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

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.