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用SCF MO ab initio计算,基于3—21G基组,用能量梯度法优化了顺式和反式甲酸分子的平衡几何构型。用反应途径法(reaction pathway)得到了甲酸顺反异构化的过渡态。将模型势函法和MNDO方法结合起来,得到了顺、反甲酸分子的完整力场。利用对角力常数校正因子的可迁性,获得了过渡态的力场,进而求得了各体系的简正振动频率和相应的振动模式。根据RRKM单分子反应理论,得到了相应的频率因子和活化熵值。计算结果与实验观测值进行比较,对分子的性质进行了讨论。
Using SCF MO ab initio calculations, the equilibrium geometry of the cis- and trans-formic acid molecules was optimized using the energy gradient method based on the 3-21G basis set. The transition state of cis-trans isomerization of formic acid was obtained by reaction pathway. Combining the model potential function method and the MNDO method, we get the complete force field of cis and trans formic acid molecules. The force field of the transition state was obtained by using the correctability of the correction factor of the diagonal force constants, and the normal vibration frequency and the corresponding vibration mode of each system were obtained. According to RRKM theory of single molecule reaction, the corresponding frequency factor and activation entropy were obtained. The calculated results are compared with the experimental observations, and the properties of the molecules are discussed.