been found that the carbon dioxide obtained from the decarboxylation of singly-carboxyl labeled malonic acid is impoverished in the C{sup 14} label and that the acetic acid formed is correspondingly enriched.
Date: July 13, 1948
Creator: Yankwich, Peter E. & Calvin, Melvin
Partner: UNT Libraries Government Documents Department