Short-Chain Branching in Polyethylene and Poly(vinyl Chloride) Using Pyrolysis Hydrogenation Gas Chromatography and 13C Nuclear Magnetic Resonance Analysis

This study compares the use of pyrolysis hydrogenation gas chromatography (PHGC) and ¹³C Fourier transform nuclear magnetic resonance (FTNMR) methods for the analysis of reference polyethylene (PE) samples, ethylene-α-olefin copolymers, and specially prepared poly(vinyl chloride) (PVC) samples which were reduced to their PE skeletal structures. The nature and relative quantities of the short branches along the polymer chains were determined using both techniques. Improved high-resolution PHGC data, obtained with a fused silica capillary separation column, gave results which were in satisfactory agreement with the ¹³C FTNMR data. This approach confirms that detailed microstructural information can be obtained with these methods by using carefully controlled experimental conditions and appropriate reference systems.     

Tentative Identification of the Reactive Species in the Reaction of Cp2 TiCl2 with Salts of Diacids

The tentative identification of the reactive species in the condensation of Cp₂ TiCl₂ with salts of diacids to form titanium polysters is made. The reactive species are believed to be the same for both aqueous solution and interfacial systems, i.e., R -CO₂ ^? and Cp₂ Ti²⁺ with reaction occurring in the aqueous phase. The condensation of Cp₂ TiCl₂ with disodium terephthalate in interfacial systems occurs via a pseudo-first-order reaction:

Rate = K[Cp₂ TiCl₂] The rate-determining step (s) is believed to be diffusion of CP₂ TiCl₂ into the aqueous layer and/or hydrolysis of Cp₂ TiCl₂     

Structure selective ion molecule interactions (SSIMI) in ion mobility spectrometry

The overall objective of this project was to develop an analytical method that utilizes structure selective ion molecule interactions (SSIMI) in ion mobility spectrometry (IMS) to shift the mobility of a targeted analyte through the addition of a gas phase modifier to the buffer gas. IMS is a sensitive, rapid method for the detection of harmful chemicals; however false alarm responses do occur and a reduction in their frequency decrease both the cost and time required for detection. The investigation reported here probed the effects of a series of buffer gas modifiers on the mobilities of chemical warfare agent simulants (CWAs), toxic industrial chemicals (TICs) and a known interference (butyl carbitol) found in fire extinguishing agents. The major finding of this research was that a modifier with a proton affinity similar to, but not greater than, the target analyte produced the greatest changes in mobilities due to the formation of an ion cluster between the neutral modifier and target analyte ion. Mass spectrometry was utilized to confirm the formation of ion-neutral clusters that caused the target ion to shift its mobility. While a number of modifiers were screened, acetonitrile and isobutyronitrile were found to have sufficiently selective SSIMI with the target compound. For example, in the presence of acetonitrile modifier, the protonated response ion of the CWA simulant DMMP, [DMMP]H⁺, had a mobility shift of 10.8 %, but the mobility was unchanged for the interferent, butyl carbitol. The mobility of the simulant DMMP decreased with the introduction of modifiers, while the mobility of the interference did not change, demonstrating the potential of the SSIMI technique for reducing false alarm rates.     

Applicability of generic linear scoring mosels in the U.S. credit-union environment

Although credit-scoring models represent a widely used managerialaid for large financial intermediaries, the vast majority ofU.S. credit unions—relatively small cooperatively ownedretail intermediaries, constrained by sample and funding limitations—haveyet to adopt such techniques. Lovie & Lovie (1986) havetheorized that the flat-maximum effect or curve of insensitivityassociated with linear scoring models could be advantageousin areas of applied prediction such as credit scoring. In thiscontext, we reported the relative predictive power of genericcredit-scoring models versus customized models in an earlierpaper (Overstreet et al. 1992). Unfortunately, these findingswere not readily adaptable to the credit-union industry dueto a dated sample with incomplete credit-bureau information.Consequently, from 1988 to 1991, we gathered a refined databasefrom which to further develop and field-test generic scoringmodels in the credit-union environment. The results reportedherein not only confirm, but amplify, the relative predictivepower of such models found earlier. Relative costs and benefitsof generic versus customized models are modelled for a representativecredit union. Future research directions are set forth in theconclusions.     

Isomer-specific spectroscopy of the (H2O)8- cluster anion in the intramolecular bending region by selective photodepletion of the more weakly electron binding species (isomer II)

The vibrational predissociation spectra of the two more strongly electron binding forms of the (H2O)8- anion are obtained in the HOH intramolecular bending region. This is accomplished by deconvoluting the overlapping spectra obtained from a mixed ensemble using a population modulation scheme in which the low electron binding isomer (II) is removed from the ion packet prior to spectroscopic analysis. By choosing the energy of the photodepletion laser to lie between the vertical detachment energies of the two isomers, the contribution from isomer II can be quantitatively eliminated, leaving the population of I largely unaffected. The low binding energies involved in the application of the method to the water cluster anions necessitate that this should be carried out in the midinfrared, thus requiring two tunable ir laser systems for implementation. The isolated spectrum of isomer 1 displays a strong, redshifted feature associated with a double H-bond acceptor (AA) water molecule in direct contact with the excess electron and a large gap before higher energy features appear that are typically associated with (acceptor/donor) AD and ADD binding sites in the network. The more weakly binding isomer II does not display the AA feature and instead contributes broad structure at intermediate redshifts that merges with the region associated with neutral water cluster networks.     

