Equivalent protons

Number of equivalent protons to which nucleus is coupled Appearance of multiplet Intensities of lines in multiplet... 

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

The quotation marks indicate that the coupling constants are virtually the same for non-equivalent protons. C-p should, for example, split into a doublet Ucnto a-H) of quartets UchIo p-CHs) since both couplings have the same value (7.5 Hz), a pseudoquintet qui is... 

Since fragment A was clearly assigned with the help of HH coupling constants, all of the C atoms not included in A, which, according to the CH COLOC plot, are two or three bonds apart from the equivalent protons at Sh = 6.72 (Table 43.1), belong to the benzene ring B. 

Integrated area (Section 13.6) The relative area of a signal in an NMR spectrum. Areas are proportional to the number of equivalent protons responsible for the peak. 

Rule 1 Chemically equivalent protons do not show spin-spin splitting. The equivalent protons may be on the same carbon or on different carbons, but their signals don t split. 

Yet another complication in 1H NMR spectroscopy arises when a signal is split by two or more nonequivalent kinds of protons, as is the case with trans-cinnamaldehyde, isolated from oil of cinnamon (Figure 13.19). Although the n + l rule predicts splitting caused by equivalent protons, splittings caused by nonequivalent protons are more complex. 

Four dichloro isomers, the 1,6-, 2,3-, 2,7-, and 2,8-dichlorodibenzo-p-dioxins, were studied. These compounds also dissolve in TFMS acid, forming cation radicals in the absence of oxidizing agents or UV irradiation. The 2,8-isomer (Figure 5) exhibited a three-line spectrum, in agreement with the two equivalent protons in the 3,6 positions. The 2,7-isomer should also exhibit a three-line spectrum, similar to the 2,8-... 

Fig. 3. Rapid Mo(V) EPR signals obtained on reducing xanthine oxidase at pH 10 with 15 moles of xanthine for 1 min. at about 20 °. The upper four spectra are at 9.1 GHz and the lower four at 34.4 GHz. a, a, c, 8 refer to H2O as solvent and b, b, d, d to D2O. a, b, c, d are computer simulations of the experimental spectra, a, b, c, d, respectively. The interpretation is that two species, each having exchangeable protons which interact with Mo(V), are responsible for the signals. For one of these (dotted complex type II) there are two equivalent interacting protons and for the other (dashed complex type I), two non-equivalent protons. These species are believed to correspond to two different complexes of reduced xanthine oxidase with xanthine. (Reproduced from ref. 78 see also Table 2 for the parameters of the signals.)...

The experiment took 11 min, and the spectrum shows quite clearly the correlation signals for the acetal 5 cross peaks between the methyl and methylene signals from the ethyl group and between the magnetically non-equivalent protons of the ethylene bridge. CH correlation experiments can easily also be carried out, even though in the case of the two acetals 4 and 5 they require between two and three hours ... 

There are two other important consequences of spin-spin coupling. First, n equivalent protons will split another signal into n + 1 lines (hence three methyl protons split a methylene CH2 into 3 + 1 = 4 lines). Second, the relative sizes of peaks of a coupled multiplet can be calculated from Pascal s triangle (Figure 1.5). 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

When several magnetically equivalent nuclei are present in a radical, some of the multiplet lines appear at exactly the same field position, i.e., are degenerate , resulting in variations in component intensity. Equivalent spin-1/2 nuclei such as 1H, 19F, or 31P result in multiplets with intensities given by binomial coefficients (1 1 for one nucleus, 1 2 1 for two, 1 3 3 1 for three, 1 4 6 4 1 for four, etc.). One of the first aromatic organic radical anions studied by ESR spectroscopy was the naphthalene anion radical,1 the spectrum of which is shown in Figure 2.2. The spectrum consists of 25 lines, a quintet of quintets as expected for hyperfine coupling to two sets of four equivalent protons. 

Figure 2.1 Splitting pattern for one of the electron spin energy levels coupled to four equivalent protons. Note that the degeneracies of the levels are not shown. See Section 2.5 for details.

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

B) How many lines are expected from this model The total number of nuclear spin states is (2 f + 1) x (2I2 + 1) x (2/3 + 1). Thus, if the model structure has six protons (I = 1/2), there should be (2 x 1/2 + l)6 = 26 = 64 nuclear spin states. If some of the nuclei are expected to be equivalent, then the number of lines will be less than the number of spin states, i.e., some of the spin states will be degenerate (to first-order in perturbation theory). Thus, if the six protons are in three groups of two, it is as if you had three spin-1 nuclei and you expect (2 x 1 + l)3 = 33 = 27 distinct lines. If there is one group of four equivalent protons and another group of two, then it is as if you had one spin-2 nucleus and one spin-1 nucleus and you expect (2x2+ 1)(2 x 1+1) =15 lines. 

Lucken and co-workers32 subjected a single crystal of methylene diphosphonic acid to X-irradiation. The ESR spectrum indicated that the radicals produced were those pictured in Tables 4.7 and 4.8. The spectra were analyzed as described above and the results are also summarized in the tables. The species shown in Table 4.7 is the more abundant of the two. The methylene group freely rotates at room temperature but is stationary at 77 K, where splitting from two non-equivalent protons is observed for some orientations of the crystal. 

The radical cation of benzene has been produced by y irradiation of benzene adsorbed on silica gel (62, 63). The seven-line spectrum shown in Fig. 24 is expected for a molecule having six equivalent protons. An experimental coupling constant of 4.4 G, compared to a value of 3.75 G for the negative ion, gives strong support for attributing the spectrum to a positive ion. The g value is also consistent with this assignment. 

Although it is very hard to observe the absorption spectrum of eh when metal is dissolved in water because of its high reactivity, some attempts were made in water and ice (Jortner and Stein, 1955 Benett et al., 1964, 1967). Furthermore ESR (electron spin resonance) studies revealed that the trapped or solvated electron in ice interacts with six equivalent protons, thus ruling out H20-. 

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