Two-dimensional vibrational spectroscopy of the amide I band of crystalline acetanilide: Fermi resonance, conformational substates, or vibrational self-trapping?

摘要

Two-dimensional infrared (2D-IR) spectroscopy) is applied to investigate acetanilide, a molecular crystal consisting of quasi-one-dimensional hydrogen bonded eptide units. The amide-I band exhibits a double peak structure, which has been attributed to different mechanisms including vibrational self-trapping, a Fermi resonance, or the existence of two conformational substates The 2D-IR spectrum of crystalline acetanilide is compared with that of two different molecular systems: (i) benzoylchloride, which exhibits a strong symmetric Fermi resonance and (ii) N-methylacetamide dissolved in methanol which occurs in two spectroscopically distinguishable conformations. Both 2D-IR spectra differ significantly from that of crystalline actanilide, proving that these two alternative mechanisms cannot account for the anomalous spectroscopy of crystalline acetamilide. On the other hand, vibrational self-trapping of the amide-I band can naturally explain the 2D-IR response.