Trends in the reactions of gaseous phenylpnictogen radical cations C6H5EH2˙+ (E = N, P, As)

Abstract

In context of an analysis of the effect of the central atom E of gaseous radical cations of phenyl pnictogens C₆H₅EH₂, E = N (1), P (2), and As (3), the mass spectrometric reactions of phenyl phosphane 2 have been re-investigated by D-labeling and by using methods of tandem mass spectrometry. The 70 eV mass spectrum of 2 shows the base peak for ion [M-2H]˙⁺ and significant peaks for ions [M-H]⁺, [M-(2C,3H)]⁺, [M-PH]˙⁺, and [M-(C,P,2H)]⁺. Metastable 2˙⁺ fragments exclusively by loss of H₂, and the investigation of deuterated 2-d₂ shows that excessive H/D migrations occur before fragmentation. Other significant fragment ions in the mass spectrum of 2 arise by losses of C₂H_2,P, or HCP from the ion [M-H]⁺. This mass spectrometric behavior puts the radical cation 2˙⁺ in between the fragmentation reactions of aniline radical cation 1˙⁺ (loss of H and subsequent losses of C₂H_2, or HCN) and phenyl arsane radical cation 3˙⁺ (elimination of H₂ and loss of As from ion [M-H]⁺). The fragmentation mechanisms of the radical cations 1˙⁺–3˙⁺ and of related ions were analyzed by calculations of the enthalpy of relevant species at the stationary points of the minimum enthalpy reaction pathways using the DFT hybrid functionals UBHLYP/6–311+G(2d,p)//UBHLYP/6–311+G(d). The results show that, in contrast to ionized aniline 1˙⁺, the reactions of the derivatives 2˙⁺ and 3˙⁺ of the heavier main group elements P and As are characterized by an easy elimination of H₂via a reductive elimination of group C₆H₅-E (E = P, As) and by a special stability of bicyclic isomers of 2˙⁺ and 3˙⁺. Thus, while 1˙⁺ rearranges by ring expansion and formation an 7-aza-tropylium cation by loss of H·, the increased stability of bicyclic intermediates in the rearrangement of 2˙⁺ and in particular of 3˙⁺ results in separate rearrangement pathways. The origin of these effects is the more extended and diffuse nature of the 3p and 4p AO of P and As.