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Iowa State University

1992

Articles 1 - 12 of 12

Full-Text Articles in Physical Sciences and Mathematics

Π-Bond Strengths Of H2x=Yh2: X = Ge Or Sn, And Y = C, Si, Ge, Or Sn, Theresa L. Windus, Mark S. Gordon Nov 1992

Π-Bond Strengths Of H2x=Yh2: X = Ge Or Sn, And Y = C, Si, Ge, Or Sn, Theresa L. Windus, Mark S. Gordon

Mark S. Gordon

The molecular structures and 1r-bond strengths are determined using both MP2 and MCSCF + CI energies for a series of H2X=YH2 compounds, where X = Ge or Sn and Y = C, Si, Ge, or Sn. These strengths are estimated both by evaluating the rotation barriers and by investigating the appropriate thermochemical cycles. The results show that C > Si ,.., Ge > Sn in their ability to form The molecular structures and 1r-bond strengths are determined using both MP2 and MCSCF + CI energies for a series of H2X=YH2 compounds, where X = Ge or Sn and Y = C, Si, …

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Uncatalyzed Peptide Bond Formation In The Gas Phase, Jan H. Jensen, Kim K. Balgridge, Mark S. Gordon Oct 1992

Uncatalyzed Peptide Bond Formation In The Gas Phase, Jan H. Jensen, Kim K. Balgridge, Mark S. Gordon

Mark S. Gordon

Several levels of electronic structure theory are used to analyze the formation of a peptide bond between two glycine molecules. Both a stepwise and concerted mechanism were considered. The energetic requirements for the stepwise and concerted mechanisms are essentially the same within the expected accuracy of the methods used. A simpler model system comprised of formic acid and ammonia is found to provide a good representation of the essential features of dipeptide formation. Total electron densities and localized molecular orbitals are used to interpret the mechanisms.

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Strategies Tor Designing A High-Valent Transition-Metal Silylidene Complex, Thomas R. Cundiari, Mark S. Gordon Sep 1992

Strategies Tor Designing A High-Valent Transition-Metal Silylidene Complex, Thomas R. Cundiari, Mark S. Gordon

Mark S. Gordon

The electronic structure of complexes arising from the formation of a double bond between a silylene ligand (Si(R)R') and a high-valent transition-metal fragment have been investigated using ab initio wave functions including the effects of electron correlation. Using the analogy of carbon, these complexes may be referred to as Scbrock-type silylenes or silylidenes. A prime motivation for this work is that complexes of this type, unlike their carbon analogues, have so far eluded attempts at experimental characterization. Several conclusions are reached as a result of these calculations. (a) The inclusion of electron correlation is necessary to adequately describe the MSi …

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Spatial Variation Of A Short-Lived Intermediate Chemical Species In A Couette Reactor, R. Dennis Vigil, Q. Ouyang, Harry L. Swinney Apr 1992

Spatial Variation Of A Short-Lived Intermediate Chemical Species In A Couette Reactor, R. Dennis Vigil, Q. Ouyang, Harry L. Swinney

R. Dennis Vigil

We have conducted experiments and simulations of the spatial variation of a short-lived intermediate species (triiodide) in the autocatalytic oxidation of arsenite by iodate in a reactor that is essentially one dimensional-the Couette reactor. (This reactor consists of two concentric cylinders with the inner one rotating and the outer one at rest; reagents are continuously fed and removed at each end in such a way that there is no net axial flux and there are opposing arsenite and iodate gradients.) The predictions ?f a one-dim~ns~onal . reaction-diffusion model which has no adjustable parameters, are In good qualItative (and, In some …

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Pinski Et Al. Reply, F. J. Pinski, B. Ginatempo, Duane D. Johnson, J. B. Staunton, G. M. Stocks, B. L. Gyorffy Mar 1992

Pinski Et Al. Reply, F. J. Pinski, B. Ginatempo, Duane D. Johnson, J. B. Staunton, G. M. Stocks, B. L. Gyorffy

Duane D. Johnson

With our calculations [I], we uncovered the electronic mechanism responsible for inducing atomic short-range order (SRO) in the disordered solid solution of NiPt as it is cooled. Usually, but not always, SRO, whether derived theoretically or measured experimentally, indicates the nature of the long-rangeordered (LRO) state that will stabilize at low temperature. Our calculation of the atomic SRO, while agreeing with experiments [2], neglected the relativistic effects in the electronic structure. Lu, Wei, and Zunger (LWZ) [3,4], on the other hand, have calculated the L lo-ordered alloy formation energy and find that only when relativistic effects are included is the …

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Hypercoordination In Group Iv Mh5 And Mh5- Systems, Marshall T. Carroll, Mark S. Gordon, Theresa L. Windus Mar 1992

Hypercoordination In Group Iv Mh5 And Mh5- Systems, Marshall T. Carroll, Mark S. Gordon, Theresa L. Windus

Mark S. Gordon

The energetics and bonding of the group IV hypervalent MH5 and MH5- D3h structures (M = C, Si, Ge, Sn) are examined in this paper. Ab initio all-electron calculations are used to predict the energies and geometries of the systems. The resulting electron densities are analyzed using the topological theory of atoms in molecules. It is found that the anion energetically is more stable than the neutral radical for M = Si, Ge, and Sn but not for M = C. Further distinguishing carbon from the other members of the group is the fact that the CH bond mostly is …

