We introduce a novel calculation to the transmission coefficient of molecules with single impurity. The study employs tight binding Green's function method and new derived analytical formula to investigate chromene-like (even) and doped azulene-like (odd) model systems. Under certain circumstances, these specific even and odd models are respectively shown single and double critical points, at which the transmission becomes independent of the heteroatom position. Moreover, a counterintuitive correlation between the transmission and the coupling strength has analytically and numerically been approved: the transmission decreases with strengthening lead-nodes couplings.