Miscellaneous Problems

15. Betweenanenes are bis-trans-cycloalkenes fused at the double bond. That is, they are two fused cycloalkenes which share a common double bond, but in which that double bond is trans in both rings. What factors control the stability of the [n,n] compounds ranging from n = 8 to n = 11? [If someone else does Problem 12, a comparison of those answers with these would be informative; see the literature citation from Problem 1, and references therein.]

84. Unlike its planar "vinylog" benzene, cyclooctatetraene is a tub-shaped molecule. Do MM calculations on the parent compound, on its monocarboxy derivative, and on its two types of fused derivatives with n = 2, 3, and 4. Compare the MMX energies of the isomeric 1,2-bridged and 1,8-bridged compounds. Determine the "flap angle" (a measure of non-planarity; see the cited reference for its definition) of each compound; compare your results with those reported for MMX and X-ray crystal structures.

105B. Do calculations on the various cis-fused bicyclo[n.1.0]alkanes (A, n = 1, 2, 3, 4, 5); on those molecules which "might" be able to have trans ring fusion/s (B, n = 3, 4, 5); and on the cis- and trans-fused alkenes C; compare your answers with those in the article.

126. The question of whether 1,4-cyclohexadiene is planar or bent has been studied experimentally and by computation. Calculate the energies and geometries of four compounds: 1,4-cyclohexadiene itself (1); its mono- and di-benzo fused derivatives (i.e., benzene rings fused at one or both of the double bond positions); and hexahydrotriphenylene (2). Compare your answers with those in the cited reference. Suggestion: when doing this sort of MMX problem, you should always begin with a non-planar structure; often a planar structure will minimize as a local minimum, not the most stable structure.

145. A remarkable (and lengthy article) entitled "Molecular LEGO" described the syntheses of molecules such as those depicted here. Note: even though these are shown as if they were "planar," their shapes are really quite different; [12]Beltene, for example, has an inner and an outer periphery of 12 CH₂ units in the shape of a nearly circular belt. An X-Ray structure of "Kohnkene" (the first drawing) is shown in Fig. 7 of the article. Do MMX calculations (energy, structure, interesting structural features) of all three compounds shown here; compare your results with those in the article, both experimental and calculated.

[Ashton, P. R.; Brown, H. R.; Isaacs, N. S.; Giuffrida, D.; Kohnke, F. H.; Mathias, J. P.; Slawin, A. M. Z.; Smith, D. R.; Stoddart, J. F.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 6330.]

154. A recent article describes the synthesis of a series of "molecular rods" ranging from bicyclo[2.2.2]octane itself (n = 0) to the compound with n = 3. Do MMX calculations on these four molecules; obtain structural information (bond distances, twist angles, etc.) and compare your results with the calculated and experimental results in the article. Comment on any differences.

158. Here is a problem that is reminiscent of Problems 126 and 135. The central ring in 9,10-dihydroanthracene (R = H) might be planar or might be bent; 9-substituted derivatives have the option of having the R group pseudo axial or pseudo equatorial, and might turn out to be more or less bent than the parent. Do MM calculations on R = H and on both axial and equatorial conformations of R = Me, iPr, t-Bu, TMS. Determine the and alpha values (see the article for definitionsof these terms) and compare them with those reported in Table 1.

166. A review article related to the subject matter of Problem 145 has recently been published. Four isomeric structures (compounds 14 to 17, shown below) are discussed in the article. Using the aromatic carbon notation (rather than the much slower pi-calculation), calculate the relative energies of these isomers. Compare your results with those in the article. Discuss why these four compounds differ in stability.

[Girreser, U.; Giuffrida, D.; Kohnke, F. H.; Mathias, J. P.; Philip, D.; Stoddart, J. F. Pure Appl. Chem. 1993, 65, 119.]

188. Shown to the right are cubane (C₈H₈) and "homocubane" (C₉H₁₀). There are five possible "bishomocubanes" (C₁₀H₁₂) that are derived from homocubane by inserting a CH₂ group into one of the numbered bonds of the remaining cubane nucleus (these are called 1,2- or 1,3- or 1,3', or 1,4- bishomocubane) or into the same bond of cubane that had already been inserted in the production of C₉H₁₀ (1,1- bishomocubane). Note: this latter compound with a CH₂CH₂ bridge is also called "basketane." Compute the relative energies of these C₁₀H₁₂ isomers; compare your results with those in the cited reference.

195. Successful syntheses of several methano-bridged [5]- helicenes have recently been reported. Unlike the helicenes themselves, which adopt a helical structure, these compounds are constrained by the bridge to adopt a different sort of non-planar arrangement. Do calculations on structure and energy of the parent compound, R = R' = H and the mono-methyl derivative R = CH₃; for the latter, compare the bond distances, bond angles, and dihedral angles with those found in the x-ray structure. Then, calculate the energies of the dimethyl compounds, R = R' = CH₃, in which the methyl on the bridge can be either exo or endo.

213. The exo-fused "ladderanes" are an interesting class of molecules. Do MMX calculations on [10]ladderane (1) and on the two derivatives 2 and 3 in which CH₂ groups are inserted into the C-C bonds of the parent. Compare the shapes and structures that you calculate with those found in the article. Use these three as prototypes for others that may strike your fancy.

255. Removal of H's from adjacent carbons of neopentane and replacement of them with - CH=CH- units leads to a series of compounds called "centroquinacenes." Do MMX calculations on centrodiquinacene (shown), and the remaining tri-, tetra-(shown), penta- and the beautifully symmetric hexaquinacene.

[Kuck, D.; Schuster, A.; Gestmann, D.; Poteher, F.; Pritzkow, H. Chem. Eur. J. 1996, 2, 58.]

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