HIGH RESOLUTION INFRARED AND MICROWAVE SPECTRA OF NH$_{3}$-HCCH AND NH$_{3}$-OCS COMPLEXES: STUDIES OF WEAK C-H$\cdots$N HYDROGEN BOND AND ELECTRIC MULTIPOLE INTERACTIONS

Abstract

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$_{3}$-HCCH, at the vicinity of the $\nu$4 band of NH$_{3}$. The spectrum has been recorded using an infrared spectrometer equipped with an astigmatic multipass cell aligned for 366 passes and a room temperature external cavity quantum cascade laser at the 6\,$\mu$m region. The perpendicular band spectrum of symmetric top rotor observed is consistent with the previous microwave and infrared studies at 3\,$\mu$m. We also extended the previous microwave measurement to higher $J$ and $K$. For the related NH$_{3}$-OCS complex, microwave spectrum of $J$ up to 6 and infrared spectrum at the vicinity of the $\nu$4 band of NH$_{3}$ have been recorded and analyzed for the first time. Comparison has been made with the previously studied isoelectronic complexes such as NH$_{3}$-N$_{2}$O and NH$_{3}$-CO$_{2}$. The source of the difference will be discussed with the aid of \textit{ab~initio} calculations.