Two-dimensional nuclear magnetic resonance studies on a single crystal of l-alanine. Separation of the local dipolar fields; and 2D exchange spectroscopy of the 14N relaxation processes

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Abstract
Two types of 2D NMR techniques, namely, separated local field 2D NMR (SLF 2D NMR), and 2D exchange NMR spectroscopy were applied to a single crystal of l‐alanine at room temperature. In the SLF 2D NMR experiments, we found that the 13C–1H dipolar field at the Cα carbon nucleus could be separated not only from the chemical shift interaction, but also from the 13Cα–14N dipolar field. The angular variation of the 13Cα–1H dipolar splitting was measured when the static magnetic field was rotated about three orthogonal axes (a, b, and c axes). The 13Cα‐1H dipolar coupling tensor was determined and the Cα–H bond length was evaluated to be 1.073 Å. In the 2D exchange NMR experiment for Cα carbon nucleus, the off‐diagonal cross peaks due to the single quantum and the double quantum transitions for the spin‐lattice relaxation processes of the adjacent 14N nucleus were observed. The single quantum transition rate constant was evaluated to be 0.8 s−1, and the double quantum transition rate constant was estimated to be much smaller. Inspection of the experimental results of the 2D exchange NMR, together with the theory, indicates that, (1) the double quantum cross peaks which appeared when a long mixing time (τm=1.0 s) was used, is brought about by two consecutive single quantum processes, and (2) the main spin‐lattice relaxation process of the NH+3 nitrogen nucleus is the fluctuation of 14N–1H dipolar interaction rather than the fluctuation of 14N quadrupole interaction.