Disentangling the Complex Vibrational Spectra of Hydrogen-Bonded Clusters of 2-Pyridone with Ab Initio Structural Search and Anharmonic Analysis

Abstract

Vibrational spectra of 1:1 clusters of 2-pyridone (2PY) with water, ammonia, and other hydrogen bond-forming molecules have been measured by several experimental groups over the past two decades. Complex vibrational signatures associated with the N–H stretching fundamental at 3 μm are often observed. Several anharmonic coupling schemes have been proposed; however, the origin of these commonly seen complex features remains unclear. In this work, we present our theoretical analysis on the structure and vibrational spectra of these clusters using ab initio random search and ab initio anharmonic algorithms, respectively. Low-energy conformers were found to be hydrogen-bonded clusters and their vibrational spectra at 3 μm were simulated with ab initio anharmonic algorithms. We demonstrate that simple anharmonic mechanisms of Fermi resonance (FR), coupling between NH stretching modes, and overtone/combinations of skeleton modes of 2PY can lead to the complex vibrational signatures observed experimentally. Since this vibrational coupling scheme is inherent to the cis-amides with adjacent N–H and CO groups when a hydrogen bond is formed with 2PY as the donor and acceptor, we believe that such a phenomenon is general to other hydrogen-bonding systems with the same functional group.

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“…However, these signals are detected when replacing CF3CO2D with DMSO-d6, as was shown for compound 11a. This is due to the pyridone-pyridol tautomeric equilibrium [43][44][45] between the 11 A and 11 B structures (Scheme 2), which depends on the pH environment, the solvent nature, and other factors. 46 A similar behavior for 2-pyridones has been reported by us earlier.…”