Shape-selective isopropylation of naphthalene over mordenite catalysts: Computational analysis using MOPAC

In our experimental work on shape-selective isopropylation of naphthalene, the selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN) and the ratio of 2,6-DIPN to 2,7-DIPN were increased by mordenite catalyst dealumination. However, it was not clear whether the differentiation between the two isomers was caused by their differences in molecular dimensions or in electronic properties. In this work we performed a computational analysis of the molecular dimensions and frontier electron density f_r(E) using MOPAC program for naphthalene, isopropyl-and diisopropylnaphthalene. The f_r(E) value for electrophilic substitution reaction represents the density of electrons in the highest occupied molecular orbital (HOMO). According to the frontier molecular orbital theory, the most reactive position (the carbon atom on which electrophilic attack occurs most likely) has the highest frontier electron density. The calculation shows that 2,6-DIPN has a slightly smaller critical diameter. More importantly, position 6 in 2-IPN has higher f_r(E) value than that of position 7 in 2-IPN. This suggests that during 2-IPN isopropylation inside a mordenite channel, the formation of 2,6-DIPN is electronically more favored than that of 2,7-DIPN.