Conformational Transition of Diphenylthiourea upon Hydrogen-Bond Formation

Unraveling the fine details of hydrogen-bond structure and conformation within catalyst-substrate complexes is key to advancing organocatalysis. By employing two-dimensional infrared spectroscopy, this work provides evidence for a conformational transition of diphenylthiourea, induced by hydrogen-bond formation to 1,3-diphenyl-2-propenone in solution. Spectral shifts and coupling patterns indicate only one N–H (probed here via N–D) donor participating in hydrogen bonding. The vibrational spectra reveal a striking inhomogeneity in hydrogen-bond strength, and cross-peak analysis is consistent with single-site donor–acceptor geometry, rendering bifurcated interactions unlikely. Comparison of experimental spectral shifts upon H-bonding to harmonic frequencies from density functional theory calculations suggests a transition of diphenylthiourea from the cis–trans to the trans–trans conformation upon H-bond formation. These results highlight the interplay between molecular geometry and vibrational coupling, thereby helping to resolve the molecular structure of reactive intermediates in thiourea catalysis.