The far infrared spectrum of the NO dimer in the gas phase has been observed for the first time, using a low temperature ͑100 K͒ long-path ͑180 m͒ absorption cell. The spectroscopic results are used, together with mid-infrared observations and high level CCSD͑T͒ ab initio calculations, to locate the four low frequency intermolecular vibrational modes of (NO) 2 : 2 ͑symmetric bend͒ϭ239.361 cm Ϫ1 , 3 ͑intermolecular stretch͒ϭ134.503 cm Ϫ1 , 4 ͑out-of-plane torsion͒Ϸ117 cm Ϫ1 , and 6

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... bend͒ϭ429.140 cm Ϫ1 . These values agree poorly with previous determinations based on condensed-phase spectra, indicating that there are large shifts in going from the isolated gas-phase dimer to the liquid, solid, or matrix-isolated environments.