Vibrational pressure tuning spectroscopy of the polymethylene chain. 1. Various n-alkanes from C8H18 to C36H74

Abstract

In this paper, we present the effect of pressure on the vibrational spectra of a series of 11 crystalline paraffin hydrocarbons from n-C{sub 8}H{sub 18} to n-C{sub 36}H{sub 74} plus n-C{sub 8}D{sub 18} and a few branched-chain compounds. The focus is on the increase in intensity of the CH{sub 2} wagging and C-C stretching vibrations with pressure. The sum of the intensities increases in a manner that depends primarily on chain length. The distribution between the two vibrations depends on the crystal structure. The results imply that the primary event is an increase in the wagging vibration intensity due to increased intermolecular coupling. This intensity is redistributed to the C-C stretching vibration via some lattice mode (phonon-assisted coupling or phonon-assisted resonance). This resonance is most efficient for the orthorhombic structure with 4 molecules/unit cell and least efficient for the triclinic structure with 1 molecule/unit cell. For n-C{sub 30}H{sub 62} and n-C{sub 36}H{sub 74}, which have a monoclinic structure with 2 molecules/unit cell, the behavior is intermediate.