H NMR Spectroscopy and Proton Equivalence

Having looked at spectra, let s now focus on 1H NMR spectroscopy. Since 

The H NMR spectrum of 2,3-d1methyl-2-butene. Since all 12 protons in the molecule are chemically equivalent, there is only one peak In the spectrum. 

The H NMR spectrum of 2-methyl-2-butene. There are four kinds of protons and four different signals. 

Problem 13.13 How many kinds of nonequivalent protons are present in each of the follow compounds  

Problem 13.14 Howmany signals would you expect the following compound to have in its lHl spectrum  

We saw in Section 9.3 that addition of HBr to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use NMR to identify the product of the addition of 1 equivalent of HBr to 

For relatively small molecules, a quick look at the structure is often enough to decide how many kinds of protons are present and thus how many NMR absorptions might appear. If in doubt, though, the equivalence or nonequivalence of two protons can be determined by comparing the structures that would be formed if each hydrogen were replaced by an X group. There are four possibilities. 

A second possibility is that the protons are chemically identical and thus electronically equivalent. If so, the same product would be formed regardless of which H is substituted by X. In butane, for instance, the six CH3 hydrogens on Cl and C4 are identical, would give the identical stmcture on substitution by X, and would show the identical NMR absorption. Such protons are said to be homotopic. 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic  

How many kinds of electronically nonequivalent protons are present In each of the following compounds, and thus how many NMR absorptions might you expect in each  

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A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

Having looked at spectra, let s now focus on H NMR spectroscopy, each chemically distinct hydrogen in a molecule normally has its own uniqui absorption, one use of NMR is to find out how many kinds of non equivalent hydrogens are present. In the NMR spectrum of meth acetate shown previously in Figure 13.3a, for example, there are two si nals, corresponding to the two nonequivalent kinds of protons present, CHgCO- protons and -OCHg protons. 

Mixing a 3-substituted 1,2,4,5-tetrazine with one equivalent of a ketene A,A-aminal is followed by the formation of a heavy yellow-orange precipitate which unfortunately could not be crystallized, so x-ray diffraction was not possible. Adding CF3CO2H at low temperature transforms the yellow compound to a protonated species analogous to (292) which could be analyzed by H and C NMR spectroscopy and shown to be the proton adduct of the dipolar yellow compound (295) (Scheme 52). In solution the (4 + 2) cycloaddition leads to the pyridazine derivatives (300) and (301), which proves that the yellow compound (295) must dissociate before the cycloaddition step. 

Some antihistaminic and analgesic agents have become amenable to chiral analysis by H-nmr spectroscopy after formation of cyclodextrin inclusion complexes. In many cases equivalent protons of enantiomeric pairs showed different chemical shifts after inclusion. ... 

Some recent papers permit an exciting outlook on the degree of sophistication of experimental techniques and on the kind of data which may be available soon. In the field of NMR spectroscopy, a publication by Hertz and Raedle 172> deals with the hydration shell of the fluoride ion. From nuclear magnetic relaxation rates of 19F in 1M aqueous solutions of KF at room temperature, the authors were able to show that the orientation of the water molecules in the vicinity of fluoride ions is such that the two protons are non-equivalent. A geometry is proposed for the water coordination in the inner solvent shell of F corresponding to an almost linear H-bond and to an OF distance of approximately 2.76 A, at least under the conditions chosen. 

As seen in the gitonic and vicinal systems, ammonium and related cationic centers may be components of superelectrophiles and reactive dications having the 1,3-dicationic structure. Several types of superelectrophilic aza-carbo dications have been studied in which protonated nitro groups are involved. For example, it was found that nitroethylene reacts with benzene in the presence of 10 equivalents of CF3SO3H to give deoxybenzoin oxime in 96% yield (eq 58).71 Since the reaction does not occur with only one equivalent of CF3SO3H, the formation of the /V./V -dihydroxy iminium-methyl dication 197 was proposed. In spectroscopic studies, the stable dication (199) can be directly observed by H and 13C NMR spectroscopy from solutions of l-nitro-2-methyl-l-propene (198) in CF3SO3H (eq 59). 

The overall blending stability of SMA in the material bulk and the surface grafting stability on material surfaces were examined by leaching tests and evaluated respectively with proton nuclear magnetic resonance spectroscopy [ H-NMR] and quantitative ATR-FT-IR. Firstly, SMA-MSPEO and SPEO with equivalent amounts of PEG components were respectively blended into PEU matrix materials. The initial quantity of PEG was measured and recorded by integrating the PEG-specific I-NMR peak areas at S = 3.52 ppm (- O - CH2 - CH2 - 0 -). The integral values were normalized... 

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