Differences in the IR Methylene Rocking Bands between the Crystalline Fatty Acids and n-Alkanes:  Frequencies, Intensities, and Correlation Splitting

Detailed low-temperature infrared spectra in the methylene rocking−twisting progression band region from 700 to 1000 cm-1 are presented for the C-phase crystalline fatty acids with even numbers of carbons from 16 through 22. There are significant differences between these spectra and those of the corresponding crystalline n-alkanes. The differences in band frequencies, intensities, and splitting are relevant to the interpretation of infrared spectra of complex assemblies of chain molecules such as biomembranes. They are due to chain end and chain-packing differences and can be accounted for using simple models developed in earlier studies, particularly those on the n-alkanes. Band frequency differences result from shifts imposed on the unperturbed frequency by the end groups. Shifts associated specifically with the methyl and acid end groups were estimated from the observed frequencies of the fatty acids, fatty diacids, and n-alkanes and unperturbed frequencies obtained from the dispersion curve for the infinite polymethylene chain. The shifts observed for rocking band frequencies are found to be the sums of the shifts assigned to the end groups. The differences in intensity and intensity distribution can be explained using a model in which the contribution of the individual methylenes to the dipole moment derivative associated with a given band is assumed to be the same except for the terminal methylenes. These methylenes are distinguished by the adjoining chain end group. The intensity differences between a fatty acid and an n-alkane occur because the contribution from the acid-end methylene is much greater than that from the other methylenes. The methylene bands in the spectra of the orthorhombic and monoclinic n-alkanes and C-form fatty acids are split because of interchain vibrational coupling. Their splitting patterns depend on the chain tilt angle. The three different patterns can be accurately reproduced using a simple coupled oscillator model, the different tilts, and three methylene−methylene interchain interaction constants.