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

The phenomenological theory of spin thermodynamics based on the spin temper ature hypothesis was employed to describe the cross-polarization (CP) dynam ics between two abundant nuclei, F-19 and H-1, when the number of fluorine atoms is not substantially less than the number of hydrogens. The influence of T-1 rho's of both nuclei and the relative magnitude (heat capacity) of the two spin baths must be incorporated explicitly into the analysis in ord er to derive values for the parameters involved in the CP dynamics. Numeric al calculations were performed to clarify the difference in the evolution o f magnetization in variable contact time CP experiments between the H-1 --> C-13 and H-1 --> F-19 cases. A new type of CP-drain experiment was develop ed for observing the residual H-1 magnetization after H-1 --> F-19 CP. F-19 direct polarization magic-angle spinning (MAS), H-1 --> F-19 CP, and H-1 - -> F-19 CP-drain MAS NMR spectra have been measured for a fluorinated polyi mide, 6FDA/ODA, The CP dynamics between H-1 and F-19 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 effec tive CP parameters, which were directly obtained from the CP and CP-drain e xperiments, together with independently measured values of T-1 rho(H) and T -1 rho(F). (C) 1999 Academic Press.