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C-H stretching regions of both the Raman and i.r. spectra of the extended polymethylene chain have broad secondary maxima. In the case of the Raman spectrum, line shapes are dependent on the environment of the chain, a fact which has been previously exploited in the study of biosystems. We have explained this phenomenon in terms of Fermi resonance interaction between the methylene symmetric C-H stretching mode and appropriate binary combinations involving the methylene bending mode. It is emphasized that appropriate binary states are to be found throughout the Brillouin zone and not just at its center. It is the resulting continuum of binary states which leads to broad secondary bands. The shapes of these bands depend on the dispersion of the bending mode fundamental. For the isolated chain only parallel dispersion is involved, but in the case of the crystal perpendicular dispersion is equally important and leads to the observed dependence on crystal structure. All secondary bands have been accounted for in these terms. The ratio of the Raman intensities of the symmetric C-H stretching fundamentals to the antisymmetric is found to be about 5 and is independent of environment. The relevance of these results to studies on biosystems is briefly discussed.

A vibrational and attendant conformational analysis of the liquid n-paraffins and molten polyethylene is presented. For the purposes of the analysis a valence force field was derived which is applicable to both planar and nonplanar chains. The force field was evaluated from observed frequencies of trans (T) and gauche (G) n-C4H10; TT and GT n-C5H12; TTT, GTT, and TGT n-C6H14; and (T)∞ polyethylene, all of whose infrared spectra were assigned in detail. Infrared spectra of the liquid-n-paraffins n-C4H10 through n-C17H36 were measured at room temperature and n-C4H10 through n-C12H26 also at a temperature just above their melting point. Frequencies and normal coordinates were calculated for the extended forms and for forms having one gauche bond of n-C4H10 through n-C8H18. These quantities were also calculated for the conformations of n-C5H12 through n-C7H16 having two gauche bonds and for the nonplanar but regular conformations (TG)∞ and (G)∞ of polyethylene. Some bands attributable to forms of n-C5H12 and n-C6H14 having two gauche bonds were found. In the case of n-C5H12 the energy difference between the GT and TT states was found to be nearly the same as that between the GG and GT states. Bands in the region 1400–1300 cm−1 were found to be characteristic of specific conformations involving sequences of five or fewer methylenes, such as —GTTG— (1338 cm−1), —GTG (1368 and 1308 cm−1), —GG— (1352 cm−1), and terminal —TG groups (1344 cm−1). All these bands together with two broader ones centered near 1270 and 1080 cm−1 owe their intensity to the wagging of methylenes adjoining gauche bonds. An interpretation of the general features of the C–C-stretching, methylene-rocking, and methylene-scissoring regions is given. Bands associated with molecules or chains having trans sequences involving at least four methylene groups are found. In the region 1300–1150 cm−1 there are chain-length-dependent band progressions resembling those observed for the crystalline n-paraffins. These indicate the presence of molecules with gauche bonds, but these gauche bonds are few in number and are located near the ends of the chains. It is shown that for certain kinds of vibrations, particularly totally symmetric C–C stretching and ∠CCC bending, there is very little change in frequency in going from a fully extended chain to one having one or even two or more gauche bonds. Hence, it is very difficult in the case of the longer n-paraffins to distinguish spectroscopically between fully extended and almost fully extended conformations.