Electron states of benzene…HCl and toluene…HCl hydrogen bonded complexes by ultraviolet photoelectron spectroscopy

Electron states of the benzene...HCl complex have been studied by UV photoelectron spectroscopy. Two vertical ionization energies of the complex at 10.84 and 12.10 eV have been assigned to π C6H6 and nCl orbitals, respectively. While the π C6H6 orbital in the complex is shifted by 1.57 eV to higher binding energy, the nCl MO is shifted by 0.63 eV to lower binding energy with reference to the corresponding free benzene and HCl molecules. Ab initio MO calculations confirm these assignments. The toluene...HCl complex has also been studied. The larger shifts in the π(1.90 eV) and nCl(0.88 eV) orbital energies in the toluene ...HCl complex compared to those of the benzene…HCl complex are consistent with stronger π hydrogen bonding in the case of toluene.