Trends in the Periodic System: the mass spectrum of dimethylphenyl phosphane and a comparison of the gas phase reactivity of dimethylphenyl pnictogene radical cations C(6)H(5)E(CH(3))(2)(*+), (E = N, P, As)(dagger).

The mass spectrometric reactions of dimethylphenyl phosphane, 1, under electron impact have been studied by methods of tandem mass spectrometry and by using labeling with deuterium. The results are compared to those for the previously investigated dimethylaniline, 2, and dimethylphenyl arsane, 3, to examine the effects of heavy main group heteroatoms on the reactions of radical cations of the pnictogen derivatives C(6)H(5)E(CH(3))(2). Decomposition of the radical cation 1(*+) gives rise to large peaks in the 70 eV electron impact (EI) mass spectrum for loss of a radical, *CH(3), which is followed by abundant loss of a molecule, H(2), and formation of ion C(7)H(7)(+), and the 70 eV EI mass spectrum of the deuterated derivative 1d(3) shows that excessive positional hydrogen/deuterium (H/D) exchange accompanies all fragmentation reactions. This is confirmed by the mass analyzed kinetic energy (MIKE) spectrum of the molecular ion 1d(6)(*+) which displays a group of signals for the loss of all isotopomers, *C(H/D)(3), and three signals for formation of ions C(7)H(5)D(2)(+), C(7)H(4)D(3)+ and C(7)H(3)D(4)(+). The intensity distribution within this latter group of ions corresponds to a statistical positional exchange ("scrambling") of all six D atoms of the methyl substituents with only two H atoms of the phenyl group. In contrast, the intensity distribution of the signals for loss of *C(H/D)(3) uncovers a bimodal reaction. About 39% of metastable molecular ions 1(*+) eliminate *CH(3) after scrambling of the six H atoms of the methyl substituents with two H atoms of the phenyl group, while the remaining 61% of metastable 1(*+) lose specifically a CH(3) substituent without positional H exchange. Further, the metastable ion [M-CH(3)](+) eliminates, almost exclusively, a molecule H(2), which is preceded by excessive positional H/D exchange in the case of metastable ion [M-CD(3)](+). The formation of ion C(7)H(7)(+) from metastable ion [M-CH(3)](+) is not observed and this is a minor process, even under the high energy condition of collision-induced dissociation (CID). The mechanisms of these fragmentation and exchange reactions have been modeled by theoretical calculations using the DFT functionals at the level UHBLY/6- 311+G(2d,p)//UHBLYP/6-31+G(d). The key feature is a rearrangement of molecular ion 1(*+) to an alpha-distonic isomer 1dist1(*+) by a 1,2-H shift from the CH(3) substituent to the P atom in competition with a direct loss of a CH(3) substituent . The distonic ion 1dist1(*+) performs positional H exchange between H atoms of both CH(3) substituents and H atoms at the ortho-positions of the phenyl group and rearranges readily to the (conventional) isomer benzylmethyl phosphane radical cation 1bzl(*+). The ion 1bzl(*+) undergoes further positional H exchange before decomposition to ion C(7)H(7)(+) and a radical CH(3)P*H or by loss of a radical *CH(3). Finally, ions [M-CH(3)](+) of methylphenyl phosphenium structure 1a(+) and benzyl phosphenium structure 1b(+) interconvert easily parallel to positional H exchange involving all H atoms of the ions. Eventually, a molecule H(2) is lost by a 1,1-elimination from the PH(2) group of the protomer 1b-H(+) of 1b(+). The trends observed in the gas-phase chemistry of the pnictogen radical cations dimethylaniline 2(*+), dimethylphenyl phosphane 1(*+) and dimethylphenyl arsane 3(*+) can be comprehended by considering the variation of the energetic requirements of three competing reaction: (i) alpha-cleavage by loss of *H from a methyl substituent, (ii) rearrangement of the molecular ion to an alpha-distonic isomer by a 1,2-H shift and (iii) loss of *CH(3) by cleavage of the C-heteroatom bond. 2(*+) exhibits a strong N-C bond and a high activation barrier for 1,2-H shift and fragments far more predominantly by alpha-cleavage. Both 1(*+) and 3(*+) eliminate *CH(3) by cleavage of the weak C-heteroatom bond. The barrier for a 1,2-H shift is also distinctly smaller than for 2(*+) and, for the P-derivative 1(*+), the generation of the alpha-distonic ion is able to compete with loss of *CH(3).

Isomerization and fragmentation reactions of gaseous dimethyl phenylarsane radical cations and methyl phenylarsenium cations. A study by tandem mass spectrometry and density functional theory calculations.

The unimolecular reactions of the radical cation of dimethyl phenylarsane, C6H5As(CH3)2, 1*+ and of the methyl phenylarsenium cation, C6H5As+CH3, 2+, in the gas phase were investigated using deuterium labeling and methods of tandem mass spectrometry. Additionally, the rearrangement and fragmentation processes were analyzed by density functional theory (DFT) calculations at the level UBHLYP/6- 311+G(2d,p)//UBHLYP/5-31+G(d). The molecular ion 1*+ decomposes by loss of a .CH3 radical from the As atom without any rearrangement, in contrast to the behavior of the phenylarsane radical cation. In particular, no positional exchange of the H atoms of the CH3 group and at the phenyl ring is observed. The results of DFT calculations show that a rearrangement of 1*+ by reductive elimination of As and shift of the CH3 group is indeed obstructed by a large activation barrier. The MIKE spectrum of 2+ shows that this arsenium cation fragments by losses of H2 and AsH. The fragmentation of the trideuteromethyl derivative 2-d3+ proves that all H atoms of the neutral fragments originate specifically from the methyl ligand. Identical fragmentation behavior is observed for metastable m-tolyl arsenium cation, m-CH3C6H4As+H, 2tol+. The loss of AsH generates ions C7H7+ which requires rearrangement in 2+ and bond formation between the phenyl and methyl ligands prior to fragmentation. The DFT calculations confirm that the precursor of this fragmentation is the benzyl methylarsenium cation 2bzl+, and that 2bzl+ is also the precursor ion fo the elimination of H2. The analysis of the pathways for rearrangements of 2+ to the key intermediate 2bzl+ by DFT calculations show that the preferred route corresponds to a 1,2-H shift of a H atom from the CH3 ligand to the As atom and a shift of the phenyl group in the reverse direction. The expected rearrangement by a reductive elimination of the As atom, which is observed for the phenylarsenium cation and for halogeno phenyl arsenium cations, requires much more activation enthalpy.

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

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 (DeltaH(r)(298) = -64 kJ mol(-1)) isomer 1 sigma*+ with a structure best represented as a distonic radical cation sigma complex between AsH and benzene. (ii) The six H atoms of the benzene moiety of 1 sigma*+ become equivalent by a fast ring walk of the AsH group. (iii) A reversible isomerization 1+<==>1 sigma*+ scrambles eventually all H atoms over all positions in 1*+. The distonic radical cation 1*+ 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.