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

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.