MASS-SPECTROMETRY OF TERT-BUTYLNAPHTHALENES - A COMPARISON WITH THE UNIMOLECULAR FRAGMENTATION OF TERT-BUTYLBENZENE

The unimolecular fragmentation of ionized 1-and 2-tert-butylnaphthalen es 1 and 2 have been investigated using mass analyzed ion kinetic ener gy (MIKE) spectrometry and specifically deuterated and C-13 labeled de rivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3, M etastable molecular ions of 1, 2 and 3 lose exclusively a methyl radic al yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2-propyl cations 3a, Further fragmentations of la and 2a include, as the main processes, the elimination of C2H4 and of CH3., The loss of C2H4 is as sociated with a large kinetic energy release giving rise to a broad fl at-topped peak in the MIKE spectra (as in the case of 3a) while loss o f CH3 gives rise to a Gaussian shaped signal, The latter reaction is n ot observed for metastable 2-phenyl-2-propyl cations 3a, The fragmenta tions of labeled derivatives of 1 and 2 show that H/D exchange in la a nd 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C2H4 and of CH3., is much more extensiv e than in 3a, This H/D exchange decreases for metastable 1a and 2a dec omposing in the second field-free region, and the interchange of the C atoms within the C3H6 is less than for 3a, These observations are exp lained by H migrations between the side chain and the naphthalene ring s of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2- methyl shift known for the reactions of 3a, The differences observed f or the reactions of 1a and 2a and of 3a are explained by the more basi c aryl group of 1a and 2a, These fragmentation mechanisms are corrobor ated by an AM1 calculation of the minimum energy reaction pathways (ME RP).