Raman noncoincidence effect on OH stretching profiles in liquid alcohols

Abstract

A detailed analysis of the temperature evolution of polarized and depolarized Raman spectra in the OH stretching region of pure liquid alcohols has been performed in order to gain more insights on the nature of the noncoincidence effect (NCE) in inhomogeneously broadened OH profiles. For 1-octanol temperature-induced difference spectra, wavenumber-dependent depolarization ratios and H-bond energy dispersion have been evaluated. The results allow for a consistent interpretation of the overall NCE in terms of distinct spectral components. In practice, it results that different classes of oscillators can be singularly characterized by specific NCEs. The major contribution to the overall NCE is experimentally separated and arises from OH oscillators embedded within the H-bonded chain structures characterized by an average H-bonding enthalpy of ca 20 kJ/mol. The study has been extended to Raman OH profiles of liquid ethanol, 2-octanol and tert-butanol to understand the effects of the hydrophilic and hydrophobic molecular moieties in determining the strength on the vibrational coupling, responsible for the resonant transfer of vibrational energy (RET), in these liquid systems.

Liquid Mixtures Involving Hydrogenated and Fluorinated Alcohols: Thermodynamics, Spectroscopy, and Simulation

“…In the spectra of pure alcohols, the broadness of the OH stretching band (νOH, with maximum in the range of 3330− 3370 cm −1 ) is indicative of a diversity of H-bonds: 41 intermolecular in all of the alcohols and possibly also intramolecular in the case of fluorinated ones, where they can be established between the OH and CF 3 groups of gauche conformers, although weaker. 42 The band maximum appears at lower wavenumbers for hydrogenated alcohols when compared to the corresponding fluorinated ones (∼20 cm −1 for EtOH vs TFE and ∼30 cm −1 for BuOH vs HFB), which suggests that the dominant interactions involving the OH groups are stronger in the former.…”

The temperature dependent structure of liquid 1-propanol as studied by neutron diffraction and EPSR simulations

“…While H-bonds are playing important roles in a wide variety of materials based on complex chemistries, such as biological macromolecules or supramolecular systems, the much a) Electronic mail: per.sillren@chalmers.se simpler mono-hydroxy alcohols provide excellent model systems since the length of the alkyl part of the molecule and thus the H-bond density can be systematically controlled. The structure of alcohols has been investigated by experimental techniques such as X-ray and neutron diffraction, [5][6][7][8][9][10] vibrational spectroscopy, [11][12][13] and NMR. 14,15 In fact, one of the first X-ray diffraction studies on liquids was performed on mono-hydroxy alcohols.…”

“…The structures proposed in the literature range from ring structures, with a fixed number of 4-8 molecules in each cluster, [21][22][23][24][25] to more elongated, sometimes branched, chaintype structures. 6,8,11,12,14,19,20,[26][27][28][29][30][31][32][33][34][35][36][37][38] The lack of a widely accepted structural model for mono-hydroxy alcohols clearly motivates further investigations.…”

A statistical model of hydrogen bond networks in liquid alcohols

“…The structure of alcohols has been studied by experimental techniques, such as vibrational spectroscopy, [8][9][10][11][12] neutron-, 5,13 and x-ray diffraction, 6,7,14 as well as computational methods, 4,15 but the lack of an established quantitative model has made it difficult to reconcile the different results and to connect the structures to thermodynamics and dynamics. A family of models that has been used to describe the intermolecular structure of water, as well as other networkforming systems, are the "patchy particle" models, 16 wherein typically hard spherical particles are decorated with attractive patches of one or several kinds.…”