Structural connectivity of 18-Crown-6/H+/L-tryptophan noncovalent complexes in gas phase and in solution: Delineating host-guest-solvent interactions in solution from gas phase structures

We investigate the structural correlation of noncovalent crown ether/H+/L-tryptophan (CR/TrpH(+)) host-guest complexes in the solution phase with those in the gas phase generated through electrospray ionization/mass spectrometry (ESI/MS) techniques. We perform quantum chemical calculations to determine their structures, relative Gibbs free energies, and infrared spectra. We compare the calculated infrared (IR) spectra with the IR multiphoton dissociation (IRMPD) spectra observed for the 18-Crown-6/TrpH(+) complex by Polfer and co-workers [J. Phys. Chem. A 2013, 117, 1181-1188] for assigning the IR bands. We observe that the carboxyl group remains "naked," lacking hydrogen bonding with the CR unit in the gas phase, and that this most stable conformer originates from the corresponding lowest Gibbs free energy structure in solution. Based on these findings, we propose that gas phase host-guest complexes directly correlate with those in solution, reinforcing the possibility of obtaining invaluable information about host-guest-solvent interactions in solution from the structure of the host-guest pair in the gas phase.