Polymorphic transitions in cyanide crystals and the calculation of the lattice energies of cyanide salts. Thermodynamic parameters for the gaseous cyanide ion

Abstract

The lattice energies of the cubic and orthorhombic forms of NaCN and KCN are calculated. Using experimental data on the enthalpy of transition between the polymorphic forms we estimate: U pot(NaCN) ortho = 732 kJ mol −1, U pot(NaCN) cubic = 729 kJ mol −1, U pot(KCN) ortho = 665 kJ mol −1 and U pot(KCN) cubic = 664 kJ mol −1. Using these results we estimate Δ H° ƒ( CN −)(g) = 33 kJ mol −1 , ΔH° solv(CH −)( g) = −326 kJ mol −1, the proton affinity of the gaseous CN − ion to be −1434 kJ mol −1 and the electron affinity of the cyanide radical to be 402 kJ mol −1. The charge distribution we assign compares extremely well with quantum mechanical calculations.