Isomerization and fragmentation reactions of gaseous phenylarsane radical cations and phenylarsanyl cations. A study by tandem mass spectrometry and theoretical calculations

Abstract

The unimolecular reactions of radical cations and cations derived from phenylarsane, C₆H₅AsH₂ (1) and dideutero phenylarsane, C₆H₅AsD₂ (1-d₂), were investigated by methods of tandem mass spectrometry and theoretical calculations. The mass spectrometric experiments reveal that the molecular ion of phenylarsane, 1˙⁺, exhibits different reactivity at low and high internal excess energy. Only at low internal energy the observed fragmentations are as expected, that is the molecular ion 1˙⁺ decomposes almost exclusively by loss of an H atom. The deuterated derivative 1-d₂ with an AsD₂ group eliminates selectively a D atom under these conditions. The resulting phenylarsenium ion [C₆H₅AsH]⁺, 2⁺, decomposes rather easily by loss of the As atom to give the benzene radical cation [C₆H₆]˙⁺ and is therefore of low abundance in the 70 eV EI mass spectrum. At high internal excess energy, the ion 1˙⁺ decomposes very differently either by elimination of an H₂ molecule, or by release of the As atom, or by loss of an AsH fragment. Final products of these reactions are either the benzoarsenium ion 4˙⁺, or the benzonium ion [C₆H₇]⁺, or the benzene radical cation, [C₆H₆]˙⁺. As key-steps, these fragmentations contain reductive eliminations from the central As atom under H–H or C–H bond formation. Labeling experiments show that H/D exchange reactions precede these fragmentations and, specifically, that complete positional exchange of the H atoms in 1˙⁺ occurs. Computations at the UMP2/6-311+G(d)//UHF/6-311+G(d) level agree best with the experimental results and suggest: (i) 1˙⁺ rearranges (activation enthalpy of 93 kJ mol⁻¹) to a distinctly more stable (ΔH_r²⁹⁸ = −64 kJ mol⁻¹) isomer 1σ˙⁺ with a structure best represented as a distonic radical cation σ complex between AsH and benzene. (ii) The six H atoms of the benzene moiety of 1σ˙⁺ become equivalent by a fast ring walk of the AsH group. (iii) A reversible isomerization 1⁺ ⇌ 1σ˙⁺ scrambles eventually all H atoms over all positions in 1˙⁺. The distonic radical cation 1σ˙⁺ is predisposed for the elimination of an As atom or an AsH fragment. The calculations are in accordance with the experimentally preferred reactions when the As atom and the AsH fragment are generated in the quartet and triplet state, respectively. Alternatively, 1˙⁺ undergoes a reductive elimination of H₂ from the AsH₂ group via a remarkably stable complex of the phenylarsandiyl radical cation, [C₆H₅As]˙⁺ and an H₂ molecule.