ROTATIONALLY RESOLVED FLUORESCENCE EXCITATION-SPECTRA OF PHENOL AND 4-ETHYLPHENOL IN A SUPERSONIC JET

The rotationally resolved fluorescence excitation spectra of the electronic origin transitions of phenol and 4-ethylphenol in a supersonic jet have been measured. Analysis of the rotational structure allowed the determination of the rotational constants of the ground and excited electronic states and the measurement of the direction of the transition moment with respect to the inertial axes. From the measured rotational constants possible geometries of the ground and excited states were determined. The geometry of phenol exhibits an appreciable quinoidal character following electronic excitation. The contraction of the carbon-oxygen bond upon electronic excitation supports the theory that the interaction between the lone-pair electrons of the oxygen atom and the π-electrons of the benzene ring is greater in the excited state than in the ground state. The geometrical computations of the molecular conformer structure of 4-ethylphenol confirm our previous assignment and indicate that 4-alkyl-substitution causes an additional shortening of the molecule along the A inertial axis. These experiments provide the groundwork for future high-resolution experiments on para-substituted phenols including the amino acid tyrosine. © 1992.