ELECTRONIC SUBSTITUENT AND AMINO TORSIONAL COUPLING EFFECTS ON INTRAMOLECULAR PROTON TUNNELING IN 5-AMINOTROPOLONE

Fluorescence excitation, dispersed fluorescence, and population labeling spectra of six isotopes of 5-aminotropolone (-OH/-NH2,-OH/-NHD,-OH/-ND2,-OD/NH2,-OD/-NHD, and -OD/-ND2) are reported. A primary purpose of this study is to investigate the effect of the strong electron donation of the amino group on the proton tunneling. Single vibronic level tunneling splittings in the S1 <-- S0 transition of the supersonic-jet cooled molecules are used to probe these effects. Amino substitution produces a modest increase in the origin tunneling splitting but has a much more dramatic effect on the 26n0 progression involving a low-frequency out-of-plane vibration of the molecule. This progression, which dominates the spectrum of tropolone itself, is much reduced in intensity in 5-aminotropolone. At the same time, the excited-state frequency Of nu26 is increased by almost a factor of 2 relative to tropolone. Both features point to a stiffening of the pseudoaromatic ring induced by the amino group. Accompanying these changes is a much reduced sensitivity of the tunneling splitting to Q26 excitation. D2O concentration studies and population labeling spectra show unusual secondary isotope effects in -OH/-NHD and -OD/-NHD which produce a 7.8-cm-1 asymmetry splitting in the ground state, large enough to effectively quench proton tunneling. A model two-dimensional (2D) potential along proton tunneling and amino torsional coordinates is presented which accounts for the observations. In the ground state where the amino group is nonplanar, intramolecular proton tunneling between equivalent minima must be accompanied by internal rotation of the amino group through 180-degrees, efficiently coupling the motions. By contrast, in the excited state, -NHD isotopic substitution has no effect on the tunneling splittings. It is postulated that the lack of quenching in the excited state is a consequence of the planarity of the -NHD group which removes the major sources of asymmetry in the minima of the 2D torsion/tunneling potential energy surface.