IR and Raman spectra of Glycerol: a computational study

Deep Eutectic Solvents (DESs) are an emerging class of low-cost solvents based on a halide salt (organic or inorganic) and an alcohol/organic acid acting as hydrogen bond acceptor (HBA) and as hydrogen bond donor (HBD) respectively [1]. When their applicability as efficient and more sustainable electrolyte in electrochemical energy storage systems is concerned, DESs obtained by combining glycerol (Gly) and NaCl as respectively HBD and HBA should be considered [2,3]. To better characterize such system, NaCl/Gly mixtures (liquid state) at variable NaCl:Gly ratio were deeply investigated in the past by Raman spectroscopy, showing then a peculiar behavior of some Gly vibrational features when DES composition is reached [4]. The Raman spectrum of liquid Gly presents a very complex structure to which different Gly conformations, together with the inter-molecular hydrogen bond network, contribute. To disclose in part such complexity, we adopt a combined computational approach in which conformational search has been performed. We start our investigation from Gly molecule in the vapor phase which the experimental IR spectrum recorded at 498 K is available [5]. The conformational search has been performed by adopting the Global Optimization Algorithm (GOAT) [6] as developed in the ORCA (release 6.0.1) code [7] coupled to the GFN2-xTB method [8]. The obtained conformers have then been fully re-optimized at r2SCAN-3c level [9] and both IR and Raman spectra have been computed for each conformer (IRconf). The final 498 K IRtotal spectrum (see Figure 1, red line), obtained as a combination of IRconf each weighted for the relative abundance (the Gibbs Free Energy at the given temperature T has been used) of the associated conformer, showed a quite nice correspondence with the experimental one.