系列化合物C—H伸缩频率的自然杂化轨道研究

Natural Hybrid Orbital Study of the C-H Stretching Frequencies in Hydrocarbons and Heterosubstituted Hydrocarbons

在计算C—H核白旋-自旋偶合常数的新公式及其与C—H伸缩频率之间的关系的基础上,得出了计算C—H伸缩频率的新的一般关系式.并利用CNDO/2分于轨道和自然杂化轨道方法,具体计算了三种不同系列化合物的原子净电荷和自然杂化轨道.给出了计算不同系列化合物C—H伸缩频率的良好线性关系式.结果表明,碳氢化合物中的C—H伸缩频率主要由碳原子的轨道杂化作用所决定,而对于含杂原子的取代碳氢化合物,C—H键的极性成为影响伸缩频率的重要因素.

As a result of earlier interest in the calculation of the C-H nuclear spin-spin coupling constants and its correlation with the C-H stretching frequencies in hydrocarbons and heterosubstituted hydrocarbons, a novel general relationship for the calculation of the C-H stretching frequencies is proposed in this paper. The proposed relationship has been employed to calculate the C-H stretching frequencies (VCH) for a series of hydrocarbons, fluorosubstituted hydrocarbons and substituted aldehydes by use of the CNDO / 2 molecular orbital approximation and the natural hybrid orbital scheme. It is shown that the C-H stretching frequencies are directly proportional to the s-character of the corresponding C atom in hydrocarbons. But for heterosubstituted hydrocarbons, the net charges of atoms C and H, or the polarity of the C-H bond, become a significant factor. The calculated results may also be utilized to account for other factors affecting VCH, such as steric effect, cis-trans geometry and substituted effects etc.. Some discussions concerning the calculated results are also reported.