Аннотация:Substances, which molecules contain one or more polymethylene chains, are widely used in numerous fields of human activity, including petrochemical, pharmaceutical and food industries. Physical properties of these substances, such as viscosity, density, and dielectric constant, depend on both length and number of chains in molecule.
The band of the symmetric stretching vibrations of C-C bonds is observed in the spectral region 1100 -1160 cm-1 in Raman spectra of many substances, containing polymethylene chains. The wavenumber and intensity of this band depend not only on molecule length, but also on the chain conformation. At present, it is established that for normal alkanes this band corresponds to the most probable all trans-conformation, and its wavenumber directly relates to the molecule length. However, this dependence is still unknown for other substances with polymethylene chains.
In our contribution, we present experimental study in conjunction with DFT calculations of a behavior of this mode in Raman spectra of a wide collection of substances with polymethylene chains. We determined the dependence of the symmetric C-C stretching mode wavenumber on the chain length for 1-alkanols and fatty acids. For alkylammonium salts and branched alkanes we observed splitting of the C-C mode due to the presence of several polymethylene chains with various lengths. We showed that structure of terminal groups, chain branching, presence of strong intermolecular interactions dramatically influence the dependence of the symmetric C-C stretching mode wavenumber on the chain length. However, for all the substances under study the wavenumber of the C-C stretching mode is a universal characteristic of length of polymethylene chains.
We are grateful to the Joint Supercomputer Center of the RAS for the possibility of using their computational resources for our calculations.
