Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4+(NH3)1-4.

The gas phase vibrational spectroscopy of the protonated ammonia dimer N2H7+, a prototypical system for strong hydrogen bonding, is studied in the spectral region from 330 to 1650 cm-1 by combining infrared multiple photon dissociation and multidimensional quantum mechanical simulations. The fundamental transition of the antisymmetric proton stretching vibration is observed at 374 cm-1 and assigned on the basis of a six-dimensional model Hamiltonian, which predicts this transition at 471 cm-1. Photodissociation spectra of the larger protonated ammonia clusters NH4+(NH3)n with n=2–4 are also reported for the range from 1050 to 1575 cm-1. The main absorption features can be assigned within the harmonic approximation, supporting earlier evidence that hydrogen bonding in these clusters is considerably weaker than for n=1. [ABSTRACT FROM AUTHOR]