5-卤代尿嘧啶质子转移异构化的密度泛函理论和拉曼光谱实验

Density Functional Theory and Raman Spectroscopy Study of the Proton Transfer Tautomerism of 5-Halogenouracils

采用量子化学中的密度泛函理论，在6-31C（d）计算水平上，考察了5-卤代尿嘧啶（5-XU，其中X=F，Cl，Br，Ⅰ）的质子转移异构化反应的7种可能途径、相关异构体和过渡态的结构，以及质子转移异构化的势能面，并对5-碘尿嘧啶和尿嘧啶的拉曼光谱频率进行了讨论。研究表明，取代基X的吸电子能力对5-卤代尿嘧啶各质子转移异构体的稳定性和某一步的质子转移异构化能垒的大小几乎没有影响，尽管各步质子转移异构化反应能垒之间有明显的差异，如第一步的质子转移异构化能垒最大，约为188～214kJ·mol^-1，第二步其次，约为158～171kJ·mol^-1，第三步最小，约为145～155kJ·mol^-1。获取了5-碘嘧啶和尿嘧啶的傅立叶拉曼光谱。拉曼光谱显示，碘对尿嘧啶的5位氢的取代明显改变了尿嘧啶的C4-C5／C5-C5伸缩振动、C4—O10伸缩振动和N3-H9弯曲振动的拉曼活性．袁明光或电磁场对C4-O10和N3—H9局部的电子云分布产生重要影响。

Seven proton transfer tautomerization channels of 5-halogenouracil （5-XU, X = F, Cl, Br, I） were investigated using density functional theory computations at the B3LYP/6-31 G（d） level of theory. The geometry structures and the relative energies for all tautomers as well as the corresponding transition-states are reported. The results indicate that the electron-withdrawing capacity of substituent （X） for 5-halogenouracil have little effect on the stability of proton transfer tautomers, though the energy barrier for each proton transfer tautomerizations differs. There are three proton transfer steps for 5-halogenouracils that display three different energy barrier： 188 -214 kJ·mol^-1 for the first step, 158-171 kJ·mol^-1 for the second step, and 145 - 155 kJ · mol^-1 for third step. The FT-Raman spectra of 5-iodouracil and uracil are measured and their similarities and the differences were discussed in Raman frequency and Raman intensity. The Raman activity of C4-C5/C5-C6 stretching mode, C4-O10 stretching mode and N3-H9 wagging mode for 5-iodouracil differs significantly from uracil. It shows that light or electromagnetic field has great effect on the electron cloud distribution of C4-O10 bond and N3-H9 bond and might selectively favor the XU1→XU3 proton transfer process.