Raman Spectroscopy Study of Structurally Uniform Hydrogenated Oligomers of α-Olefins

Abstract

The expansion of the range of physico-chemical methods in the study of industrially significant α-olefin oligomers and polymers is of particular interest. In our article, we present a comparative Raman study of structurally uniform hydrogenated dimers, trimers, tetramers, and pentamers of 1-hexene and 1-octene, that are attractive as bases for freeze-resistant engine oils and lubricants. We found out that the joint monitoring of the disorder longitudinal acoustic mode (D-LAM) and symmetric C–C stretching modes allows the quantitative characterization of the number and length of alkyl chains (i.e., two structural characteristics), upon which the pour point and viscosity of the hydrocarbons depend, and to distinguish these compounds from both each other and linear alkanes. We demonstrated that the ratio of the contents of CH2 and CH3 groups in these hydrocarbons can be determined by using the intensities of the bands in the spectra, related to the asymmetric stretching vibrations of these groups. The density functional theory (DFT) calculations were applied to reveal the relations between the wavenumber and bandshape of the symmetric C–C stretching mode and a conformation arrangement of the 1-hexene and 1-octene dimers. We found that the branched double-chain conformation results in the splitting of the C–C mode into two components with the wavenumbers, which can be used as a measure of the length of branches. This conformation is preferable to the extended-chain conformation for hydrogenated 1-hexene and 1-octene dimers.

Using Raman Spectroscopy and Molecular Dynamics to Study Conformation Changes of Sodium Lauryl Ether Sulfate Molecules

“…Assigning peaks within this region has historically been controversial, with different authors assigning peaks as originating from different vibrational modes. It is generally agreed that a mode at around 2850 cm –1 originates from ν s (CH 2 ) vibrations and the mode at 2960 cm –1 from ν as (CH 3 ) vibrations. ,− A peak also usually manifests at around 2880 cm –1 , which across the literature has been assigned differently. One common assignment attributes this peak to ν s (CH 3 ) ,− vibrations, while others categorize it as having ν as (CH 2 ) origin. − DFT calculations , support the assignment of ν as (CH 2 ), and therefore, this is the choice we make in this work.…”

“…It is generally agreed that a mode at around 2850 cm –1 originates from ν s (CH 2 ) vibrations and the mode at 2960 cm –1 from ν as (CH 3 ) vibrations. ,− A peak also usually manifests at around 2880 cm –1 , which across the literature has been assigned differently. One common assignment attributes this peak to ν s (CH 3 ) ,− vibrations, while others categorize it as having ν as (CH 2 ) origin. − DFT calculations , support the assignment of ν as (CH 2 ), and therefore, this is the choice we make in this work. We follow the same general peak assignment as Shemouratov et al, assigning modes at around 2850 and 2880 cm –1 to the symmetric and antisymmetric stretching vibrations of the CH 2 group in the trans -conformers.…”

Exploring the Potential of Raman Spectroscopy for Characterizing Olefins in Olefin-Containing Streams

“…Based on our current knowledge, there have been limited studies published on the use of Raman spectroscopy to analyze olefins in different materials. Most of these studies focus on using Raman spectroscopy to monitor the phase and conformational composition of various polyolefin oligomers. − However, there is only one paper that specifically investigates the application of Raman spectroscopy to hydrocarbon streams that could potentially be used as transportation fuels. In this particular study conducted by Kuptsov et al, Raman spectroscopy was used to comprehensively analyze Fischer–Tropsch streams containing olefins.…”