For a methyl group with (ideal) tetrahedral geometry, θ ij = 109.5°and P 2 (cos θ ij ) = -1/3, leading to S 2 f = 1/9. Relaxation rates are determined by spectral density functions at specific Larmor frequencies. Cross-correlated relaxation (CCR) rates for methyl groups originate from the interference between dipolar interactions of C-H bond vectors. For example, the C CCR rate Γ of the methyl groups is given in terms of the cross-SDF 31,32,49 J CH,CH (ω) ,