Density functional calculations were performed to study the active sites of M4+-substituted zeolites (M = Ti, Zr, Ge, Sn, Pb) and find a descriptor to correlate the olefin structures and epoxidation activities. The active-site formation is the most thermodynamically favorable for M = Ti. Transition elements (Ti and Zr) form the bidentate M(eta(2)-OOH) active sites whereas main group elements (Ge, Sn and Pb) are apt to form the monodentate M(eta(1)-OOH) species. The charge density of the C=C double bond (Q(CC)) was found to be closely correlated with the activation free energy (Delta G(not equal)), and the linear functions were fitted as Delta G(not equal) = 32.5 * Q(CC) + 54.4 (R-2 = 0.89) and Delta G(not equal) = 41.2 * Q(CC) + 55.5 (R-2 = 0.95) for the ethylene- and styrene-related systems, respectively. Several exceptions to these functions have been detected suggesting the presence of other influencing factors. Nonetheless, the Q(CC) descriptor proves to be applicable not only for the common electrophilic reactions, but also for those that may fall outside the scope of "electrophilic" as discussed in this work. (c) 2013 Elsevier B.V. All rights reserved.