P. T. Inglefield

The application of a simultaneous model for multisite exchange to solid-state NMR lineshapes

Abstract A multisite exchange treatment is developed to simulate NMR powder lineshapes in cases where motional processes of more than one type occur simultaneously. An example is presented where one type involves rotational diffusion, libration, or oscillation over a limited range whereas the other includes jumps of larger amplitudes between potential minima, the axis of motion being the same for both types. Restricted rotation of lower angular amplitude is conceived in terms of jumps between sites within one well with the amplitude of this restricted rotation being the key parameter which can be the same or different for each potential minimum. The second motional process of larger amplitude involves a series of jumps between sites in one well to sites in the other well. In the cases treated, the first process is always considered too fast to exhibit rate-dependent effects. One rate parameter is used for all jumps. This rate is always kept sufficiently large to keep the first process in the rapid exchange limit. At very large rates, both motions exhibit rate-independent effects. At slower rates, the rate dependence of the second process leads to conspicuous changes in the lineshape. The model sets an equal probability for all jumps and represents an alternative to the conventional treatment where one deals with the processes sequentially. The method is illustrated for the case of bisphenol-A polycarbonate (BPAPC), where results show an improvement over the previously published treatment based on a sequential approach.

Multifield, temperature and concentration dependence of spin relaxation and local motion in 2,2-propane diyl-bis(4-hydroxyphenyl) polyformal

Abstract 13 C and proton spin-lattice relaxation data were determined as a function of field, temperature and concentration for 2,2-propane diyl-bis(4-hydroxyphenyl) polyformal in C 2 D 2 Cl 4 . The concentration of polymer was varied from a few per cent by weight up to the bulk rubber and temperature was varied from −20°C to +120°C. The relaxation data is interpreted in terms of several local motions including segmental motion, phenylene group rotation, methyl group rotation and formal group rotation. Segmental motion is described by a correlation function developed by Hall and Helfand which allows for cooperative and single bond transitions. The phenylene group rotation is described by the Woessner anisotropic stochastic diffusion model except at high concentration (⩾50 wt%) where the phenylene data is simulated by restricted rotation. Methyl group rotation is described by the Woessner three fold jump model, and the formal group rotation is described by double trans-gauche rotations about the C-O axes. At a concentration of 5 wt%, the time scale of segmental motion is slightly slower than in bisphenol-A-polycarbonate. However, the time scales of phenylene group and methyl group rotations are nearly identical for both polymers. A comparison is also made with chloral polyformal, another similar polyformal. The effect of concentration on local motion was monitored by 13 C spin-lattice relaxation measurements at three Larmor frequencies over the range of 5 to 100 wt% polymer at a temperature 40°C above the glass transition of this polyformal. The rate of local chain motion decreases as the concentration of polymer increases with the exception of methyl group rotation which remains constant. The correlation times for several local backbone motions are fit to free volume theory which yields a fractional free volume of 0.38 for the solvent (C 2 D 2 Cl 4 ) and 0.28 for the polymer.

Further studies of multiple nuclear spin relaxation and local motions is dissolved 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene polyformal

Abstract Deuterium, carbon-13 and proton spin-lattice relaxation times at two fields are reported for dilute solutions of 1,1-dichloro-2,2,-bis(4-hydroxyphenyl)ethylene polyformal. The carbon-13 and proton relaxation measurements were made at a concentration of 10% (w/w) in deuterated s-tetrachloroethane and as a function of temperature. A partially deuterated analog with deuterated methylene groups was used in order to remove cross-relaxation effects from the phenylene proton relaxation. In addition, deuterium relaxation measurements were made on this sample at a concentration of 10% (w/w) in tetrachloroethane as a function of temperature. The data are interpreted in terms of segmental motion arising within the bisphenol units and anisotropic internal rotations of the other structural components. Motions of the phenylene groups in the backbone are described by the Hall-Helfand segmental correlation function plus the Woessner anisotropic internal-rotation correlation function. Motions of the formal linkage are described by the same segmental correlation function plus an internal correlation function based on restricted double rotation about the two carbon/oxygen bonds. The local motion of the formal group is discussed in terms of confomational transitions that are likely in a polyformal in view of the conformational energy surface. A Helfand Type II motion of the formal group corresponding to a transition from gg′ to tg is identified as the most plausible rearrangement of this unit.

Chemical Shift Reference for MASSCP-13C NMR Studies of Reagents Immobilized on Silica Gel

Abstract This report demonstrates that immobilized trimethylsilane is an appropriate reference standard for MASSCP-13C NMR studies of reagents immobilized on silica gel. An example is presented which shows that the immobilized trimethylsilane exhibits many of the advantages as a reference for these solid samples as does tetramethylsilane for liquid samples. It is further shown that pure solids may interact with an immobilized phase and produce significant spectral changes and thus are not appropriate as reference standards.