Physical Chemistry I Laboratory Manual

https://orc.library.atu.edu/atu_faculty_books/1074/thumbnail.jp

General Chemistry II: CHEM 2134

https://orc.library.atu.edu/atu_oer/1005/thumbnail.jp

Electron solvation in water-ammonia mixed clusters: structure, energetics, and the nature of localization states of the excess electron

The structure and energetics of water-ammonia mixed clusters with an excess electron, [(H2O)n(NH3)m]- with m = 1, n = 2-6 and m = 2, n = 2, and also the corresponding neutral clusters are investigated in detail by means of ab initio quantum chemical calculations. The authors focus on the localization structure of the excess electron with respect to its surface versus interiorlike states, its binding to ammonia versus water molecules, the spatial and orientational arrangement of solvent molecules around the excess electron, the changes of the overall hydrogen-bonded structure of the clusters as compared to those of the neutral ones and associated dipole moment changes, vertical detachment energies of the anionic clusters, and also the vertical attachment energies of the neutral clusters. It is found that the hydrogen-bonded structure of the anionic clusters are very different from those of the neutral clusters unlike the case of water-ammonia dimer anion, and these changes in structural arrangements lead to drastically different dipole moments of the anionic and the neutral clusters. The spatial distribution of the singly occupied molecular orbital holding the excess electron shows only surface states for the smaller clusters. However, for n = 5 and 6, both surface and interiorlike binding states are found to exist for the excess electron. For the surface states, the excess electron can be bound to the dangling hydrogens of either an ammonia or a water molecule with different degrees of stability and vertical detachment energies. The interiorlike states, wherever they exist, are found to have a higher vertical detachment energy than any of the surface states of the same cluster. Also, for interiorlike states, the ammonia molecule with its dangling hydrogens is always found to stay on top or on a far side of the charge density of the excess electron without participating in the hydrogen bond network of the cluster; the intermolecular hydrogen bonds are formed by the water molecules only which add to the overall stability of these anionic clusters

Microscopic solvation of a lithium atom in water-ammonia mixed clusters: solvent coordination and electron localization in presence of a counterion

The microsolvation structures and energetics of water-ammonia mixed clusters containing a lithium atom, i.e., Li(H2O)n(NH3), n = 1-5, are investigated by means of ab initio theoretical calculations. Several structural aspects such as the solvent coordination to the metal ion and binding motifs of the free valence electron of the metal are investigated. We also study the energetics aspects such as the dependence of vertical ionization energies on the cluster size, and all these structural and energetics aspects are compared to the corresponding results of previously studied anionic water-ammonia clusters without a metal ion. It is found that the Li-O and Li-N interactions play a very important role in stabilizing the lithium-water-ammonia clusters, and the presence of these metal ion-solvent interactions also affect the characteristics of electron solvation in these clusters. This is seen from the spatial distribution of the singly occupied molecular orbital (SOMO) which holds the ejected valence electron of the Li atom. For very small clusters, SOMO electron density is found to exist mainly at the vicinity of the Li atom, whereas for larger clusters, it is distributed outside the first solvation shell. The free dangling hydrogens of water and ammonia molecules are involved in capturing the SOMO electron density. In some of the conformers, OH{e}HO and OH{e}HN types of interactions are found to be present. The presence of the metal ion at the center of the cluster ensures that the ejected electron is solvated at a surface state only, whereas both surface and interiorlike states were found for the free electron in the corresponding anionic clusters without a metal ion. The vertical ionization energies of the present clusters are found to be higher than the vertical detachment energies of the corresponding anionic clusters which signify a relatively stronger binding of the free electron in the presence of the positive metal counterion. The shifts in different vibrational frequencies are also calculated for the larger clusters, and the results are discussed for some of the selective modes of water and ammonia molecules that are directly influenced by the location and hydrogen bonding state of these molecules in the clusters

A Comparative Study of Specific Enthalpy of Aromatic Hydrocarbons with Simple Carbohydrates

Calorimetry is an aspect of chemistry primarily focused on determining the enthalpy of reactions (∆Hrxn). In the bomb calorimetry technique, the heat of combustion of chemical compounds can be measured experimentally. From this data and the application of Hess’s Law, ∆the Hrxn of several chemical reactions can be determined. The technique of bomb calorimetry can be applied to food, fuels, pharmaceuticals, and many other fields. The objective of the present project is to determine the specific enthalpy of various simple carbohydrates (naturally occurring sugars) through bomb calorimetry and compare it with that of aromatic hydrocarbons. By performing benzoic acid standardization reactions with the Parr™ Model 1341 Oxygen Combustion Vessel, the calorimeter constant was found to be 10.2717 ± .0565 KJ/°C with a 95% confidence interval, allowing the accurate determination of specific enthalpy for each of the sugars at constant volume in a pure O2 vessel. As we proceed with the experiment several aromatic hydrocarbons (e.g Naphthalene, etc) and several simple carbohydrates (e.g Sucrose, Glucose, etc.) will be tested to obtain the enthalpy of combustion (∆Hcomb). We will perform quantum chemical calculations on the reactant and product molecules to determine the ∆Hcomb and compare the experimental values with computational data

