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Iowa State University

1989

Articles 1 - 23 of 23

Full-Text Articles in Physical Sciences and Mathematics

Substituent Effects On The Gas-Phase Acidity Of Silane, Mark S. Gordon, David E. Volk, David R. Gano Dec 1989

Substituent Effects On The Gas-Phase Acidity Of Silane, Mark S. Gordon, David E. Volk, David R. Gano

Mark S. Gordon

In a previous paper, the gas-phase acidities of XHn compounds (X= C, N, 0, F, Si, P, S, Cl) were predicted with ab initio wave functions. At the MP4 level of theory with extended basis sets (6-311++G(3df,2pd)3 for second-period atoms and 6-3l++G- (3df,2pd) for third-period atoms], the calculated gas-phase acidities for these species were determined to be within 2 kcaljmol of experimental values. Similar results for the second period were obtained by DeFrees and McLean.

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Toxicology Of Synthetic Pyrethroids In Aquatic Organisms: An Overview, Steven P. Bradbury, Joel R. Coats, D. M. Symonik, S. D. Dyer, L. K. Timson, G. J. Atchison Aug 1989

Toxicology Of Synthetic Pyrethroids In Aquatic Organisms: An Overview, Steven P. Bradbury, Joel R. Coats, D. M. Symonik, S. D. Dyer, L. K. Timson, G. J. Atchison

Steven P. Bradbury

The aquatic toxicology of the photostable synthetic pyrethroid insecticides as it affects two important groups of susceptible organisms — fish and aquatic insects — is discussed. The sensitivity of these aquatic species to the pyrethroids is dependent on several factors, including toxicokinetics, target site (nervous system), sensitivity and possible secondary mechanisms of action, as well as chemical and physical properties of the aquatic medium that influence toxicity and bioavailability. Uptake rates and routes of fenvalerate greatly affected the toxicity of fenvalerate to mosquito larvae. LD50 values were determined for cuticular and dietary exposure routes by utilizing radiolabeled fenvalerate at the …

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Predicted Heats Of Formation For Methylsilylene And Dimethylsilylene, Mark S. Gordon, Jarry A. Boatz Aug 1989

Predicted Heats Of Formation For Methylsilylene And Dimethylsilylene, Mark S. Gordon, Jarry A. Boatz

Mark S. Gordon

Ab initio calculations at the MP216-31G(d,p) level of computation, combined with a sequence of isodesmic reactions, have been used to predict the heats of formation of methylsilylene and dimethylsilylene to be 49.2 and 32-33 kcallmol, respectively. These results are discussed in the light of recent experimental determinations in the accompanying experimental papers.

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Structures Of Two Organosilyl Azides, Steven S. Zigler, Kenneth J. Haller, Robert West, Mark S. Gordon Jul 1989

Structures Of Two Organosilyl Azides, Steven S. Zigler, Kenneth J. Haller, Robert West, Mark S. Gordon

Mark S. Gordon

X-ray crystal structures were determined for trimesitylazidosilane (3) and 1,1-dimesityl-2,2-diphenyl- 2-tert-butylazidodisilane (4). The NcN2 and N2-N3 bond lengths (pm) in both compounds are nearly equal, being 115.8 and 117.0 for 3 and 117.0 and 114.7 for 4, respectively. These silyl azides thus differ markedly from organic azides, in which N2-N3 is much shorter than NcN2• Molecular orbital calculations predict N2-N3 to be 6 pm shorter than NcN2 in H3SiN3 (5). The N-N-N angle is 173.7° for 3 and 174.9° for 4, in good agreement with the calculated value for 5.

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Intrinsic Frequency Analysis Of The Generalized Normal-Mode Vibrations For The Reaction H 2 + Ch3 - H + Ch4, Jerry A. Boatz, Mark S. Gordon Jul 1989

Intrinsic Frequency Analysis Of The Generalized Normal-Mode Vibrations For The Reaction H 2 + Ch3 - H + Ch4, Jerry A. Boatz, Mark S. Gordon

Mark S. Gordon

Vibrational energy distributions and intrinsic frequencies are computed at selected points along the H 2 + CH 3 - H + CH4 intrinsic reaction coordinate, calculated at the UHF /ST0-3G level. The energy distributions and intrinsic frequencies are shown to provide a clear and intuitively pleasing picture of the evolution of generalized normal coordinates along the reaction path. Furthermore, these quantities aid in the identification of avoided crossings between distinct generalized normal modes and between generalized normal modes and the reaction coordinate.

