A CAS study of the S0, T1, S1, T2, and S2 states of the trans-HONO molecule

The S0 (X1A′), T1 (a3A″), S1 (A1A″), T2 (b3A′), and S2 (B1A′) states of the (trans-)HONO molecule were studied by using the CASSCF and CASPT2 methods. The CASPT2(//CASPT2) adiabatic and vertical excitation energy values are in good agreement with available experimental data. The CASPT2//CASSCF potential energy curves (PECs) calculations indicate that: (i) all the five states correlate with the products of OH (X2Π) + NO (X2Π); (ii) along each of the T1, S1, and T2 PECs there is a minimum followed by a transition state (barrier); and (iii) the repulsive S2 PEC crosses the T2, S1, and T1 PECs. The geometries and relative energies for the stationary points along these PECs were calculated at the CASPT2(//CASPT2) level, and the calculations predict that the barrier height value for S1 is negligibly small (0.0018 eV).