Comparison of Model Chemistry and Density Functional Theory Thermochemical Predictions with Experiment for Formation of Ionic Clusters of the Ammonium Cation Complexed with Water and Ammonia; Atmospheric Implications

The G2, G3, CBS-QB3, and CBS-APNO model chemistry methods and the B3LYP, B3P86, mPW1PW, and PBE1PBE density functional theory (DFT) methods have been used to calculate ΔH° and ΔG° values for ionic clusters of the ammonium ion complexed with water and ammonia. Results for the clusters NH4+(NH3)n and NH4+(H2O)n, where n = 1−4, are reported in this paper and compared against experimental values. Agreement with the experimental values for ΔH° and ΔG° for formation of NH4+(NH3)n clusters is excellent. Comparison between experiment and theory for formation of the NH4+(H2O)n clusters is quite good considering the uncertainty in the experimental values. The four DFT methods yield excellent agreement with experiment and the model chemistry methods when the aug-cc-pVTZ basis set is used for energetic calculations and the 6-31G* basis set is used for geometries and frequencies. On the basis of these results, we predict that all ions in the lower troposphere will be saturated with at least one complete first hydration shell of water molecules.

Accurate Predictions of Water Cluster Formation, (H2O)n=2-10

An efficient mixed molecular dynamics/quantum mechanics model has been applied to the water cluster system. The use of the MP2 method and correlation consistent basis sets, with appropriate correction for BSSE, allows for the accurate calculation of electronic and free energies for the formation of clusters of 2−10 water molecules. This approach reveals new low energy conformers for (H2O)n=7,9,10. The water heptamer conformers comprise five different structural motifs ranging from a three-dimensional prism to a quasi-planar book structure. A prism-like structure is favored energetically at low temperatures, but a chair-like structure is the global Gibbs free energy minimum past 200 K. The water nonamers exhibit less complexity with all the low energy structures shaped like a prism. The decamer has 30 conformers that are within 2 kcal/mol of the Gibbs free energy minimum structure at 298 K. These structures are categorized into four conformer classes, and a pentagonal prism is the most stable structure from 0 to 320 K. Results can be used as benchmark values for empirical water models and density functionals, and the method can be applied to larger water clusters.

Where Is "Home"? Value Conflicts, Identity Crises and the Formation of Cosmopolitan Identity in Myanmar Students Studying in the United States

Fully engaging in a new culture means translating oneself into a different set of cultural values, and many of the values can be foreign to the individual. The individual may face conflicting tensions between the psychological need to be a part of the new society and feelings of guilt or betrayal towards the former society, culture or self. Many international students from Myanmar, most of whom have little international experience, undergo this value and cultural translation during their time in American colleges. It is commonly assumed that something will be lost in the process of translation and that the students become more Westernized or never fit into both Myanmar and US cultures. However, the study of the narratives of the Myanmar students studying in the US reveals a more complex reality. Because individuals have multifaceted identities and many cultures and subcultures are fluctuating and intertwined with one another, the students¿ cross-cultural interactions can also help them acquire new ways of seeing things. Through their struggle to engage in the US college culture, many students display the theory of ¿cosmopolitanism¿ in their transformative identity formation process and thus, define and identify themselves beyond one set of cultural norms.