Standard molar enthalpies of formation and lattice energies of 1-methylethyl, n-butyl and 2-methylpropyl ammonium halides

Standard molar enthalpies of formation of 1-methylethyl, (CH3)(2)CHNH3X, n-butyl, CH3(CH2)(3)NH3X and 2-methylpropyl, (CH3)(2)CHCH2NH3X, ammonium halides (X=Cl, Br, I) have been determined by solution-reaction calorimetry at 298.15+/-0.02 K. Heat capacities for solid and liquid phases were determined for the means of differential scanning calorimetry and estimated by gaseous phase through the generalized vibrational assignment method. Starting from the equivalent methyl cation derivative, the molar standard enthalpies of both cations were estimated by a group contribution method. From these values, the lattice energies of the salts have been calculated via a thermochemical cycle. The lattice energies decrease from chloride to iodide for all three series of compounds. Semi-empirical methods were used to determine the thermochemical cation radii. These values depend on the number of carbon atoms in the organic chain and the position of the attached methyl group on the branched carbon chain. (C) 1998 Elsevier Science B.V.313217518