Microwave spectrum of the ¹⁵N¹⁶O¹⁵N¹⁶O dimer

The microwave transitions for the (¹⁵NO)₂ dimer were reinvestigated following a report by McKellar et al. (1995, Molecular Physics, 86, 273) of a discrepancy between a previously measured microwave transition frequency and the corresponding frequency calculated from their molecular constants. The present microwave measurements confirm the previous observation of a rotational transition at 11159•297(4) MHz, but this transition is apparently not due to the cis N—N bonded NO dimer, since a mixture of ¹⁵NO in argon gas is required for observation of the 11159•297 MHz transition. A new doublet was located at 11128•434(4) MHz and this transition is now assigned to the 3₀₃ ← 2₁₂ transition of the ¹⁵N—¹⁵NO dimer, in agreement with predictions from the rotational constants of McKellar et al. This transition can be observed with either argon or neon as the expansion gas. New structural parameters obtained from the microwave data are in excellent agreement with those based on the FTIR data.