Ma. Natal-santiago et al., DFT study of the isomerization of hexyl species involved in the acid-catalyzed conversion of 2-methyl-pentene-2, J CATALYSIS, 181(1), 1999, pp. 124-144
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.