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Differences in the IR Methylene Rocking Bands between the Crystalline Fatty Acids and n-Alkanes: Frequencies, Intensities, and Correlation Splitting
journal contribution
posted on 2004-08-12, 00:00 authored by Hung-Wen Li, Herbert L. Strauss, Robert G. SnyderDetailed 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.