Structurally nonplanar

Condensation reactions on nickel(II) matrices are sensitive to steric effects. X-Ray structm analysis showed [310, 320-328] that H2L263 and its metal complexes are saddle-like, rather than planar. The sources of structural nonplanarity are steric repulsions between methyl groups R, 4 and orr/K>-aromatic protons of the Schiff bases. The increase in the bulk of the groups in these positions hinders macrocyclisation. For example, 3,5-heptanedione, 4,6-nonanedione and 1-phenyl-2,4-pentanedione form cyclic products [Ni(L274)]-[Ni(L276)J, whereas dibenzoyl-methane, 2,6-dimethyl-3,5-heptanedione and 2,2,6,6-tetramethyl-2,5-heptanedione do not give macrocyclic compounds in the presence of nickel(II) and o-phda [300, 320]. In the case of dibenzoylmethane, the nickel(II) )ff-diketonate bis-diamine adduct is isolated (Eq. 2.164) [319]. 

Structural studies show allene to be nonplanar As Figure 10 7 illustrates the plane of one HCH unit is perpendicular to the plane of the other Figure 10 7 also portrays the reason for the molecular geometry of allene The 2p orbital of each of the terminal car bons overlaps with a different 2p orbital of the central carbon Because the 2p orbitals of the central carbon are perpendicular to each other the perpendicular nature of the two HCH units follows naturally... 

Cyclopentane is nonplanar, with a structure that resembles an envelope (see Fig. 1.5). Four of the carbon atoms are in one plane, and the fifth is out of that plane. The molecule is in continual motion so that the out-of-plane carbon moves rapidly around the ring. 

An important issue in the thermodynamics of confined fluids concerns their symmetry which is lower than that of a corresponding homogeneous bulk phase because of the presence of the substrate and its inherent atomic structure [52]. The substrate may also be nonplanar (see Sec. IV C) or may consist of more than one chemical species so that it is heterogeneous on a nanoscopic length scale (see Sec. VB 3). The reduced symmetry of the confined phase led us to replace the usual compressional-work term —Pbuik F in the bulk analogue of Eq. (2) by individual stresses and strains. The appearance of shear contributions also reflects the reduced symmetry of confined phases. 

To illustrate the effects of nonplanarity of the substrate on fluid structure, a hard-sphere fluid exposed to a periodic array of wedges (see Fig. 13) is... 

Spectroscopic methods have been successfully applied to the elucidation of some details of the fine structure of isoxazole derivatives. Thus IR spectra revealed steric hindrance in the case of some 3,4,5-trisubstituted isoxazoles for phenylisoxazoles this results in the nonplanarity of the benzene and isoxazole rings and decreasing mutual interaction. 

An acetyl group in the 2-position favors the monocyclic structure presumably because of the resonance stabilization.12 The same observation was made with oxepin-2,7-dicarbaldehyde, oxepin-2,7-dicarboxylic acid, and oxepin-2,7-dicarbonitrile.23 Substituents in the 4- and 5-positions of the oxepin such as methyl or methoxycarbonyl groups shift the equilibrium towards the epoxide.12 24 Low temperature 1H NMR studies on 7-ethyloxepin-2-carbonitrile and ethyl 7-ethyloxepin-2-carboxylate established a nonplanar boat geometry with a ring-inversion harrier of 6.5 kcal mol-1.25... 

X-ray structures of 2,7-uiphenyloxepin,34 3,7-di-te/7-butyl-5-phenyl-2-(4-tolyl)oxepin,35 and tert-butyl oxepin-2-carboxylate25 and various 3,6-alkanooxepin derivatives36-38 confirmed a nonplanar boat geometry that had already been assumed from spectroscopic data. 

Surprisingly, X-ray structural analysis of the dark-green, hygroscopic tetrabutylam-monium salt of 5//-dibenz[6,/]azepine reveals that the potentially antiaromatic 5//-dibenz-azepine anion has a less pronounced nonplanar butterfly structure (161° vs. 144 ) than 5H-dibenz[6,/]azepine.243... 

Benzopentathiepins are stable solids. Varacin, a cytotoxic metabolite, was isolated from a marine ascidian.398 The parent benzopentathiepin exhibits infrared absorptions at 1620, 1570, 1235 and 1020 cm" 1.399 X-ray structure analyses of benzopentathiepin and 6-methylbenzopen-talhicpin show that the seven-membered ring in both compounds is nonplanar and that it adopts a chair conformation. The S-S bond lengths are in the range 204-206 pm.400... 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

In the 1,4-dihelerocine scries 1 with different heteroatoms only the 1,4-oxazocines 2 are known. As in the case of the 1,4-diazocines, depending upon the substituent on the nitrogen, these systems exhibit either nonplanarity with a localized -electron system for acceptor substituents or planar delocalized structures for donor-substituted or the unsubstituted systems.10... 

Although [34]octaphyrin 80 fulfills Hiickel s rule, the II NMR spectrum indicates by the high-field shift of the methine protons that the system is nonaromatic. The X-ray structure analysis demonstrates clearly the reason for the lack of aromatic stabilization, namely the nonplanar loop conformation in which the whole macrocycle is twisted similarly to the [32]octaphyrin structure and which is also found for [36]octaphyrin and [40]decaphyrin structures (vide infra). 

The (Z)-isomers do not show a clear sequence (Table 7-2). This result is consistent with a planar structure for the ( >diazoates and a nonplanar, sterically hindered structure for the (Z)-diazoates, which is an obvious cause for their lower chemical stability. 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

Density functional calcnlations showed transitions from planar to nonplanar strnctnres at n = 5 with increasing size of Al and G clnsters [47], Both Si and A1 tend to build three-dimensional structures rather than two- or one-dimensional strnctnres, except for n = 3 or 4 [48],... 

---