THE EXPERIMENTAL INVESTIGATION OF [C5H12O]+. ION STRUCTURES RELATED TO NEOPENTYL ALCOHOL AND ITS METHYL-ETHER

The energetics and mass spectral dissociation characteristics of ionized neopentyl alcohol have been investigated in detail. Metastable ion, collisional activation and neutralization-reionization mass spectra were recorded and C-13 and D-labelled analogues were studied in the attempt to identify the isomeric ion structures accessed by the molecular ion prior to its metastable fragmentation by loss of CH3OH. It was concluded, in agreement with an earlier proposal, that the reacting configuration is best represented as an ion/molecule complex between methanol and ionized methyl propene. The involvement of this complex in the partial positional loss of identity among H atoms which preceedes fragmentation could not be established and, as an alternative, distonic and ylid ions were favourably considered. In particular the distonic ion (CH3)2CCH2O+(H)CH3 was prepared; its DELTA-H(f) value, 592 kJ mol-1, lies 28 kJ mol-1 below that of neopentyl alcohol and although it fragments via the same transition state as the alcohol it does not rearrange to the neopentyl alcohol structure. D-labelled analogues did not show behaviour identical with that of neopentyl alcohol. It was also concluded that the molecular ions of neopentyl alcohol and methyl-t-butyl ether do not fragment via common intermediates. The losses of CH3OCH3 and CH3OH from metastable, ionized methyl neopentyl ether were briefly studied, again with the aid of isotopic labelling. The former reaction was considered to have a dimethylether-ionized isobutane complex as its transition state. Loss of CH3OH, however, was proposed to involve an ion/radical complex between the methoxy radical and a neopentyl cation. The primary alkyl cation rearranges in situ to the t-amyl cation and beta-H atom transfer therefrom yields methanol and the [C5H10]+. daughter ion, whose structure was shown to be that of trimethylethene.