CHEM2090-LAB-EXP6-2010 - EXPERIMENT 6 Molecular Shape and...

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6 – 1 EXPERIMENT 6 Molecular Shape and Polarity Objective: Determine the shapes and polarities of a number of molecules using the VSEPR model. The shapes and polarities of these same molecules will also be determined via a software program that will perform the necessary quantum mechanical calculations. The agreement between the VSEPR model and the quantum mechanical calculations will be examined. Following this, a drug molecule that will interact favorably with a known receptor site will be created. Introduction: Molecular shape and polarity are factors that influence both the chemical and physical properties of compounds. Chemical reactivity, especially in biological settings, is often especially sensitive to molecular shape and polarity. Physical properties such as solubility and melting and boiling points are also strongly dependent on molecular shape and polarity, as is the lattice arrangement in molecular solids. Molecular shape is determined by energy considerations; a molecule assumes the geometry that gives it the lowest possible potential energy. The most accurate means of determining the geometry of lowest energy for a particular molecule or ion involves sophisticated quantum mechanical calculations that consider numerous possible geometric arrangements for a molecule, calculate the total energy of the molecule for each arrangement, and identify the arrangement with the lowest potential energy. The valence shell electron–pair repulsion (VSEPR) model accomplishes this qualitatively by arranging electron groups about a central atom in such a manner as to maximize their separation, a geometry that should be approximately that of minimum potential energy. Molecular polarity is determined by summing the individual bond polarities according to the known shape of a molecule. For example, each of the carbon–oxygen bonds in carbon dioxide is polar covalent, but because carbon dioxide is a linear molecule the two bond polarities are of the same magnitude and directed oppositely to one another, causing the net polarity of the molecule, the molecular dipole moment ( μ ), to be zero (see Figure 6.1). O C O μ = 0 Figure 6.1 Contrast carbon dioxide with water (which also has two identical polar covalent bonds). Owing to the bent shape of water these two bond polarities do not sum to zero; instead they sum to a nonzero molecular dipole moment: H O H μ = Figure 6.2
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Experiment 6: Molecular Shape and Polarity 6 – 2 In this experiment, you will compare predictions based on the VSEPR model with results of quantum mechanical calculations to determine the shapes and polarities of a number of molecules. You will make predictions based on the VSEPR model as part of the pre–laboratory assignment. You will perform the quantum mechanical calculations in lab on a computer using the software program Spartan ® . Following this, you will then apply your knowledge of the VSEPR model and your nascent ability in performing calculations via Spartan ®
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CHEM2090-LAB-EXP6-2010 - EXPERIMENT 6 Molecular Shape and...

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