P3_Reaction Energies

Singlet and triplet carbenes molecules incorporating

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Unformatted text preview: (n=1-4). Repeat your calculations and analyses on both carbon and silicon compounds substituted with chlorine instead of fluorine, CClnH4-n and SiClnH4-n (n=1-4). Singlet and Triplet Carbenes: Molecules incorporating divalent carbon are referred to as carbenes or methylenes. The parent compound, CH2 (methylene), is known to possess a triplet ground state, with one unpaired electron residing in an in-plane σ orbital and the other in an out-of-plane π orbital. The lowest-energy singlet state (with both electrons in the σ orbital) is known experimentally to be approximately 42 kJ/mol higher in energy. Because triplet methylene and other triplet carbenes have one fewer electron pair than the corresponding singlets, Hartree-Fock models will always bias in favor the former. Thus, the (estimated) Hartree-Fock limiting (cc-pVQZ basis set) energy difference for triplet and singlet methylene (triplet favored) is 118 kJ/mol. B3LYP and MP2 models show no such bias. Limiting (cc-pVQZ basis set) singlet-triplet energy splittings are 48 and 60 kJ/mol (in favor of the triplet). These two models appear to correctly assign the ground state, and (in the very few cases for which experimental data exist) provide a good account of singlet-triplet energy differences. Use the B3LYP/6-31G* model to obtain equilibrium geometries of both singlet and triplet states of methylene, difluoromethylene (CF2), dichloromethylene (CCl2) and dibromomethylene (CBr2). Adjust the calculated singlet-triplet energy differences for CF2, CCl2 and CBr2 to account for the error in the singlet-triplet difference for CH2 and assign the ground state for each. Rationalize any change in preferred ground state (relative to the parent compound) that you uncover. 24 Singlet and Triplet Cyclobutadiene: While cyclobutadiene, C4H4, is a very short-lived molecule, the cyclobutadienyl ligand is common throughout organometallic chemistry. Here, the coordinated metal may change the number of π electrons from 4 (meaning that cyclobutadiene is formally an antiaromatic molecule) to a lesser or greater number. Does cyclobutadiene itself possess a singlet or triplet ground state? To tell, obtain equilibrium geometries for both singlet and triplet cyclobutadiene using the B3LYP/6-311+G** model, starting from structures that are not square. Describe the geometries. Is either or both square? If not, provide a rational as to why not? Calculate the singlet-triplet energy difference in cyclobutadiene, and correct it to account for the error in the corresponding difference in methylene (experimentally the triplet is favored by 42 kJ/mol). Which state is favored and by how much? Cyclopropylidene and Tropylidene Use the B3LYP/6-31G* model to obtain equilibrium geometries for both singlet and triplet state of cyclopropylidene. Correct the calculated singlet-triplet energy difference to account for the error in the corresponding difference in methylene (experimentally the triplet is favored by 42 kJ/mol). Is the ground state different from that for methylene? Examine both the carbon-carbon bond distances in singlet cyclopropylidene as well as its highest-occupied molecular orbital (use cyclopropene as a reference). Speculate on the cause behind the state preference Repeat the calculations and analysis for tropylidene. Use cycloheptatriene as a reference and locate the molecular orbitals in the carbene that correspond to the three occupied π orbitals in cycloheptatriene. Dissociation of Krypton Difluoride: While numerous compounds of xenon are known, the only neutral krypton compound to be reported is the difluoride, XeF2. Is such a species thermodynamically stable with regard to dissociation into xenon atom and fluorine molecule? To tell, use the B3LYP/6-31G* model to obtain equilibrium structures of XeF2, F2 and (the energy of) Xe. Sulfur-Sulfur Linkages in Proteins: Nearby cysteine residues in proteins may form sulfur-sulfur linkages. In so doing they impose geometrical constraints on the protein which in turn affects its secondary structure. How strong are sulfur-sulfur bonds? To tell, consider the model reaction. H3CS-SCH3 2 CH3S Use the B3LYP/6-311+G* model to calculate equilibrium geometries for dimethyl disulfide and thiomethoxy radical, and calculate the energy of SS bond cleavage. How does it compare with other bond energies (see Excel spreadsheet AH and AB bond dissociation energies for typical examples)? 25 The next comparison involves energy differences among structural isomers. Here, the total number of electron pairs is conserved but the numbers of individual bond types are not maintained. G3(MP2) serves as a reference. As with the previous bond energy comparisons, only a summary of mean absolute errors is provided (Table P3-4). An Excel spreadsheet containing structural isomer energies for Hartree-Fock, B3LYP and MP2 models with 6-31G*, 6-311+G**, cc-pVTZ and cc-pVQZ basis sets is provided on the CD-ROM accompanying this text (structural isomer energies). Hartree-Fock models with the 6-31G* and 6...
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