Crowded molecules

Sterically crowded molecules, like neopentane, are too unstable. 

Variable-temperature H and 13C NMR spectroscopic study of this seemingly crowded molecule 38 revealed that the ring rotation about the iron ring axis occurs freely down to — 95 °C, although at lower temperatures the rotation of the... 

Cp Sm 59 (it can be regarded as a vinylsamarium when a Cp coordinates Sm in an -fashion) is an extremely crowded molecule, which displays high reactivity with substrates including CO, nitriles, isonitriles and isocyanates.31 The reactions proceed according to Scheme 20. 

AMI. While MNDO was widely accepted and extensively used, there were still some deficiencies in the model. In particular, excessive repulsions were observed in MNDO potential energy surfaces just outside chemical bonding distances. This deficiency manifested itself (5,7) in the inability of MNDO to model hydrogen bonding, as well as in large positive errors in the AHf of sterically crowded molecules and in heats of activation. Again Dewar set off to correct this deficiency. 

Limitations of MNDO. From its inception, some important limitations of MNDO were apparent. Sterically crowded molecules were calculated too unstable for example, the AHf of neopentane is predicted by MNDO to be —24.6 kcal/mol, compared with the observed -40.3 kcal/mol. On the other hand, four—membered rings were predicted to be too stable, this reaching a limit in cubane, which was predicted to be 49.6 kcal/mol too stable. Later on, other limitations were discovered, the most important from a biochemical standpoint being the virtually complete lack of a hydrogen bond. Other deficiencies included the extreme instability of hypervalent molecules. This effectivdy precluded the application of MNDO to organophosphorus compounds of biologic interest. Finally, activation barriers were predicted to be too high. 

Strengths of AMI. AMI is the first semiempirical NDDO method to accurately reproduce the heat of dimerization of water. This was achieved by modifying the core-core interaction. This change also corrected the instability of stericaUy crowded molecules, and the excessive stability of four membered rings. Average errors in AHf obtained using AMI are reduced by almost 40%, relative to MNDO. 

The formation of benzene (or substituted benzene derivatives) is a common transformation catalyzed by numerous homogeneous and heterogeneous metal catalysts, mainly Co, Rh, Pd, and Ni.63-69 Even highly crowded molecules, such as hexaisopropylbenzene, could be synthesized in the presence of metal carbonyls such as [Co(CO)4]2.70 A very simple catalyst system, Me3SiCl and Pd on carbon in refluxing tetrahydrofuran, has been shown to transform symmetrical alkynes into hexaalkylbenzenes in excellent yield.71... 

Characteristic of f-olefin and cyclopropene complex formation are the upfield shifts observed in the NMR resonances of the olefinic protons (<5 =3.30-5.29 ppm) and carbons (<5 = 59.9-72.4 ppm). For the phosphite complexes, these upfield shift resonances decrease as the steric bulk of the imido ligand increases, corresponding to weaker binding of the cyclopropene in the more sterically crowded molecule. Difference NOE spectroscopy... 

Other kinds of molecules besides 1 (which has unusual bond stereochemistry) for which these methods might fail to give good results are hypercoordinate molecules like NF5, molecules with noble gas atoms, particularly those of helium and neon, molecules with highly twisted C=C bonds, extraordinarily crowded molecules like hexaphenylethane, unknown dimers, trimers etc. of small familiar molecules, like C202 and N6, and very highly strained molecules. All these cases are discussed in a book on exotic molecules [4],... 

The reaction of I with P(0)Cl3 gave a very crowded molecule, bis(2,4,6-tri-t-butylphenyl)phosphinic chloride (II, mp 210 - 210.5° C) in 49 % yield, together with 2,4,6-tri-t-butylchlorobenzene (III, 29 % yield) (9), while the reaction of I with P(S)Cl3 gave mainly III in 59 % yield. Compound II has two very bulky 2,4,6-tri-t-butylphenyl groups attached geminally on one element. The chlorine atom of II resists toward the further nucleophilic attack and II is quite insensitive to moisture. The X-ray analysis of II... 

There is one method of direct alkylation of a nitrogen nucleophile. Preliminary FGI (with reduction in mind again) to an alkyl azide 52 allows C-N disconnection to the alkyl halide and azide ion 54. This interesting species is linear and can slip into crowded molecules like a tiny dart. But there is a drawback all azides are toxic and POTENTIALLY EXPLOSIVE. 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

A gra[. is defined as a set of elements V, with a binary relation E defin on the set- Usually circles represent vertices ( elements) and lines, usually referred to as edges, represent binary relations- The binary relation is both symmetric and anti reflexive- The classical text of Harary s Graph Theory [ll] collects several of the more popular types of graphs- One observes the inherent generality of the definition of graphs There are no restrictions on what the set of elements, V, represents or to what the binary relation, E, corresponds- If we model a chemical structure, V, may be the set of atoms, while E the set of bonds present, but equally E may represent "close" atomic contacts (such as those occuring in crowd molecules, etc-)- Atlernatively V... 

Table II shows some literature examples of the breakdown in additivity which can occur in flexible molecules (Compounds 1-4) or in sterically crowded molecules such as the diphenylmethanes (Compounds 5 and 6). In the first four cases, a dipole is probably interacting with polarizable 7r-electrons of the aromatic ring. In Compound 5, the overall shape probably prevents water molecules from forming a solvate iceberg between the two rings—a situation which can be considered an intramolecular hydrophobic bond. In Compound 6 a combination of intramolecular hydrophobic effects and possibly interaction of the side chain dipole with one or both aromatic rings leads to a wide discrepancy between experimentally determined and calculated log F values.

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