Isomers of N ₂ O ₃: Observation of trans-cis N₂O₃ in solid Ar

Two isomers of N₂O₃, asym-N₂O₃ and sym-N₂O₃, are identified with infared (IR) absorption when mixtures of NO-NO₂-Ar or NO-O₂-Ar are deposited onto a target at 13 K. Upon irradiation with a XeCl excimer laser at 308 nm, asym-N₂O₃ is converted to sym-N₂O₃ and a new form of N₂O₃, trans-cis N₂O₃; the latter is readily converted to sym-N₂O₃ upon further irradiation with red light. Assignments of IR absorption lines to each conformer in its isotopic variants are based on ¹⁸O-isotopic substitution and photoconversion experiments. For asym-N₂O₃, we observed ¹⁸O-isotopic shifts of a few vibrational modes previously unresolved in a N₂ matrix, confirming that O-atoms in the NO₂ moiety are inequivalent. For sym-N₂O₃, a more nearly complete set of isotopic shifts for absorption lines at 1688.6, 971.0, and 704.6 cm^-1 enables us to provide refined assignments. In addition, lines at 1722.5 and 1721.1 cm^-1 are assigned to symmetric stretching modes of two terminal NO groups of ¹⁸ON¹⁶ON¹⁶O and ¹⁸ON¹⁸ON¹⁶O; they gain IR activity because C_2V symmetry is broken. Trans-cis N₂O₃ with an asymmetric ONONO structure is identified with absorption lines at 1704.5, 1665.7, 877.8, and 243.0 cm^-1. Isotopic experiments indicate that this species contains two nearly isolated NO groups. Spectral assignments are supported by theoretical calculations with density-functional theories (BLYP and B3LYP); previous assignments of low-energy vibrational modes of asym-N₂O₃ and sym-N₂O₃ are revised based on comparison with calculations. Photoconversion among these isomers is discussed.