GAS-PHASE DECOMPOSITION OF CONJUGATE ACID IONS OF SIMPLE TERT-BUTYL ALKYL ETHERS

Unimolecular decompositions of protonated methyl (MTBE), ethyl (ETBE), n-propyl, isopropyl, and isobutyl tert-butyl ethers have been examined experimentally and the results compared with the outcome of ab initio calculations. Extensive hydrogen transposition between the hydrogen on oxygen and the nine hydrogens of the tert-butyl is revealed by mass-resolved ion kinetic energy spectroscopy (MIKES) experiments on deuterated ions from MTBE and ETBE. Mechanistic possibilities are probed with the help of FT-ICR, and isotope effects are interpreted by comparison with the MIKES of protonated ethyl tert-amyl ether and its deuterated analogues. Protonated MTBE displays a single unimolecular decomposition product, tert-butyl cation. Of the protonated MTBE ions that decompose, 30% do so without any hydrogen transposition, while 70% completely randomize the non-methoxy hydrogens. This calls for the intervention of at least two noncovalent intermediates. In the case of MTBE, Hartree-Fock-based SCF computations exhibit only one plausible candidate that corresponds to an energy minimum, the hydrogen-bonded complex between protonated methanol and isobutene. The other intermediate is inferred to be the ion-neutral complex [MeOH tBu(+)], even though that does not correspond to a well on the SCF potential energy surface. Protonated ETBE yields a pair of unimolecular decomposition products, tert-butyl cation and protonated acetaldehyde. Here the ab initio results display two noncovalent potential energy minima, but the observed ion intensities cannot be accounted for without at least three intermediates. Again, the conclusion is that the additional intermediate corresponds to an ion-neutral complex, [EtOH tBu(+)], which does not correspond to a potential energy minimum. The transient ion-neutral complexes have non-zero lifetimes because their collapse is prevented by entropic (rather than energetic) barriers.