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Ab Initio Reaction Paths And Direct Dynamics Calculations, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler, Donald G. Truhlar Jun 1989

Ab Initio Reaction Paths And Direct Dynamics Calculations, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler, Donald G. Truhlar

Mark S. Gordon

A detailed study of methods for generating the minimum energy path of a chemical reaction using ab initio electronic structure calculations is presented; the convergence with respect to step size of the geometry and energy along this path is studied with several algorithms. The investigations are extended to the calculation of chemical reaction rate coefficients by interfacing the POLYRATE code for variational transition-state theory and semiclassical tunneling calculations with a locally modified GAUSSIAN82 electronic structure package that now contains reaction path following capabilities at both the Hartree-Fock and perturbation theory levels. This combined package is used to study the kinetics …

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Nife Invar Alloys: Theoretical Insights Into The Underlying Mechanisms Responsible For Their Physical Properties, Duane D. Johnson, F. J. Pinski, J. B. Staunton, B. L. Gyorffy, G. M. Stocks Jun 1989

Nife Invar Alloys: Theoretical Insights Into The Underlying Mechanisms Responsible For Their Physical Properties, Duane D. Johnson, F. J. Pinski, J. B. Staunton, B. L. Gyorffy, G. M. Stocks

Duane D. Johnson

What are the "driving forces" responsible for various physical properties of alloys? What causes alloy systems to order chemically and/or magnetically? To answer these questions, we have been using a quantum mechanical (QM) method to calculate cohesive energies, magnetic properties, and thermodynamic phase instability of alloys. This scheme directly incorporates the inherent disorder of the high-temperature solid solution and in this sense goes beyond traditional "band theory." Although approximations have been made at various steps in the thermodynamical averaging, and in the QM treatment of the electrons, adjustable parameters have not been used. Recently, for FCC NiFe (INVAR) alloys, we …

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Toxicokinetics And Toxicodynamics Of Pyrethroid Insecticides In Fish, Steven P. Bradbury, Joel R. Coats May 1989

Toxicokinetics And Toxicodynamics Of Pyrethroid Insecticides In Fish, Steven P. Bradbury, Joel R. Coats

Steven P. Bradbury

The pyrethroid insecticides are extremely toxic to fish, with 96—h LC50 values generally below 10 μg/L and i.p. and i.v. LD50 values below 20 mg/kg. Corresponding LD50 values in mammals and birds are in the range of several hundred to several thousand milligrams per kilogram. This review examines pyrethroid toxicokinetics and toxicodynamics in fish as critical factors associated with species selectivity. Studies with permethrin, cypermethrin and fenvalerate have established that rates of metabolism and elimination in rainbow trout are significantly lower than those reported for birds and mammals. Comparatively low lethal brain pyrethroid concentrations and nonneural aspects of pyrethroid intoxication …

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Molecular And Electronic Structure Of Siladiimide And Other Allenic X=Y=X Compounds, Mark S. Gordon, Michael W. Schmidt, S. Koseki May 1989

Molecular And Electronic Structure Of Siladiimide And Other Allenic X=Y=X Compounds, Mark S. Gordon, Michael W. Schmidt, S. Koseki

Mark S. Gordon

The preferred geometric structures of the allene analogues CH2=Si=CH2 (1), NH=Si=NH (2), PH=Si=PH (3), NH= C=NH (4), and NH=Si=CH2 (5) are examined at the RHF /6-31 G(d) level of theory, and that of 2 is further examined at the MP2/6-31G(d) computational level. The prediction that at the SCF level compounds 1, 3, 4, and 5 prefer orthogonal geometries like allene, while 2 prefers a planar structure, is examined with the aid of localized molecular orbitals, correlated energies, and, in the case of 2, a detailed analysis of the potential energy surface. The latter is very flat, and at the highest …

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Phase Transitions In A Driven Lattice Gas With Repulsive Interactions, K.-T. Leung, Beate Schmittmann, R. K. P. Zia Apr 1989

Phase Transitions In A Driven Lattice Gas With Repulsive Interactions, K.-T. Leung, Beate Schmittmann, R. K. P. Zia

Beate Schmittmann

We study a lattice gas with repulsive nearest-neighbor interactions driven to steady state by an external electric field E. Using Monte Carlo techniques on a two-dimensional system, we find, in the E-T plane, a line of second-order transitions joining a line of first-order ones, at a point which is probably tricritical. From a field theoretical model, we show that the operator associated with E is naively irrelevant for critical behavior. This expectation is borne out by the Monte Carlo result β=(1/8.

