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

Abstract

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 NH3, acetone, and CH3I, 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, RMSR. Excited state calculations using configuration interaction (singles) with the 6-31+G* basis set (augmented in the case of CH3I) were performed. Calculations are also reported for CH2O, SO2, CS2, 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(RMSR) values range between 0.23 Å (for ABCO) and 0.95 Å (for NH3). The calculations on ABCO and acetone indicate the involvement of C atom 3s orbitals in the lowest excited (Rydberg) states. These findings are consistent with the experimental results obtained from pressure perturbation spectroscopy.