Women in STEM

Like many issues regarding acknowledgment of women, there is a lack of appreciation and recognition for the women in Science, Technology, Engineering, and Mathematics (STEM). Despite all disparities, women have made tremendous progress in STEM education, research, and workspace during the past 50 years. Women in the past few centuries and in the current century have worked hard to earn their positions in STEM fields. In this research paper, the authors present some of the women scientists who have made notable contributions to their fields of study; significant women like Rosalind Franklin and Andrea Ghez are mentioned. The authors compare recent, relevant data from Tennessee Technological University science departments of Chemistry and Biochemistry (undergraduate, masters, and graduate students as well as faculty) – findings show that science students are 54% female and science faculty are 31% female. The rate of STEM courses taken by female students dop off significantly at higher education levels. The authors conclude by describing propositions to bridge the gender gap and increase women's representation and desire for STEM careers. Solutions include researching areas where women are less represented and increasing the amount of female role models for younger generations. Overall, there needs to be more educational and employment opportunities for women in STEM, and society today can make that change a reality

Unimolecular dissociation of peptides: statistical vs. non-statistical fragmentation mechanisms and time scales

International audienceIn the present work we have investigated mechanisms of gas phase unimolecular dissociation of a relatively simple dipeptide, the di-proline anion, by means of chemical dynamics simulations, using the PM3 semi-empirical Hamiltonian. In particular, we have considered two activation processes that are representative limits of what occurs in collision induced dissociation experiments: (i) thermal activation, corresponding to several low energy collisions, in which the system is prepared with a microcanonical distribution of energy; (ii) collisional activation where a single shock of hundreds of kcal mol−1 (300 kcal mol−1 in the present case) can transfer sufficient energy to allow dissociation. From these two activation processes we obtained different product abundances, and for one particular fragmentation pathway a clear mechanistic difference for the two activation processes. This mechanism corresponds to the leaving of an OH− group and subsequent formation of water by taking a proton from the remaining molecule. This last reaction is always observed in thermal activation while in collisional activation it is less favoured and the formation of OH− as a final product is observed. More importantly, we show that while in thermal activation unimolecular dissociation follows exponential decay, in collision activation the initial population decays with non-exponential behaviour. Finally, from the thermal activation simulations it was possible to obtain rate constants as a function of temperature that show Arrhenius behaviour. Thus activation energies have also been extracted from these simulations

Chemical Dynamics Simulations of Intermolecular Energy Transfer: Azulene + N₂ Collisions

Chemical dynamics simulations were performed to investigate collisional energy transfer from highly vibrationally excited azulene (Az*) in a N₂ bath. The intermolecular potential between Az and N₂, used for the simulations, was determined from MP2/6-31+G* ab initio calculations. Az* is prepared with an 87.5 kcal/mol excitation energy by using quantum microcanonical sampling, including its 95.7 kcal/mol zero-point energy. The average energy of Az* versus time, obtained from the simulations, shows different rates of Az* deactivation depending on the N₂ bath density. Using the N₂ bath density and Lennard-Jones collision number, the average energy transfer per collision ⟨Δ<i>E</i>_c⟩ was obtained for Az* as it is collisionally relaxed. By comparing ⟨Δ<i>E</i>_c⟩ versus the bath density, the single collision limiting density was found for energy transfer. The resulting ⟨Δ<i>E</i>_c⟩, for an 87.5 kcal/mol excitation energy, is 0.30 ± 0.01 and 0.32 ± 0.01 kcal/mol for harmonic and anharmonic Az potentials, respectively. For comparison, the experimental value is 0.57 ± 0.11 kcal/mol. During Az* relaxation there is no appreciable energy transfer to Az translation and rotation, and the energy transfer is to the N₂ bath

Chemical Dynamics Simulation of Low Energy N₂ Collisions with Graphite

A chemical dynamics simulation was performed to study low energy collisions between N₂ and a graphite surface. The simulations were performed as a function of collision energy (6.34 and 14.41 kcal/mol), incident polar angle (20–70°) and random azimuthal angle. The following properties were determined and analyzed for the N₂ + graphite collisions: (1) translational and rotational energy distributions of the scattered N₂; (2) distribution of the final polar angle for the scattered N₂; (3) number of bounces of N₂ on the surface before scattering. Direct scattering with only a single bounce is dominant for all incident angles. Scattering with multiple collisions with the surface becomes important for incident angles far from the surface normal. For trajectories that desorb, the parallel component of the N₂ incident energy is conserved due to the extremely short residence times of N₂ on the surface. For scattering with an incident energy of 6.34 kcal/mol, incident polar angle of 40°, and final polar angle of 50° the percentage incident energy loss is 29% from the simulations, while the value is 27% for a hard cube model used to interpret experiment (J. Phys.: Condes. Matter 2012, 24, 354001). The incident energy is primarily transferred to surface vibrational modes, with a very small fraction transferred to N₂ rotation. An angular dependence is observed for the energy transfer, with energy transfer more efficient for incident angles close to surface normal