NATURAL HYBRID ORBITAL STUDY OF THE C-H S TRETCHING FREQUENCIES IN HYDROCARBONS AND HETEROSUBSTITUTED HYDROCARBONS

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 stretchi ng frequencies in hydro and heterosubstituted hydrocarbons, a novel ge neral relationship for the calculation of the C-H stretching frequenci es is proposed in this paper. The proposed relationship has been emplo yed to calculate the C-H stretching frequencies (nu(CH)) for a series of hydrocarbons, fluorosubstituted hydrocarbons and substituted aldehy des by use of the CNDO/2 molecular orbital approximation and the natur al hybrid orbital scheme. It is shown that the C-H stretching frequenc ies are directly proportional to the s-character of the corresponding C atom in hydrocarbons. But for heterosubstituted hydrocarbons, the ne t 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 acc ount for other factors affecting nu(CH), such as steric effect, cis-tr ans geometry and substituted effects etc.. Some discussions concerning the calculated results are also reported.