Reinvestigation of the aluminum hydride (AlH+ and AlD+) cation radicals by ESR in argon matrices at 4 K: Generation by reactive laser sputtering

The ESR spectra previously assigned to the AlH+ radical ion (X 2Σ) [J. Chem. Phys. 71, 3991 (1979)] actually belong to the divalent neutral aluminum radical AlHOH whose charge distribution and electronic structure can be described as AlH+OH− with 90% of the unpaired electron on the AlH+ part of the molecule. Reactive laser sputtering and photoionization of AlH(g) were used to generate the AlH+ and AlD+ cation radicals, whose ESR spectra have been observed for the first time. A detailed analysis of the ESR results for argon matrices at 4 K reveal unusually large Al hyperfine interaction (hfi) with Aiso and Adip values of 1586(2) and 49(1) MHz, respectively. The H hfi is essentially isotropic with Aiso=442(2) MHz. The observed nuclear hyperfine A tensors for Al and H show excellent agreement with ab initio CI theoretical calculations. The results for AlH+ are compared with the isoelectronic neutral radical MgH, and the similar AlF+ cation radical. The Al hfi is slightly larger in AlD+ relative to AlH+. This interesting isotopic effect is qualitatively explained on the basis of electronic structure dependence on small changes in the bond distance for the two isotopic radicals.