William B. Moniz

Zero quantum NMR in the rotating frame: J cross polarization in AXN systems

An analysis of liquid state cross polarization in AXN spin systems is presented. A two‐level geometrical formalism derived from symmetry considerations provides a closed form description of the quantum dynamics for arbitrary N. A parallel discussion using a classical vector model explains the significance of the quantum mechanical solutions and illustrates the nature of the spin–spin correlations accompanying magnetization transfer. General expressions for the J cross polarization A signal with and without X decoupling are given. A discussion of RJCP and PCJCP pulse sequences introduced previously shows how manipulation of Hartmann–Hahn mismatch can advantageously modify cross‐polarization dynamics. The quantum formalism demonstrates that efficient polarization transfer is equivalent to population inversion of an ensemble of fictitious spins subjected to different rf field intensities, and that pulse sequences for cross polarization can be constructed by analogy with conventional spin inversion techniques.

Electron Spin Resonance Study of the Photolysis of Polystyrene and Poly(α‐Methylstyrene)

The photolysis of polystyrene and poly (α‐methylstyrene) was studied by ESR techniques using highly concentrated glassy solutions of the polymers near liquid‐nitrogen temperature. Every attempt was made to insure detection of the earliest free‐radical species. The use of poly (1‐vinylnaphthalene) and polystyrene‐2,3,4,5,6‐d5 verified that the primary radical species is located on the polymer chain and not the ring. The ESR spectra obtained for both polystyrene and poly (α‐methylstyrene) are consistent with the structure HH–C–Ċ–C–HφH. Interpretation of the observed spectra is made on the basis of the stereochemistry of the polymer chain.

NMR Subspectral Analysis Applied to Polyfluorobenzenes C6HnF6−n. III. Pentafluorobenzene

The nuclear magnetic resonance spectra (1H and 19F) of pentafluorobenzene at ν0 = 6.14, 94.1, and 100.0 MHz have been analyzed. Subspectral analysis, perturbation theory, and direct analysis were applied to the low‐frequency spectra in order to obtain all the magnitudes and relative signs of the parameters. It was demonstrated that although at high frequencies a loss of information due to weak coupling should in principle arise, it is still possible to obtain the same results by exploitation of high‐precision measurements through computer iteration and by double irradiation (tickling).

Application of Low‐Field NMR to the Unambiguous Assignment of Spin‐Coupling Parameters. III. Other Pentafluorobenzenes

The 6.14‐MHz fluorine NMR spectra were employed in removing ambiguities in the signs and assignments of coupling constants in C6F5–Cl, –Br, –CN, –COOH, –COCl, –OH, –NH2, and –SO2Cl. Except for C6H5OH, selection of the correct parameter set entailed only a visual comparison of the low‐field spectrum and computed spectra based on permutations of the nonassignable parameters from high‐field studies. The correct parameter sets were subsequently computer refined to obtain best fits to the 6.14‐MHz spectra. However, in the case of C6F5OH, the solvent dependence of the spectrum necessitated computer iterative analysis of the 6.14‐MHz data to arrive at the correct parameter set. For C6F5COCl, high‐precision 94.1‐MHz spectra were of use in eliminating all but one uncertainty in the assignments. The Abraham–MacDonald–Pepper linear relationship involving the ortho–ortho substituent contribution and the Hammett σp constant was modified and extended to yield for the compounds C6F5–X the relationship J26(Hz) = − 9.5σp(...