Cyclic inorganic compounds. Part IV. The mass spectra of some bromophosphonitriles

Abstract

The behaviour of the bromophosphonitriles (PNBr₂)_n(n= 3, 4, 5, or 6) on electron impact is described. The most intense ion for the first three compounds is that obtained by loss of a bromine atom from the molecular ion. For the hexamer, however, the base peak is the P₃N₃Br₅⁺ ion although the P₆N₆Br₁₁⁺ ion is also intense. The mode of fragmentation can be rationalised on the basis of the stability of P_nN_n cyclic systems, as in all cases a high percentage of the positive-ion current results from ions of this type. Ions based on the parent ring system play a decreasingly important role as ring size of the bromophosphonitriles increases, reflecting the possibility for the tetramer and higher homologues to yield smaller cyclic systems of comparable stability on fragmentation. Metastable transitions indicate that ring contraction probably occurs by loss of PN units and that both bromine atoms attached to a given phosphorus are lost before fragmentation at a second phosphorus occurs, i.e., bromine loss follows a geminal path.