P3_Reaction Energies

Obtain equilibrium geometries for these two compounds

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Unformatted text preview: H H H HN O N H N N H O N N N CH 3 N NH H N N O H H HN N CH3 NH2 N N N N H CH3 N O N N CH3 N N N H HN O N H3 C O C H3 C H3 N N O Metal-Ligand Binding in Transition-Metal Organometallics Discussion to be written Metal-Ligand Binding Energies in Iron Carbonyls: Both ethylene and acetylene bind to iron tetracarbonyl to form stable organometallics. Obtain equilibrium geometries for these two compounds as well as for their component fragments (ethylene, acetylene and iron tetracarbonyl), and calculate metal-ligand binding energies. Use the B3LYP/6-31G* model. Which is more tightly bound? Is your result consistent with changes to the C=C and C≡C bonds in ethylene and acetylene that result from binding to the metal carbonyl? Elaborate. Perform analogous calculations on butadiene iron tricarbonyl and cyclobutadiene iron tricarbonyl as well as 1,3-butadiene, cyclobutadiene and iron tricarbonyl. Which ligand is more tightly bond to iron tricarbonyl, butadiene iron or cyclobutadiene? Is this result in line with the relative bond length changes in the two dienes? Elaborate. Metal-Ligand Binding Energy in Benzene Chromium Tricarbonyl: Stable πcomplexes between chromium tricarbonyl and arenes are common and are sufficiently stable to allow “chemistry” to be performed on the ligand. Cr OC CO CO Use the B3LYP/6-31G* model to obtain equilibrium geometries for benzene chromium tricarbonyl as well as for benzene and chromium tricarbonyl. Is the metal-ligand binding energy of the same order of magnitude as a normal covalent bond (250-400 kJ/mol), or of the same order of magnitude as a hydrogen bond (20-30 kJ/mol) or somewhere in between? Compare the binding energy in benzene chromium tricarbonyl to those in complexes considered in the previous problem. Does benzene appear to be a better (more tightly bound) ligand than ethylene or acetylene? Than butadiene or cyclobutadiene? Rationalize you results. Tebbe Reagent: Early transition metal carbenes, LnM=CRR’, catalyze a variety of related processes involving olefins, including metathesis and polymerization. The Tebbe reagent, Cp2Ti=CH2 Me2AlCl, exhibits similar behavior suggesting that it is nothing more than a weak complex between a titanium methylidene and a Lewis acid. 50 Use the B3LYP/6-31G* model to establish the equilibrium geometry for the Tebbe reagent as well as for its methylidene and Lewis acid components, and calculate the energy of complexation. is both an excellent olefin metatesis catalyst for olefin 51 Using Approximate Equilibrium Geometries Is it actually necessary to obtain geometries with the same theoretical model required to accurately calculate reaction energies, or are geometries from a simpler (lower computation cost) model sufficient? This is an important question because determining equilibrium geometry may easily require one or two orders of magnitude more computation than calculating energy at a fixed geometry, due to the need of multiple calculations at different geometries. It is also straightforward to answer. Table P3-6 compares energies for structural isomers obtained from B3LYP/6-311+G** calculations using four different sets of geometries: 3-21G, 6-31G*, B3LYP/6-31G* and “exact” (B3LYP/6-311+G**). Also provided are reference isomer energies obtained from G3(MP2) calculations. The conclusion is clear. The magnitude of the error introduced as a result of using approximate geometries is insignificant in comparison with that inherent to the underlying B3LYP/6-311+G** model. Even the HF/3-21G model provides satisfactory geometries. Some properties will be more sensitive to small changes in geometry, for example, the dipole moment. Also, the infrared spectrum of a molecule (discussed in Chapter P2) requires that the geometry be calculated using the same theoretical model. However, for reaction energy calculations, the savings in computation cost achieved by the use of approximate geometries more than offsets any small changes in results. 52 Table P3-6: Effect of Use of Approximate Geometries on the Energies of Structural Isomers from B3LYP/6-311+G** (kJ/mol) 3-21G reference propyne 6-31G* B3LYP/6-31G* B3LYP/6-311+G** G3(MP2) isomer allene -7 -7 -7 -7 3 cyclopropene 102 101 101 101 102 cyclopropane 38 38 38 38 38 2-butyne 36 37 37 37 3 bicyclo[1.1.0]butane 132 130 130 130 119 cyclobutane 47 46 47 47 48 1,4-pentadiene 63 63 63 63 65 methyl isocyanide 100 100 100 102 102 vinyl alcohol 45 43 43 43 43 oxirane 124 122 121 121 116 ethanol dimethyl ether 48 48 47 47 49 acetic acid methyl formate 69 69 68 68 70 methyl vinyl ketone cyclobutanone 24 23 24 24 23 divinyl ether 101 98 98 98 102 1 0 0 - - propene 1,3-butadiene isobutene cyclopentene acetonitrile acetaldehyde mean absolute deviation from “exact” mean absolute error from G3(MP2) - 53 Silaolefins: Only a very few compounds incorporating a carbon-silicon double bond (“silaolefins”) have been characterized, among them compounds 1-4. On the contrary, stable compounds with silicon-carbon single bonds (“silanes”) are common. There is indirect evidence that suggests that the lack of silaolefins is due at least in part to the high reactivity of silicon-carbon double bonds. Specifically...
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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 Berkeley.

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