n -pentanol at high pressures: Rotational isomerism in the liquid phase and the liquid-solid phase transition

Abstract

The vibrational spectrum of liquids constituted of chain molecules is difficult to analyze because it may have contributions of different rotational isomers. In turn, with a proper vibrational assignment, this feature allows us to extract information about the effect of temperature or pressure on the molecular conformations in the liquid state. In this regard, the information on the vibrational spectrum in the solid phase greatly simplifies the vibrational analysis of the different rotational conformers existing in the liquid, as the molecules usually present all-trans conformations in the crystalline state. Here we report room-temperature Raman experiments on n-pentanol performed in a sapphire-anvil cell up to 3 GPa. A detailed analysis of the liquid-solid phase transition occurring at 1.3 GPa is provided. The analysis of the Raman spectrum in the solid phase allows the identification of the bands due to the different rotational isomers present in the liquid. The analysis of the spectral region corresponding to skeletal vibrations of the carbon chain (800-1200 cm(-1)) indicates that gauche conformers are promoted by the application of pressure. The analysis of the intensity ratio of those bands assigned to trans and gauge conformations is used to calculate the change in molecular volume ascribed to the trans-gauge isomerization process. We find a value similar to that found in n-alkanes, i.e., -0.88 cm(3) mol(-1). In addition, we find indication that pressure varies the proportions of the different gauge conformers. Thus, it appears that the GTTt to TGTt transition in the carbon chain is favored at high pressures. As expected, a smaller change in the molecular volume accompanies this conformation change.

Identification of Alcohol Conformers by Raman Spectra in the C–H Stretching Region

“…30 The CC stretching bands in Raman spectra were applied to identify the transconformers associated with the CCCC axis. 49 Recently, the skeleton deformation and stretching bands in Raman spectra helped to identify the TTT, TTG, and TGG conformers. 50 To the best of our knowledge, the spectra of the n-pentanol conformers were ignored in the C−H stretching region.…”

“…The −OH stretching band in FTIR absorption spectra was used to distinguish the trans- and gauche- conformer associated with CO rotation in the pentanol/CCl 4 solution . The CC stretching bands in Raman spectra were applied to identify the trans- conformers associated with the CCCC axis . Recently, the skeleton deformation and stretching bands in Raman spectra helped to identify the TTT, TTG, and TGG conformers .…”

High Pressure Raman Scattering and Synchrotron X-ray Diffraction Studies of Benzyl Azide

“…Benzyl azide possesses a relatively simple molecular structure among organic azides with several different conformations, all of which are easily affected by environmental conditions, such as pressure . In addition, benzyl azide presents unique properties due to the attractive interaction between the C–H bond and π electron system (C–H···π interaction), a feature similar to hydrogen bonding. , In our previous studies on NH 4 N 3 , we found that hydrogen bonds significantly affect the behavior of the azide ions under high pressure. , Therefore, a study of benzyl azide can provide more insight into not only the formation mechanism of polymeric nitrogen but also the influence of C–H···π interactions in the process of compression and the behaviors of organic azides under high pressure.…”

Probing the balance of attraction and repulsion in binary mixtures of dimethyl sulfoxide and n-alcohols

“…Here, we focus on two characteristic peaks, assigned with reference to a number of sources: [20][21][22][23][24] the OH stretching and the CO torsion of the alcohol compounds. The frequencies of these vibrations observed in the pure liquids are summarized in Table 1.…”