The enthalpy of formation of gaseous tetra-n-propylammonium cations Dedicated to the memory of the late Professor Manuel Ribeiro da Silva

The standard molar enthalpies of formation of crystalline tetraalkylammonium halides, Δ_fH (R₄NX, c), R = Me, Et, Pr, Bu and X = Cl, Br, I, and the corresponding lattice energies, U _L(R₄NX) were obtained from the literature as far as available for T = 298.15 K. From consistent values of these two quantities and values for Δ_fH (X^-, g) the standard molar enthalpies of formation of the gaseous cations were obtained from Δ_fH (R₄N⁺, g) = Δ_fH (R₄NX, c) + U_L(R₄NX) + 2RT - Δ_fH (X^-, g). The value for the hitherto unreported Δ_fH (Pr₄N ⁺, g) = (307 ± 7) kJ · mol is here derived. The value for the absolute standard molar enthalpy of hydration Δ_hH (R₄N⁺, aq) is -210 kJ · mol^-1, but may have an appreciable uncertainty, reflecting that of Δ_fH (R ₄N⁺, aq), obtained from interpolation of values for the Me, Et, and Bu analogues.