| Abstract
| - Cracking of an all-trans n-alkane, via idealized Lewis acid and Bronsted acid catalysis, was examined usingdensity functional theory. Optimized geometries and transitions states were determined for catalyst−reactantcomplexes, using AlCl3 and HCl·AlCl3 as the Lewis and Bronsted acids. For the Lewis acid cycle, hydride-transfer steps are seen to have large barriers in both forward and reverse directions, and an unstable physisorbedcarbenium ion (lying 20 kcal mol-1 above the chemisorbed intermediate) is the launching point for the β-scissionthat leads to products. For the Bronsted acid cycle, proton-transfer steps have smaller barriers in both forwardand reverse directions, and a semistable physisorbed alkanium ion is the launching point for the alkaniumα-scission that leads to products. In the idealized Lewis cycle, formation of HCl units (and hence Bronstedacids) was found to be a common side reaction. A recent ionic-liquid catalysis study is mentioned as motivation,although our study is not a computational modeling study; we are more interested in the fundamental differencesbetween Brosnted and Lewis mechanisms rather than merely mimicking a particular system. However, resultsof exploratory optimizations of various intermediates with Al2Cl7- as the catalyst are presented to providethe first step for future modeling studies on the ionic liquid system.
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