A new ab initio method was developed to supply reliable molecular dimensions for catalytic studies. For the DIPN isomers, calculations at correlated level revealed that the 2,6-isomer has the best molecular shape and dimensions concerning molecular transportation in the channel system of mordenite (M) zeolite. Adsorption rate measurements supported this theoretical finding According to the ab initio calculations performed at correlated level, the2,6- and 2,7-DIPN molecules may transform into each other via 1,2-isopropyl shift at an appropriate temperature in the main channel of mordenite. Isomerization reactions of 2,6-DIPN carried out over HM at high temperatures resulted in 2,7-DIPN in the reaction mixture supporting the theoretical results. Theoretical and experimental studies revealed that the selective formation of 2,6-DIPN over HM zeolite is the result of diffusion-controlled shape-selective catalysis, i.e., product selectivity is operative in this case.