Structural aspects of the effect of pressure on the Raman and infrared spectra of <i>n</i>-hexadecane

Raman and infrared spectra of neat n-hexadecane were measured as a function of hydrostatic pressure up to 54 and 83 kbar, respectively. The application of high pressure leads to anomalous frequency shifts and band splittings, and induces drastic changes in the band shapes and in the distribution of intensities within the spectra. The pressure-induced changes in the vibrational spectra of n-hexadecane are discussed in terms of phase transitions, molecular distortions, interchain interactions, frequency dispersions, and Fermi resonance interactions. The effect of pressure on the Raman and infrared spectra of n-hexadecane is used to test conclusions drawn from theoretical Fermi resonance calculations concerning the origin of several features in the Raman spectra of n-alkanes. Evidence is given to show that some assumptions made in these calculations are incorrect and that the model used is too simple to account for the intensity distribution in the CH stretching region of the Raman spectra of n-alkanes. It is also shown that the peak heights of the 2930 and 2850 cm−1 Raman bands can be affected by such factors as the magnitude of interchain interactions which renders questionable the use of the peak height ratio between these bands for the quantitative estimation of the gauche/trans population in biomembranes and polymers. While hydrostatic pressure initially causes a conformational ordering of the chains in n-hexadecane, at high pressures the compression along the chain direction is predominant over the lateral compression, leading to a distortion of the methyl end groups and a conformational disordering of the methylene chains.

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