Trends in the Periodic System: The Mass Spectrum of Dimethylphenyl Phosphane and a Comparison of the Gas Phase Reactivity of Dimethylphenyl Pnictogene Radical Cations C6H5E(CH3)2•+, (E = N, P, As)
Authors: Kirchhoff Dirk , Grützmacher Hans-Friedrich , Grützmacher Hansjörg 
Published: March 2009

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 C6H5E(CH3)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, •CH3, which is followed by abundant loss of a molecule, H2, and formation of ion C7H7+, and the 70 eV EI mass spectrum of the deuterated derivative 1d3 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 1d6•+ which displays a group of signals for the loss of all isotopomers, •C(H/D)3, and three signals for formation of ions C7H5D2+, C7H4D3+ and C7H3D4+. 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 •CH3 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 CH3 substituent without positional H exchange. Further, the metastable ion [M – CH3]+ eliminates, almost exclusively, a molecule H2, which is preceded by excessive positional H/D exchange in the case of metastable ion [M – CD3]+. The formation of ion C7H7+ from metastable ion [M – CH3]+ 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 α-distonic isomer 1dist1•+ by a 1,2-H shift from the CH3 substituent to the P atom in competition with a direct loss of a CH3 substituent. The distonic ion 1dist1•+ performs positional H exchange between H atoms of both CH3 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 C7H7+ and a radical CH3P•H or by loss of a radical •CH3. Finally, ions [M – CH3]+ 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 H2 is lost by a 1,1-elimination from the PH2 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) α-cleavage by loss of •H from a methyl substituent, (ii) rearrangement of the molecular ion to an α-distonic isomer by a 1,2-H shift and (iii) loss of •CH3 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 α-cleavage. Both 1•+ and 3•+ eliminate •CH3 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 α-distonic ion is able to compete with loss of •CH3.