Overcrowded Molecules

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

A close-packed monolayer of long, planar molecules must necessarily contain at least "islands" of local order in which molecules are packed plane-to-plane. One notices that crystals of these dyes tend to pack in sheets, within which the molecules have their long axes parallel to one another and parallel to the plane of the sheet. The molecular short axes are nearly perpendicular to the plane of the sheet, and the intermolecular contacts within the sheet tend to occur at the graphitic distance, except for molecules which are bent or twisted out of planarity. Twisted molecules ("overcrowded" in Brooker s classification) generally make poor sensitizers, probably because they have a fast route of deexcitation directly from the excited singlet state to the ground state opened by the existence of the twisted, stressed structure. 

Part 27 of Structures of Sterically Overcrowded and Charge Perturbed Molecules, for part 26 see H. Bock, C. Nather, Z. Havlas, A. John and C. Arad, Angew. Chem. in print. 

Results from X-ray studies of three annulenes are presented In Table 8. According to Hiickel s rule [14]annulene (14-ANN) and [18]annulene (18-ANN) should be aromatic and most probably planar molecules, while [16]annulene (16-ANN), as a [4n]annulene, should be antiaromatic. The [14]annulene molecule is nonplanar, with a structure that approaches C2h symmetry. The cause of the nonplanarity is the steric overcrowding in the center of the molecule. While the spread of the individual bond lengths implies possible significant differences, there is no significant pattern to the values obtained. 

In contrast, the structurally similar molecules 66 and 67 are strictly monomeric in the solid state, with large distances to the lead atoms of neighboring plumbylene molecules. The same holds for the diarylplumbylene 6885 and for the sterically extremely overcrowded plumbylenes 69,86 70,87 as well as 71.88... 

Van der Waals deshielding In big molecules where there is overcrowding, it is possible that some proton may be occupying sterically hindered position. Thus such a proton will be deshielded and will resonate at a higher value of 8 than expected. 

Thermal cleavage of C—C bonds has been studied in cyclopropanes and cyclophanes, and particularly extensively in highly substituted alkanes. Riichardt and his school discovered a linear correlation between the experimental activation enthalpy for the homolysis of the weakest bond in overcrowded ethanes and the strain in the ground states of these molecules in accordance to MM2 calculations (284). 

Figure 9. A view of the P4(NMe)/jS4 molecule with the two locations Si and S2 found for the sulfur atoms at 3 °C (11). This disorder is shown on one phosphorus atom only to avoid overcrowding.

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

Much is known empirically on B-NMR data (39). Restricting boron compounds to noncyclic molecules with no or one nitrogen atom and two, three, or four carbon atoms bonded to boron and omitting sterically overcrowded alkyl groups R and R, we find four classes with the following typical chemical shift ranges (Et20 BF3 as the external standard) ... 

McKervey and coworkers have developed an elegantly simple pathway for the conversion of m-bicyclo[3.3.0]octane-2,6-dione to (C2)-C2o-hexaquinane diketone 858 (Scheme CII) Twofold cyclopentenone annulation and hydrogenation rapidly led to the tetraquinane 857. Repetition of the same steps then afforded 858, the x-ray analysis of which showed the molecule to have an opened out conformation as the direct result of severe intramolecular overcrowding. 

X-Ray diffraction studies of complexes of 2,9-dimethyl-1,10-phenanthroline (cf. e.g. Preston and Kennard33 and Power34) and 2,4,7,9-tetramethyl-l,10-phenanthroline (e.g., Canty and Gatehouse35) and the overcrowded molecule, l,10-dichloro-3,8-dimethyl-4,7-phenanthroline,36 have also been determined. 

Overcrowded Molecules.—Many condensed aromatic molecules that are forced into nonplaruir configurations by steric repulsion have been studied during recent years. An example is 3,4-5,6-dibenzophenan-threne,... 

Another aspect of steric overcrowding which has not attracted much attention, although obviously of fairly general potential occurrence, is illustrated by the dipyridothiophene (14). In a hypothetical planar molecule the distance between the two peri-chlorine atoms would be 2.05 A. In practice this is reduced to a more acceptable 3.06 A, principally as a result of displacement of the peri -chlorines by about 0.6 A from the least-squares plane (74JCS(P2)1925). 

Figure 9.3. Basis vectors used to describe the atomic displacements in the MLg molecule, showing the numbering system used for the seven atoms. (Labeling of the unit vectors at atoms 4, 5, and 6 is not shown explicitly to avoid overcrowding the figure.)...

Taking phenanthrene (Fig. 2) as a very simple example, Senent and Herraez (1957a) calculated the deformation energy for relief of the overcrowding, implied by the cf5-planar conformation of the molecule,... 

Type of deformation Deformation energy for overcrowded H-atoms only Deformation energy for whole molecule... 

The consequences of intermolecular interactions are of importance when considering the results of X-ray analysis of crystals the apparent deviations from planarity of molecules having no obviously overcrowded atoms may be wholly caused by crystal forces. Anthracene is an interesting example in this category. Cruickshank (1956a) showed that... 

The dipole moments found in perylene and 3,4-benzophenanthrene can be explained by assuming that intramolecular overcrowding causes these molecules to be non-planar. The resulting differences in electronegativities of the various carbon atoms in the molecules could then quite easily be responsible for the weak but measurable dipole moments which were found. 

In spite of these difficulties great progress has been made by Pinsker (1953) and Weinstein (1956) and by Cowley and Rees (1958). In this work the Fourier method has been used to delineate potential distribution in molecules with considerable clarity. The results are of interest in the present context because it is already clear that electron diffraction is much more powerful than X-ray diffraction for the detection of hydrogen atoms in organic molecules. Future results may therefore be important in the study of overcrowded aromatic molecules. 

For the type of structures with which we are concerned in this article this is the important application. The detection of hydrogen by X-ray diffraction is always difficult and never accurate. It is important to know the precise positions of the hydrogen atoms in many aromatic molecules where some degree of overcrowding must exist, and this information can probably only be obtained by neutron diffraction. Already accurate studies of hydrogen positions have been made for... 

IV. Classification of Compounds In the discussion of planar and non-planar aromatic molecules which constitutes Section V of this review, the criterion used in arranging the molecules into the various categories is simply that of the nature of the overcrowding which they exhibit. Biphenyl-like systems are not discussed as they were reviewed not too long ago (Wepster, 1958). The classification used, which differs slightly from that used in the review by Hamik et al. (1954), is as follows ... 

Class G. Examples of ethylenic overcrowding. Two molecules, difluorenylidene and dianthronylidene (dianthrone) are discussed. 

---