A theoretical study on the electronic states and O-loss photodissociation of the NO2(+) ion.

CASPT2 (multiconfiguration second-order perturbation theory) calculations were performed at the molecular geometry for 17 low-lying singlet and triplet states of the NO(2)(+) ion. The CASPT2 vertical relative energies (T(v)') were obtained and the characters of these ionic states (primary or shake-up ionization states) were determined. For the eight low-lying states, we performed CASPT2 geometry optimization calculations and obtained the CASPT2 adiabatic relative energies (T(0)). We conclude that the 1(1)A(1), 1(3)B(2), 1(3)A(2), 1(1)A(2), 1(1)B(2), 1(3)A(1), 2(3)B(1), and 3(3)B(2) states of NO(2)(+) correspond to the X(1)Sigma(g)(+), a(3)B(2), b(3)A(2), A(1)A(2), B(1)B(2), c(3)A(1), d(3)B(1), and (3b(2))(-1) (3)B(2) states (the eight ionic states below 20 eV observed in the photoelectron spectra of Brundle et al.1 and Baltzer et al.2), respectively. The 1(1)A(1), 1(3)B(2), 1(3)A(2), 1(1)A(2), 1(1)B(2), 1(3)A(1), and 3(3)B(2) states are primary ionization states, and the CASPT2 T(v)' and T(0) values of these states are close to the corresponding experimental values from refs. [1] and [2]. The 2(3)B(1) state is not a typical primary ionization state, and the CASPT2 T(v)' and T(0) values for 2(3)B(1) are in reasonable agreement with the experimental values for d(3)B(1) from refs. [1] and [2] (the CASPT2 T(0) value for 1(3)B(1) is more than 2.5 eV smaller than the experimental values). Based on our CASPT2 T(0) calculations, we comment on the assignments of the d(3)A(1), C(1)B(1), and D(1)B(2) states below 20 eV observed by Jarvis et al. and on the MRCI T(0) values of Hirst for the 1(3)B(1), 1(1)B(1), and (3)A(1) states. On the basis of the CASPT2 potential energy curve (PEC) and CASSCF singlet/triplet minimum-energy crossing point (MECP) calculations, we reach the following conclusions concerning O-loss photodissociation from the X(1)Sigma(g)(+), a(3)B(2), b(3)A(2), A(1)A(2), and B(1)B(2) states, which are in line with the experimental facts. The adiabatic dissociation process of the X(1)Sigma(g) (+) state to the second limit [NO(+)(X(1)Sigma(+))+O((1)D)] cannot occur due to a high energy barrier (>5.0 eV) along the PEC, and the nonadiabitic process of X(1)Sigma(g)(+) to the first limit [NO(+)(X(1)Sigma(+))+O((3)P)] via the triplet states is unlikely since the MECPs lie very high above X(1)Sigma(g)(+). For the a(3)B(2) and b(3)A(2) states, adiabatic dissociation processes to the first limit may occur. Both the A(1)A(2) and B(1)B(2) states can undergo processes of predissociation to the first limit by a repulsive 2(3)A'' state, since the MECPs lie low above A(1)A(2) and B(1)B(2) and the calculated spin-orbit couplings at the MECPs are not small.