Sublimation enthalpies study for barbituric, tiobarbituric, and selenobarbituric acids from their vapor pressure measurements

The vapor pressures for solid barbituric acid and tiobarbituric acid were m easured by the torsion method; for selenobarbituric acid the vapor pressure s were measured in both the solid and liquid phases. The results were fit t o the following linear equations: barbituric acid (solid), log(p/kPa) = (11 .18 +/- 0.20) - (5920 +/- 200)(K/T) (from 392 to 493 K); tiobarbituric acid (solid), leg(p/kPa) = (10.78 +/- 0.20) - (5770 +/- 200)(K/T) (from 400 to 461 K); selenobarbituric acid (solid), log(p/kPa) = (12.00 +/- 0.20) - (739 3 +/- 200)(WT) (from 449 to 486 K); selenobarbituric acid (liquid), log(p/k Pa) = (8.33 +/- 0.20) - (5532 +/- 200)(K/T) (from 490 to 557 K). Knudsen me asurements of the molecular weight of the vapor show that barbituric and ti obarbituric acids vaporize predominantly in the monomeric form, while the v apor above selenobarbituric acid is prevalently a dimer. The sublimation en thalpies at the middle temperatures [Delta(sub)H degrees(442 K) = (113 +/- 4) kJ.mol(-1), Delta(sub)H degrees(430 K) = (110 +/- 4) kJ.mol(-1), and Del ta(sub)H degrees(466 K) = (141 +/- 4) kJ mol(-1)] for solid barbituric, tio barbituric and selenobarbituric acids respectively were derived. The enthal py of vaporization [Delta(vap)H degrees(523 K) = (106 +/- 4) kJ.mol(-1)] an d the enthalpy effusion [Delta(fus)H degrees(486 K) = 18 kJ mol(-1)] for se lenobarbituric acid were also derived.