Born-oppenheimer symmetry breaking in the (C)over-tilde state of NO2:: Importance of static and dynamic correlation effects

A systematic theoretical treatment is performed with highly correlated ab initio theoretical methods to establish the structural nature of the (C) over tilde state of NO2. We predict the (C) over tilde state to have an asymmetric structure (point group C,). Spin-restricted and spin-unrestricted configuration interaction (CISD), coupled cluster [CCSD and CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods were used in conjunction with very large correlation-consistent polarized valence zeta cc-pVXZ and aug-cc-pVXZ [X = T, Q, 5] basis sets. The asymmetric (C) over tilde (2)A '' state is predicted to lie T-e = 47.5 kcal/mol (2.06 eV, 16600 cm(-1)) above the (X) over tilde (2)A(1) state at the aug-cc-pV5Z/UCCSD(T) level of theory, with T-0 = 46.0 kcal/mol (2.00 eV, 16100 cm(-1)), in good agreement with the experimental values of 46.77 kcal/mol (2.028 eV, 16360 cm(-1)) by Weaver and 46.42 kcal/mol (2.013 eV, 16234 cm(-1)) by Aoki. The symmetric structure (in C-2 upsilon symmetry) with r(e)(NOs) = 1.274 angstrom and theta(e) (ONO) = 109.9 degrees is a transition state between the two equivalent asymmetric (in C-s symmetry) structures and is located only 1.53 kcal/mol (0.066 eV, 540 cm(-1)) above the asymmetric structure. The asymmetric structure is predicted to have structural parameters r(e)(NO1) = 1.489 angstrom, r(e)(NOs)= 1.169 angstrom, and theta(e)(ONO)= 109.7 degrees with the same method, aug-cc-pV5Z/TJCCSD(T). The averaged NO bond distance is 1.329 angstrom, and the difference between the two NO bond distances is 0.320 angstrom. The three harmonic vibrational frequencies for the (C) over tilde (2)A '' state are 1656 (in-phase stretch), 759 (bend), and 378 (out- of-phase stretch) cm(-1). While these theoretical results further corroborate the previous predictions concerning the asymmetric nature of the (C) over tilde state, there remains discrepancy between the theoretical and experimental symmetric stretching mode omega(1) (1656 and 923 cm(-1), respectively). It is possible, however, that this disagreement could be resolved by a reassignment of the corresponding lines in the experimental spectrum, though additional vibronic simulations of the spectrum are required to confirm this proposition.