Sublimation enthalpies of some methyl derivatives of uracil from vapor pressure measurements

The standard sublimation enthalpies for uracil, 1-methyluracil, 3-methylura cil, and 1,3-dimethyluracil, Delta(sub)H degrees(298 K) = (128 +/- 2), (124 +/- 5), (121 +/- 4), and (118 +/- 4) kJ mol(-1), respectively, were determ ined from their vapor pressures measured by the torsion method. The results of vapor pressure measurements were fit to the following linear equations: uracil, log(p/kPa) = (12.29 +/- 0.15) - (6634 +/- 100) K/T (from 384 K to 494 K); 1-methyluracil, log(p/kPa) = (13.75 +/- 0.15) - (6357 +/- 150) K/T (from 343 K to 428 K); 3-methyluracil, log(p/kPa) = (13.59 +/- 0.10) - (621 0 +/- 100) K/T (from 344 K to 419 K); 1,3-dimethyluracil; log(p/kPa) = (15. 10 +/- 0.10) - (6049 +/- 100) K/T (from 311 K to 364 K). The different subl imation behavior of some compounds can be due to the presence of hydrogen b onds in their solid state.