title: Hydrogen-bonded structures for self-aggregates of 2,5-dimethylpyrrole and its binary clusters with pyrrole studied by IR cavity ringdown spectroscopy.
abstract: N-H···π hydrogen-bonded (H-bonded) structures were studied by applying vibrational spectroscopy to self-aggregate clusters of 2,5-dimethylpyrrole (DMPy) and its binary clusters with pyrrole (Py). The NH stretching vibrations of jet-cooled clusters were observed by IR cavity ringdown spectroscopy. A combination of experiments and density functional theory calculations revealed the stable structures, intermolecular binding energies, and harmonic vibrational frequencies. The IR spectrum of the DMPy self-aggregate clusters was very similar in spectral features to that of the Py clusters in a previous work. The observed NH stretching vibrations at 3505, 3420, 3371, and 3353 cm(-1) are simultaneously red-shifted by ∼25 cm(-1) from the Py monomer, dimer, trimer, and tetramer, respectively. Based on a spectral analogy of DMPy with Py, and a consistency of the calculated harmonic frequencies with experiments, the H-bonded structures of the DMPy clusters were determined to be of a T-shape for a dimer and a cyclic for a trimer and a tetramer. For the DMPy-Py binary clusters, we discussed the stability and geometry of the N-H···π interactions in the T-shaped dimer and the cyclic trimer. The binary dimer showed the only single NH stretch at 3419 cm(-1) in the IR spectrum. A vibrational analysis of the H-bonded NH stretches as well as the calculated stabilization energies deduced that only the binary dimer by DMPy as an acceptor and Py as a donor can exist in a supersonic jet. For binary trimers, NH stretches were observed due to both (DMPy)(2)-(Py)(1) and (DMPy)(1)-(Py)(2). They were found to have different vibrational patterns from each other; the former showed three dispersed NH stretches, and the other had two quasi-degenerate NH stretches. Throughout this study, we also considered the intermolecular geometries, such as the H-bond distance and the angle in terms of the methyl group substitution effect.
authors.affiliation: Department of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori, Hyogo 678-1297, Japan. matsu@sci.u-hyogo.ac.jp
authors.forename: Yoshiteru
authors.initials: Y
authors.lastname: Matsumoto
authors.forename: Kenji
authors.initials: K
authors.lastname: Honma
mesh_terms: D019281:Dimerization; D006860:Hydrogen Bonding; D011758:Pyrroles / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D011758:Pyrroles; C067286:2,5-dimethylpyrrole