THE EVAPORATION THERMODYNAMICS OF LITHIUM IODIDE - MASS-SPECTROMETRICAND AB-INITIO STUDIES

The vaporization of UI was investigated in the range between 583 and 7 26 K by Knudsen effusion mass spectrometry (KEMS), The ionization or t he appearance energies (IE or AE) of all kinds of ions formed from the equilibrium vapour over solid lithium iodide were determined using Vo gt and Pascual's deconvolution method. For the determination of vapour pressure two methods, viz, mass-loss Knudsen effusion and Knudsen eff usion mass spectrometry were applied. The mean natural logarithms of t he equilibrium vapour pressures (in Pa) of monomer and all kinds of ol igomers, that can be detected using KEMS, as a function of temperature , can be expressed as follows: ln (p(LiI)) = -(22240 +/- 450)/T + (28. 03 +/- 0.66) between 583 and 723 K ln (p(Li2I2(g))) = -(21940 +/- 390) /T + (28.46 +/- 0.60) between 583 and 726 Kln (p(Li3I3(g))) = -(27300 +/- 660)/T + (31.5 +/- 1.1) between 643 and 726 K ln (p(Li4I4(g))) = - (24820 +/- 540)/T + (25.65 +/- 0.66) between 668 and 726 K. The molecu lar structure and the harmonic vibrational frequencies of (LiT)(n) spe cies (n = 1,2,3,4) were predicted using ab initio molecular orbital me thods, Both the bond dissociation energies and the enthalpy changes of dissociation of (LiI)(n) oligomers were evaluated and compared with t he measured enthalpy change data. Using the calculated geometry and th e vibration frequencies, the thermodynamic functions of (LiI)(n) could be calculated, making it possible to compare the second and third law thermodynamic data. (C) 1998 John Wiley & Sons, Ltd.