| Abstract
| - Threshold collision-induced dissociation techniques are employed to determine bond dissociation energies(BDEs) of mono- and bis-complexes of alkali metal cations, Li+, Na+, K+, Rb+, and Cs+, with indole, C8H7N.The primary and lowest energy dissociation pathway in all cases is endothermic loss of an intact indoleligand. Sequential loss of a second indole ligand is observed at elevated energies for the bis-complexes.Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures,vibrational frequencies, and rotational constants of these complexes. Theoretical BDEs are determined fromsingle point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31G* geometries.The agreement between theory and experiment is very good for all complexes except Li+(C8H7N), wheretheory underestimates the strength of the binding. The trends in the BDEs of these alkali metal cation−indolecomplexes are compared with the analogous benzene and naphthalene complexes to examine the influence ofthe extended π network and heteroatom on the strength of the cation−π interaction. The Na+ and K+ bindingaffinities of benzene, phenol, and indole are also compared to those of the aromatic amino acids, phenylalanine,tyrosine, and tryptophan to elucidate the factors that contribute to the binding in complexes to the aromaticamino acids. The nature of the binding and trends in the BDEs of cation−π complexes between alkali metalcations and benzene, phenol, and indole are examined to help understand nature's preference for engagingtryptophan over phenylalanine and tyrosine in cation−π interactions in biological systems.
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