This is the dipole dipole effect noted previously for

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Unformatted text preview: ch that they will subtract (180o) and not add (0o). This is the dipole-dipole effect noted previously for 1,2-difluoroethane. 8 The two-fold term favors coplanar (0o, 180o) over perpendicular (90o, 270o) arrangements. This may be interpreted as evidence for electron donation from the nitrogen lone pair into an empty σ* orbital associated with the CF bond. The resulting delocalization leads to stabilization. The three-fold term reflects the preference for staggered (60o, 180o, 300o) over eclipsed (0o, 120o, 240o) conformers. 9 Conformational Energy Profiles for Simple Molecules We next examine the conformational energy profiles about single bonds for a selection of simple molecules, fitting these to three-term Fourier series, and interpreting the results. The energy curves have all been provided from HF/6-31G* calculations. Higher-order terms typically do not contribute significantly for bond involving first and second-row main-group elements, although they may be important where heavier elements an in particular transition metals are involved. However, a three-term Fourier series is not able to … Methylamine and Ethylamine The geometry about nitrogen in methylamine, ethylamine and other organic amines is roughly tetrahedral. Three of the four tetrahedral directions point toward the directly-bonded atoms, while the fourth direction is occupied by a non-bonded pair of electrons (a lone pair). As a result, the nitrogen centers in amines are pyramidal and not planar. The energy plot for CN bond rotation in methylamine is quite similar to that shown previously for ethane; over 360o rotation, both show three identical energy minima corresponding to staggered structures and three identical energy maxima corresponding to eclipsed structures. That is to say, the CH single bonds in methylamine prefer to stagger the nitrogen lone pair in the same way that the CH single bonds in ethane prefer to stagger each other. The only significant difference between the two energy curves is a 50% reduction in the energy difference between staggered and eclipsed conformers (~8 kJ/mol in methylamine vs. ~12 kJ/mol in ethane). 10 As was the case of ethane, only the three-fold term in the Fourier series contributes significantly. Given the similarity of energy profiles for ethane and methylamine, it is not unexpected that the plot for CN bond rotation in ethylamine is qualitatively similar to that for n-butane. There are, however, significant differences. For one, the anti and gauche minima in methylamine have nearly the same energy. Also, the energy barrier connecting the two gauche conformers in methylamine (through a syn structure) is much smaller than the analogous barrier in n-butane. This suggests that destabilizing interaction of the nitrogen lone pair and a CC single bond is of less consequence than that between two CC bonds. Finally note that the three-fold term dominates the Fourier series describing CN bond rotation in ethylamine. E(HCN:) = 1 [1-cos(HCN:)] + 1 [1-cos2(HCN:)] -5 [1-cos3(HCN:)] Energy Profiles for 1-Chloropropane and 1,2-Dichloroethane: As described earlier, rotation about the central carbon-carbon in n-butane by 360o leads to an energy curve with three minima. The anti conformer (CCCC torsion angle = 180o) is ~4 kJ/mol lower in energy than the pair of identical gauche conformers (CCCC torsion angles ~120o and ~240o). The usual explanation for the preference is that a methyl group is larger than hydrogen, and unfavorable non-bonded contacts (steric effects) will be smaller for the anti conformer than for the gauche conformers. Finally, note that energy barriers separating anti and gauche conformers are small enough to ensure their rapid equilibration. 11 Use the HF/6-31G* model to obtain an energy profile for rotation about the central carbon-carbon bond for 1-chloropropane. 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 similar to that for n-butane? Which conformer (anti or gauche) is preferred and by how much? What is the roomtemperature equilibrium distribution of the two conformers? What does the similarity or difference say about the non-bonded interaction of chlorine and methyl (in 1chloropropane) relative to the interaction of two methyl groups (in n-butane)? Use the HF/6-31G* model to obtain an energy profile for rotation about the carboncarbon bond in 1,2-dichloroethane. Is what you find consistent with previous results for n-butane and 1-chloropropane? Elaborate. Energy Profile for CO Bond Rotation in Ethanol: Obtain an energy profile for rotation about the central carbon-carbon bond in ethanol. 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.) Which conformer (anti or gauche) is preferred and by how much? What is the roomtemperature eq...
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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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