Knowledge Bank

The non-polar N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O dimer has four low frequency intermolecular vibrational modes: van der Waals stretch (A symmetry), symmetric and asymmetric in-plane bends (A and B symmetry, respectively), and out-of-plane torsion (A symmetry). There are two possible intramolecular dimer vibrations which correlate with the $\nu 1$ monomer stretch. One of these is an in-phase vibration of the two monomers, which has A symmetry and is not infrared active. The other is an out-of-phase vibration, which has B symmetry and gives rise to the infrared active dimer band at 2229.48 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. In the present work, we report the observation of an N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O dimer band with c-type rotational structure, assigned as a combination of the A intramolecular N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O $$ stretching vibration and the A intermolecular out-of-plane dimer torsional vibration. The vibrational origin for this band is measured to be 2249.360 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, giving an estimated torsional frequency of 21.5 cm$$. A second combination band with a- and b-type rotational transitions has also been measured and assigned as the combination of the same A intramolecular vibration and the B asymmetric van der Waals bend. The measured band center is 2264.37 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, giving an asymmetric bending frequency of about 36.5 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. A search for the other two combination bands is currently underway.