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Ozonation For Non-Filamentous Bulking Control In An Activated Sludge Plant Fuel Synthesis Waste Transfer, Johannes Van Leeuwen Apr 1989

Ozonation For Non-Filamentous Bulking Control In An Activated Sludge Plant Fuel Synthesis Waste Transfer, Johannes Van Leeuwen

Johannes van Leeuwen

A zero discharge mode of operation at the South African fuel synthesis plants necessitates the infinite recycle of treated industrial waste water. The treatment process incorporates activated sludge units in which non-filamentous bulking and carry-over of solids often occurs. The use of ozone for the control of this non-filamentous binding was investigated on a 33 l/d small pilot-scale plant. An ozone dosage of 1 g per kg mixed liquor suspended solids per day could reduce the diluted sludge volume index from an average of 125 to about 70 ml/g mainly by preventing zoogleal growths, the main contributor to the non-filamentous …

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Theoretical Studies Of Three-Membered Ring Compounds Y2h4x (Y = C, Si; X = Ch2, Nh, O, Sih2, Ph, S), Jerry A. Boatz, Mark S. Gordon Apr 1989

Theoretical Studies Of Three-Membered Ring Compounds Y2h4x (Y = C, Si; X = Ch2, Nh, O, Sih2, Ph, S), Jerry A. Boatz, Mark S. Gordon

Mark S. Gordon

The heats of formation of the three-membered ring compounds Y 2H4X (Y = C, Si; X = CH2, NH, 0, SiH2, PH, S) and the thermodynamics of the insertion reactions X+ YH2=YH2 -- c-Y2H4X are predicted by using MP2/6-31G(d) energies at the 6-31G(d) geometries. Bent bond lengths are calculated by tracing the path of maximum electron density connecting two nuclei, with the 6-31G(2d) basis set at the 6-31G(d) structures. The short Si-Si internuclear distances in Si2H4X (X = CH2, NH, 0, PH, S) apparently are the result of severe bond bending rather than significant 1r character in the Si-Si bonds.

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Theoretical Studies Of Bond Stretch Isomerism In Silabicyclobutanes, Jerry A. Boatz, Mark S. Gordon Apr 1989

Theoretical Studies Of Bond Stretch Isomerism In Silabicyclobutanes, Jerry A. Boatz, Mark S. Gordon

Mark S. Gordon

The structures and relative energies of the bond stretch isomers of tetrasilabicyclo[l.l.O]butane (1), 1,2,3-trisilabicyclo[ l:l.O]butane (2), and 1,3-disilabicyclo[l.l.O]butane (3) are predicted at the GVB/3-21G* level, with the u and u* bridge bond molecular orbitals correlated in the GVB wave function. The transition-state structure and intrinsic reaction coordinate connecting each pair of bond stretch isomers are calculated. Two transition states joining the bond stretch isomers of 3 are located. The more stable isomer in all three molecules is the one with the longer bridge bond.

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Use Of Respiratory-Cardiovascular Responses Of Rainbow Trout (Salmo Gairdneri) In Identifying Acute Toxicity Syndromes In Fish: Part 3. Polar Narcotics, Steven P. Bradbury, Tala R. Henry, Gerald J. Niemi, Richard W. Carlson, Virginia M. Snarski Mar 1989

Use Of Respiratory-Cardiovascular Responses Of Rainbow Trout (Salmo Gairdneri) In Identifying Acute Toxicity Syndromes In Fish: Part 3. Polar Narcotics, Steven P. Bradbury, Tala R. Henry, Gerald J. Niemi, Richard W. Carlson, Virginia M. Snarski

Steven P. Bradbury

The physiological responses of rainbow trout to acutely lethal aqueous concentrations of the suspected polar narcotics phenol, 2,4-dimethylphenol, aniline, 2-chloroaniline and 4-chloroaniline were examined. Visible signs of intoxication included tremors that progressed to whole-organism clonic seizures, followed by general depression and respiratory-cardiovascular collapse. Tremors and seizures were usually initiated with coughs. The most striking changes in the respiratory-cardiovascular parameters for all five toxicants included elevated cough frequency (coincident with seizures), ventilation frequency and hematocrit, and depressed gill oxygen uptake efficiency, arterial pH, total arterial oxygen and total arterial carbon dioxide. The physiological responses noted were attributed to the initial muscular …

