In the exploration of synthesizing graphitic carbon nitride (g-C3N4), the existence of secondary anime bridging units shows the incompleteness of related theories. Thus, taking the thermal synthesis of melamine as an example, this work finds a possible reaction path with Density Functional Theory (DFT) for forming melem during the thermal synthesis of g-C3N4. Combined with transition state theory (TST), it indicates that the formation of melem results from the condensation of melamine and isomerization of melam. Meanwhile, the weak signal near 2135 cm-1 in the Fourier Transform Infrared Spectra (FTIR) corresponds to the vibration of carbodi-imines (-N[double bond, length as m-dash]C[double bond, length as m-dash]N-), which further proves the proposed reaction path. Thus, this work can explain the formation of g-C3N4 and its monomer, which may contribute to the successful formation of ideal g-C3N4 in the future.
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