Endo or Exo ? Structures of Gas‐Phase Alkali Metal Cation/Aromatic Half‐Belt Complexes

1,1,9,9‐Tetramethyl[9](2,11)teropyrenophane (TM9TP), a belt‐shaped molecule, has a sizable cavity that molecules or ions could occupy. In this study, the question of whether TM9TP forms gas‐phase ion‐molecule complexes with metal cations (K + , Rb + , Cs + ) situated inside or outside the TM9TP cavity was addressed using both experimental and computational methods. Complexes were trapped in a Fourier transform ion cyclotron resonance mass spectrometer and their structures were explored by some novel physical chemistry/mass spectrometry methods. Blackbody infrared radiative dissociation kinetics reveal two populations of ions, a fast dissociating fraction and a persistent fraction. Infrared multiphoton dissociation spectra (vibrational spectra) provide very strong evidence that the most abundant population is a complex where the metal cation is inside the TM9TP cavity, endo ‐TM9TP. Red‐shifted C−H stretching bands present in the gas‐phase vibrational spectra of these ionic complexes show that there is an interaction between the metal cation and bridge C−H bonds due to the cation sitting inside the cavity of TM9TP. B3LYP/6‐31+G(d,p) calculations showed the endo complexes to be the lowest in energy; about 60 kJ mol −1 more thermodynamically stable and more than 120 kJ mol −1 kinetically more stable than the exo complex.