P-FLUOROTOLUENE .1. METHYL (CH3 AND CD3) INTERNAL-ROTATION IN THE S1 AND S0 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 less-than-or-equal-to m less-than-or-equal-to 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 DELTAE3, the observed splitting of the 3a1" and 3a2" rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6 = -2-Delta-E3 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 flurescence 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 greater-than-or-eqaul-to 5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations.