Studies of the internal rotation about the C—N bond of the dimethylaminomethylene (DMAM) group of DMAM-adenosine,1, DMAM-guanosine,2, and DMAM-cytidine,3, was determined by high-resolution1H NMR spectroscopy. Arrhenius plots (lnk = lnA − Ea/RT) were used to determine the barriers to internal rotation,Ea, of the compounds. They were found to be 58.0 ± 2.4 kJ mol−1, 71.2 ± 1.4 kJ mol−1, and 62.8 ± 2.4 kJ mol−1for1,2, and3, respectively. At the same time the coalescence temperatures,tc, for the compounds were determined and found to be 93.5 °C for1, 124.7 °C for2, and 119.6 °C for3in DMSO. In D2O, compound1was found to have atcof 104.5 °C. The compound decomposed so rapidly at this temperature that the barrier to internal rotation could not be determined. The results indicate that the oxo function in the nucleobase has a considerable effect on the barrier to internal rotation and coalescence temperature.