FIG. 6.

Simulated evolution of ¹³C, ¹⁵N singlet in acetonitrile in solution. Starting with 100% singlet, the free species is evolved at 0.05 µT. The black curve is the evolution of a 2-spin system ignoring the methyl protons; the heteronuclear singlet is protected since |¹J_NC| is much greater than the resonance frequency difference between ¹³C and ¹⁵N. The green curve is the evolution of the 5-spin system, including the methyl protons. The singlet state remains mostly protected, but slow population flow into (shorter lived) hydrogen magnetization will reduce the maximum buildup of singlet molecules in solution.