Solvent-Induced Frequency Shifts of 5-Hydroxymethylfurfural Deduced via Infrared Spectroscopy and ab Initio Calculations

Solvent-induced frequency shifts (SIFS) of the carbonyl stretching vibration ν(C═O) of 5-hydroxymethylfurfural were measured in protic, polar aprotic, and nonpolar solvents. The Gutmann acceptor number (AN) was found to correlate with the measured frequency shifts. The SIFS in six solvents were 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) of solvation predicted that ν(C═O) shifted according with the dielectric constant as (e – 1)/(2e + 1), in agreement with the analytical predictions of the Kirkwood–Bauer–Magat (KBM) theory for a dipole in a dielectric continuum, but in disagreement with the experimental trend. The experimental SIFS were best predicted using gas-phase complexes of HMF and explicit solvent–ligand. Natural bond orbital (NBO) analysis and Bader’s atoms in molecules theory were used to investigate the electronic structure of the...