DFT study of the isomerization of hexyl species involved in the acid-catalyzed conversion of 2-methyl-pentene-2

Quantum-chemical calculations were conducted on the basis of density-functi onal theory to study reactions of hexyl species involved in the acid-cataly zed isomerization of 2-methyl-pentene-2. The production of 4-methyl-pentene -2 and 3-methyl-pentene-2 involves 1,2-migrations of hydrogen atoms and met hyl groups whose activation energies are lower than 30 kJ/mol for gaseous c arbenium ions. The activation energy for branching rearrangements of gaseou s hexyl cations to form 2,3-dimethyl-butene-2 is 94 kJ/mol. Transformations of hexyl species were studied in the presence of gaseous water and an alum inosilicate site to simulate reactions on acidic oxides. In the presence of these oxygenated (conjugate) bases, the cationic center in the carbenium i ons bonds with oxygen to form alkoxonium ions and alkoxy species, respectiv ely. The relative energies of these species are fairly insensitive to their secondary or tertiary nature. Reactive intermediates of the same order are stabilized more than the corresponding transition states upon interaction with an oxygenated base, thus leading to an increase in the activation ener gies of isomerization reactions. Transition states have greater separation of electronic charge than the corresponding alkoxonium ions and alkoxy spec ies;The transition state for branching rearrangement requires the greatest separation of electronic charge in the aluminosilicate cluster; the transit ion state for methyl migration requires the second greatest separation of e lectronic charge; transition states for hydride-shifts require a smaller se paration of electronic charge; and transition states for the protonation of alkenes to form alkoxy species require the least separation of electronic charge in the aluminosilicate cluster These observations imply the existenc e of a correlation between the positive charge localized in the hydrocarbon fragment of a transition state and the sensitivity of the corresponding re action pathway to changes in the acidity of the catalyst. Lastly, activatio n energies for alkene isomerization reactions over aluminosilicates are det ermined by the energies of transition states with respect to the gaseous re actants plus the acid site and not by the relative stabilities of the alkox y intermediates in the reaction scheme. (C) 1999 Academic Press.