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Decomposition Of Normal-Coordinate Vibrational Frequencies, Jerry A. Boatz, Mark S. Gordon Mar 1989

Decomposition Of Normal-Coordinate Vibrational Frequencies, Jerry A. Boatz, Mark S. Gordon

Mark S. Gordon

A decomposition scheme for the partitioning of normal-coordinate vibrational frequencies into individual internal-coordinate "intrinsic~ frequencies is presented. This scheme assigns to each internal coordinate a frequency that is representative of a vibration described solely by motion of that particular coordinate. This facilitates the interpretation of harmonic vibrational frequencies and their associated normal-coordinate displacements, especially in those cases where the normal coordinates are best described by the coupling of several internal coordinates. The utility of the method is demonstrated via application to several simple hydrides and cycloalkanes, using ab initio SCF molecular structures and Cartesian force constant matrices.

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Theoretical Studtes Of The Insertion Reactions Of Atomic Carbon And Silicon Into Methane And Silane, Shogo Sakai, John Deisz, Mark S. Gordon Mar 1989

Theoretical Studtes Of The Insertion Reactions Of Atomic Carbon And Silicon Into Methane And Silane, Shogo Sakai, John Deisz, Mark S. Gordon

Mark S. Gordon

The mechanisms for the insertions of atomic (ID and 3P) carbon and silicon into the C-Hand Si-H bonds of methane and silane are investigated by ab initio SCF methods, many body perturbation theory, and a localized molecular orbital (LMO) analysis. The LMO analysis shows that the insertion of 1D atoms into CH4 and SiH4 may be classified into two types: cationic hydrogen transfer and anionic hydrogen transfer. For the triplet atoms the LMO analysis suggests two different insertion reaction mechanisms: the near abstraction and the pull-push mechanisms.

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Effects Of Pollution On Critical Population Dynamics, R. Kree, B. Schaub, Beate Schmittmann Feb 1989

Effects Of Pollution On Critical Population Dynamics, R. Kree, B. Schaub, Beate Schmittmann

Beate Schmittmann

We investigate the effects of pollution on a population that is on the brink of extinction. In the vicinity of the associated critical point, the temporal scales of the population density fluctuations are found to be completely governed by the diffusive behavior of the pollution density fluctuations. Moreover, the mean value of the population density is found to vanish with a larger power-law exponent in the presence of pollution density fluctuations. Results are obtained within a renormaliza- tion-group calculation to O(ε) (ε=4-d, d being the spatial dimension).

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Mechanistic Studies Of The Far-Uv Photochemical Ring-Opening And Cleavage Reactions Of 1, 1-Dimethyl-1-Silacyclobut-2-Ene, Mark G. Steinmetz, B. S. Udayakumar, Mark S. Gordon Feb 1989

Mechanistic Studies Of The Far-Uv Photochemical Ring-Opening And Cleavage Reactions Of 1, 1-Dimethyl-1-Silacyclobut-2-Ene, Mark G. Steinmetz, B. S. Udayakumar, Mark S. Gordon

Mark S. Gordon

Direct photolyses of 1,1-dimethyl-1-silacyclobut-2-ene (1) at 214 nm in tert-butyl alcohol gave 29% tert-butoxydimethyl(2-propenyl)silane (2), 4.3% (Z)-tert-butoxydimethyl(1-propenyl)silane [(Z)-3], 6.9% (E)-tert-butoxydimethyl(1-propenyl)silane [(E)-3], and 5.9% tert-butoxytrimethylsilane (4) at 49% conversion. Products 2-4 were primary, and the total quantum yield of formation was 0.11. Use of tert-butyl alcohol-0-d led to >98% monodeuteration: 2-dl had 93.5% of the label at cl and 6.5% at c3 of the 2-propenyl group, (E)- and (Z)-3-d1 [(E),(Z)-3-dd were labeled >95% at C3 of the 1-propenyl group, and 4-d1 was deuterated at a silyl methyl. Cleavage of a silacyclopropylmethylene intermediate to 1,1-dimethyl-1-silaethene accounts for the labeling of 4-d1 and …

