Biomedical subjects

RK Harris

Publications and source records attributed to RK Harris.

3 recordsLinked to original sources

PubMed1999

Fluorine-19 solid-state NMR magic-angle-turning experiments using multiple-pulse homonuclear decoupling

For compounds giving "crowded" 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as 13C and 15N and under 1H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as 1H, 19F, and 31P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F1 and F2 dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence. Copyright 1999 Academic Press.

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PubMed1999

Analysis of cross-polarization dynamics between two abundant nuclei, (19)F and (1)H, based on spin thermodynamics theory

The phenomenological theory of spin thermodynamics based on the spin temperature hypothesis was employed to describe the cross-polarization (CP) dynamics between two abundant nuclei, (19)F and (1)H, when the number of fluorine atoms is not substantially less than the number of hydrogens. The influence of T(1rho)'s of both nuclei and the relative magnitude (heat capacity) of the two spin baths must be incorporated explicitly into the analysis in order to derive values for the parameters involved in the CP dynamics. Numerical calculations were performed to clarify the difference in the evolution of magnetization in variable contact time CP experiments between the (1)H --> (13)C and (1)H --> (19)F cases. A new type of CP-drain experiment was developed for observing the residual (1)H magnetization after (1)H --> (19)F CP. (19)F direct polarization magic-angle spinning (MAS), (1)H --> (19)F CP, and (1)H --> (19)F CP-drain MAS NMR spectra have been measured for a fluorinated polyimide, 6FDA/ODA. The CP dynamics between (1)H and (19)F for the polyimide were analyzed on the basis of the spin thermodynamics theory. The constant for polarization transfer (T(HF)) was determined by the analysis using the effective CP parameters, which were directly obtained from the CP and CP-drain experiments, together with independently measured values of T(H)(1rho) and T(F)(1rho). Copyright 1999 Academic Press.

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PubMed1997

Studies of an Isolated 15 N-15 N Spin Pair. Off-Angle Fast-Sample-Spinning NMR and Self-Consistent-Field Calculations for Diazo Systems

An off-magic-angle spinning study of the nonassociated molecular solid, doubly 15 N-labeled 5-methyl-2-diazobenzenesulphonic acid hydrochloride (I ) is reported. The validity of the off-magic-angle spinning approach under fast-spinning conditions is verified by average Hamiltonian theory. Ab initio SCF calculations were performed on the simpler molecule, C6 H5 N2 + , to provide the shielding parameters, the dipolar coupling between the two nitrogen nuclei, and the electric field gradient existing at both the alpha-nitrogen and beta-nitrogen sites. The calculated values are in good agreement with the shielding and effective dipolar coupling data elucidated in the present investigation, and with a previous study of the two singly 15 N-labeled isotopomers in which information concerning the electric field gradient at the alpha and beta sites was deduced.

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