Theoretical And Experimental Study Of The Protonated Ammonia Clusters

The hydrogen bonded clusters have been investigated extensively in recent years for their important position in physical, chemical and biological fields. There is a high-negative N atom in ammonia molecule, and ammonia is a small molecule that can form hydrogen bond like water. So the research on ammonia clusters has very important scientific value.In this paper, mass spectra of ammonia cluster ions are obtained by multi-photon ionization (MPI) technology using the nanosecond lasers which have the wavelengths of 355nm and 523nm respectively ,and then analyzed. Besides, considering the theoretic study of ammonia cluster is few recently, we get a series of stable structures and infrared vibration spectra of the protonated ammonia clusters (NH₃)_nH⁺ (n=1～9) by using the Gaussian 03W suite of programs with the density functional theory of B3LYP method and the 6-31+G(d) basis set.The main innovation and conclusion of this paper are as follows.1. It is the first time that the unprotonated ammonia cluster ions of n=2～6 are detected at the 355nm laser. It shows that there is an absorption-ionization-dissociation mechanism (AID) in the experiment.2. The stable structure of protonated ammonia clusters (NH₃)_nH⁺ (n=1～9) is that a proton and an ammonia molecule form a NH₄⁺ core ion, and the other ammonia molecules are bound to four H of this core ion by hydrogen bonds in the form of ammonia chains.3. The stability of the clusters is studied with the second difference of the energies. The structure of n=5 should be the magic number structure. This conclusion coincides with the result in references.4. Average binding energies of an ammonia molecule in optimized cluster are defined in this paper and the result shows that they decrease with the increasing of clusters size.5. The proton affinities of the most stable structures of neutral ammonia clusters are also calculated. The proton affinity increases with the increasing of cluster size.6. In the infrared vibration spectra, all the strongest peaks correspond to the asymmetric stretching vibration modes of N-H bonds in NH₄⁺ core ion except n=2.