The S1-S0 fluorescence excitation spectra of 5-chlorotropolone-M1 (M = Kr, Xe, CH4) van der Waals (vdW) complexes in the region near the electronic origin have been measured in a supersonic free jet to investigate the effect of the vdW interactions on proton tunneling. Tunneling splittings have been observed in the vdW vibrations as well as in the 000
transitions of the Kr and Xe complexes. The 000 tunneling splitting of the 5-chlorotropolone-(CH4)1 complex is significantly smaller than those of the Kr and Xe complexes. It has been suggested that the vdW vibrations couple with intramolecular motions, leading to a higher potential energy barrier to tunneling in the CH4 complex. The results of the 5- chlorotropolone complexes have been compared to those of the tropolone complexes.
