Interactions steric

Between hydrophobic surfaces a completely different interaction is observed. Hydrophobic surfaces attract each other [184], This attraction is called hydrophobic interaction. The first direct evidence that the interaction between solid hydrophobic surfaces is stronger than the van der Waals attraction was provided by Pashley and Israelachvili [185,186], With the surface force apparatus they observed an exponentially decaying attractive force between two mica surfaces with an adsorbed monolayer of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Since then the hydrophobic force has been investigated by different groups and its existence is now generally accepted [189]. The origin of the hydrophobic force is, however, still under debate. 

Non-DLVO forces also occur when the aqueous medium contains surfactants, which form micelles, or polyelectrolytes. A discussion of the complex interaction is, however, beyond the scope of this book. We recommend Ref. [199], 

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Monte Carlo computer simulations of spheres sectioned into a disc [104, 105] show tliat steric interactions alone can produce a nematic phase of discotic molecules. Columnar phases are also observed [104, 105]. 

The autliors analyse tliese results in considerable detail, demonstrating tliat botli tlie stmcture of tlie surface and steric interactions between F atoms on neighbouring SiF groups influence the reaction progress. 

For acyclic fragments and molecules, the principle of longest pathways has been implemented in CORINA (sec Figure 2-95) i.e., since no ch configuration is specified, all torsions arc set to anti in order to minimize steric interactions. 

Copper is clearly the most selective metal-ion catalyst. Interestingly, proton catalysis also leads to high selectivities. This is a strong indication that selectivity in this catalysed Diels-Alder reaction does not result from steric interactions. 

The molecular overlay experiment orients the molecules to hnd the best RMS or held ht. The held ht is based on electrostatic and steric interactions. The application can hnd either the best total alignment of all molecules or the best match of all molecules to a specihed target molecule. Alignment can include a database search for conformers that show the best alignment based on the molecules under study. 

Attack on the electrophilic C-2 may occur as in the 2-aminothiazoles series, which probably explains the rearrangements observed in acidic medium (121, 711, 712, 723, 724), in aqueous medium with NaOAc (725), or with aqueous NaHCOj (725) (Scheme 232). That the initial attack probably involves the C-2 atom is substantiated by the fact that this rearrangement occurs under extremely mild conditions for 2-iinino-3-substituted-5-nitro-4-thiazolines (725). As the whole mechanism proposed (see p. 92) is reversible, when imino derivatives are submitted to such rearrangement conditions the rearrangement is expected to occur faster if steric interaction between 3- and 4-substituents exists in the 2-imino isomer. Another reaction may occur in acidic medium phenylimino-2-bipheny]-3,4-4-thiazoline hydrolyzed with hydrochloric acid gives the corresponding 4-thiazoline-2-one and aniline (717). 

Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

In the cationic intermediate (180), when the steric interaction between R, and R2 becomes too great, Ri is eliminated as an olefin leaving the nitrogen-unsubstituted A-4 thiazoline-2-thione (182). 

Catalytic asymmetric hydrogenation was one of the first enantioselective synthetic methods used industrially (82). 2,2 -Bis(diarylphosphino)-l,l -binaphthyl (BINAP) is a chiral ligand which possesses a Cg plane of symmetry (Fig. 9). Steric interactions prevent interconversion of the (R)- and (3)-BINAP. Coordination of BINAP with a transition metal such as mthenium or rhodium produces a chiral hydrogenation catalyst capable of inducing a high degree of enantiofacial selectivity (83). Naproxen (41) is produced in 97% ee by... 

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymeri2ation (see Table 1) (24). When corrected for the heat of fusion, it is significantly less than the theoretical value of —83.7 kJ/mol (—20 kcal/mol) for the process of converting a double bond to two single bonds. The steric strain apparentiy is not important in the addition step, because VDC polymeri2es easily. Nor is it sufficient to favor depolymeri2ation the estimated ceiling temperature for poly (vinyhdene chloride) (PVDC) is about 400°C. 

Rotation about the 1,1 -bond is resisted by van der Waals interactions between the hydrogens shown in the structures. These hydrogens crowd each other when the two naphthyl groups are coplanar, and the racemization process requires tjie hydrogens to move past each other. The existence of enantiomeric substituted biphenyls also depends on steric interactions between substituents. The relationship between the rate of racemization and... 

Allyl strain influences the conformation of Z-alkenes. A 4-substituted 2-alkene will prefer conformation C over D or E to minimize the steric interaction with the C—1 methyl group. ... 

Because of the same preference for coplanarity in the enamine system, a alkyl substituents adopt an axial conformation to minimize steric interaction with the amino groi. ... 

According to this concept, the aldol condensation normally occurs through a chairlike transition state. It is further assumed that the stmcture of this transition state is sufficiently similar to that of chair cyclohexane to allow the conformational concepts developed for cyclohexane derivatives to be applied. Thus, in the example above, the reacting aldehyde is shown with R rather than H in the equatorial-like position. The differences in stability of the various transition states, and therefore the product ratios, are governed by the steric interactions between substituents. 

Reductions by NaBKt are characterized by low enthalpies of activation (8-13kcal/mol) and large negative entropies of activation (—28 to —40eu). Aldehydes are substantially more reactive than ketones, as can be seen by comparison of the rate data for benzaldehyde and acetophenone. This relative reactivity is characteristic of nearly all carbonyl addition reactions. The reduced reactivity of ketones is attributed primarily to steric effects. Not only does the additional substituent increase the steric restrictions to approach of the nucleophile, but it also causes larger steric interaction in the tetrahedral product as the hybridization changes from trigonal to tetrahedral. 

Aimulene offers a particularly significant test of the Hiickel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

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