中文摘要：

采用密度泛函理论的B3LYP交换火联能泛函，在6-311＋G（2df，2p）基组水平上，对尿嘧啶及硫尿嘧啶分子进行优化计算，发现硫尿嘧啶两个互变异构体中，硫取代7位置氧（U7S）的总能量为-20080．0636eV，硫取代3化置氧（U3s）的总能量为20079．9896eV，U？s的总能量较小，较为稳定．在同样基组水平上对这些分子进行振动频率分析，发现尿嘧啶分子C-O健伸缩振动频率约为1770cm，而形成硫尿嘧啶时，与之相对应的S-O键伸缩振动频率约为1160cm^-1，发生了约600cm^-1的红移，且红外活性减弱．对应于该振动模式的力常数从16N·cm^-1。降低至3N·cm^-1，折合质苗由8amu降至3．8amu，力常数的减弱是产生这一振动红移的主要原因．

英文摘要：

A density functional investigation of uracil and thiouracil tautomers was performed to determine geometric coordinates and characteristic vibrations at B3LYP/6-311 ＋ G （2df, 2p） level. The geometry optimizations of thiouracil tautomers have been carried out. According to the result, the total energy 3-thiouraeil（U3s） is -20 079. 989 6 eV,and 7-thiouracil（U7s） is -20 080. 063 6 eV. The total energy for U7s is lower than U3s, so UTs is more stable than U3s. What＇s more, the frequencies of optimized structures are calculated for characters of infrared vibration. We find that the stretching vibration frequencies of C-O for the three molecules is about 1 770 cm^-1 and changes a little, but the stretching vibration of C-S for two thiouraeil tautomers have more than 600 cm 1 redshift compared with the stretching vibration of C-O at the same position for uracil, which is about 1 160 cm-1. The reason for the redshift is that the sulfur atom taking place of the oxygen atom in uracil changes the force constants and the reduced masses. The force constant reduces from 16 N-cm^-1 to 3 N.cm^-1, and the reduced mass from 8 ainu to 3.8 amu, so the decrease of the force constants is the main reason for redshift.