Calculations of lattice sums and heats of sublimation of long-chain even n-alkanes

Crystal energies at 0 K and 25°C of three polymorphic forms of the normal alkanes C₂₈H₅₈ and C₃₆H₇₄ have been calculated from crystal structure data with a Lennard-Jones 6-12 potential. Taking into account the side packing and end packing energies, a general expression for the heat of sublimation at 25°C has been derived, viz ΔH⁰_sub = 8.24n_C + 4.10 kJ mo^-1, where n_C is the number of C atoms in the molecule. The agreement with the two known experimental values (for C₁₈H₃₈ and C₃₂H₆₆) is within 1%.