The enthalpy of formation of gaseous tetra-n-propylammonium cations

The standard molar enthalpies of formation of crystalline tetraalkylammonium halides, ΔfH°(R4NX, c), R = Me, Et, Pr, Bu and X = Cl, Br, I, and the corresponding lattice energies, UL(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 ΔfH°(X−, g) the standard molar enthalpies of formation of the gaseous cations were obtained from ΔfH°(R4N+, g) = ΔfH°(R4NX, c) + UL(R4NX) + 2RT° − ΔfH°(X−, g). The value for the hitherto unreported ΔfH°(Pr4N+, g) = (307 ± 7) kJ · mol is here derived. The value for the absolute standard molar enthalpy of hydration ΔhH°(R4N+, aq) is −210 kJ · mol−1, but may have an appreciable uncertainty, reflecting that of ΔfH°(R4N+, aq), obtained from interpolation of values for the Me, Et, and Bu analogues.