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Heats Of Formation Of Alkylsilanes, Mark S. Gordon, Jerry A. Boatz, Robin Walsh Feb 1989

Heats Of Formation Of Alkylsilanes, Mark S. Gordon, Jerry A. Boatz, Robin Walsh

Mark S. Gordon

Theoretical heats of formation at 298 K for several alkylsilanes, predicted at the MP2/6-31G(d) level of theory, are compared with recently obtained experimental and additivity values. Excellent agreement is obtained between the ab initio and additivity values and with the more reliable experimental values for acyclic alkylsilanes. The ab initio heats of formation for the silacycloalkanes permit the evaluation of strain energy increments for the additivity scheme. Comparison is made with limited experimental data.

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Polar Narcosis In Aquatic Organisms, Steven P. Bradbury, Richard W. Carlson, Tala R. Henry Jan 1989

Polar Narcosis In Aquatic Organisms, Steven P. Bradbury, Richard W. Carlson, Tala R. Henry

Steven P. Bradbury

The majority of industrial organic chemicals lack identifiable structural characteristics that result in specific biological activity. These nonpolar-nonelectrolytes are acutely toxic to aquatic organisms via a nonspecific mode of action termed narcosis. The toxicity of industrial chemicals eliciting nonpolar narcosis can be reliably predicted by log P (baseline toxicity models). Using single chemical and joint toxic action models, several research groups have reported classes of polar compounds (for example, esters, phenols, and anilines) that elicit a narcosis-like syndrome; however, they are more acutely toxic than what is predicted using baseline toxicity models. An assessment of rainbow trout (Salmo gairdneri) in …

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Comparative Toxicology Of The Pyrethroid Insecticides, Steven P. Bradbury, Joel R. Coats Jan 1989

Comparative Toxicology Of The Pyrethroid Insecticides, Steven P. Bradbury, Joel R. Coats

Steven P. Bradbury

The toxic effects elicited by synthetic pyrethroids in animals are varied in degree and nature. Their relative safety to birds and mammals contrasts sharply with their acute effects on fish and arthropods. Explantation of their differences in toxicity depends on examination of all factors of their comparative toxicology. Routes of exposure are important, as are metabolism and elimination rates, especially for mammals and birds with their considerable capabilities for biotransformation. Significant differences in sensitivity at the sites of toxic action may also play a role in differential responses to these insecticides. Finally, physical properties that influence the environmental disposition and …

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Intrinsic Reaction Coordinate Calculations For Very Flat Potential Energy Surfaces: Application To Singlet Si2h2 (Disilenylidene) Isomerization, Shiro Koseki, Mark S. Gordon Jan 1989

Intrinsic Reaction Coordinate Calculations For Very Flat Potential Energy Surfaces: Application To Singlet Si2h2 (Disilenylidene) Isomerization, Shiro Koseki, Mark S. Gordon

Mark S. Gordon

Si2H2 is an important species that may appear in the chemical vapor deposition of silicon. This paper reports an intrinsic reaction coordinate (IRC), or a minimum energy path (MEP), of the isomerization from silasilene to bridged disilyne obtained by using the local cubic and quadratic approximations. These. new approximations generate a correct IRC for this isomerization, while some conventional methods fail to predict a reasonable IRC because of the very flat potential energy surface. This reaction path bifurcates to two identical IRCs to reach bridged disilyne. The activation energy for this isomerization is predicted to be less than 3 kcalfmol. …

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Transition State Structure, Barrier Height, And Vibrational Frequencies For The Reaction Cl+Ch4→Ch3+Hcl, Thanh N. Truong, Donald G. Truhlar, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler Jan 1989

Transition State Structure, Barrier Height, And Vibrational Frequencies For The Reaction Cl+Ch4→Ch3+Hcl, Thanh N. Truong, Donald G. Truhlar, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler

Mark S. Gordon

We have carried out a b i n i t i o calculations using second‐ and fourth‐order Mo/ller–Plesset perturbation theory, scaled electron correlation, and several basis sets for the reaction Cl+CH4→CH3+HCl. We found that including electron correlation is essential for obtaining accurate barrier heights and vibrational frequencies. Furthermore, scaling the correlation energy further improves the barrier height predictions provided that the basis set being used is correlation balanced for both bonds involved in the reaction. Geometries and transition state frequencies calculated at the MP2 and MP‐SAC2 levels with the most extensive and best balanced basis set are in good agreement …

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