Asymmetric (13)C-(13)C polarization transfer under dipolar-assisted rotational resonance in magic-angle spinning NMR

A two-dimensional (2D) homonuclear exchange NMR spectrum in solids often shows an asymmetric cross-peak pattern, which disturbs a quantitative analysis of peak intensities. When magnetization is prepared using cross polarization (CP), the asymmetry can naively be ascribed to nonequilibrium initial magnetization. We show, however, that the CP effect cannot fully explain the observed mixing-time dependence of the peak intensities in 2D (13)C-(13)C exchange spectra of [2,3-(13)C] l-alanine (2,3-Ala) under (13)C-(1)H dipolar-assisted rotational resonance (DARR) recoupling, which has recently been proposed for a broadband recoupling method under magic-angle spinning. We develop a theory to describe polarization transfer in a two-spin system under DARR recoupling. By taking into account the effects of the partial spectral overlap among (13)C signals, which is a unique feature of DARR recoupling, and (1)H-(1)H flip-flop exchange, we can successfully explain the observed mixing-time dependence of the peak intensities of 2D (13)C-(13)C DARR exchange spectra of 2,3-Ala. A simple initial-rate analysis is also examined.