Heat capacities and molar enthalpies and entropies of fusion for anhydrous 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline

Abstract

Heat capacities ( C p,m ) of solid anhydrous 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmphen) were determined by differential scanning calorimetry using the three step method over the temperature range from 298 K to about 5 K (for phen) and 40 K (for dmphen) below their melting temperatures. No further solid–solid phase transition occurs in the temperature range examined. In addition, the average melting temperature, molar enthalpy and molar entropy of fusion for phen and dmphen were also determined to be (391.1 ± 0.1 K, 11.8 ± 0.1 kJ mol −1, and 30.0 ± 0.3 J K −1 mol −1) and (435.9 ± 0.3 K, 17.6 ± 0.3 kJ mol −1, and 40.3 ± 0.6 J K −1 mol −1), respectively. The experimental heat capacity data were obtained and the respective second-order polynomial fitting equations were derived: C p , m ( phen , cr ) / ( J K − 1 mo l − 1 ) = ( − 3905 ± 16 ) + ( 22.0 ± 1.0 ) ( T / K ) − ( 0.0289 ± 0.0015 ) ( T / K ) 2 from 298.2 to 360.2 K , R 2 = 0.9935 ; C p , m ( dmphen , cr ) / ( J K − 1 mo l − 1 ) = ( − 2808 ± 14 ) + ( 17.1 ± 1.0 ) ( T / K ) − ( 0.0229 ± 0.0014 ) ( T / K ) 2 from 298.2 to 393.2 K , R 2 = 0.9953 . In particular the C p,m values at T = 298.15 K for phen and dmphen were found to be (115 ± 18 and 253 ± 30) J K −1 mol −1, respectively, where the associated uncertainties are twice the standard deviations of the mean.