Tunneling splittings in the S1 electronic states of symmetrically substituted 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone

The S1–S0 electronic spectra of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone have been measured in a supersonic free jet to investigate the effect on proton tunneling of symmetrical substitution of Cl or Br atoms for H atoms of tropolone. The 000 tunneling splittings in S1 of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone are 45, 31, and ≲1 cm−1, respectively. The increase of the tunneling splitting for 3,7-dichlorotropolone compared to that for tropolone (19 cm−1) is attributed to a decrease in the O–H...O heavy atom separation, leading to smaller potential barrier height. The decrease of the 000 tunneling splitting for 3,5,7-trichlorotropolone compared to 3,7-dichlorotropolone and remarkable decrease of the 000 tunneling splitting for 3,7-dibromotropolone are ascribed to significant changes of the normal coordinates compared to those of tropolone.