(600s) Ab Initio Study of Solvent-Induced Frequency Shifts of 5-Hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) is a promising platform-chemical for sustainable fuels and plastics. A major issue in the production of HMF is the side reactions that lead to significant loss of sugars and high cost. Use of polar aprotic solvents improves HMF yields from sugar dehydration by preventing HMF side reactions, but the influence of solvent on the structure of HMF is not fundamentally understood. In an effort to understand solvent effects on HMF stability and yield, solvent-induced frequency shifts (SIFS) of the carbonyl stretching vibration ν(C=O) of 5-hydroxymethylfurfural were measured in seven protic, polar aprotic, and non-polar solvents. The Gutmann Acceptor Number (AN), an empirical measure of Lewis acidity, was found to correlate with the measured frequency shifts. The SIFS were then investigated using ab initio electronic structure calculations, treating the solvent implicitly and with an explicit solvent ligand interacting with the carbonyl. The conductor polarizable continuum model (CPCM) solvation model predicted that ν(C=O) shifted as a function of the dielectric constant, in agreement with the analytical predictions of the Kirkwood-Bauer-Magat (KBM) theory for a dipole in a dielectric continuum, but in disagreement with experimental ν(C=O). Experimental ν(C=O) were best predicted using gas-phase complexes of HMF and explicit solvent ligand. NBO analysis and Bader’s Atoms in Molecules theory were used to investigate the electronic structure and hydrogen bonding in these complexes. SIFS arose from H-bonding interactions, as observed in delocalization of carbonyl lone-pair electrons to H-bonding solvent σ*(X-H) orbitals, and in increased charge density and decrease in local potential energy at the H-bond (3,-1) critical point in the charge density. Consequently, by predicting the experimental SIFS and examining the electronic structure, we find the first theoretical evidence for treating Gutmann’s solvent AN as a measure of solvent Lewis acidity.