p5_conformations

What does the similarity or difference say about the

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Unformatted text preview: uilibrium distribution of the two conformers? What does the similarity or difference say about the non-bonded interaction of the OH and CC bonds relative to the interaction of two CC bonds (in n-butane)? Hydrazine The energy profile for rotation about the NN bond in hydrazine by 360o closely resembles the curve for rotation about the OO bond in hydrogen peroxide presented at the beginning of this chapter. It incorporates two identical energy minima, corresponding roughly to the two possible arrangements with the non-bonded electron pairs on nitrogen (lone pairs) perpendicular, and two different energy maxima corresponding to arrangements in which they are coplanar. Both NH bonds eclipse as do the lone pairs in the higher-energy maximum (~44 kJ/mol above the minima). The other energy maximum is very broad and only ~13 kJ/mol 12 above the minima. It corresponds to a range of conformers centering on a structure in which the two lone pairs are anti to each other. E(ϕ ) = 11 (1-cosϕ ) + 14 (1-cos2ϕ ) + 4 (1-cos3ϕ ) The Fourier series is dominated by the one and three-fold terms. The former shows that the anti coplanar geometry (:NN: torsional angle = 180o) is favored over the corresponding syn coplanar arrangement (:NN: torsional angle = 0o). This both minimizes unfavorable steric interactions in the syn coplanar arrangement and leads to a canceling of local dipole moments associated with the nitrogen lone pairs. H HN H N H HN H N H H The large two-fold term in the Fourier fit shows that the way to minimize lone pair-lone pair interaction is to keep the lone pairs perpendicular. H HN H H H N HN H N H Energy Profile for NN Bond Rotation in Methylhydrazine: Use the HF/6-31G* model to obtain an energy profile for rotation about the nitrogen-nitrogen bond in methylhydrazine. Step from 0 to 360o in 20o increments and fit to a Fourier series. Compare and contrast the energy profile (and the fit) with that of hydrazine. How many energy minima does the curve contain? How many unique conformers of methylhydrazine are there? If there is more than one conformer, are any of the nonlowest-energy conformers likely to be sufficiently abundant at room temperature to actually be observed. (Use >1% as a cutoff.) Tetrafluorohydrazine: Obtain an energy profile for rotation about the NN bond in tetrafluorohydrazine and obtain a Fourier fit. Use the HF/6-31G* model and step (:NN: dihedral angle) from 0 to 180o in 20o increments. (It is not necessary to step all 13 the way to 360o to identify the unique energy minima and to obtain the connecting barriers.) Is this profile qualitatively similar to that for for 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. Dinitrogen Tetroxide: Dinitrogen tetroxide (O2NNO2) is an intermediate oxidation product of hydrazine. Are the two nitrogen centers coplanar or perpendicular (or somewhere in between)? To tell, obtain the equilibrium geometry using the B3LYP/6-31G* model. Start with a twisted geometry. Rationalize your result. Hydrogen Peroxide We return to hydrogen peroxide, the molecule that opened this chapter. As shown earlier, the energy curve for 360o rotation about the OO bond shows a pair of identical minima with HOOH torsional angles around 120o and 240o. These may be interconverted either via an anti (HOOH torsion angle = 180o) energy maximum that is only ~5 kJ/mol above the minima, or by a syn (HOOH torsion angle = 0o) energy maximum that is ~40 kJ/mol above the minima. The location (value of the dihedral angles) of the two energy minima in hydrogen peroxide warrants comment. Most chemists would picture the molecule as incorporating two sp3 hybridized (“tetrahedral”) oxygen atoms (as in water). Two of the hybrids would be used to construct the single bonds to other oxygen and to hydrogen, and the two remaining hybrids would be used to hold the two “equivalent” lone pairs. Such a hybrid model is consistent with the HOO bond angle of 102o obtained from HF/6-31G* calculations. However, same model also suggests that the observed 120o dihedral angle should lead to an eclipsing interaction involving one lone pair on each oxygen, and a pair of eclipsing interactions involving the other oxygen lone pair and an OH bond. Such an arrangement would, therefore, not be expected to be favorable. 14 H H The Fourier fit provides a clue to what is going on. E(ϕ ) = 16 (1-cos ϕ ) + 9 (1-cos2 ϕ ) + 1 (1-cos3 ϕ ) First, it reveals that the three-fold term (corresponding to the difference between staggered and eclipsed structures) is not very important. Rather, the fit is dominated by the one and two-fold terms. The one-fold term (favoring of an anti coplanar geometry over a syn coplanar arrangement) has the same origin as the analogous one-fold term in hydrazine, that is, the desire to minimize the sum of local dipole moments. The two-fold term (favoring a perpendicular over a planar geometry) may be rationalized by...
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