Use the b3lyp6 31g model to calculate energies for

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Unformatted text preview: borate. Order the aromaticity of thiophene, pyrrole and furan. Aromaticiy of Borazine and Boraphosphazine: Borazine, B3N3H6, is isoelectronic with benzene and like benzene is known to be a planar molecule with six π electrons (originating from the three out-of-plane nitrogen lone pairs). HN HB H B N H NH BH Does this mean that borazine is aromatic? Use the B3LYP/6-31G* model to calculate energies for successive addition of one, two and three equivalents of hydrogen (analogous to what has previously been done for benzene). HN HB H B N H NH H2 BH H2N HB H2 B N H NH H 2 H2 N BH H2B H2 B N H2 NH H2 H2N BH H2 B H2 B N H NH BH Is the first hydrogenation step significantly more difficult (more endothermic) than the second and third steps? If it is, estimate the “aromaticity” of borazine and compare this to the value for benzene obtained from the same analysis. Use the B3LYP/6-31G* model to calculate energies for the individual steps in the hydrogenation of the phosphorous analogue of borazine (boraphosphazine). HP HB H B P H PH BH H2 H2P HB H2 B P H PH BH H2 H2 P H2B H2 B P H2 P H H2 H2P BH H2 B H2 B P H PH BH Rank the aromatic stabilization of benzene, borazine, and boraphosphazine. Cyanuric Acid: Heating urea leads to loss of ammonia and production of hydrogen cyanate. This in turn trimerizes to cyanuric acid, which may either take on a keto or enol structure. Both are aromatic molecules insofar as both involve six π electrons in a planar six-membered ring. Which is preferred and why? 44 O HN O • H2N NH3 + HN C O NH 2 O HO NH N H O N N OH N OH Use the B3LYP/6-31G* model to obtain equilibrium geometries for urea, ammonia, hydrogen cyanate and the keto and enol forms of hydrogen cyanate trimer. Is loss of ammonia from urea an endothermic or exothermic process? Is it is endothermic, what do you think drives the reaction? Is trimerization to the more stable of the two forms of cyanuric acid endothermic or exothermic? Is the overall reaction endothermic or exothermic? What is the preferred structure of cyanuric acid? Cyclobutadiene: Whereas a molecule with 4n+2 π electrons in a cycle is thought to be unusually stable (aromatic), a cyclic arrangement of 4n electron has the potential of being unusually unstable (antiaromatic). A good example of this is singlet cyclobutadiene. Compare the energy of hydrogenation of cyclobutadiene (to cyclobutene) with that of hydrogenation of cyclobutene (to cyclobutane). Use the B3LYP/6-311+G** model. The two reactions are similar in that each trades an HH bond and a CC π bond for two CH bonds. However, only the first reaction removes interaction of the coplanar double bonds. Aromaticity of Cyclopentadienyl and Cycloheptatrienyl Radicals: Cyclopentadienyl radical assumes a planar geometry with a total of five π electrons. Cycloheptatrienyl radical is also planar but has seven π electrons. Do these radicals benefit from aromatic stabilization? To tell, use the same test previously employed for benzene and related systems, that is, compare energies for the first step in an overall hydrogenation reaction with that of subsequent steps. Use the B3LYP/6-31G* model to obtain equilibrium geometries for all the molecules in the above two reactions. For each, calculate the sequence of hydrogenation energies. Would you conclude that either cyclopentadienyl and/or cycloheptatrienyl radical is aromatic? Elaborate. Aromaticity of Cyclopentadienyl Anion and Cycloheptatrienyl (Tropylium) Cation: The cyclopentadienyl anion is among the most common ligands found in transition-metal organometallic compounds. Here it maintains a planar (or nearly planar) geometry and, is generally thought to contribute six electrons to the valence shell of the metal to which it is bonded. Is cyclopentadienyl anion aromatic? Certainly it fits the obvious criteria of possessing a planar cyclic arrangement of 4n+2 π electrons (as does benzene). However, these electrons are spread over only five carbons (and not six as in benzene). 45 The opposite situation exists for cycloheptatrienyl cation (commonly known as tropylium). It is also planar with six π electrons, but these are spread over seven carbons. Do these ions benefit from aromatic stabilization? To tell, use the same test previously employed for benzene and related systems, that is, compare energies for the first step in an overall hydrogenation reaction with that of subsequent steps. – + H2 H2 – + H2 – H2 + + Use the B3LYP/6-31G* model to obtain equilibrium geometries for all the molecules in the above two reactions. For each, calculate the sequence of hydrogenation energies. Would you conclude that either cyclopentadienyl anion and/or cycloheptatrienyl cation is aromatic? Elaborate. Ring Strain A molecule with an sp3 center incorporated into three-membered rings would be expected to be energetically disfavored. The main reason is that the centers cannot adopt their ideal tetrahedral values (bond angles of 109.5o). Another reason is that planarity of the skeleton leads to eclipsing...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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