Is it use the hf6 31g model to obtain an energy

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Unformatted text preview: ring to be coplanar. As with 1,3-butadiene, most organic chemists assume that styrene is a planar molecule. Is it? Use the HF/6-31G* model to obtain an energy profile for rotation about the CC single bond. You only need to step from 0 to 90o (in 15o increments). Biphenyl: Is biphenyl (C6H5-C6H5) a planar molecule or does it assume a twisted geometry? Obtain the equilibrium geometry using theHF/6-31G* model starting from a structure that is slightly distorted from planarity. If you find that the two rings are not coplanar, obtain the geometry of molecule where they are coplanar (that is, start with a coplanar 20 structure) and calculate the energy barrier. How does it compare with the corresponding energy barriers in 1,3-butadiene and in styrene? λ max vs. Diene Conformation: A very simple way to model the energy of an electronic transition from ground to excited state (λmax in the UV/vis spectrum) is to assume that it parallels the difference in energy between the highest-occupied and lowest-unoccupied molecular orbitals (the HOMO-LUMO gap). To what extent does this gap (and λmax) for a diene depend conformation? To what extent does the change in the gap parallel the change in energy of the ground-state molecule with change in conformation? Obtain energy profiles for 1,3-butadiene and 2-methyl-1,3-butadiene, varying the CCCC dihedral angle in each from 0° to 180° in 20° steps. Use the HF/6-31G* model. For each diene, plot both the energy and the HOMO/LUMO gap as a function of dihedral angle and answer the following questions: At what dihedral angle is the HOMO/LUMO gap the largest? At what dihedral angle is it the smallest? Is there much difference in the HOMO/LUMO gap between cis and transplanar diene conformers? Does the variation in total energy closely follow the HOMO-LUMO gap or are the two uncorrelated? Molecules Incorporating Transition Metals Ferrocene and bis-Benzene Chromium: The two cyclopentadienyl ligands in ferrocene can either eclipse or stagger one another. `` The preference would be expected to be small both because the ligands are far apart and because the two conformers differ by a torsion angle of only 36o. Similarly, the two benzene ligands in bis-benzene chromium can either eclipse or stagger. Use the B3LYP/6-31G* model to obtain equilibrium geometries of both eclipsed and staggered conformers for both ferrocene and bis-benzene chromium. For each, indicate which conformer is favored. Are the energy conformer energy differences in these two molecules of the same order of magnitude as previously noted for main-group compounds, for example, ethane, or are they smaller or larger? CO OC C0 OC OC CO CO C0 CO CO CO CO OC CO O C Mn Mn CO OC OC OC CO 21 Conformational Changes from Constrained Rotation, Inversion, Pseudorotation In addition to “free” rotation about single bonds there are several other ways available for changes in molecular shape to occur. These fall into three broad catagories: constrained rotation, inversion and pseudorotation. Constrained Rotation Constrained rotation involves changes in the dihedral angles involving atoms in a ring. Six and seven-membered rings typically exhibit two or three distinguishable conformers, and larger rings almost always exhibit three or more conformers. Four and five-membered rings may also undergo constrained rotation, but the different conformers that result are typically very similar and difficult to distinguish experimentally. Constrained rotation generally involves changes in several dihedral angles either stepwise or in concert in order to pass from one conformer to another. As such, it is not as straightforward to describe the motion pathway connecting ring conformers as it is the motion connecting single-bond conformers. With few exceptions, the best that chemists have been able to do is to construct “cartoons” that satisfy what is known about the energies of the different conformers and the barriers separating them. More to the point of the present text, it will normally not be possible to construct the kinds of simple diagrams and fitting functions used up to this point to describe and interpret free rotation about single bonds. By far, the most famous example cyclohexane, a molecule that is known to exist in a so-called chair geometry (that drawn in all organic chemistry textbooks). Here, all six carbons are equivalent and the twelve hydrogens are divided into two sets of six equatorial hydrogens and six axial hydrogens. A “normal” temperatures only a single resonance at 1.36 ppm relative to tetramethylsilane is seen in the proton NMR spectrum of cyclohexane, despite the fact that the equatorial hydrogens are chemically distinct 22 from the axial hydrogens. Only when the sample is cooled to -90oC does the spectrum show the expected pair of resonances (at 1.12 and 1.60 ppm). These two observations suggest that a low-energy process exists, leading to the interconversion of equatorial and axial positions in cyclohexane. ax eq eq* ax* In addit...
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