Knowledge Bank

Continuing our earlier $study1$ of $(14N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O)2$. We have now obtained spectra of the $\nu 1$ bands (symmetric N-O stretch) of $(15N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O)2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O-\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O$. These were recorded at a resolution of $0.005cm-1$ using a Bomem FTIR spectrometer and a new 5 m cooled multiple-traversal cell. The absorption path was 200 m with T = 84 K and P = 0.5 Torr. The $\nu 1$ band origins are found to be 1835.359 and $1854.869cm-1$ for the $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}N$ and mixed isotopes, respectively, and a full set of rotational and quartic centrifugal distortion parameters is obtained for each. Observed predissociation line widths, which show no strong rotational dependence, are $0.0056cm-1$ for $(14N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O)2$, and $0.0044cm-1$ for $(15N\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O)2$. This is somewhat narrower than reported by $Matsumotoetal.,1$ who did not detect any significant difference in line width between the two isotopes. Our observed width of $0.014cm-1$ for the mixed dimer is significantly larger than those of the symmetric dimers. $Nouretal.3$ have determined a harmonic force field for $(NO)2$ based on observed frequencies and isotope shifts in the Raman spectrum of solid NO. However, their result is not compatible with our observed centrifugal distortion parameters. We are thus investigating further the force field and its consequences for the assignment of the low frequency intermolecular vibrational modes of the dimer, which are yet to be detected in the gas phase.