Knowledge Bank

Infrared spectra of N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by R.E. Miller and L. Pedersen [J. Chem. Phys. \textbf{108}, 436 (1998)] in the N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O \nub{1}+\nub{3} combination band region (~3480 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) showed the trimer structure to be noncyclic, with three inequivalent N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O monomer units which could be thought of as an N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O fundamental band regions \nub{3} (~1280 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) and \nub{1} (~2230 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$). In the \nub{3} region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the \nub{1}+\nub{3} region, but in the \nub{1} region all three possible trimer bands are observed. Relationships among the various bands such as rotational intensity patterns, vibrational shifts, and the properties of the related N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O dimer, generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully$15$N-substituted species in the \nub{1} region, and these results should aid in detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer. Finally, three combination bands involving the intermolecular van der Waals modes at 2253.7, 2255.5, and 2269.4 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ have been measured. The analyses of these bands and the identification of the nature of the intermolecular modes involved are currently underway.