CASSCF STUDY OF THE GROUND-STATE AND LOWEST LYING 3S RYDBERG STATES OF ABCO

An ab initio CASSCF study of the ground and first two excited 3s Rydberg states (A(1), E) of azabicyclo[2.2.2]octane (ABCO) is presented. The calculations support a previous assignment of the lowest energy transition of ABCO as 2A(1) <-- 1A(1). The excited Rydberg state molecular orbital 2A(1) is composed of 2p nitrogen, 3s nitrogen, and 3s carbon orbitals. An examination of the HOMO and LUMO natural orbitals for this state suggests that the atomic 3s carbon orbitals located on the carbon atoms bonded to the tertiary nitrogen atom contribute roughly 80% to each orbital. In fact, on the basis of these calculations, a more accurate description of this Rydberg transition is as a two-electron excitation from S-0[2p(z)(N)](2) to a partially delocalized R(1)[3s(C)3s(N)](2). The transition 2A(1) <-- 1A(1) is found experimentally at 39 080 cm(-1) and is calculated to appear at 36 067 cm(-1). An extended basis set calculation employing [3s plus 3p(x,y,z)(N)] orbitals does not alter these conclusions significantly. The second excited Rydberg state (of E symmetry) is calculated to lie ca. 4000 cm(-1) above the first. This state is calculated at the 3s active space level to be composed of a 2p(z) nitrogen orbital (HOMO) and carbon atomic 3s orbitals located on the carbon atoms bonded to the nitrogen atom (LUMO). Employing a [3s plus 3p(N)] active space again has only a small effect on the orbital composition of the excited E Rydberg state. On the basis of the energy separation between the 2A(1) and E Rydberg states, one can assign the second observed Rydberg transition to the E <-- 1A(1) excitation. Geometry-optimized structures show that the C-N-C apex angle increases upon electronic excitation R(2), R(1) <-- S-0, supporting a previous assignment of cage compression vibrational modes in the vibronic activity of these transitions. The effect of Rydberg state excitation is analyzed in terms of charge redistribution in the ABCO molecule.