EXTENDED ROTAMERS IN LIQUID ALKANES

$^{1}$J. Mazur and B. Fanconi. To be published. $^{2}$P. J. Flory. Statistical Mechanics of Chain Molecules, Interscience, 1969.Author Institution:Raman spectra of the liquid n-alkanes, $C_{g}$ through $C_{20}$, show weak bands that closely car-respond in frequency to the longitudinal acoustic mode (CAM-1) bands observed for the fully extended n-alkanes in the solid state. From a measurement of the intensities of the LAM-1 bands of liquid n-alkanes an average energy difference, $\Delta $E, between gauche and trans CC bonds, can be determined. With the aid of a normal coordinate $calculation^{1}$, the LAM-1 bands in liquid $C_{11}$ to $C_{14}$ have been assigned to rotamers having only 0, 1 or 2 gauche bonds. The Raman intensities of these bands were measured as a function of temperature. From a comparison of the intensities of LAM-1 bands (CH stretching bands used for an internal standard) of solid and liquid n-alkanes, the absolute concentration of these extended molecules was determined. Using well known statistical mechanical relationships between DLE and rotamer $concentration^{2}$, we find DLE to have a value of $550 \pm 25$ cal/mole for $n-C_{12}H_{26}$. This energy difference, which is of great importance in the computation of the properties of the polymethylene $chain^{2}$, has previously been estimated only from small molecules. The present value is more precisely determined and is derived from a molecule that is closely related to larger chains