Gordon K. Hamer

Application of High-Resolution Solid-State NMR with Cross-Polarization/Magic-Angle Spinning (CP/MAS) Techniques to Cellulose Chemistry

Abstract Vegetation is thought to produce an estimated 100 billion tons of cellulose a year, that is, approximately 25 tons of cellulose for every person on earth This vast quantity of naturally occurring, readily available fiber has helped to maintain cotton as the most important textile fiber in the world and made wood the commodity material of the world. Enormous research effort has been directed toward the study of cellulosic materials and has led to a wealth of information in the literature pertaining to the morphology, biosynthesis, and varied reactions of this important substance.

An Investigation of Substituent Effects on Coupling Constants to Vinyl Protons in Styrene Derivatives

Experimental long-range phenyl proton–vinyl proton coupling constants in 4-substituted styrenes are substituent independent. This is also predicted by INDO–finite perturbation theory calculations of these coupling constants. Comparison with calculated and experimental long-range coupling constants for 4-substituted benzaldehydes suggests that the previously reported substituent dependence for the latter coupling constants arises from substituent-induced changes in molecular geometry.Geminal vinyl coupling constants in 4-substituted styrenes, α-methylstyrenes, and α-t-butylstyrenes are substituent dependent with substituent effects increasing as phenyl and vinyl groups are twisted out of planarity. These trends are reproduced by INDO–FPT calculations. It is concluded that the substituent effects are primarily transmitted through space.Both experimental and calculated vinyl 13C–1H coupling constants show strong stereospecific substituent effects. From the pattern of results (particularly the greater field d...

The relative merits of single and dual substituent parameter scales for the interpretation of substituent-induced chemical shifts in styrene derivatives

Abstract The relative abilities of single and dual substituent parameter scales to reproduce substituent effects on magnetic resonance parameters in 4-substituted styrenes are compared, using both σp and the θ scale of Edlund and Wold andthe σ1, σR0 scales of Taft. Various statistical criteria demonstrate that the σ1, σR0 correlations are significantly more precise, even though the θ parameters were estimated from the chemical shifts of the compounds being tested. It is further shown that there are statistically significant variations in the relative proportions of field/inductive and resonance effects for the different parameters, demonstrating that no single substituent parameter scale could reproduce the experimental data. Finally, principal-component analysis is used to demonstrate that a dual substituent parameter scale is both necessary and sufficient to account for substituent-induced changes in magnetic resonance parameters in styrene derivatives. Problems associated with the use of pattern recognition methods for classifying structures are commented upon.

Diffraction studies on metal free phthalocyanines (β-H2PC and X-H2Pc)

The crystal structure of the β-polymorph of metal free phthalocyanine (β-H2Pc) was redetermined from single crystals to obtain precise placements of the hydrogen atoms and a precise model for the X-polymorph (X-H2Pc) for which only limited data from different diffraction techniques are available. Partial occupancy for the hydrogens fixed to the nitrogens in the inner ring was found leading to a highly symmetric phthalocyanine molecule. Careful evaluation of X-ray, electron and neutron diffraction data for X-H2Pc results in an orthorhombic unit cell with a=2.10 nm, b=0.491 nm, c=2.31 nm, space group Pna21 containing four molecules in contrast to a recent determination of Oka and Okada who proposed a monoclinic unit cell with two molecules. As expected the phthalocyanine molecules pack in stacks. The overlap of molecules within one stack matches the current ideas for photosensitive phthalocyanine pigments.

Insights into the Lattice Structure of Cellulose II From the High Resolution CP/MAS Solid State 13C NMR Spectrum of Cellotetraose

Abstract The chemical shifts and multiplicities of the high-resolution 13C CP/MAS NMR spectrum of cellulose II are quite diagnostic of the lattice structure of this polymorph. Particularly important is the chemical shift of C-1 and its clear splitting into two lines of equal intensity. Similar chemical shifts and multiplicities are seen in the spectrum of cellotetraose. Thus cellotetraose is considered to be a good model for the lattice structure of the polymer. A detailed investigation of the multiplicity of the C-1 resonance of cellotetraose shows that the two peaks are of equal intensity in this case also. Because of the limited number of repeat units in the tetramer, this observation implies that the unit cell contains two independent chains rather than a “double” repeat unit. This gives support for a similar lattice structure, with two independent chains, for cellulose II itself.

Carbon-13 nuclear magnetic resonance study of the styrene–platinum(II) bond

13 C N.m.r. and i.r. spectroscopic studies of a series of para-substituted styrene–platinum(II) complexes indicate a significant ionic contribution to the styrene–platinum bond.

