The Low-Lying Electronic States of NO₂: Potential Energy and Dipole Surfaces, Bound States, and Electronic Absorption Spectrum


Nitrogen dioxide, NO2, is a free radical composed of the two most abundant elements in Earth’s atmosphere, nitrogen and oxygen, and is relevant to atmospheric and combustion chemistry. The electronic structure of even its lowest-lying states is remarkably complex, with various conical intersections and Renner−Teller pairings, giving rise to complex and perturbed vibronic states. Here we report some analysis of the 18 molecular states of doublet spin-multiplicity formed by combining ground-state N(4Su) and O(3Pg) atoms. Three-dimensional potential energy surfaces were fit at the MRCI(Q)-F12/VTZ-F12 level, describing the lowest four (X̃, Ã, B̃, and C̃) electronic states. A properties-based diabatization procedure was applied to accommodate the intersections, producing energies in a quasidiabatic representation and yielding couplings that were also fit into surfaces. The low-lying vibrational levels on the ground X̃ state were computed and compared with experimental measurements. Compared to experiment, the lowest 125 calculated vibrational levels (up to 8500 cm-1 above the zero-point energy) have a root-mean-squared error of 16.5 cm-1. In addition, dipole moments for each of the lowest four electronic states -- and the transition dipoles between them -- were also computed and fit. With the coupled energy and dipole surfaces, the electronic spectrum was calculated in absolute intensity and compared with experimental measurements. Detailed structure in the experimental spectrum was successfully reproduced, and the total integrated intensity matches experiment to an accuracy of ∼1.5% with no empirical adjustments.



Research Center/Lab(s)

Center for High Performance Computing Research


Published as part of The Journal of Physical Chemistry virtual special issue “125 Years of The Journal of Physical Chemistry

R.D. is supported by the U.S. Department of Energy (DOE) (Award No. DE-SC0019740). Computing resources were supported by the National Science Foundation under Grant No. OAC-1919789.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.1c03482.

Keywords and Phrases

Group theory; Electrical energy; Chemical calculations; Equilibrium; Mathematical methods

International Standard Serial Number (ISSN)

1089-5639; 1520-5215

Document Type

Article - Journal

Document Version


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© 2021 American Chemical Society (ACS), All rights reserved.

Publication Date

11 Jun 2021