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The compounds X0 and X (except for C, C0 , Si, Si0 , Ge and Ge0 ) have 6n-electrons in the ring. It was proposed that the overlap between the lone pair (AO) of heteroatom and the Pz (AOs) of the double bonds carbon atoms (p-11 overlap) improve homo-aromatic character for those compounds. In this work, the homo-aromatic character of those compounds is estimated. Homo-aromatic character is not a directly measurable or computable quantity. Homo-aromaticity is generally evaluated on the basis of energetic, geometric, and magnetic criteria. For the basis of energetic, it usually require to comparisons with non-aromatic model. There are some routes for the estimation of homo-aromatic stabilization energies (HASE) through various isodesmic reactions [25]. Isodesmic reactions consist of equal numbers of formal single and double bonds in the products and reactants. Systems with negetive HASE are homo-aromatic, while those with positive HASE are non-aromatic. Five isodesmic reactions and HASE are presented (Scheme 2). Equations I and 2 obviously show that the system with one

Scheme 2. Five isodesmic reactions for 1-hetero-2,5-cyclohexadiene-1-oxide, X0 and 1-hetero-2,5cyclohexadiene, X.