IR Cavity Ringdown Spectroscopy and Density Functional Theory for Jet-Cooled Pyrrole-Cyclopentanone Binary Clusters: Effect of Pseudorotation on N-H···O═C Hydrogen Bonds.

The geometry and energetics of the N-H···O═C hydrogen bond (H-bond) are important to understand the stability and flexibility of biomolecules, such as protein and DNA. Jet-cooled pyrrole-cyclopentanone (Py-Cp) binary clusters are appropriate models to investigate the N-H···O═C H-bond from a microscopic point of view. In this study, NH stretching vibrations of the Py-Cp binary clusters were observed by IR cavity ringdown spectroscopy. Furthermore, density functional theory calculations revealed geometric structures, harmonic vibrations, intermolecular energies, and donor-acceptor interactions for various sizes of binary clusters. The IR spectra of the Py-Cp binary clusters were measured under various conditions of the vapor pressures of Py and Cp in He buffer gas for a supersonic expansion. The dependence of the IR band intensities on the vapor pressure provides vibrational assignments of the NH stretching vibrations, which were reproduced by calculated frequencies of Py1-Cp1, Py1-Cp2, and Py2-Cp1. An admixture of Ar in He buffer gas for a supersonic expansion was also applied to produce Py1-Cp2 in order to differentiate several NH stretches of isomeric structures due to the pseudorotation of Cp molecules. Py1-Cp1 is formed by the N-H···O═C H-bond. Py1-Cp2 has a cyclic structure that is formed by the N-H···O═C H-bond and stacking interactions among Py and two Cp molecules. Py2-Cp1 also has a cyclic structure that is formed by not only the N-H···O═C H-bond but also a N-H···π H-bond between two Py molecules and a stacking interaction between Py and Cp. A comparison of the H-bond geometries between Py2-Cp1 and the corresponding pyrrole-acetone binary cluster reveals that the stacking interaction between Py and Cp strengthens the N-H···O═C H-bond through a cooperative effect.