CHAIN-LENGTH DEPENDENCE OF INTERLAYER INTERACTION IN CRYSTALLINE N-ALKANES FROM RAMAN LONGITUDINAL ACOUSTIC MODE MEASUREMENTS

An analysis of the observed frequencies of the Raman-active longitudin al acoustic mode (LAM) bands of room-temperature crystalline n-alkanes in the chain-length range C33 to C246 indicates that, as the chains b ecome longer, there is a significant decrease in interlayer interactio n, this interaction being measured by the vibrational coupling between the ends of chains in adjoining layers. This conclusion is based main ly on LAM measurements on n-alkanes longer than 100 carbons that have recently become available. In the present analysis, the n-alkane cryst als are modeled as collinear monatomic chains having end-to-end intera ctions to simulate interlayer interaction. One intrachain force consta nt (F) and one interchain coupling force constant (f ) were evaluated from the observed frequencies of the LAM-1 and LAM-3 bands for each of seven n-alkanes: C48, C62, C70, C72, C94, C150, and C192. As expected , the values found for F are essentially independent of chain length. However, the values of f were found to decrease in going to longer cha ins, contradicting the generally held view that the interlayer force c onstant is independent of chain length. The value of f is proportional to 1/n within experimental error. If we assume this proportionality, the observed Raman frequencies of the LAM-m modes can be accounted for quantitatively. This is not the case if f is assumed independent of c hain length. The decrease in the interchain interaction with increasin g chain length is attributed to an increase in the layer separation re sulting from an increase in chain-end amplitudes associated with the l ow-frequency longitudinal vibrations of longer chains.