Knowledge Bank

C-H$\cdots$N weak hydrogen bond is of much current interest. We report the first high resolution infrared spectroscopic study of a prototypical C-H$\cdots$N bonded system, i.e. NH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$-HCCH, at the vicinity of the $\nu$4bandofNH${3}$.Thespectrumhasbeenrecordedusinganinfraredspectrometerequippedwithanastigmaticmultipasscellalignedfor366passesandaroomtemperatureexternalcavityquantumcascadelaseratthe6$\mu$mregion.Theperpendicularbandspectrumofsymmetrictoprotorobservedisconsistentwiththepreviousmicrowaveandinfraredstudiesat3$\mu$m. We also extended the previous microwave measurement to higher $J$ and $K$. For the related NH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$-OCS complex, microwave spectrum of $J$ up to 6 and infrared spectrum at the vicinity of the $\nu$4bandofNH${3}$ have been recorded and analyzed for the first time. Comparison has been made with the previously studied isoelectronic complexes such as NH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$-N$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O and NH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$-CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$. The source of the difference will be discussed with the aid of \textit{ab~initio} calculations.