Raman spectra, ab initio molecular orbital calculations, vibrational analysis, and thermodynamic functions for NH3:AlX3 (X=F, Cl, Br)a)

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Abstract
Raman spectra of gaseous NH3:AlCl3 and NH3:AlBr3 were recorded at 400 °C. The observed Raman frequencies in combination with reported infrared frequencies for NH3:AlCl3 and ab initio molecular orbital calculations on NH3:AlCl3 and NH3:AlF3 were used to derive force constants for all three NH3:AlX3 complexes (X=F, Cl, Br) based on an adjusted valence force field (AVFF) concept. The resulting force constant calculations produced complete sets of A1 and E mode frequencies for each complex. Statistical mechanical analyses were then performed using the A1 and E mode frequencies together with estimated values for the torsional mode of each complex and published enthalpy data. From these analyses, the relative thermodynamic stability of each complex was determined. At 700 and 1000 K, NH3:AlCl3 was found to be more stable than NH3:AlBr3. It was further predicted that the reaction of gaseous NH3 with solid AlF3 to form NH3:AlF3 is not favored in this temperature range, which provides an explanation for the lack of success in prior efforts to produce NH3:AlF3.