p-fluorotoluene. I. Methyl (CH3 and CD3) internal rotation in the S1 and S states

Supersonic jet S1-S0 spectroscopy (resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the S1 and S0 internal rotation energy level structure of p-fluorotoluene with a CD3 methyl rotor as well as to extend observations of the CH3 rotor structure. The observed rotor energy levels 2≤m≤8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of ΔE3, the observed splitting of the 3a″1 and 3a″2 rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6=−2ΔE3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and S1-S0 fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm−1 for the S0 state are B=5.46 (2.82) and V6=−4.77 (−4.77) for CH3 (CD3) rotors. The S1 values are B=4.90 (2.54) and V6=−33.0 (−25.2). The 20% barrier height reduction occurring on transformation from a CH3 to a CD3 rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the S1-S0 rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy-level perturbations often occur for states with m≥5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations.