Vibrational force fields for strained hydrocarbons involving exo-double bonds. An ab initio study

Abstract

Ab initio calculations of stretching force constants for the carbon-carbon double bond in a series of molecules characterized by different amounts of strain in the neighbourhood of the double bond were performed. The stretching force constants for bonds adjacent to the double bond have also been calculated as well as all relevant coupling terms. The variation in the force constant for the double bond is in agreement with the variations in bond distance predicted previously. Furthermore, it appears that the stretching force constants are linearly related to the product of s-character of the hybrids forming the bond in question, if other effects remain roughly constant. This seems to be the case in a series of studied molecules. It turns out that the strength of an exo-double bond is proportional to the amount of the angular strain inherent in the ring fragments attached to the double bond.