Fig. 4.

(A) Dynamic processes of the 1,4-BCO rotators that may contribute to the spin-lattice relaxation of the ¹H nuclei. Conformational twisting has the characteristics of a normal mode with frequencies that are too high (∼10¹² s⁻¹) to contribute to ¹H spin-lattice relaxation in the megahertz (10⁶–10⁸ s⁻¹) regime. By contrast, inertial rotation and rotation by a site-exchange mechanism has a small barrier and may be slowed down to megahertz regime and constitute an efficient spin-lattice relaxation mechanism. (B) ¹H T₁ relaxation data of the natural abundance BODCA-MOF from T = 2.3–80 K, plotted with a semilog scale for T₁⁻¹ vs. 1/T. The red circles correspond to data collected at a Larmor frequency ν_L= 29.49 MHz and the green triangles correspond to ν_L = 13.87 MHz. The high-temperature data from both high- and low-field experiments can be fit well to the Kubo–Tomita fit (dotted lines), both revealing an activation energy, E_a = 0.185 kcal mol⁻¹, and a preexponential factor, τ₀⁻¹ = 4.7 × 10¹⁰ s⁻¹.