Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 16, Issue 8, Pages 3798-3806Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c3cp54127d
Keywords
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Funding
- TU Berlin
- Deutsche Forschungsgemeinschaft [DO 729/4]
- German-Israeli Foundation [G.I.F. 1164-158.5/2011]
- Ministry of Education, Sports, Science and Technology in Japan (MEXT) [23685005]
- JSPS Core-to-Core Program Photoionization-induced switch in aromatic molecule-solvent recognition''
- Cooperative Research Program Network Joint Center for Materials and Devices''
- Grants-in-Aid for Scientific Research [23685005, 13J02937] Funding Source: KAKEN
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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.
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