title: 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.
authors.affiliation: Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan. sekiya@chem.kyushu-univ.jp.
authors.forename: Kenji
authors.initials: K
authors.lastname: Sakota
authors.forename: Markus
authors.initials: M
authors.lastname: Schütz
authors.forename: Matthias
authors.initials: M
authors.lastname: Schmies
authors.forename: Raphael
authors.initials: R
authors.lastname: Moritz
authors.forename: Aude
authors.initials: A
authors.lastname: Bouchet
authors.forename: Takamasa
authors.initials: T
authors.lastname: Ikeda
authors.forename: Yuuki
authors.initials: Y
authors.lastname: Kouno
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sekiya
authors.forename: Otto
authors.initials: O
authors.lastname: Dopfer
mesh_terms: D002412:Cations / Q000737:chemistry; D019281:Dimerization; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007536:Isomerism; D008024:Ligands; D008958:Models, Molecular; D018377:Neurotransmitter Agents / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014363:Tryptamines / Q000737:chemistry*
chemical_list: D002412:Cations; D008024:Ligands; D018377:Neurotransmitter Agents; D014363:Tryptamines; C030820:tryptamine; D009584:Nitrogen