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Experimental And Theoretical Evidence In Support Of An Intermediate Complex In The Insertion Reaction Of Silylene Into Silane, R. Becerra, H. M. Frey, B. P. Mason, R. Walsh, Mark S. Gordon Mar 1992

Experimental And Theoretical Evidence In Support Of An Intermediate Complex In The Insertion Reaction Of Silylene Into Silane, R. Becerra, H. M. Frey, B. P. Mason, R. Walsh, Mark S. Gordon

Mark S. Gordon

The reaction between silylene, SiH2, and silane, SiH4, viz. SiH2 + SiH4 - Si2H6 (1) may reasonably be considered the prototype Si-H insertion process of silicon hydride chemistry. This reaction, which is also the second step in the thermolysis of silane1 leading ultimately to solid silicon and silicon hydride deposition, is known to be fast. Experimental measurements of the rate constant2-4 show the reaction to proceed at close to the collisional limit. Theoretical calculations of the energy surface for the decomposition of Si2H6 show that it possesses no energy barrier at the threshold of dissociation,5 which suggests a zero barrier …

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Tem Observations Of The Mechanism Of Delamination Of Chromium Films From Silicon Substrates, D. Goyal, Alexander H. King Feb 1992

Tem Observations Of The Mechanism Of Delamination Of Chromium Films From Silicon Substrates, D. Goyal, Alexander H. King

Alexander H. King

We have observed the complete delamination of polycrystalline chromium films from single crystal silicon substrates during deposition due to the formation of high internal stresses. These intrinsic stresses can give rise to interfacial defects which assist in the separation of the film from the substrate. Stresses in the film are balanced by stresses in the substrate, which cause mechanical failure in the substrate near the interface. Extensive arrays of dislocations and cracking of the substrate have been observed. We find that the delamination of the films from the substrate is initiated by the formation of damage in the substrate, rather …

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Micro And Macromorphology Of Recycled Fiber And Wood, Douglas D. Stokke Jan 1992

Micro And Macromorphology Of Recycled Fiber And Wood, Douglas D. Stokke

Douglas D. Stokke

Wood is a major industrial raw material, with U.S. consumption approaching that of aluminum, plastics, cement, and steel combined. Partially as a result of the magnitude of wood and wood products in use, these products constitute a substantial portion of the solid waste stream. In order to reduce the amount of wood and wood fiber disposed in landfills, efforts to recycle these materials into useful products such as structural composites are needed. The success of such conversion depends in part on knowledge of the morphological characteristics of various sources of secondary wood and wood fibers, and the influence of wood …

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Theoretical Investigatlons Of Olefin Metathesis Catalysts, Thomas R. Cundiari, Mark S. Gordon Jan 1992

Theoretical Investigatlons Of Olefin Metathesis Catalysts, Thomas R. Cundiari, Mark S. Gordon

Mark S. Gordon

An ab initio analysis of the electronic structure of high-valent, transition-metal alkylidenes as models for olefm metathesis catalysts is presented. The catalyst models studied fall into three categories: "new" metathesis catalyst models-tetrahedral M(0Hh(XH)(CH2) complexes; "old" metathesis catalyst models-tetrahedral MCMY)(CH2) complexes and alkylidene-substituted Mo metathesis catalysts, Mo( OHMNH)(=C(H)Z). The effect on the bonding caused by modification of either the metal, ligands, or alkylidene substituents is considered. For the new models the minimum energy structures result from maximum metal d1rligand p11' bonding and minimum competition among the ligands for the same d11' AO. Rotation about the MC axis increases this competition. The …

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Nature Of The Transition Metal-Silicon Double Bond, Thomas R. Cundiari, Mark S. Gordon Jan 1992

Nature Of The Transition Metal-Silicon Double Bond, Thomas R. Cundiari, Mark S. Gordon

Mark S. Gordon

The nature of the metal-silicon double bond is investigated using ab initio wavefunctions. Electron correlation is specifically included in the calculations at the FORS-MCSCF (full optimized reaction space multiconfiguration self-consistent field) level of theory. Silylene complexes of the form MSiH2 +,containing metals from the first transition series (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni), are chosen for this study. An analysis of the bonding and electronic structure among the silylene complexes is presented. The silylene complexes are compared to representative car bene analogues, MCH2 +, and to CrGeH2 + and CrSnH2+.

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High-Valent Transition-Metal Alkylidene Complexes: Effect Of Ligand And Substituent Modification, Thomas R. Cundiari, Mark S. Gordon Jan 1992

High-Valent Transition-Metal Alkylidene Complexes: Effect Of Ligand And Substituent Modification, Thomas R. Cundiari, Mark S. Gordon

Mark S. Gordon

An ab initio investigation into the effects of ligand and substituent modification on the metal-carbon double bond is reported. Prototypical group IVB (Ti, Zr, HO and Group VB (Nb, Ta) alkylidenes are chosen for this study. The MC/LMO/CI (multiconfigurationjlocalized molecular orbital/configuration interaction) procedure is used to examine the electronic structures of these complexes in terms of the prime resonance contributors to the ground-state wave function. The main conclusion drawn from this work is that the intrinsic nature of the metal-carbon double bond can typically be changed only within certain limits by modification of the electronegativity of the ligands (L) and …

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