Raman spectra of liquidnhyphen;alkanes. II. Longitudinal acoustic modes and thegauchendash;transenergy difference

摘要

Lowhyphen;frequency bands in the Raman spectra of the liquidnhyphen;alkanes,n=9ndash;20, have been identified with extended and certain nearly extended rotameric forms of these molecules. The bands are associated with the intense longitudinalhyphen;acoustichyphen;mode (LAMhyphen;1) band that appears in the Raman spectra of crystallinenhyphen;alkanes. For liquids, the intensities of these bands are highly temperature dependent. This dependence was measured for then=11ndash;14nhyphen;alkanes and was used to determine an average value for Dgr;E, the energy difference between thetransandgauchestates of the CC bond. The value determined for Dgr;E, 508plusmn;50 cal/mole, is very near the value commonly assumed in statistical calculations based on the rotational isomeric model. No evidence for local order was found even at temperatures just above the melting point ofnhyphen;C13H28andnhyphen;C14H30.