Nonequivalence

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

In the NMR spectrum of cis-l,2-bis[2-diethylamino-5-nitrothiazol-4-yl] ethylene (17) (1570), the nonequivalence of olefinic protons requires that the rotation of the NO2 group be hindered. 

Chemical shift nonequivalence can occur when two environments are stereochem ically different The two vinyl protons of 2 bromopropene have different chemical shifts... 

We often see splitting patterns in which the intensities of the individual peaks do not match those given in Table 13 2 but are distorted in that the signals for coupled protons lean toward each other This leaning is a general phenomenon but is most easily illus trated for the case of two nonequivalent vicinal protons as shown m Figure 13 18... 

The protons in 1 chloro 1 cyanoethene are diastereo topic (Section 13 6) They are nonequivalent and have dif ferent chemical shifts Re member splitting can only occur between protons that have different chemical shifts... 

Rotation about the carbon-nitrogen bond is slow in amides The methyl groups of NJ dimethylformamide are nonequivalent because one is cis to oxygen the other cis to hydrogen... 

Decoupling has two effects all multiplets are coUapsed into singlets, one for each nonequivalent atom and the transfer of NOE from the excited H to... 

Diastereomeric derivathation of a chiral alcohol (111) with an enantiopure compound such as Mosher s reagent [20445-33-4] (a-ttifluoromethyl-a-methoxy-a-phenylacetjichloride) (112) (91) results in two distinct compounds (113) and (114) with nonequivalent chemical shifts in the H-nmr spectmm (92). 

Structure. The thiosulfate sulfur atoms have been shown to be nonequivalent by radioactive sulfur exchange studies (1). When a sulfite is treated with radioactive sulfur and the resulting thiosulfate decomposed to sulfur and sulfite by acids, the radioactivity appears in the sulfur ... 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

SV40 and polyomavirus shells are constructed from pentamers of the major coat protein with nonequivalent packing but largely equivalent interactions... 

The two protons at C-1 are topologically nonequivalent, since substitution of one produces a product tiiat is stereochemically distinct fiom that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the more precise term enantiotopic also applies. If a chiral assembly is generated when a particular ligand is replaced by a new ligand, the original assembly is prochiral. Both C-1 and C-3 of 1,3-propanediol are prochiral centers. 

Fig. 2.8. Equivalent benzyl CHj protons in 1-benzyl-cis-2,6-dimethylpiperidine compared with nonequivalent protons in the tmns isomer. [Reproduced from Tetrahedron 21 2015 (1965) by permission of Elsevier Science.]...

If R and R are different, the two faces of the double bond become nonequivalent, permitting stereoselective reactions at the double bond. These effects have been explored, for example, using 4-silyl-2-pentenes. Reactions such as epoxidation and hydroboration proceed by preferential addition fiom the face opposite the bulky silyl substituents. 

Chirality center (Section 7.2) An atom that has four nonequivalent atoms or groups attached to it. At various times chirality centers have been called asymmetric centers or stereogenic centers. 

Spin-spin splitting (Section 13.7) The splitting of NMR signals caused by the coupling of nuclear spins. Only nonequivalent nuclei (such as protons with different chemical shifts) can split one another s signals. 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Consequently rotation about the carbon-nitrogen bond constitutes exchange of the methyl protons between nonequivalent sites, analogous to cis-trans isomerization ... 

The study of the NMR spectra of thiophenes has attracted considerable interest, 22,24-3sb partly because the spectra of substituted thiophenes containing only a few ring hydrogens are quite suitable for complete analysis and partly because in a series of related compounds the chemical shifts observed are related to differences in the electron distribution about chemically nonequivalent hydrogens (for review, see reference 39), especially for hydrogens far removed from the substituent. 

Tire reduction of TAF 100 by metallic potassium resulted in the formation at room temperature of the stable anion radical 109, which yielded a simple nine-line ESR pattern caused by the two sets of two equivalent nitrogens with Ani = 3.40 and An2 = 0.81 G (79JOC3211).Tlie nonequivalency of the nitrogens was explained by the association of the potassium cation with one of the two diazacylopentadienyl moieties (Scheme 44). 

To obtain information about the cyclization pathway, an analogous reaction was carried out with phenylhydrazine (in this case, positions 3 and 5 of the forming pyrazole are nonequivalent) (69ZOR1179). The only reaction product (yield 65%) appeared to be l-phenyl-5-methylpyrrazole (108). 

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