Knowledge Bank

Rotationally resolved infrared spectra of the $\nu 1$ and $\nu 3$ N-H stretching vibrations of the $Ar-HNH+$ radical ionic complex have been observed by means of photodissociation $spectroscopy.a$ The analysis of the rotational structure shows that the complex has a $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma -$ ground electronic state with a linear or quasi-linear proton-bound structure $Ar-H-N-H+$ characterized by an intermolecular center of mass separation of 3.085 {\AA}. The origins of the $\nu 1$ and $\nu 3$ bands were determined as 2803.65(2) and $3287.36(2)cm-1$, and the frequency of the intermolecular stretch vibration, $\nu s$, as $170.4(6)cm-1$. Ab initio calculations performed at the UMP2 level of theory confirm that the quasi-linearity and the diradical character of $HNH+$ in its electronic ground state are not changed upon Ar complexation. The calculated properties of the intermolecular bond of the complex $(Dc=1773cm-1,RAr-H\sim 2.01\backslash AA,\nu s\sim 185cm-1)$ and the predicted complexation induced frequency shifts for $\nu 1$ and $\nu 3$ are in good agreement with the experimental results.