Theoretical Analysis On The Infrared, Raman Spectra And SRES Of Melamine And Its Analogues

Food safety is closely related to people’s health and life safety and influence directly the development of national economy and social stability. Now food safety becomes the focus of the current international and domestic public opinion. Animal experiments and clinical studies have shown that intake of food contaminated by melamine will affect the health of the people, result in pediatric calculi and renal failure. The main reason for this is melamine and cyanuric acid combination through intermolecular hydrogen bonds formed insoluble melamine cyanurate in the renal tubule. Melamine analogues are considered equal potential toxicity by WTO。Therefore, how to choose the simple operation, fast, low cost, convenient and effective methods to detect melamine is very necessary. Vibrational spectra, especially the surface enhanced Raman spectrum has its unique advantages in the rapid detection. In this thesis, in order to provide the certain theoretical foundation for the detection of melamine and its analogues, density functional theory (DFT) methods have been performed to calculate the vibration spectra of melamine and its analogues, get frequencies of relevant vibrational modes. We will focus on the intermolecular hydrogen bond effect on the vibrational spectra. The specific contents are as follows:Firstly, studies on the structures and vibrational spectra of melamine monomer and dimer have been performed by density functional theory calculations with functions B3LYP, M06-2x, PBE, PBEO, X3LYP, and6-31++G**basis set. Geometrical structure obtained is in accordance with X-ray diffraction data. The vibrational spectra calculated can also reflect well the results of the experimental spectra. The calculated binding energy including BSSE correction of the dimmer through each hydrogen bond is about20kJ/mol, indicating a moderate hydrogen bond. This section focus on the influence of the intermolecular hydrogen bond on infrared spectrum, and the formation of hydrogen bond leads the decrease about230cm-1for the stretching vibration frequency of N-H bond. The NBO analysis shows that the second-order stabilization energies E(2) for both nN-σ*NH interaction are62.91kJ/mol, respectively, in which electrons transfer from the proton acceptor N atom to the a*antibonding orbital of the proton donor N-H bond (nNâ†'σ*NH) resulting in the weakening of the N-H bond strength, then the red-shift of the corresponding stretching vibrational frequency.In this section, the vibrational spectra for ammeline, ammelide and cyanuric acid, which are the structural analogues of melamine, have also been calculated, since they may be produced in the metabolic process of melamine. The calculated results agree well with the experimental results.Secondly, DFT method with functional B3LYP and6-31++G**basis set has been used to study the vibrational spectra of keto-form cyanuric acid and melamine cyanurate (MCA) fromed by intermolecular hydrogen bonding. The assignment of normal vibrational modes for cyanuric acid and melamine cyanurate have been studied via PED analysis, and obtain the changes for the vibrational frequencies after the formation of the complex. The calculated results show that MCA molecule is very stable; the binding energy including zero point vibrational energy correction and BSSE correction is more than60kJ/mol. And the NBO analysis shows that the second-order stabilization energy E(2)for nN-σ*NH interaction is148.35kJ/mol. And electrons transfer from proton acceptor atom N to the a*antibonding orbital of the proton donor N-H bond (nNâ†'σ*NH), leading to the red-shift for the stretching vibrational frequency.Thirdly, the hybrid density functional theory B3LYP method combined with the relativistic effective core potential (RECP) and the basis set6-31G**for atom C, H, O, N and modified Lanl2dz coupled with f polarization for Ag have been performed to calculate the SERS scattering activities of melamine and keto-form cyanuric acid and melamine cyanurate. The Raman activity around680cm-1for melamine, which belongs to the breathing vibration of triazine ring, is strong. And it depends on the environment of silver clusters, the electron transfer between absorbent molecule and Ag clusters lead to the redistribution of atomic charge, resulting in the changes of polarizability and the enhancement of the Raman intensity. This means that the frequency at680cm-1for SERS detection as a qualitative or quantitative tool is reasonable. For SERS detection spectra of keto-form cyanuric acid and melamine cyanurate have not been reported in detail, the frequencies around1460cm’1and1550cm-1calculated can act as the foundation for keto-form cyanuric acid and melamine cyanurate detection.