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

Abstract

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 1-alkanol 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 1-alkanols 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 (ν) 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:ν=5590n+12+198cm-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 (νC–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:νC-C=1134.7-103n+1cm-1.

Keywords

• Raman spectroscopy
• Longitudinal acoustic mode
• Disorder longitudinal acoustic mode
• C–C mode
• Length-dependence
• DFT