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

Size-selected clusters of the tryptamine cation with N-2 ligands, TRA(+)-(N-2)(n) with n = 1-6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the omega B97X-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(+)-N-2 dimer, namely the TRA(+)-N-2(H) global minimum, in which N-2 forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA(+)-N-2(pi) local minima, in which N-2 binds to the aromatic pi electron system of the indolic pyrrole ring. The IRPD spectrum of TRA(+)-(N-2)(2) is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA(+)-(N-2)(2)(2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N-2 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(+)-(N-2)(2)(H/pi), has a single and nearly linear H-bond of the first N-2 ligand to the indolic NH group, whereas the second ligand is pi-bonded to the pyrrole ring. The natural bond orbital analysis of TRA(+)-(N-2)(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 N-2 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(+)-(N-2)(n) clusters with n = 3-6 are composed of the strongly bound TRA(+)-(N-2)(2)(2H) core ion to which further N-2 ligands are weakly attached to either the pi electron system or the indolic NH proton by stacking and charge-quadrupole forces.