How Large Are Low-Lying Molecular Rydberg States? Comparisons Between Experiment and Theory

The "sizes" of low-lying molecular Rydberg states, estimated experimentally from the results of pressure perturbation spectroscopy, are compared with the results of ab initio molecular orbital calculations. Lennard-Jones 6-12 parameters associated with absorber-perturber pairs were assigned to the ground and excited states on the basis of pressure perturbation spectroscopy. These data are reported for the lowest Rydberg transitions of NH 3 , acetone, and CH 3 I, as well as for the first two transitions of 1-azabicyclo[2.2.2]octane (ABCO). Increases in the sizes of these excited states relative to the respective ground states were compared with the results of ab initio calculations of the root-mean-square values of the electronic displacement, RMS R . Excited state calculations using configuration interaction (singles) with the 6-31+ G * basis set (augmented in the case of CH 3 I) were performed. Calculations are also reported for CH 2 O, SO 2 , CS 2 , and 1,4-diazabicyclo[2.2.2]octane. Results show that increases in molecular size for intravalence transitions are nearly zero, while for the lower Rydberg transitions, Δ(RMS R ) values range between 0.23 A (for ABCO) and 0.95 A (for NH 3 ). The calculations on ABCO and acetone indicate the involvement of C atom 3 s orbitals in the lowest excited (Rydberg) states. These findings are consistent with the experimental results obtained from pressure perturbation spectroscopy.