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)

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 α-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 •+