Statistical mechanics of small chain molecules in liquids. I. Effects of liquid packing on conformational structures

摘要

When a chain molecule can be viewed as a collection of overlapping spherical groups, the effect of a solvent on the conformational structure of the chain molecule is described by the distribution function for the cavity particles associated with the spherical groups. This article discusses the calculation of the cavity distribution function fornhyphen;butane disolved in various apolar solvents: the liquids carbon tetrachloride,nhyphen;butane, andnhyphen;hexane. We consider the common picture where the CH3and CH2groups innhyphen;butane are simple spheres. For that model, the cavity distribution function is a fourhyphen;point correlation function. We find that the superposition approximation for the fourhyphen;point function, while qualitatively correct, overestimates the effects of the solvent. An alternative scheme, which is called the two cavity model, yields results that agree quantitatively with a computer simulation study of liquidnhyphen;butane. We find that a solvent medium produces significant shifts in the conformational equilibrium ofnhyphen;butane from that found in the gas phase. This phenomenon is the result of the nature of the local packing of solvent molecules neighboring the solute species under investigation. The conformational equilibrium is sensitive to this packing. The bulk packing fractions (molecular density times the volume of the molecule) of the liquids CCl4and C4H10are nearly identical. Even so there are noticeable differences between the intramolecular structure ofnhyphen;butane in liquid carbon tetrachloride and in the neat liquid. Previous ideas on conformational equilibria have ignored the importance of steric (i.e., liquid packing) effects, and have assumed that solvent shifts in conformational structures can be attributed entirely to dielectric effects. Our calculations show that this assumption is wrong. Thenhyphen;butane molecule contains no significantly polar groups. yet solvent media produce substantial shifts. For example, thetransndash;gaucheequilibrium constant,xg/xt, fornhyphen;butane in the gas phase at room temperature is roughly 0.5, while we find it is about 1.0 whennhyphen;butane is dissolved in liquid CCl4at the same temperature and 1 atm pressure. We discuss why the phenomenon has been overlooked, and suggest experiments to document its existence.