Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine+–(N2)n clusters (n ≤ 6)

Abstract

Size-selected clusters of the tryptamine cation with N₂ ligands, TRA⁺–(N₂)_n with n = 1–6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the ωB97X-D/cc-pVTZ level to characterize the microsolvation of this prototypical aromatic ethylamino neurotransmitter radical cation in a nonpolar solvent. Two types of structural isomers exhibiting different interaction motifs are identified for the TRA⁺–N₂ dimer, namely the TRA⁺–N₂(H) global minimum, in which N₂ forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA⁺–N₂(π) local minima, in which N₂ binds to the aromatic π electron system of the indolic pyrrole ring. The IRPD spectrum of TRA⁺–(N₂)₂ is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA⁺–(N₂)₂(2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N₂ ligands are attached to the indolic NH group in the aromatic plane via H-bonding and charge–quadrupole interactions. The second isomer, denoted as TRA⁺–(N₂)₂(H/π), has a single and nearly linear H-bond of the first N₂ ligand to the indolic NH group, whereas the second ligand is π-bonded to the pyrrole ring. The natural bond orbital analysis of TRA⁺–(N₂)₂ reveals that the total stability of these types of clusters is not only controlled by the local H-bond strengths between the indolic NH group and the N₂ ligands but also by a subtle balance between various contributing intermolecular interactions, including local H-bonds, charge–quadrupole and induction interactions, dispersion, and exchange repulsion. The systematic spectral shifts as a function of cluster size suggest that the larger TRA⁺–(N₂)_n clusters with n = 3–6 are composed of the strongly bound TRA⁺–(N₂)₂(2H) core ion to which further N₂ ligands are weakly attached to either the π electron system or the indolic NH proton by stacking and charge–quadrupole forces.