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

Abstract

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.

Hyperfine coupling constants, electron-spin g-factors and vertical spectra of the X2Σ+radicals BeH, MgH, CaH and BZ+, AlZ+, GaZ+(Z = H, Li, Na, K). A theoretical study

“…That is, for XH + at equilibrium, some s(X + ) density is expected to be partially transferred to H. However, because of the overall positive charge, the structure X 2+ (s)H À (s 2 ) is most probably less dominant in XH + than Y + (s)H À (s 2 ) in YH. These arguments are supported by an experimental A iso (H)[in MHz] of 442 in AlH + , 32 versus 296 in isovalent MgH, 6,33 i.e., the s(H) density in AlH + is about 1.5 times larger than in MgH. Conversely, the weight of the Al 2+ H À structure in AlH + is about 1.5 smaller than that of Mg + H À in MgH.…”

“…For the XH + ions, we are aware of only one experimental ESR study, namely that dealing with AlH + . 32 (ESR data originally assigned 47 to AlH + were later reassigned to AlHOH. 32 ) Theoretical hfcc's are also available for BH + , 41,45 and AlH + .…”

Cationic aluminum hydride complexes: reactions of carbene–alane adducts with trityl-borate

“…A matrix isolation study at 4 K described a [AlH] + radical cation which was studied by EPR spectroscopy. 12,13 The first fully characterized Al )2 )][AlH 4 ] (Scheme 1) were prepared and reported by Atwood and coworkers in 1991. 14,15 These species exhibited six-and five-coordinate aluminum centers, respectively.…”

Theoretical and electron spin resonance studies of the H⋯H, H⋯D, and D⋯D spin-pair radicals in rare gas matrices: A case of extreme singlet–triplet mixing

“…For example, the pulsed laser vaporization of metals in the presence of hydrogen gas ͑experiments designed to produce MH, MH 2 , and MH ϩ radicals͒ has produced intense ESR signals of the H¯H and D¯D spin-pairs. 25,33,34 The essential requirement to observe these hydrogen spin-pairs by ESR is the creation of rigid, well formed rare gas matrices that contain extremely intense and relatively narrow absorptions ͑ϳ2 G peak-to-peak linewidths͒ of well isolated hydrogen atoms. From a practical perspective, the H atom ESR hyperfine doublet absorption must be sufficiently intense as to be readily detectable at an amplifier gain setting of ''one,'' with the other ESR spectrometer conditions as described in the Experiment.…”

“…[22][23][24][25][26] A basic diagram of the specific arrangement utilized in these hydrogen spin-pair trapping experiments is shown in Fig. 1.…”