Vacuum Ultraviolet Photoionization Induced Proton Migration and Formation of a New C–N Bond in Pyridine Clusters Revealed by Infrared Spectroscopy and Mass Spectrometry

Abstract

The structures and reactions of pyridine (Pyd) cluster cations in a supersonic molecular beam generated upon photoionization at 9.2–9.4 eV were investigated by infrared (IR) action spectroscopy. The mass spectrum showed prominent peaks of (Pyd) m + and H+(Pyd) m , m = 1–5. In the pyridine/pyridine-d 5 mixture, the mass pattern indicated that H+ and D+ migrated during the formation and dissociation of the cluster cations. The IR photodissociation spectra of both (Pyd)2 + and H+(Pyd)2 revealed a N–H stretching band near 3400 cm–1, indicating that their structures are 1-(2-pyridyl)­pyridin-1-ium and pyridinium–pyridine, respectively. Observation of the former product implies that the reaction proceeds via an α-distonic cation intermediate, while the latter product is formed via proton migration. The IR spectra of (Pyd) m + and H+(Pyd) m , m ≥ 3, suggested that these clusters consist of a covalently bound (Pyd)2 + or H+(Pyd)2 core, respectively, with additional pyridines attached to them via hydrogen bonds and/or weak dispersive interactions.

Structures of Pyridine–Water Clusters Studied with Infrared–Vacuum Ultraviolet Spectroscopy

“…in our previous paper. 16 Therefore, we are not able to distinguish which parent species, (Pyd) m (H 2 O) n + or (Pyd) m + , the H + (Pyd) m−1 fragment is generated from under the present condition.…”

“…+ has either a hemibonded structure or a covalently bonded structure. 16 It is clear that the (Pyd) 2…”

“…. 16 In the present study, we report an IR−VUV spectroscopic investigation of (Pyd) m (H 2 O) n clusters. Among the many chemical properties of pyridine, the intermolecular interaction with protic solvents, especially with water, is of special interest.…”

Neutral Dissociation of Pyridine Evoked by Irradiation of Ionized Atomic and Molecular Hydrogen Beams

“…However, calculations concerning interactions of pyridine with various hydrides suggested that interactions in the σ N -type complexes are predominantly electrostatic, while the dispersion connects the π-type clusters [ 119 ]. Moreover, ab initio calculations identified propeller-shaped isomers of (Benzene·Pyridine) + covalently bonded via a C–N bond [ 120 ], which has been recently observed experimentally in pyridine clusters [ 121 ]. These works show that pyridine can create complexes, which would probably also be formed during present collisions.…”

“…The protonated pyridine (C 5 H 5 NH + ) where the protonation site is at the nitrogen atom has been observed employing cold ion infrared action spectroscopy [ 122 ]. A high proton affinity (9.7 eV) of pyridine [ 121 ] may be a factor in forming such a complex. However, thus far, there has been no direct spectroscopic evidence confirming the occurrence of the complexes in the collisions with H + projectiles [ 26 , 27 , 28 , 30 ].…”