NO 2 radical spectroscopy: Vibrational frequencies, dipole moment, and the energy difference between the bent and linear stationary points on the ground state potential surface

Abstract

A broad range of ab initio quantum mechanical methods is applied to the NO 2 molecule in conjunction with basis sets ranging in size from double-zeta plus polarization (DZ+P) to quadruple-zeta plus double polarization plus f functions (QZ+2P+f). As many as 54423 configurations in MCSCF methods and 894241 configurations with the CISDT method have been included in full variational rigor. The problem of symmetry breaking in NO 2 is found to be qualitatively unimportant for bond angles 130°–180°. For both the vibrational frequencies and the dipole moment, the complete active space self-consistent-field (CASSCF) method provides much better agreement with experiment than does the energetically superior configuration interaction method including all single and double excitations (CISD). The predicted energy difference Δ E between bent and linear NO 2 is 39.4 kcal/mole without zero-point vibration and 38.4 kcal/mole when corrected for zero-point vibrational energies.