Crystalline structures in large ammonia clusters studied by IR cavity ringdown spectroscopy and a density functional theory calculation

We observed the NH stretching vibrations of large ammonia clusters by IR cavity ringdown spectroscopy. The degenerate stretching vibration (ν3) showed an increase in the absorption intensity and a narrowing of the bandwidth with increasing cluster size. The calculated crystalline model clusters, (NH3)n (n = 7, 19, and 31), revealed that the ν3 vibrations of the interior and surface NH3 molecules show blue- and red-shifts with increasing cluster size, respectively. These results suggest that the ν3 frequencies in large clusters converge, and the observed ν3 narrowing is ascribed to this convergence in the crystalline hydrogen bonded clusters.