Formation of AlO, AlOH, and Al(OH)3 in the Interstellar Medium and Circumstellar Envelopes of AGB Stars

Abstract

AlO, AlOH, and Al(OH)₃ can be formed in the gas-phase starting from nothing more than simple aluminum hydrides (AlH and AlH₂) and water molecules. All three products are probable precursors to aluminum oxide clusters that may initiate the nucleation of dust grains in the interstellar medium. Chemically accurate CCSD(T)-F12b/cc-pVTZ-F12 computations provide exothermic energetic values for these pathways. For example, the fully submerged formation of AlO is exothermic by 51.3 kcal mol^–1, and this should also lead to favorable kinetics. To aid in the detection of the recurring intermediate cis/trans-HAlOH with instrumentation located on the James Webb Space Telescope, among other observatories, rotational and vibrational spectroscopic data are reported by utilizing a highly accurate quartic force field methodology. The ν₂ stretching frequency at 1807.9 cm^–1 exhibits an anharmonic intensity of 185 km mol^–1 and the antisymmetric bend (ν₄) at 534.2 cm^–1 exhibits an anharmonic intensity of 213 km mol^–1 for the cis and trans isomers, respectively. These are roughly three times the antisymmetric stretch intensity of water. The cis isomer has a smaller dipole moment of 0.83 D, while the trans isomer contains a moderate dipole moment of 1.49 D. These properties indicate that both of these isomers can be observed through vibrational and rotational spectroscopic techniques.