This is reflected in a reduction in the total dipole

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Unformatted text preview: o the σ lone pair on the OH group act to cancel rather than to reinforce each other. This is reflected in a reduction in the total dipole moment from ~4.7 debyes in the anti conformer where the two local dipoles point in the same direction, to ~1.8 debyes in the syn conformer where they point in opposite directions. H O C H O O C H3C O H3 C The two-fold term reflects the fact that interconversion of syn and anti is difficult, and that arrangements in which the carbonyl group and the OH bond are coplanar are strongly preferred over arrangements in which they are perpendicular. This may be rationalized by suggesting that electron donation from the high-energy π lone pair on the OH group into the electron deficient carbon of the carbonyl group is energetically beneficial and is maximized where the two are coplanar. O C O The three-fold term is much smaller than the one and two-fold terms, suggesting that single bond staggering (and staggering of bonds and lone pairs) is less important than other factors. Methyl Acetate: Use the HF/6-31G* model to obtain an energy profile for rotation about the central CO single bond in methyl acetate, CH3C(=O)OCH3. 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 acetic acid 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. Carbonic Acid: Three conformers in which all six atoms lie in one plane can be drawn for carbonic acid. 18 H H O H O C O C O H O O O C H O O H Use the HF/6-31G* model to obtain equilibrium geometries for all three. Which conformer is preferred? Is your result consistent with the preferences previously noted for acetic and methyl acetate? Elaborate. Does the ordering of dipole moments for the three conformers parallel the ordering of energies? Does what you find support or refute the previous interpretation given to the one-fold terms in acetic and methyl acetate? Elaborate. 1,3-Butadiene The single bond in 1,3-butadiene involves two sp2 carbon hybrids. Organic chemists would assume that the two (“conjugated”) double bonds are coplanar, either cis or trans to each other. They would be half right! While trans-planar 1,3-butadiene is the global minimum on the energy curve, the cis-planar conformer is actually an energy maximum. There is a nearby energy minimum (CCCC torsional angle ~40o) that is ~12 kJ/mol higher in energy than the trans form. This structure represents a compromise between the desire for the two double bonds to be coplanar and the need to minimize non-bonded steric interaction of the terminal methylene groups. E(φ) = -4 (1-cosφ) +8 (1-cos2φ) -5 (1-cos3φ) 19 First, note that the overall quality of the Fourier fit is not as good as we have seen in previous examples. All three terms contribute significantly. The one-fold term likely reflects unfavorable interaction between the terminal CH2 groups and is consistent with the fact that, whereas the trans conformer is planar, the corresponding “cis” structure is twisted. The two-fold reflects the desire for π systems to be coplanar (conjugation). No simple “chemical” interpretation may be attached to the three-fold term. 2,3-Dimethyl-1,3-butadiene: Use the HF/6-31G* model to obtain an energy profile for rotation about the central CC single bond in 2,3-dimethyl-1,3-butadiene. 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 1,3-butadiene insofar as the location of the energy minima? Specifically, is there a trans coplanar minimum? Is there a cis minimum that is actually slightly distorted from planarity? Is the trans minimum lower in energy than the cis minimum? Which term(s) dominate the Fourier fit? Acrolein and Glyoxal: Use the HF/6-31G* model to obtain an energy profile for rotation about the central CC single bond in acrolein (H2C=C(H)-C(H)=O). 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 1,3-butadiene insofar as the location of the energy minima? Specifically, are the CC and CO double bond in the “cis” structure of acrolein coplanar or (as in the case of 1,3-butadiene) 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. Repeat your calculations for glyoxal (O=(H)C-C(H)=O) and answer the analogous questions. Styrene: Styrene (C6H5-C(H)=CH2), while larger than 1,3-butadiene is simpler, as there is only a single way for the CC double bond and phenyl...
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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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