ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
An analysis of the observed frequencies of the Raman-active longitudinal acoustic mode (LAM) bands of room-temperature crystalline n-alkanes in the chain-length range C33 to C246 indicates that, as the chains become longer, there is a significant decrease in interlayer interaction, this interaction being measured by the vibrational coupling between the ends of chains in adjoining layers. This conclusion is based mainly on LAM measurements on n-alkanes longer than 100 carbons that have recently become available. In the present analysis, the n-alkane crystals are modeled as collinear monatomic chains having end-to-end interactions to simulate interlayer interaction. One intrachain force constant (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 chains, contradicting the generally held view that the interlayer force constant 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 chain length. The decrease in the interchain interaction with increasing chain length is attributed to an increase in the layer separation resulting from an increase in chain-end amplitudes associated with the low-frequency longitudinal vibrations of longer chains.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.467159
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