P3_Reaction Energies

Which cation is lower in energy does it appear that

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Unformatted text preview: econdary carbocation center. Use the B3LYP/6-31G* model to obtain equilibrium geometries for the two isomers of adamantyl cation. Are the CCC bond angles at the cation center in 2-adamantyl cation significantly distorted from ideal (120o)? Which cation is lower in energy? Does it appear that the need to accommodate a planar cation is more important that the benefit achieved in going from a secondary to tertiary center? Elaborate. The final comparisons (Table P3-5) exemplify reactions in which both the total number of electron pairs and the numbers of individual bond types are conserved, specifically comparisons of regio and stereoisomers. As with previous comparisons, data from G3(MP2) calculations serve as a reference. 34 Table P3-5: reference Deviations from G3(MP2) of Hartree-Fock, B3LYP and MP2 Energies of Regio and Stereoisomers (kJ/mol) isomer Hartree-Fock B3LYP MP2 6-31G* 6-311+G** 6-31G* 6-311+G** 6-31G* 6-311+G** G3 1,3-butadiene 1,2-butadiene 50 53 39 43 47 49 49 2-butyne 1-butyne 28 24 33 27 23 18 22 cyclobutene methylenecyclopropane 29 26 25 20 32 31 44 isobutene trans-2-butene cis-2-butene 1-butene 1 8 13 2 9 13 1 7 18 1 7 15 4 11 18 5 11 16 5 11 17 cyclopentene methylenecyclobutene 86 85 79 79 89 89 85 2-methyl-1, 3-butadiene 23 48 58 21 51 60 34 40 53 29 42 53 30 45 60 27 49 62 29 55 62 methyl vinyl trans-2-butenal ketone 2-methyl-2-propanal 7 9 14 9 3 5 10 5 7 4 13 1 7 4 mean absolute error 5 5 8 8 4 4 – 1,4-pentadiene 1,1-dimethylallene 1,2-pentadiene 35 Isomers of Pentavalent Phosphorus Halides, PFnCl5-n: While pentavalent phosphorus halides adopt trigonal bipyramidal geometries with distinct axial and equatorial positions, these positions rapidly interconvert via pseudorotation (see discussion in Chapter P5). Therefore, observed properties, for example, the dipole moment, will be those of an equilbrium mixture of all possible isomers and depend on temperature. Use the B3LYP/6-31G* model to obtain equilibrium geometries for the two isomers of SF4Cl (with Cl in either an equatorial or axial position). Which isomer is lower in energy? Why? Calculate the dipole moment of a sample at room temperature. Is it different at 50 K? At 1000 K? Repeat the calculations for SF3Cl2 (3 isomers), SF2Cl3 (3 isomers) and SFCl4 (2 isomers). Are the results consistent with those for SF4Cl? Elaborate. Compute dipole moments at 50 K, room temperature and 1000 K. Oxy Acids of Phosphorus: The oxy acids of phosphorus can exist in one of two isomeric (tautomeric) forms, one in which the phosphorus is trivalent and the other in which it is pentavalent. O R' P R OH R' P R H The experimental evidence points strongly to the pentavalent species as the dominant form. Specifically, the infrared spectra of phosphorus oxy acids contain lines characteristic of P=O and PH bond stretches and no evidence of OH stretches (except where R or R’ is OH). The one exception appears to be the bis(trifluoromethyl) compound (R=R’=CF3). Despite the apparent preference for pentavalent forms, the known “chemistry” of the phosphorus oxy acids demands involvement of the trivalent structure. This suggests that the two must be in equilibrium, meaning that they two are close in energy. Use the B3LYP/6-31G* model to obtain equilibrium geometries for both trivalent and pentavalent forms of dimethylphosphonate (R=R’=OMe). Precede your calculations by a search of possible conformers using molecular mechanics (discussion of conformational searching will be provided in Chapter P5). Which form is lower in energy? Is your result consistent with the experiment? Elaborate. What temperature would be required for the higher energy structure to be present as 5% of an equilibrium mixture? Repeat your calculations with the bis(trifluoromethyl) compound (R=R’=CF3). If, as the experimental data suggest, there is a change in preferred structure, propose a reason why. Adamantene: The six atoms involved in a carbon-carbon double bond prefer to lie in the same plane, for example, the two carbons and four hydrogens in ethylene lie in the same plane. This does not appear to be possible for adamantene, the olefin formed from lss of hydrogen from the stable hydrocarbon, adamantane. - H2 36 Use the B3LYP/6-31G* model to obtain the equilibrium geometry for adamantene. Does the molecule actually incorporate a double bond, that is, with a carbon-carbon distance in the usual range of 1.30 to 1.34Ǻ? Is one or both of the carbons involved in the bond puckered? Are they twisted relative to each other? Display the HOMO of adamantene, and describe how it differs from the HOMO of a “normal” alkene. Relate the hydrogenation energy of adamantene to that of 2-methyl-2-butene, a molecule with similar “substituents” on the double bond. (Use the B3LYP/6-31G* model to obtain geometries for 2-methylbutane and 2-methyl-2-butene.) adamantene + 2-methylbutane adamantane + 2-methyl-2-butene What does this tell you about the strengths of the two π bonds? Bond Separation Reactions and Interaction of Substituents: A bond-separation reaction relates any molecule that can be written in terms of a Lewis structure to th...
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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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