1. Department of Physics, School of Mathematics, Physics and Data Science, Chongqing University of Science and Technology, Chongqing 401331, China
2. Department of Applied Chemistry, School of Chemistry and Chemical Engineering, Chongqing University of Science and Technology, Chongqing 401331, China
Abstract：Fourier transforms infrared (FT-IR) spectra in the range of 400~4 000 cm-1 was ollected for 2,5-dichloropyrimidine (2,5-DCP) in solid phase as well as in liquid phase using four sample preparation methods: KBr pressed (KBr), mineral oil (Nujol), attenuated total reflection (ATR) and melting (Liquid), while Fourier transforms Raman (FT-Raman) and laser Raman (Laser-Raman) spectra in the range of 80~3 200 cm-1 was also recorded. To correctly interpret the experimentally obtained vibrational spectra, the geometry of 2,5-DCP was first optimized by applying 14 methods from density function theory (DFT) as well as second-order perturbation method (MP2), based on which its harmonic frequency, infrared intensity and Raman activity were obtained, followed by the conversion of Raman activity to Raman intensity. To consider the anharmonic effect, the perturbation calculation is performed near the equilibrium geometry to obtain the third and fourth-order force fields in normal coordinates, and the anharmonic vibration frequency and intensity of 2,5-DCP are obtained based on the vibration second-order perturbation (VMP2) theory. It is found that the anharmonic vibration frequencies calculated by B3LYP and B3PW91 have the smallest difference from the experimental values. Based on the preferred B3LYP method, the effect of the basis sets on the vibration frequency continued to be investigated, eight basis sets were adopted, and it was found that the difference between the 6-311++G(2pd, 2df) level and the experimental values was the smallest, with a root-mean-square error(RMSE) of 6.75 cm-1 (4.63 cm-1 under 22 vibration modes), the 6-311++G(d, p) greatly reduced the calculation time, while the accuracy of 6-311++G(d, p) is not much lost (6.79 cm-1). In summary, the anharmonic vibrational spectra calculated based on the B3LYP method combined with the 6-311++G(2df, 2pd) basis set are the best choice for assigning the experimental vibrational spectra of the 2,5-DCP. Then, according to the optimal calculation results and the vibrational fundamental frequencies obtained by the scaling factor method, combined with the anharmonic vibrational intensity, the schematic diagram of the normal coordinates analysis, the potential energy distribution (PED) of the vibrations, and compared to the experimentally acquired infrared and Raman spectra, all fundamental frequencies and some overtones of the 2,5-DCP were assigned, and two vibrational couplings were found, One is caused between 3 054 cm-1 and the combination tones of 1 554 and 1 540 cm-1; the other is from the coupling between 1 132 cm-1 and the sum frequency 793+351 cm-1 and the difference frequency 1 370～230 cm-1. Finally, the anharmonic vibrational spectra of 2,5-DCP under multiple isotopic substitutions were expected and the correctness of the attribution was checked again.
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