2,5-二氯嘧啶振动光谱的实验和理论研究

The Experimental and Theoretical Study of Vibrational Spectroscopy for 2,5-Dichloropyrimidine

采用KBr压片(KBr)、石蜡油(Nujol)、衰减全反射(ATR)和熔融(Liquid)四种样品制备方法,收集了2,5-二氯嘧啶(2,5-DCP)固相及液相、400~4000 cm^(-1)范围内的傅里叶变换红外光谱(FTIR),记录了80~3200 cm^(-1)区间内的傅里叶变换拉曼光谱(FT-Raman)和激光拉曼光谱(Laser-Raman)被记录。为正确解释实验获得的振动光谱,首先采用14种密度泛函理论方法(DFT)及二阶微扰理论(MP2)对2,5-DCP的结构进行优化,获得谐性近似下的振动频率、红外强度和拉曼活性,并将拉曼活性转换到拉曼强度。进一步考虑非谐效应,在平衡结构附近展开微扰计算,得到简振坐标下的三阶、四阶力场,以振动二阶微扰(VMP2)理论得到2,5-DCP的非谐振动频率和非谐振动强度,发现B3LYP计算的非谐振动频率与实验值差异最小。基于优选的B3LYP方法,继续考察基组对振动频率的影响,通过9种基组计算结果发现6-311++G(2pd,2df)获得的非谐振动频率最佳,其与实验值的均方根误差(RMSE)为6.75 cm^(-1)(22个振动模下为4.63 cm^(-1)),6-311++G(d,p)在大大减少计算时间的同时,精度未有多大损失(6.79 cm^(-1))。综上所述,基于B3LYP方法结合6-311++G(2df,2pd)基组得到的非谐振动光谱是归属2,5-DCP实验光谱的最佳选择。基于最优计算结果、标度因子方法获得的振动基频,结合非谐红外振动强度、谐性近似下的拉曼强度、简振坐标示意图以及振动势能分布(PED),对比到实验采集的红外和拉曼光谱,对2,5-DCP的所有基频和部分泛频进行了指认,发现两个振动耦合,一为3054 cm^(-1)与1554以及1540 cm^(-1)的和频耦合导致;二为1132 cm^(-1)与793+351 cm^(-1)和频及1370~230 cm^(-1)差频耦合引起。最后预期了多个同位素取代下的2,5-DCP振动谱,同时检验了上述归属的正确性。

Fourier transforms infrared(FT-IR)spectra in the range of 400~4000 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 80~3200 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 3054 cm^(-1) and the combination tones of 1554 and 1540 cm^(-1);the other is from the coupling between 1132 cm^(-1) and the sum frequency 793+351 cm^(-1) and the difference frequency 1370~230 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.