13C-detected 1H dipolar magic-angle-spinning nmr experiments with isotropic mixing

Abstract

Cross polarization using the WIM-24 (windowless isotropic mixing) pulse sequence and also ¹³C-detected ¹H dipolar magic-angle-spinning nuclear magnetic resonance employing the WIM-24 pulse sequence during the mixing period on a sample of the urea tridecane inclusion compound was investigated. Numerical calculations of the theoretically expected spectra were performed, using equations derived by two methods: (l) from average Hamiltonian theory and (2) the time-evolution operator obtained by multiplying together the instantaneous time-evolution operators (the multistep method). In comparisons of the calculated ¹³C-detected ¹H dipolar MAS NMR spectra from these two methods with the experimental spectra, quantitative agreement was found only with the results from the multistep method. The effects of errors in the pulse phases and amplitude of the WIM-24 pulse sequence were investigated by computer simulations based on equations derived from the multistep method. Although the WIM-24 cross-polarization curve was changed significantly by the presence of 2° errors of the pulse phases, or 10% errors in the pulse amplitudes, the ¹³C-detected ¹H dipolar MAS NMR spectra obtained by using the WIM24 mixing were found to be rather insensitive to those pulse imperfections. Cross-polarization curves were also calculated for the WALTZ-4 pulse sequence applied synchronously to both the ¹³C and the ¹H channels during the mixing period to study rotational resonance effects. The calculations were performed for dipolar and J mixing. The results show that the WALTZ-4 sequence couples with the sample rotation more intensely than does the WIM-24 sequence.