Average Length and Radius of Normal Paraffln Hydrocarbon Molecules

Abstract

The formulas previously published for the average (square) length and radius of long-chain molecules with restricted internal rotation are applied to a somewhat idealized model of the normal paraffin hydrocarbons. The potential barrier for rotation about the carbon-carbon single bond has been compounded of the usual threefold component plus a smaller onefold component which serves to stabilize the trans configuration of three successive bonds. Calculations are presented for a wide range of values of the parameters, and the most probable values of the parameters applicable to the normal paraffins, as determined from the thermodynamic and spectroscopic data, are discussed. The predicted ratios of the values of the average length and radius to the values calculated for free rotation are 1.76, 1.48, 1.36, and 1.29 at 0°, 100°, 200°, and 300°C, respectively. The effect of steric hindrance is discussed, and it is pointed out that the calculated values are probably appreciably low because of the neglect of steric hindrance.