Organic photoconductive (OPC) devices incorporating bisarylimidazole perinone pigments

Abstract The synthesis and purification of a series of four bisarylimidazole perinone photoconductive pigments is described. The optical and electrical properties of negatively charged dual-layer photoreceptors incorporating a thin 0·20μm evaporated film of the bisarylimidazole perinones and a thick (15 μm) hole transport layer were determined. The photoreceptor devices exhibit low dark decay, excellent cyclic stability and high white light photosensitivity. An improvement in the spectral response above 600 nm was obtained by varying the chemical structure of the perinone.

Apparent asymmetry in the relaxation of a strongly coupled spin system

Spin relaxation is usually investigated by disturbing the spin system and then observing it as it decays back to equilibrium. This is the familiar inversion-recovery experiment (1, 2). With a coupled spin system, the naive expectation is that all the members of a multiplet will relax at the same rate, since they all “belong” to the same nucleus. This is certainly not true in general, since systems with equivalent spins can show differential relaxation if there is correlation among the motions of the spins in the molecule (3-5). However, except in pathological cases (6-9), this differential rel,axation in a multiplet is an “inner vs outer” difference, rather than a “left vs right” difference. This is based on some fundamental symmetries in the spin relaxation (IO). Strong coupling can also disturb the simple picture of spin relaxation (II-13), and the possible flipangle dependence of the spectra may play a part ( 14-16). When the strong left-right asymmetry in the relaxation of N,N’-bis( 2-ethylhexyl)-3,4,9, lo-perylenetetracarboxylic dimide was observed (see Fig. 1)) there were several possible explanations. This paper presents the mechanism for this asymmetry, shows that it is a general phenomenon in strongly coupled systems, and confirms and illustrates some theoretical analyses in the definitive work on spin relaxation in strongly coupled systems by Vold and Vold ( I1 ). Although these spectra are unusual, it is not the first time that this effect has been reported in the literature (I 7). Some of the candidates for an explanation were as follows. First, although the spin system is apparently an AB system, it is, strictly speaking, better described as [ AB], or even [ ABId, and there may be some correlation effects in the relaxation of this spin system. Furthermore, in a strongly coupled system, not only do the x-y magnetizations alter, but the character of the z magnetizations also changes with the strength of the coupling (14, 18). Another possibility is that there may really be asymmetric cross terms in the relaxation. Finally, both strong coupling and large flip angles are known to mix transitions together (1%Z6), so the

Substituent effects of the groups CH2M(CH3)3(M = C to Pb) and M(CH3)3(M = Si to Pb) from proton and carbon-13 chemical shift measurements on 4-substituted styrenes

Proton and carbon-13 chemical shifts for 4-substituted styrenes are used to estimate σI and σR° for M(CH3)3 and CH2M(CH3)3 derivatives (M = Si, Ge, Sn, or Pb). The derived constants are consistent with previous conclusions concerning electronic effects of these groups and indicate that σ–π hyperconjugation is an important mechanism for their transmission.

Application of Rietveld analysis to crystal structures of titanyl phthalocyanine

This paper discusses the use of Rietveld analysis to solve crystal structures of titanyl phthalocyanines. Rietveld analysis is based on whole pattern fitting in which every point in the x-ray powder diffraction pattern is used as a measure of one or more Bragg diffraction peaks. Thus the refinement of relatively complicated crystal structures from x-ray patterns containing a relatively small number of resolved diffraction peaks is enabled. Various profile-fitting functions used in Rietveld analysis were parameterized and tested against known crystal structures of type I and type II titanyl phthalocyanine. It was found that a split Pearson VII function was found to best correct for preferred orientation effects observed in the x-ray patterns. The final goodness-of-fit parameters were R(Bragg) equals 0.17 and 0.13 for type I and type II structures, respectively. A computer program was used to generate several unit cells for type IV titanyl phthalocyanine. These unit cells were tested by stereochemical packing analysis to first determine which unit cells allowed for good intermolecular packing arrangements. Energy minimized models were then used as phasing models for Rietveld refinement. A triclinic structure with space group P-1 having an x-ray goodness-of-fit parameter R(Bragg) equals 0.24 was proposed as the most probable crystal structure for type IV titanyl phthalocyanine. The unit cell parameters are a equals 1.083 nm, b equals 1.312 nm, c equals 0.996 nm, alpha equals 72.28 degrees, beta equals 77.25 degrees and gamma equals 104.48 degrees. There are two molecules in the unit cell related by a center of inversion.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.