65. Calculate the
relative energies of the three 1,10- dimethyl-triquinacenes and compare
your answers with those obtained by other versions of molecular mechanics
(see Table 6 of the cited reference).The next four questions are derived from a recent review article on the possibility of creating molecules with planar (or nearly planar) sp3-hybridized carbon atoms. The structures shown in Problem 65 are called triquinacenes; those in Problems 66-68 are [5.5.5.5]-fenestranes. [This name is derived from the Latin word for window, and was originally assigned to [4.4.4.4]- fenestrane which looks like a window pane.] Be sure to use molecular models for all of these questions; the closer your input structure is to "reality," the better and quicker it will minimize,
65. Calculate the
relative energies of the three 1,10- dimethyl-triquinacenes and compare
your answers with those obtained by other versions of molecular mechanics
(see Table 6 of the cited reference).
66. For this set of
[5.5.5.5]-fenes- tratetraenes, compute their heats of formation,
strain energy, and bond angles; compare your results with those shown in
Table 8 and in Figure 7 of the cited article.
67. For this set of
[5.5.5.5]-fenestra-pentaenes, compute their energies and bond angles;
compare your results with those in Table 9 (and accompanying text) in the
cited article.
68. The first two
structures represent the most stable of the tetraenes and pentaenes
(Problems 66 and 67); the third structure is a [5.5.5.5]-fenestrahexaene.
Compute the change in strain energy which occurs as each double bond is
introduced; compare your answers with those in Table 10 of the cited article.
[Gupta, A. K.; Fu, X.; Snyder, J. P.; Cook, J. M. Tetrahedron 1991, 42, 3665.]
And speaking of fenestranes, the next two problems are concerned with the strain energy and geometry of [4.4.4.4]-fenestrane (a.k.a. "windowpane," an appropriate name with an appropriate ending for an alkane).
69.
Using molecular models (careful - they can break if they are abused
too much) to enter the approximate structure, calculate the heats of formation,
strain energies, and geometries of the various [4.4.4.4]-fenestranes. Note
that each of these structures has a central C which is forced to adopt
some rather unfavorable bond angles; compound 4 is especially interesting
in that the central C has all four bonds on one side of a plane.
Compare your energies and geometries with those in the cited articles.
[Würthwein, E.-U.; Chandrasekar, J.; Jemmis, E. D.; Schleyer, P. von R. Tetrahedron Lett. 1981, 22, 843; Ibid. 1982, 23, 3306 (correction); Wiberg, K.; Olli, L. K.; Golembeski, N.; Adams, R. D. J. Am. Chem. Soc. 1980, 102, 7467.]
70.
This exercise is concerned with assessing the increase in strain on going
from a five- to a four-membered ring in certain compounds. For each of
these molecules, compute their heats of formation, strain energies (SE),
and bond angles; compare your answers with those in Figure 3 in the cited
article. Calculate deltaSE for the pair 1/2; for the
pair 3/4; for the pair 5/6; and for the pair
6/7; compare your values with the published numbers.
[Wiberg, K.; Olli, L. K.; Golembeski, N.; Adams, R. D. J. Am. Chem. Soc. 1980, 102, 7467.]
149. There have been
numerous attempts to synthesize molecules with planar or with inverted
sp3-hybridized carbons. Consider the case of "bowlane";
the authors of a recent article allege that because MM-type calculations
are deceptive for such molecules only higher-level ab initio calculations
are valid. Nevertheless, compute the structure and energy of bowlane for
three cases: the quaternary carbon "above" its four bonds; the
same carbon "below" its four bonds; or the same carbon with an
approximately tetrahedral arrangement of bonds. Compare your results with
those in the article. You might consider also doing calculations on related
molecules having "unusual" carbon atoms.
[McGrath, M. P.; Radom, L.; Schaefer, H. F., III J. Org. Chem. 1992, 57, 4847.]
161. Problems 69
and 70 were concerned with calculations on the [4.4.4.4]-,
[4.4.4.5]-, and [4.4.5.5]-fenestranes. This new problem relates to the
stereoisomers of [5.5.5.5]-fenestrane (shown to the right). There are six
distinct stereoisomers for this molecule; perhaps the easiest way to visualize
them is by looking for the bicyclo[3.3.0]octane units within, and characterizing
these fused rings units as either cis or trans. Thus, one has isomers labeled
cccc, tccc, cctt, ctct, cttt, and tttt. These isomers differ dramatically
in strain energy and in the deviation from the ideal sp3 angle
at the central carbon. Do MMX calculations on the six isomers; compare
your energies and angles with those cited in the article.
[Hirschi, D.; Luef, W.; Gerber, P.; Keese, R. Helv. Chim. Acta 1992, 75, 1897.]
174. Over the years,
various molecules have been proposed as candidates for structures having
planar tetracoordinate carbon. A recent article suggests a class
of molecules called "alkaplanes" as candidates. Shown to the
right are partial structures A and B in which the central
carbon might be planar (or nearly so) if tight cages were built around
it. Use the A framework to construct compound 6 (from the
article) by connecting C1 to C2 to C3
to C4 and back to C1; similarly around C5
to C8. Also construct 7 (from the article) by putting
CH2 bridges into each cyclobutane bond in the first structure.
Also do structures based on partial structure B, including compound
8 from the article. Tabulate the angles around the central C and
decide whether a molecule with a planar carbon has been achieved.
[McGrath, M. P.; Radom, L. J. Am. Chem. Soc. 1993, 115, 3320.]
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