Spectroscopic characterization of the isolated SF6- and C4F8- anions: observation of very long harmonic progressions in symmetric deformation modes upon photodetachment

Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

Quantum state resolved scattering from room-temperature ionic liquids: the role of cation versus anion structure at the interface

We present results on state-resolved scattering studies for seeded CO(2) supersonically cooled molecular beams (E(inc) = 61.9(40) kJ/mol) from a series of room-temperature ionic liquids (RTILs). These RTILs are composed of C(n)-methylimidazolium cations with BF(4)(-) or Tf(2)N(-) counteranions. The final rovibrational quantum state distributions from these nonequilibrium surface scattering collisions are monitored by high-resolution diode laser absorption spectroscopy as a function of (i) cation alkyl chain length and (ii) anion size, and analyzed to yield the propensity for thermal desorption (TD) versus impulsive scattering (IS) dynamics. For a fixed BF(4)(-) or Tf(2)N(-) counteranion, the distributions reveal an increase in the TD fraction (α) with the C atom number (n) in the alkyl side chain, which provides evidence for selective preference of nonpolar groups at the gas-liquid interface with increasing chain length. Conversely, for short carbon chains (n = 4), the thermal fraction decreases when the anion is changed from a compact and less polarizable BF(4)(-) to the bulkier and more polarizable Tf(2)N(-), whereas any sensitivity to anion identity essentially vanishes for longer alkyl chains (n = 8, 12). These combined data illustrate a number of interesting trends in anion versus cation competition for interfacial sites, specifically (i) the presence of interfacial anions at the surface layer for sufficiently short alkyl headgroups, (ii) inertial "stiffening" due to increasing average surface mass, as well as (iii) a propensity for larger anion sizes in the interfacial region. Finally, the TD probabilities follow the exact opposite trend in "bulk" Henry's Law solubility constants with respect to anion size, which further highlights the intrinsically nonequilibrium dynamics sampled by hyperthermal collisions at the gas-liquid interface.     

Quantum Monte Carlo study of the CO interaction with a dimer model surface for Cr(110)

The chemisorption of CO on a Cr (110) surface is investigated using the quantum Monte Carlo method in the diffusion Monte Carlo (DMC) variant and a model Cr₂CO cluster. The present results are consistent with the earlier ab initio HF study with this model that showed the tilted/near-parallel orientation as energetically favoured over the perpendicular arrangement. The DMC energy difference between the two orientations is larger (1.9 eV) than that computed in the previous study. The distribution and reorganization of electrons during CO adsorption on the model surface are analysed using the topological electron localization function method that yields electron populations, charge transfer and clear insight on the chemical bonding that occurs with CO adsorption and dissociation on the model surface.     

Characterization of aromatic-thiol π-type hydrogen bonding and phenylalanine-cysteine side chain interactions through ab initio calculations and protein database analyses

In this study, the aromatic-thiol π hydrogen bonding and phenylalanine-cysteine side chain interactions are characterized through both molecular orbital calculations on a C₆H₆-HSCH₃ model complex and database analyses of 609 X-ray protein structures. The aromatic-thiol π hydrogen bonding interaction can achieve a stabilization energy of 2.60 kcal mol^?1, and is stronger than the already documented aromatic-hydroxyl and aromatic-amino hydrogen bonds. However, the occurrence of the aromatic-thiol hydrogen bond is rather rare in proteins. This is because most of the thiol groups participate in the formation of either disulphide bonds or stronger S—H…O (or N) ‘normal’ hydrogen bonds in a protein environment. Interactions between the side chains of phenylalanine and cysteine residues are characterized as the phenyl(Phe)(HSCH₂-)(Cys) interaction. The bonding energy for such interactions is approximately 3.71 kcal mol-¹ and is achieved in a geometric arrangement with an optimal phenyl(Phe)-(HS-)(Cys) π-type hydrogen bonding interaction. The interaction is very sensitive to the orientation of the two lone electron pairs on the sulphur atom relative to the π electron cloud of the phenyl ring. Accordingly, the interaction configurations that can accomplish a significant bonding energy exist only within a narrow configurational space. The database analysis of 609 experimental X-ray protein structures demonstrates that only 268 of the 1620 cysteine residues involve such phenylalanine-cysteine side chain interactions. Most of these interactions occur in the form of π (aromatic)-lone pair(sulphur) attractions, and correspond to a bonding energy less than 1.5 kcal mol^?1. A few were identified as the aromatic-thiol hydrogen bond with a bonding energy of 2.0–3.6 kcal mol^?1.