Infrared Spectroscopy of Imidazole Trimer in Helium Nanodroplets: Free NH Stretch Mode

We report the first vibrational band assignment of imidazole trimer (IMT) solvated in helium nanodroplets. Several congested vibrational bands of imidazole (IM) clusters were obtained in the frequency region of 3513-3515\;cm⁻¹ and vibrationally resolved due to the extremely low temperature (0.37 K) and very weak solutesolvent interaction environments of helium droplets. The unambiguous free NH band assignment was achieved with an aid of pick-up oven temperature dependence and vibrational transition moment angle (VTMA) experiments as well as density functional theory (DFT) calculations. The band at 3514.3\;cm⁻¹ is attributed to the free NH stretching mode of linear IMT clusters, easily formed by the dipole-dipole interactions of IM in ultracold helium nanodroplets.