The higher energy lone pair is perpendicular to the

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Unformatted text preview: recognizing that the two lone pairs on oxygen are different. The higher-energy (π) lone pair is perpendicular to the plane made by the two bonds to oxygen while the lower-energy (σ) lone pair lies in the plane. Minimizing interaction between the two high-energy π lone pairs is much more important that minimizing interaction between σ and π lone pairs. Lone Pairs in Water and Hydrogen Sulfide: Describe the two highest-occupied molecular orbitals of water as obtained from a HF/6-31G* calculation. Are they primarily bonding, non-bonding or antibonding? Is the highest-occupied orbital a σ orbital or a π orbital? Repeat your calculations and analysis for hydrogen sulfide. Point out (and rationalize) any significant differences between the two molecules. Dimethyl Peroxide: Obtain an energy profile for rotation about the OO bond in dimethyl peroxide and obtain a Fourier fit. Use the HF/6-31G* model and step from 0 to 180o in 20o increments. (It is not necessary to step all the way to 360o to identify the unique energy minima and to obtain the connecting barriers.) Is this profile qualitatively similar to that for hydrogen peroxide insofar as the location of the energy minimum and the locations and heights of the rotational barriers? Which term(s) dominate the Fourier fit? Point out any significant differences between the two and provide a rationale. Hydrogen Disulfide: Obtain an energy profile for rotation about the sulfur-sulfur bond in hydrogen disulfide and provide a Fourier fit. Use the HF/6-31G* model and step from 0 to 180o in 20o increments. (It is not necessary to step all the way to 360o to identify the unique energy minima and to obtain the connecting barriers.) Is this 15 profile qualitatively similar to that for hydrogen peroxide insofar as the location of the energy minimum and the locations and heights of the rotational barriers? Which term(s) dominate the Fourier fit? Point out any significant differences between the two and provide a rationale. Propene Unlike the previous examples which involved bonds connected by sp3 hybridized centers, the single bond in propene is between sp3 and sp2 centers. A plot of energy vs. the C=C-C-H torsion angle shows three identical minima and three identical maxima. This plot closely resembles that for ethane, and the associated Fourier fit like that for ethane is dominated by the three-fold term. Even the rotational barriers are similar (~8 kJ/mol vs. ~12 kJ/mol in ethane). The energy minima for both molecules correspond to arrangements in which CH bonds stagger. In the case of propene, this means that one of the methyl CH bonds eclipses the carbon-carbon double bond. The latter preference (single bonds eclipse double bonds) is quite general extending to CC, CN, CO and CS single bonds on the one hand and CN and CO double bonds on the other. 1-Butene: Use the HF/6-31G* model to obtain an energy profile for rotation about the central CC single bond in 1-butene. Step from 0 to 180o in 20o increments. (It is not necessary to step all the way to 360o to identify the unique energy minima and to obtain the connecting barriers.) Is this profile qualitatively similar to that for propene insofar as the location of the energy minimum and the locations and heights of the rotational barriers? Which term(s) dominate the Fourier fit? Point out any significant differences between the two and provide a rationale. cis-2-Butene: Starting from a structure of cis-2-butene in which both HCC=C dihedral angles to 0° (eclipsed), calculate and plot the energy with change in one of these dihedral angles from 0° to 180° in 20° steps. Use the HF/6-31G* model. Characterize the structure of the energy minima as staggered or eclipsed relative to 16 the CC double bond. Rationalize any difference between energy minima and maxima in cis-2-butene (the rotational barrier) with the corresponding quantity in propene. Acetic Acid Acetic acid incorporates to two rotatable single bonds and two different energy plots can be generated. The first corresponds to rotation of the methyl group, and assumes a structure in which the OH bond eclipses the CO double bond (see discussion following). The plot is nearly identical to that for propene. The three minima correspond to the methyl CH bonds eclipsing the CO double bond and three maxima correspond to these bonds staggering the CO double bond. The rotational barrier is <3 kJ/mol, much smaller than that in propene. The second energy plot is much more interesting. It is for rotation about the OH bond, and assumes that a CH bond eclipses the CO double bond (see previous discussion). There are two different energy minima, corresponding to syn (O=CCH torsional angle = 0o) and anti (O=CCH torsional angle = 180o) conformers, connected by two equivalent energy maxima (O=CCH torsional angles ~90o and ~270o). E(ϕ) = 14 (1-cos ϕ) + 21 (1-cos2 ϕ) +1 (1-cos3 ϕ) 17 The Fourier fit is dominated by the one and two-fold terms. The former accounts for the ~30 kJ/mol difference in energy between syn and anti conformers. The syn conformer is preferred in order that the local dipole moment of the C=O bond (+C=O-) and the local dipole moment due t...
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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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