Raman spectra of n-pentane, n-hexane, and n-octadecane: experimental and density functional theory (DFT) study

Abstract

We carried out a density functional theory (DFT) modeling of the stable conformations of n-pentane (C 5 H 12 ), n-hexane (C 6 H 14 ), and n-octadecane (C 18 H 38 ) as well as their Raman spectra. The functional/basis combination OLYP/4z provides the best agreement between the calculated and experimental characteristics (the angles and bond lengths, Raman shifts, depolarization ratios, and relative Raman intensities) among the twelve approximations considered here.Using the DFT calculations, we obtained the Raman spectra of individual conformations of n-pentane and n-hexane molecules. We modeled the n-pentane and n-hexane Raman spectra by summing the spectra of individual conformations, taking into account the conformation energies and the Boltzmann distribution. Such spectra turned out to describe the experimental spectra quite well. This fact indirectly confirms that the conformational composition of n-pentane and n-hexane molecules is well described by the Boltzmann distribution.The calculated contents of n-pentane and n-hexane molecules in the all-trans-conformation were in a good agreement with the experimental values calculated by using the integral intensities of the Raman lines. We specified the experimental Raman bands belonging to the symmetric C-C stretching mode of the n-alkane molecules in accordance to our DFT calculations. In the cases of both n-pentane and n-hexane, mostly the vibrations of molecules in the all-trans-conformation contribute to this band intensity, while the contributions of molecule vibrations in all other conformations are negligible. We also defined the assignment of the Raman bands in the range 700-900 cm −1 to the vibrations of molecules in different conformations.

Vibrational study of lead bromide perovskite materials with variable cations based on Raman spectroscopy and density functional theory

“…We have performed a band‐profile analysis using a traditional deconvolution process over the whole range; details have been described earlier. [ 53 ] The experimental data have been analysed using pseudo‐Voigt functions, where y 0 is an offset, μ is the profile shape factor, w the full width at half maximum (FWHM), and xc the line position. Varying the parameters, we have obtained quasi‐Lorentzian profiles for all bands.…”

“…We have performed a band-profile analysis using a traditional deconvolution process over the whole range; details have been described earlier. [53] The experimental data have been analysed using pseudo-Voigt functions,…”

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

“…The wavenumbers and FWHMs for the dimers were determined by the spectrum deconvolution using two lines and nonlinear (second-order polynomial) background. The profile of each line was described as a weighted sum of Gaussian and Lorentzian functions [ 25 ]. For other oligomers, the wavenumber values, obtained using the spectrum deconvolution, demonstrated a poor accuracy (about 5 cm −1 ).…”

“…In the Raman spectra of liquid linear alkanes, the C–C mode related to the vibrations in the trans -sequences [ 17 , 19 , 25 , 31 , 32 ] demonstrates narrow and symmetric band with FWHM = 7–13 cm −1 ( Table 1 , Figure 5 ). The observed wavenumbers for the band follow the theoretical law (2) at ν ∞ = 1130.5 cm −1 and β = 313 cm −1 ( Figure 6 ).…”

“…The features of the DFT calculations of molecular geometry and Raman spectra, including justification of the choice of the functional and basis, were described in detail in our recent DFT study of Raman spectra of linear alkanes (pentane, hexane, and octadecane) [ 25 ], presented in the Supporting Information (SI) . The calculations employed the OLYP functional [ 26 ], the Gaussian orbitals 4z basis ( Table S1 in the SI ), and the PRIRODA software (M.V.…”