Study of the self‐association of methylated pyrimidines in aqueous solutions by 1 H NMR chemical shifts

The self‐association of pyrimidine and its methylated derivatives was studied by measuring the concentration‐dependent changes in proton chemical shifts in deuterium oxide at 35°C. The data were analysed by a dimer model with two parameters. The equilibrium constants give an increasing tendency for self‐association within the series pyrimidine (0.091 ° 0.006 l mol −1 ) < 5‐methylpyrimidine (0.165 ° 0.006 l mol −1 ) ≈ 4‐methylpyrimidine (0.183 ± 0.005 l mol −1 ) < 4,6‐dimethylpyrimidine (0.410 ± 0.011 l mol −1 ), and the values of the dimer shifts suggest a stacking‐like arrangement for the associated molecules.