STABILITY OF GASEOUS AMMONIUM CHLORIDE MOLECULE

The equilibrium vapor effusing from a Knudsen cell, in which solid ND 4Cl was evaporated, was analyzed by a quadrupole mass spectrometer. In the temperature range considered (335°-485°K) the main vaporization process corresponds to the decomposition of the solid towards gaseous ND 3 and DCl; the measured enthalpy change ΔH°407 dec[ND4Cl(s)]=42.6±2.0 kcal mole-1 is very close to that measured for NE4Cl: ΔH°407 dec[NH4Cl(s)]=42.5 kcal mole-1. Apart from the main gaseous species, small proportions (5×10-5-2×10 -3) of ND4Cl molecules were also observed. The ND 4Cl molecule fragments (>99%) under electron impact and yields the ND4+(m/e=22) ion. From the observed ion intensity ratios, as well as their dependence on temperature, D0°(NH 4Cl⇌NH3+HCl)= 10.0±3.0 kcal mole -1 was obtained confirming Clementi's computed prediction <14.0 kcal mole-1.