Large hydrocarbon ion/molecule complexes formed during the unimolecular fragmentation of protonated tert-butyl-substituted tri- and tetrabenzylmethane

Matthias C, Kuck D. Large hydrocarbon ion/molecule complexes formed during the unimolecular fragmentation of protonated tert-butyl-substituted tri- and tetrabenzylmethane. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. 2002;217(1-3):131-151.The reactivity of two large gaseous ion/neutral complexes generated by protonolysis of tert-butyl-substituted tribenzylmethane (1) and tert-butyl-substituted tetrabenzylmethane (3) has been studied on the metastable ion time scale. These complexes, [t-C4H9+(C6H5CH2)(3)CH] and [t-C4H9+(C6H5CH2)(4)C], undergo competitive intra-complex proton transfer from the ionic component and hydride transfer to the ionic component, generating isobutene and isobutane, respectively. The hydride transfer process involves all of the benzylic methylene groups with the same probabilities, i.e., without any regioselectivity, demonstrating the formation of truly non-covalent adducts in which the ionic constituent moves relatively freely with respect to the neutral partner. The kinetic isotope effect found ubiquitously for simpler tert-butylbenzenium ions, k(H)/k(D) = 1.6, operates in these large all-hydrocarbon complexes as well. The facile protonolytic release of the t-C4H9+ ion into the complexes does not suppress the fast and apparently complete exchange between the 15 and, respectively, 20 protons at the aromatic rings of ions [1 + H](+) and [3 + H](+). In both cases, loss of isobutane from the ion/molecule complexes is followed by fast elimination of one or even two molecules of benzene. Similarly, loss of isobutene from [3 + H](+) is followed by a fast two-fold benzene elimination. This is evident from the unusual observation that two or even three neutral hydrocarbon molecules are expelled within the same field-free region of the sector-field mass spectrometer. (C) 2002 Elsevier Science B.V. All rights reserved