Figure 3.

Simplified energy level diagrams for intramethyl ¹H−¹³C and ¹H transitions within a side-chain methyl group. In each, α and β represent the low- and high-energy states being manipulated in NMR experiments. (A) Four energy levels are shown for the simplified ¹H–¹³C spin system. The single-quantum (SQ), double-quantum (DQ), and zero-quantum (ZQ) transitions are indicated. The ZQ/DQ relaxation experiments shown in Figure 2D monitor the relaxation of the ZQ and DQ coherences depicted here. (B) Four energetic states are shown for intramethyl proton transitions,⁶⁵ where the SQ, DQ, and triple-quantum (TQ) transitions are indicated. SQ transitions between adjacent energy levels (gray arrows) are allowed. The ¹H DQ (solid lines) and TQ (dashed line) coherences are traditionally “forbidden” but can be accessed in side-chain methyl groups in macromolecular systems since their relaxation is nonzero.