Kinetic Study and NBO Analysis of the Dehydrogenation Mechanism of Five‐membered Ring Heterocyclic 2,5‐Dihydro‐[furan, thiophene, selenophene

Abstract

AbstractThe theoretical study of the dehydrogenation of 2,5‐dihydro‐[furan (1), thiophene (2), and selenophene (3)] was carried out using ab initio molecular orbital (MO) and density functional theory (DFT) methods at the B3LYP/6‐311G**//B3LYP/6‐311G** and MP2/6‐311G**//B3LYP/6‐311G** levels of theory. Among the used methods in this study, the obtained results show that B3LYP/6‐311G** method is in good agreement with the available experimental values. Based on the optimized ground state geometries using B3LYP/6‐311G** method, the natural bond orbital (NBO) analysis of donor‐acceptor (bond‐antibond) interactions revealed that the stabilization energies associated with the electronic delocalization from non‐bonding lone‐pair orbitals [LP(e)X3] to δ*C(1)H(2) antibonding orbital, decrease from compounds 1 to 3. The LP(e)X3→δ*C(1)H(2) resonance energies for compounds 1–3 are 23.37, 16.05 and 12.46 kJ/mol, respectively. Also, the LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the decrease of occupancies of LP(e)X3 non‐bonding orbitals in ring of compounds 1–3 (3>2>1). The electronic delocalization from LP(e)X3 non‐bonding orbitals to δ*C(1)H(2) antibonding orbital increases the ground state structure stability, Therefore, the decrease of LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the kinetic of the dehydrogenation reactions of compounds 1–3 (k1>k2>k3). Also, the donor‐acceptor interactions, as obtained from NBO analysis, revealed that the (C(4)C(7)→δ*C(1)H(2) resonance energies decrease from compounds 1 to 3. Further, the results showed that the energy gaps between (C(4)C(7) bonding and δ*C(1)H(2) antibonding orbitals decrease from compounds 1 to 3. The results suggest also that in compounds 1–3, the hydrogen eliminations are controlled by LP(e)→δ* resonance energies. Analysis of bond order, natural bond orbital charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and synchronous six‐membered cyclic transition state type of mechanism.