Theoretical study of cyclic radicals NOx (x = 2–6) Original Research Article

Ab initio MO calculations at the UHF and MP2 levels with 6–31G∗ basis set are carried out to examine the stability and the structure of a series of NOx (x = 2–6) radicals. We have located a series of exotic but stable radicals with a ring comprised of one nitrogen atom and two to six oxygen atoms. The stability is confirmed by evaluating the harmonic frequencies at both levels. The bonds in the cyclic radicals are discussed by comparing with both lengths and bond stretching vibrational frequencies of the related known molecules. The reaction energy for NOx → NO2 + (x − 2)/2 O2 is evaluated with the MP4SDTQ approximation, and it is shown that the cyclic NOx is a new type of energy-rich molecules. Besides, all the ring radicals of being stable not only on UHF/6–31G∗ but also on MP2/6–31G∗ are further investigated by density functional theory with nonlocal corrections (Becke3LYP). Most of them become unstable, except for three-membered NO2 ring and four-membered NO3 ring. The different stability for some NOx ring radicals on MP2 and DFT levels are illustrated.