Raman signatures of alkyl chain length: Comparative analysis of 1-alkanols and normal alkanes

The comparative experimental and density functional theory studies of Raman spectra of 1-alkanols (CnH2n+1OH with n = 2-16) and n-alkanes (CnH2n+2 with n = 5-24) are presented. Three collective vibrations (LAM, D -LAM and symmetric C-C stretching mode), which are sensitive to the molecular chain length, were found in the Raman spectra of 1-alkanols with n >= 4. The evolutions of these modes with the change of the chain length of 1alkanol molecule were analyzed. For the first time, we assigned the Raman bands of 1-alkanols in the spectral region of 100-600 cm- 1 in terms of the LAM and D -LAM bands. The LAM band is the most intense in this region of the Raman spectra of liquid 1alkanols with n = 4-6. However, it is masked by D -LAM band, which becomes the most intense for liquid 1-alkanols with n >= 7. We found that the following relation between the observed wavenumber (v) of the D -LAM band and the number of atoms in the molecule backbone, i.e. the chain length of the molecules, can be used to measure the chain length for the 1-alkanols in a liquid phase with n >= 7: v = (n + 1)2 5590 + 198 [cm -1]. In the case of liquid 1-alkanols with n = 4-6, the same dependence can be applied. However, it is important to keep in mind another assignment of the most intense band in the region of 100-600 cm- 1 in this case: LAM instead of D -LAM. For the first time, we revealed that measurement of the wavenumber (vC-C) of the symmetric C-C stretching mode, localized in the spectral region of 1100-1130 cm - 1, allows one to evaluate the chain length of the molecules of liquid and solid 1-alkanols (with n <= 16) by using the following relation: vC-C = 1134.7 103 [cm- 1]. n + 1