, where µ0 is vacuum permeability, h is the reduced Planck constant, γH and γN are the gyromagnetic constants of 1 H and 15 N, respectively, and the average cubic length is calculated as ⟨r NH 3 ⟩ = 0.101 nm. Spectral density, J(ω), is the Fourier transform of the second-order rotational correlation function of N-H bond vector, g(t). Assuming multiexponential decay for g(t) with N number of timescales τi with the weights of αi, the spectral density can be written as