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 Rydbe rg states (A(1), E) of azabicyclo[2.2.2]octane (ABCO) is presented. Th e calculations support a previous assignment of the lowest energy tran sition 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 thi s state suggests that the atomic 3s carbon orbitals located on the car bon atoms bonded to the tertiary nitrogen atom contribute roughly 80% to each orbital. In fact, on the basis of these calculations, a more a ccurate description of this Rydberg transition is as a two-electron ex citation 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 b asis set calculation employing [3s plus 3p(x,y,z)(N)] orbitals does no t alter these conclusions significantly. The second excited Rydberg st ate (of E symmetry) is calculated to lie ca. 4000 cm(-1) above the fir st. This state is calculated at the 3s active space level to be compos ed of a 2p(z) nitrogen orbital (HOMO) and carbon atomic 3s orbitals lo cated on the carbon atoms bonded to the nitrogen atom (LUMO). Employin g 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 th e energy separation between the 2A(1) and E Rydberg states, one can as sign the second observed Rydberg transition to the E <-- 1A(1) excitat ion. Geometry-optimized structures show that the C-N-C apex angle incr eases upon electronic excitation R(2), R(1) <-- S-0, supporting a prev ious 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.