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

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 C_α carbon nucleus could be separated not only from the chemical shift interaction, but also from the ¹³C_α-¹⁴N dipolar field. The angular variation of the ¹³C_α-¹H dipolar splitting was measured when the static magnetic field was rotated about three orthogonal axes (a, b, and c axes). The ¹³C_α- ¹H dipolar coupling tensor was determined and the C _α-H bond length was evaluated to be 1.073 Å. In the 2D exchange NMR experiment for C_α 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 (τ_m=1.0 s) was used, is brought about by two consecutive single quantum processes, and (2) the main spin-lattice relaxation process of the NH₃⁺ nitrogen nucleus is the fluctuation of ¹⁴N-¹H dipolar interaction rather than the fluctuation of ¹⁴N quadrupole interaction.