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Structures and dynamics of protonated ammonia clusters
Antony Fouqueau1, Markus Meuwly
1Department of Chemistry, University of Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland.
The Journal of Chemical Physics
|January 7, 2006
Summary
Protonated ammonia clusters NH(4+)(NH3)n were studied using DFT. Larger clusters (n≥5) isomerize at 100 K, with infrared spectra matching experimental data, supporting multiple isomers under experimental conditions.
Area of Science:
- Computational chemistry
- Physical chemistry
- Spectroscopy
Background:
- Protonated ammonia clusters are fundamental systems in atmospheric and interstellar chemistry.
- Understanding their structural dynamics and spectral properties is crucial for interpreting experimental observations.
Purpose of the Study:
- To investigate the structures and infrared spectra of protonated ammonia clusters NH(4+)(NH3)n for n ≤ 8.
- To determine the stability and isomerization behavior of these clusters at varying temperatures.
- To correlate calculated spectra with experimental predissociation data.
Main Methods:
- Density Functional Theory (DFT) calculations.
- Semiempirical DFT/molecular dynamics simulations.
- Ab initio molecular dynamics using the self-consistent charges density functional tight-binding method.
Main Results:
- Clusters with n < 5 are stable up to 100 K; larger clusters (n ≥ 5) isomerize.
- Calculated infrared spectra at 10 K show good agreement with experimental data for n = 3-8.
- Multiple isomers are likely present under experimental conditions for n ≥ 6.
- Only three of thirteen stable structures for n = 8 survive at 100 K.
Conclusions:
- The study provides insights into the structural dynamics and spectral signatures of protonated ammonia clusters.
- Calculated infrared spectra can help identify the number and identity of isomers in experiments.
- Clusters tend to form globular structures, but chain-like arrangements are also observed.