Issue 10, 2004

Microsolvation of the indolecation (In+) in a nonpolar environment: IR spectra of In+–Ln complexes (L = Ar and N2, n ≤ 8)

Abstract

Infrared photodissociation (IRPD) spectra of complexes composed of the indole cation (In+ = C8H7N+) and several neutral ligands (L = Ar and N2) were recorded in the vicinity of the N–H stretch vibration (ν1) of bare In+ in its 2A″ electronic ground state. The analysis of systematic size-dependent ν1 band shifts and photofragmentation branching ratios in the spectra of In+–Arn (n ≤ 5) and In+–(N2)n (n ≤ 8) provides information about the stepwise microsolvation of In+ in a nonpolar hydrophobic environment, including the existence of structural isomers and the determination of ligand binding energies. The IR spectra of the In+–L dimers reveal two transitions, which are attributed to ν1 fundamentals of the H-bound and π-bound isomers on the basis of their complexation-induced ν1 frequency shifts, Δν1. In both cases, the H-bound isomer is found to be more stable than possible π-bound isomers. The Δν1 shifts are used to derive the first experimental estimate for the proton affinity of the indolyl radical (∼920 ± 30 kJ mol−1). The IR spectra of In+–Arn (n ≤ 5) suggest that the preferred microsolvation path for this cluster system begins with the formation of the H-bound In+–Ar dimer core, which is further solvated by (n − 1) π-bound ligands. In contrast, the spectra of In+–(N2)n with n ≤ 8 suggest that this cluster grows by the formation of an In+–(N2)2 trimer core with two H-bound N2 ligands, to which (n − 2) π-bound N2 molecules are attached. The In+–Ln complexes were generated in an electron impact (EI) ion source, which predominantly produces the most stable isomer of each cluster ion. For several In+–Ln complexes, the geometry of the most stable isomer produced in this ion source differs drastically from the structures previously observed by resonant photoionization of the corresponding neutral precursors, demonstrating the severe restriction of photoionization techniques (given by the Franck–Condon principle) for the spectroscopic characterization of cluster ions. Most of the In+–Ln complexes investigated exhibit a distinct ionization-induced change in the preferred substrate–ligand recognition pattern.

Article information

Article type
Paper
Submitted
17 Oct 2003
Accepted
03 Dec 2003
First published
14 Jan 2004

Phys. Chem. Chem. Phys., 2004,6, 2732-2741

Microsolvation of the indole cation (In+) in a nonpolar environment: IR spectra of In+–Ln complexes (L = Ar and N2, n ≤ 8)

N. Solcà and O. Dopfer, Phys. Chem. Chem. Phys., 2004, 6, 2732 DOI: 10.1039/B313110F

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