Fig. 1. Schematic diagram of energy level and transition probabilities for electron–nuclear (13C) spin systems for benzene/TEMPO (above) and phenylacetylene/TEMPO (below). αS and βS are the spin states of the unpaired nitroxide electron spin, and αI and βI are for nuclear spins. WS represents the electron relaxation; W0, WD1 and WD2 are zero, single and double quantum transitions, respectively. W0 is equal to the sum of WSc0 and WD0 in eqn (3) and (4), which are transition rates of scalar and dipolar relaxation, respectively. 13C DNP enhancement of benzene is mainly due to dipolar relaxation, while scalar relaxation is the dominant mechanism for the β carbon of phenylacetylene.
