We present a new NMR method to clarify the dynamics of proton tautomerism in solid 9-hydroxyphenalenone. Two C-13 resonance lines influenced by the proton tautomerism have a chemical-shift difference between them, which increases with decreasing temperature. To depict the precise potential curve of the proton tautomerism, the chemical-shift difference when the proton tautomerism is completely frozen is necessary. For solid 9-hydroxyphenalenone and its derivatives, the freezing temperatures are often under -100 degrees C. When the freezing temperatures are below the temperature range in which standard magic angle spinning NMR probes can perform a sample spinning, it is very difficult to obtain the shift difference. The NMR experiments based on this new method are performed at a temperature significantly higher than -100 degrees C at which the proton tautomerism is still active. The new method yields the C-13 spin relaxation rates, the rates for the proton tautomerism, and the populations of the two tautomers. Using the populations and the C-13 chemical-shift difference at that temperature, we determined the chemical-shift difference at the freezing temperature. We also obtained several parameters characterizing the potential profile for the proton dynamics in solid 9-hydroxyphenalenone.