Vibrational spectra of matrix isolated Al2Cl6. Isotopic fine structure and valence force field

Abstract

High resolution i.r. and Raman spectra have been recorded for the bridged dimeric aluminium chloride molecules isolated in argon and nitrogen matrices at very low temperatures. An accurate valence force field has been calculated from the frequencies of fifteen fundamental modes and some isotopic shifts due to the ³⁵Cl, ³⁷Cl distribution. The calculated isotopic frequencies and the isotopic patterns are shown to be in agreement with the experimental results. From thermodynamic and spectroscopic considerations, the unobserved ring-puckering mode is predicted to occur at very low frequencies (ν≃ 10 cm^–1). The Al—Cl bond dissociation energies are 270 and 170 kJ mol^–1 for terminal and bridging atoms, respectively; the force constants of the same bonds are 226 and 130 N m^–1.