Ab initio studies on the NO(X²II) − O₂(X³ Σ_g⁻) van der Waals complexes in the doublet state

ABSTRACT

UCCSD(T) and CASPT2 geometry optimisations were performed on six different structures of NO–O₂ complexes in doublet states, using aug-cc-pVXZ basis sets up to X = 5. MRCI calculations were carried out at optimised UCCSD(T) and CASPT2 geometries. The most stable structure has NO and O₂ arranged nearly parallel in the ²A′′ state, with a counterpoise adjusted dissociation energy D_e of 179 cm⁻¹ (UCCSD(T) results). For the X-shaped ²A′ isomer D_e= 144 cm⁻¹. C_2v structures have D_e values between 115 and 121 cm⁻¹, whereas linear structures are least stable. The closest $\text{O}\cdots \text{O}$ and $\text{N}\cdots \text{O}$ distances are 3.1–3.4 Å. The X-shaped ²A′′ isomer is not stable. The results are discussed and compared with values for other complexes.

GRAPHICAL ABSTRACT

KEYWORDS:

• Nitric oxide
• oxygen
• NO–O₂ van der Waals complex
• doublet state
• ab initio methods
• CASSCF
• CASPT2
• MRCI
• UCCSD(T)
• vibrational frequencies

Acknowledgements

Thanks to Dr Pablo J. Bruna for reading the manuscript and commenting on the work, and to both reviewers for very helpful comments. Long-standing support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Provision of computer time by ComputeCanada is much appreciated.

Disclosure statement

No potential conflict of interest was reported by the author.