The Stable Structures And Infrared Spectra Of Protonated Ammonia Water Clusters Abstract

Protonated water complexes are key elements to dissociation and transportphenomena in aqueous chemistry and biological systems. In addition to theirfundamental chemical relevance, protonated water clusters play an important role in anumber of areas, including atmospheric sciences, astrophysics, and biology. It istherefore not a surprise that protonated water clusters have received extensiveattention from both the experimental and theoretical communities during the pastdecades.Identifying isomeric structures of atomic and molecular clusters is a subject ofcurrent interest, with interest rapidly expanding owing to the development of modernspectroscopic techniques for characterizing these species. Related research is alsofacilitated by advances in density functional theory and molecular dynamicssimulations, which can now accurately predict the structures and properties of largecomplex systems, so it is more accurate for small clusters than large.In this paper, density functional theory (DFT/B3LYP) with6-31G (d) basis sethave been used to perform the optimizations and frequency calculations of candidatestructures of200protonated ammonia water clusters NH4+(H2O)7and NH4+(H2O)8obtained from an empirical potential. Consequently, some kinds of lower energy andstable isomers without imaginary frequency have been obtained under theB3LYP/6-31+G*and B3LYP/6-311++G**level.Comparing the conformations andenergies of the resulting isomers,analyze the infrared spectrum of two clustersstructures,infrared spectrum with same framework of experimental results anddifferent frequencies can be obtained by theoretical result previously. We can knowthe connection between vibration mode and vibration intensity from the vibrationmode,bond lengths and angles at each peak of infrared spectrum of NH4+(H2O)8. The main conclusions of paper are:1） Comparing the conformations and energiesof the resulting isomers, it is found that the stablest structure is the networkconformation not the symmetric cubic geometries which ammonium ions in center ofthe structure,but the network conformation has a H2O as free water.2） In thenature, the existence of protonated ammonia clusters is not alone, it is an admixtureof a variety of different structures.3） Frequency ranging from3000cm-1～4000cm-1, the infrared spectrum of these stable structures were analyzed. We find that eachpeak of infrared spectrum is basically superposition of molecules vibration, and thepeak and vibration mode are closely relation.4） The theoretical spectrum is ainfrared spectrum of multiple stable structure with different percentages,we find thateach peak of infrared spectrum is basically superposition of vibration of molecules.Infrared spectrum with same framework of experimental results and differentfrequencies can be obtained by theoretical result previously.The main contents of this paper are as follow:Chapter one: Firstly, we simply introduce the meaning and prospects of studyprotonated ammonia clusters.Secondly,we presented the important and difficultproblems of ammonia clusters.Lastly, we show the research of atmosphereChapter two: We Introduce the theoretical calculation method of study of twoclusters,including density functional theory (DFT/B3LYP), HF, and calculation ofvibrational frequency,then briefly describe functions of Gaussian03program andGauss View.Chapter three:6-31G (d) basis set have been used to perform the optimizationsand frequency calculations of candidate structures of200protonated ammonia waterclusters NH4+(H2O)7obtained from an empirical potential. Consequently,23kinds oflower energy and stable isomers without imaginary frequency have been obtainedunder the B3LYP/6-31+G*level. We find that the most stable structure is a networkconformation which ammonium ion in center of cluster and has a free watermolecules on the structure,comparing the infrared spectrum of experimental resultsand theoretical results.Chapter four: Under the B3LYP/6-311++G**level,candidate structures of200 protonated ammonia water clusters NH4+(H2O)7obtained from an empirical potential.Consequently,14kinds of lower energy and stable isomers without imaginaryfrequency have been obtained.We discover the most stable structure still the networkstructure which has the lowest energy,and analyzing the frequency,bond lengths andangles at each peak of infrared spectrum.Chapter five: Analyze and summarize the main contents of this paper,narrateprospects of the future works,and point out the direction of future research.