Lattice Energies and Hydration Thermodynamics of Tetra-alkylammonium Halides

The lattice energies of tetramethylammonium (TMA⁺) chloride, bromide, and iodide, tetraethylammonium (TEA⁺) iodide, and tetra‐n‐propylammonium (TPA⁺) bromide are calculated from their crystal structures and an adopted potential model for the nonionic interactions between H, C, N and halogen atoms. When combined with heats of solution in water, hydration enthalpies are arrived at and a division among cation and anion is made. The lattice energies are (in kilocalories per mole): TMA⁺Cl⁻, − 132.1; TMA⁺Br⁻, − 130.1; TMA⁺I⁻, − 123.8; TEA⁺I⁻, − 109.6; TPA⁺Br⁻, − 117.9. The hydration enthalpies are: TMA⁺, − 53; TEA⁺, − 42; TPA⁺, − 48 (based on values of − 80.3, − 72.2, and − 61.8 for Cl⁻, Br⁻ and I⁻, respectively). Entropies of hydration of the TMA⁺ halides are determined from a calculation of gas‐phase entropies and experimental entropies of the aqueous solutions, leading to values of ${\overline{s}}_h°$ for TMA⁺Cl⁻, TMA⁺Br⁻, TMA⁺I⁻ of − 47.5, − 42.5, and − 38.1 cal °K⁻¹·mole⁻¹, respectively. The hydration heats of the tetra‐alkylammonium cations show a reversal in the usual trend of decreasing hydration heat with increasing size. This is due to the increasing importance of nonelectrolyte interactions relative to the charge effect as the size increases. The entropy loss on hydration of TMA⁺ is very high compared to that expected from extrapolation from smaller ions and is also apparently due to the approach to non‐electrolyte character of large ions.