The fragmentation of melamine: A study via electron-impact ionization, laser-desorption ionization, collision-induced dissociation, and density functional calculations of potential energy surface

We have studied the fragmentation of the melamine (2,4,6-triamino-s-triazine) molecule and its deuterated counterparts via electron impact ionization (ET), laser desorption ionization (LDI), and collision-induced dissociation (CID). Our EI and LDI measurements show that the dissociation of melamine is different from the concerted triple dissociation pathway of s-triazine. In EI experiments, the protonated and parent melamine ion (m/z = 127 (C3N6H7+) and 126 (C3N6H6+)) were formed initially with 20 and 70 eV electron bombardment. Other fragment ions, such as m/z = 43 (CN2H3+), 53 (C2N2H+), 56 (CN3H2+), 68 (C2N3H2+), 83 (C2N4H3+), 85 (C2N4H5+), 99 (C2N5H5+), 110 (C3N5H4+), etc., were subsequently formed from the decomposition of metastable melamine ions. This speculation was supported by our additional CID measurements. On the other hand, in the LDI experiments the melamine molecule was pumped to 1(1)A " and 2(1)A' excited electronic states, respectively, with 266 and 193 nm lasers. In view of the same fragment ions (m/z = 43, 45 (CN2H5+), 60 (CN3H6+), 85, and 127) resulting from the different excited 1(1)A " and 2(1)A' states, we conclude that the fragmentation of melamine in LDI proceeds via internal conversion to its ground potential energy surface (1(1)A') prior to dissociation. The decomposition mechanism in the ground electronic state has been investigated using the density functional B3LYP/6-31G* and B3LYP/cc-pVTZ methods. All the molecular ions observed in EI experiments can be produced from major and minor neutral fragments of melamine dissociation. The calculations demonstrate the reaction pathways leading to these fragments and predict the corresponding activation energies. The dissociation mechanism of melamine is shown to be distinct from that of s-triazine, because of the presence of mobile hydrogen atoms in the amino groups.