Thermochemistry and crystal lattice energetics of phenyl acridine-9-carboxylates and 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates

The melting enthalpies and melting points of phenyl acridine-9-carboxylate, its eleven alkyl-substituted derivatives in the phenyl fragment and eight 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates derived from them, were determined by DSC. The volatilisation enthalpies and temperatures of twelve phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium and trifluoromethanesulphonate ions, obtained by quantum chemistry methods, and the corresponding enthalpies of sublimation or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well to the experimentally obtained enthalpies of sublimation. Analysis of crystal lattice enthalpy contributions indicates that the crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whereas the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.