THEORETICAL-STUDY OF THE DI-IMIDE (N2H2) MOLECULE IN GROUND AND N-]PI-ASTERISK EXCITED-STATES

A number of ab initio approaches have been used to determine the equilibrium structures, energies, and vibrational frequencies of N2H2 in the ground state and in the excited singlet and triplet n --> pi* states. The methods included restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF), multiconfiguration self-consistent field (MCSCF), single-reference configuration interaction (SRCI) and multireference configuration interaction (MRCI) including all single and double excitations from the reference configurations, and second-order Moller-Plesset perturbation theory (MP2) using RHF and UHF orbitals for ground and excited states, respectively. Unlike the singlet excited state, for which broken-symmetry solutions were found at the RHF level, no symmetry breaking was encountered for the triplet state. The ground-state MCSCF and MP2 structures of N2H2, which have C2h (trans-planar) symmetry, are in good agreement with the experimental structure. The excited states are predicted to have nonplanar C2 structures with dihedral angles ranging from 96-degrees to 106-degrees for the triplet state and from 105-degrees to 121-degrees for the singlet state. Except for the SRCI singlet adiabatic excitation energy, the effect of configuration interaction is to increase the vertical and adiabatic excitation energies of both excited states relative to the RHF values in single-reference calculations, and to decrease these excitation energies relative to MCSCF values in multireference calculations, bringing the single-reference and multireference CI values into better agreement with each other. The MRCI vertical excitation energies are 2.6 eV for the triplet and 3.6 eV for the singlet, while the corresponding 0-0 transition energies are 1.9 and 2.9 eV, respectively.