Nitrous oxide tetramer has two highly symmetric isomers

Infrared spectra of the nitrous oxide tetramer, (N 2 O) 4 , are studied in the region of the N 2 O ν 1 fundamental band (∼2200 cm −1 ). The spectra are observed using a tunable diode laser to probe a pulsed supersonic jet expansion. Parallel (Δ K = 0) bands are observed for the previously observed isomer of (N 2 O) 4 , which is confirmed by isotopic substitution to have an oblate symmetric rotor structure with D 2 d symmetry. A distinct new isomer of (N 2 O) 4 is observed by means of a perpendicular (Δ K = ±1) band. It has a prolate symmetric rotor structure with S 4 symmetry. These isomers represent two distinct, but almost equally favorable, ways of bringing together a pair of nonpolar N 2 O dimers to form a tetramer. It is not clear at present which one represents the true ground state.