Paraffins vibrational assignments

Snyder, R.G. and Schachtschneider, J.H., Vibrational analysis of the n-paraffins. i. Assignments of infrared bands in the spectra of C3H8 through n-Ci9H4o,... 

Using assumed molecular models and force constants based on the force constants derived from the paraffin series, normal co-ordinate calculations for the simple alkylcarbonium ions were carried out. These calculations were made in order to predict the vibrational spectra. Comparison with the experimentally obtained infra-red spectra show that the main observed features can indeed be reasonably explained in terms of the modes calculated for the planar models of the ions and allowed an assignment of the fundamentals (Table 11). 

The spectra of normal alkanes (paraffins) can be interpreted in terms of four vibrations, namely, the stretching and bending of C—H and C—C bonds. Detailed analysis of the spectra of the lower members of the alkane series has made detailed assignments of the spectral positions of specific vibrational modes possible. 

Not all of the possible absorption frequencies of the paraffin molecule are of equal value in the assignment of structure. The C—C bending vibrations occur at very low frequencies (below 500 cm-1) and therefore do not appear in our spectra. The bands assigned to C—C stretching vibrations are weak and appear in the broad region of 1200-800 cm-1 they are generally of little value for identification. 

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

The infrared active v (CH2), v (CH2), 8 (CH2), and yr (CH2) fundamentals can be readily assigned as a result of the extensive spectroscopic studies on hydrocarbons which have been undertaken [Sheppard and Simpson (795)]. In addition, because of the polarized radiation studies on single crystals of normal paraffins [Krimm (95)], it is possible to assign uniquely the components of the doublets found in the spectrum for these bands to symmetry species. Similarly, the Raman active va(CH. ), vs(CH2), (CHg), v+ (0), and v+ (n) fundamentals can be unambiguously assigned, the latter two on the basis of normal vibration calculations... 

Rasmussen, R. S. Vibrational frequency assignments for paraffin hydrocarbons infrared absorption spectra of the butanes and pentanes. J. chem. Phys. 16, 712—727 (1948). 

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