Hydrogen equivalent

Splitting is most commonly caused by the interaction of protons on adjacent carbons with the proton of interest. If there are m equivalent hydrogens on an adjacent carbon, the proton of interest produces m + 1 peaks by this coupling. 

Inspection of the citrate structure shows a total of four chemically equivalent hydrogens, but only one of these—the pro-/J H atom of the pro-i arm of citrate—is abstracted by aeonitase, which is quite stereospecific. Formation of the double bond of aconitate following proton abstraction requires departure of hydroxide ion from the C-3 position. Hydroxide is a relatively poor leaving group, and its departure is facilitated in the aeonitase reaction by coordination with an iron atom in an iron-sulfur cluster. 

The 31P NMR spectrum of RhH2Cl(PBu2)2 is shown in Figure 2.69 the triplets show coupling with two equivalent hydrogens, split further by coupling with rhodium (/(Rh-P) 110.3 Hz /(P-H) 14.9 Hz). 

As a second example of molecular symmetry, consider the ammonia molecule. It has three symmetrically equivalent hydrogen atoms, but it is not... 

The benzenonium ion (62), the proposed intermediate for the formation of (63), has chemically equivalent hydrogens in the methylene group. Hence essentially equal amounts of 6-exo and 6-endo deuterium should appear in (68) if this mechanism is operative. 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

One quartet is expected because the carbon with two hydrogens is adjacent to a carbon with three (n) equivalent hydrogens (n + 1 = 4). One triplet is expected because the carbon with three hydrogens is adjacent to a carbon with two (n) equivalent hydrogens (n + 1 = 3). 

A given peak will be split into a number of peaks equal to the number of equivalent hydrogens on the adjacent carbons plus 1 (the n + 1 rule). This represents a clue about the structure that can help lead to identification of the compound. 

The lines in the spectrum of Fig. 1 are, even under optimum experimental conditions, quite broad. This is due to small unresolved hyperfine interaction with the eighteen equivalent hydrogen atoms. Either the width of the lines, or nmr measurements (La Mar et al., 1973) can reveal the magnitude of this y-hydrogen splitting (ca. 0.15 G—dependent on temperature and solvent). 

Some para-substituted anilines in strongly hydrogen bonding solvents show two non-equivalently hydrogen bonded species with differently hybridized aniline nitrogens72 rehybridization of aromatic amino nitrogens depends on the OH acidity of the solvent molecule and the basicity of the substituted anilines. 14 shows three possible modes of interaction of p-aminoacetophenone and acohol, where the amino group is simultaneously both a proton donor and an acceptor. 

The peak at m/z 100 belongs to H loss and is also due to a-cleavage as can be easily recognized (Scheme 6.13). There are three different positions to cleave off the radical, and even seven almost equivalent hydrogens are available in total (for clarity only one them has been shown at any position in the scheme). Despite this multiple chance, the peak at m/z 100 is very weak, the reason for this being the unfavorable thermodynamics of H loss as compared to methyl loss (Table 2.2). 

The equivalent hydrogens are grouped as a, b and c. The replacement of equivalent hydrogens will give the same product. 

Because proteins are made up of L-amino acids, they exhibit chirality in their structures, lacking planes or points of symmetry. Proteins also can exhibit chirality in their interactions with other chiral molecules as well as prochiral centers in other molecules. This latter point is beautifully illustrated by fumarase s catalysis of the dehydration of L-malate, a molecule containing two seemingly equivalent hydrogen atoms ... 

Coupling follows the n + 1 rule. According to this rule, a peak splits into n + 1 peaks due to neighboring hydrogen atoms, where n is the number of equivalent hydrogen atoms. The amount of splitting is expressed by the coupling constant (J). 

Nevertheless, just that kind of accidental hyperfine equivalence may occur in the case of TME (5) (see below), which shows a nine-line pattern " suggestive of eight equivalent hydrogens. Neither planar nor twisted TME can have eight tmly equivalent hydrogens, so the hyperfine pattern is due to some cause other than molecular symmetry. 

If one sums these values over all the equivalent hydrogens and all three rings, about one quarter of the spin density is in the ligands, which indicates that the ionic charge is distributed between the ligands and the iron. We have no information, of course, about the spin densities on the other four carbons, or on the nitrogens. 

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