Low-frequency Raman spectroscopy of n-alcohols. LAM vibration and crystal structure

Low-frequency Raman spectra of even and odd n-alcohols have been recorded. The longitudinal acoustic modes (LAM) are described, and the effects of hydrogen-bonding, chain length, and temperature on the frequencies of these vibrations are discussed. The frequencies of the LAMs are interpreted in terms of the one-dimensional crystal model of Minoni and Zerbi and also ab initio calculations on isolated single chains. The low-frequency LAM-1 mode of the hydrogen-bonded dimer exhibited an odd-even effect, and to gain insight into this, the crystal structures of the C₁₇ and C₂₀ alcohols were determined by single-crystal X-ray diffractometry. C₁₇H₃₅OH packs in the β form, with half the oxygen atoms trans and half gauche, with respect to the alkyl chains. C₂₀H₄₁OH adopts an all trans conformation and the packing is in the γ form. The hydrogen-bonding interactions differ in the two structures. We therefore ascribe the odd-even effect in the LAM-1 frequencies to an odd-even effect in crystal structure.