Fluorescence excitation spectra of jet-cooled 9-hydroxyphenalenone derivatives. Effects of symmetrical substitution on deuterium atom tunneling

The fluorescence excitation spectra of symmetrically substituted 5-bromo-9-hydroxyphenalenone-d, 5-chloro-9-hydroxyphenalenone-d, and 5-methyl-9-hydroxyphenalenone-d have been measured in helium jets, The 0(0)(0) tunneling doublet splittings of the three molecules have been determined, A comparison of these splittings with that of 9-hydroxyphenalenone-d suggests that electron releasing from the Br or Cl atom to the phenalenone ring significantly decreases the potential energy barrier to tunneling in the S-1 state.