Skeletal isomers

There is evidently a flat potential energy surface for these clusters as different skeletal isomers can actually be isolated in the solid state for Au9[P(C6H40Me)3]8(N03)3 a brown form has a D4d crown structure and a green form has a D2h structure. In solution, however, they share a common structure with identical31P NMR spectra either the solution cluster has a regular arrangement of phosphines, or it is fluxional [186]. 

ROP. Their concentration is lower than the one of Ethyl- and Propyl-ROP at low conversions, but with increasing conversion they become the most abundant ROP group. Nevertheless, a noteworthy trend is observed for concentrations of all ROP groups, except Butyl-ROP, which increase rapidly after reaching conversion higher than 60%. This links the ROP formation to the secondary formed skeletal isomers of decalin (DMBCO) rather than to MBCN, which are present already at lower conversions in sufficient amounts. 

Structural (skeletal) isomers differ from each other in the length of their carbon atom chains and in the length of the side chains. The carbon skeleton differs between these isomers. Positional isomers differ in the location or position where functional groups are attached to the carbon skeleton. Geometric isomers differ in whether two substituents are on the same side of the molecule or on opposite sides of the molecule from each other usually they are on opposite sides or the same side of a double bond. 

The affect of architecture on small molecular properties has been recognized since the historical Berzelius (1832) discovery that defined the following premise substances of identical compositions but different architectures - skeletal isomers - will differ in one or more properties [15]. These effects are very apparent when comparing the fuel combustion benefits of certain isomeric octanes or the dramatic property differences observed in the three architectural isomers of carbon namely graphite, diamond and buckminsterfullerene. 

C5 Cyclization of various alkanes 38, 38a) over platinum on carbon was first observed in 1954. Barron et al. 15a) postulated the formation of a surface C5 cyclic intermediate, which may desorb as a cyclopentanic hydrocarbon or may produce skeletal isomers without desorption. 

One-electron reduction of a norcaradiene derivative produces the corresponding anion-radical. The conditions of the odd-electron delocalization in this anion-radical are less favorable than in its skeletal isomer. According to calculations, the incorporation of the unpaired electron in the nonatetraenyl n system lowers the energy content by 0.62p. However, the anion-radical initially formed is less stable than the benzotropylidene anion-radical. The latter is the end product of the isomerization (Gerson et al. 1978 Scheme 6.33). 

Figure 10 The structures of [PWl20 ]3 ( Keggin , a) and four possible skeletal isomers (fi, y, S, e) formed by n/3 rotation of one, two, three and four W3013 groups, respectively. The rotated groups are shown unshaded...

Two or more compounds made up of the same number and types of atoms but with different chemical structures are known as isomers. We have already seen the difference between the two isomers -butyl and isobutyl alcohol. These are known as skeletal isomers, since they have different carbon skeletons. There are also positional isomers in which the "functional group" is located at a different position on the carbon skeleton. For example, alpha, beta, and gamma terpineol all have the same arrangement of carbon atoms but with the—OH (hydroxy) group and the double bond in different positions ... 

As well as bridging a W—W edge, the Cu(PPh3) fragment is also bonded to the carbyne C atom in the CC6H4Me-4 unit. fcIn the solid state, the cluster anion adopts the structure of isomer A (Fig. 9), but skeletal isomers A and B (Fig. 9) are both present in equilibrium in solution. 

The cluster exists as two skeletal isomers in the solid state (Fig. 8). 

Fig. 8. Structures of the two skeletal isomers of the dusters [AuRu5C(CO),3(NO)-(PEt3)], with the CO and NO ligands omitted for darity.

Fig. 9. Structures of the two skeletal isomers of the duster anions [MFe4(CO)13L] (M = Cu, L = PPh3 or CNC6H3Me2-2,6 M = Au, L = PPh3 or PEt3).

The NMR spectra of the cluster suggest that two skeletal isomers are present in solution at ambient temperature. It seems likely that these two isomers exhibit metal core structures similar to those which the two isomeric forms of the very closely related cluster [Au2Re2H6(PMe2Ph)4(PPh3)2] are thought to adopt (Fig. 11) (52). 

The clusters [Cu2Ru4(/i-H)2(CO)12 P(CHMe2)3 2], [Au2Ru6C(CO)16-(PEt3)2], and [Au2Ru5M(CO)17(PEt3)2] (M = Cr, Mo, or W) exhibit only one metal core structure in the solid state, but two skeletal isomers exist in solution at low temperatures. Interestingly, at ambient temperature in... 

Fig. 11. Structures of the skeletal isomers of the cluster [Au2Re2H6(PMe2Ph)4(PPh3)2], with the three and two hydrido ligands which bridge the Re—Re vectors of isomers A and B, respectively, omitted for clarity.

Fig. 12. Structure of one skeletal isomer of the cluster [Au2Os4(/a-H)2(CO), 2(PR3)2] (R = Ph or Et), with the hydrido ligands, which are thought to bridge two Os—Os vectors, omitted for clarity.

Fio. 13. Structures of the two skeletal isomers of the cluster [Cu2Ru4(/x3-H)2(CO) 12-[P(CHMe2)3 2]. The positions of the hydrido ligands in isomer B are not known. [Reprinted with permission of the Royal Society of Chemistry (140).]... 

Although signals arising from the two skeletal isomers can be observed at low temperatures, the Au atoms in the isomer with two inequivalent Au(PEt3) moieties are still undergoing site exchange. 

Cu2Ru4(/i3-H)2(CO)12 P(CHMe2)3 2] Two capped trigonal bipyramidal skeletal isomers, one with and one without the Cu atoms in close contact, at -90°C in solution" 2 and 1 H and 31P- 1H spectra show equivalent P atoms Yes 140... 

Gold dusters [ Au3Re( -H)2(PMe2Ph)3(PPh3)j] Two skeletal isomers, one with a tetrahedral and one... 

---