Insertion of the Al atom into the NH₃ molecule: Semiempirical SCF MO and matrix isolation ESR study

Abstract

The insertion process of the Al atom into the ammonia molecule (NH3) was examined by a semiempirical SCF molecular orbital method (AM1) and by matrix isolation ESR spectroscopy. The MO study revealed the three-step process: (1) the spontaneous formation of the dative complex Al:NH3 driven by the three-electron bonding scheme between the unpaired electron of Al and the lone pair electrons of NH3, (2) cleavage of a N-H bond upon vibrational excitation of the Al:NH3 complex, and (3) completion of the insertion process by the three-electron bonding scheme between the unpaired electron on the cleaved hydrogen atom and the lone pair electrons on Al. The ESR study revealed the spontaneous formation of the dative complex Al:NH3 upon co-condensation of Al atoms and NH3 in argon matrixes, and its conversion to the insertion radical H-Al-NH2 when the matrix was irradiated with near-IR light (λ = 700 ± 50 nm). In matrixes with higher NH3 concentration (≥2%), the insertion radical complexed with the second NH3 molecule, (H-Al-NH2): NH3, was also formed. Irradiation of matrixes containing these insertion radicals with blue light (λ = 450 ± 50 nm) effected the dissociation processes H-Al-NH2 → H· + Al-NH2 and (H-Al-NH2):NH3 → 2H· + Al(NH2)2. Well-resolved ESR spectra assigned to the dative complex Al:NH3, the insertion radical H-Al-NH2, its complex (H-Al-NH2):NH3, and the diligand species Al(NH2)2 were observed, analyzed, and discussed.