Proton environment

Figure 10.2.6. Special distance measures for the characterization of proton environments a) distance r and angle a, to double bonds b) distance and angle Oc, to single bonds c) dihedral angle a, to the third bond from the hydrogen atom.

Analyzing an NMR spectrum m terms of a unique molecular structure begins with the mfor matron contained m Table 13 1 By knowing the chemical shifts characteristic of various proton environments the presence of a particular structural unit m an unknown compound may be inferred An NMR spectrum also provides other useful information including... 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Notice the small peak labelled TMS. TMS is short for tetramethylsllane, S1(CH3) , which is used as a standard against which all absorptions due to other proton environments are measured. TMS is assigned a value of zero and the difference between the protons in TMS and the protons in other chemical environments is known as the chemical shift, which is given the symbol 5. The chemical shift is measured in parts per million (ppm). Chemical shift values for protons In different chemical environments are given on p. 16 of the SQA Data Booklet. 

The low resolution H NMR spectrum for methyl propanoate has three peaks, showing that there are three proton environments - these are circled on the structural formula. The high resolution H NMR spectrum shows each of these peaks in more detail and we can see that they are made up of multiples of peaks, known as multiplets. 

The additional detail provided by a high resolution spectrum allows more information to be determined about the structure of the compound. Analysing the multiplets allows the neighbouring proton environment to be identified. This can be done using the n + 1 rule, where n is the number of carbon atoms attached to the next-door carbon atom and n + 1 is the number of peaks that will be seen in the cluster. 

Changes in nuclear resonance characteristics due to exchange processes may be described fairly accurately. Consider a hypothetical molecule in which there are two proton environments, each containing equal numbers of hydrogen atoms. The H1 resonance spectrum (assume Ahahb—>0) will then consist of two peaks of equal intensity separated by some chemical shift va°—vi°... 

Figure 5. Proton (hydrogen) NMR spectrum of a 0.1 vol% APS in regular water. The dominant resonance of the water protons is at the left. The identities of the APS proton environments correspond to the structure and labels previously shown in Fig. 1.

Five different proton environments in 1-butanol five signals... 

Figure 8.5 Symmetry of proton environments in representative strongly hydrogen bonded complexes.

XH chemical shifts. More detailed chemical shift data for a wide range of proton environments is given in Appendix 3, Tables A3.1, A3.3 and A3.4. In particular the chemical shift values quoted in Table A3.1 show that an electronegative substituent in aliphatic systems causes a downfield shift the greater the electronegativity the more substantial the shift. When two substituents are attached to the same carbon atom there is a greater downfield shift, but not as great as the sum of the two substituents separately. The approximate position of absorption in such cases can be predicted on the basis of the empirical parameters shown in Appendix 3, Table A 3.2. 

Proton NMR spectroscopy is particularly useful for determining the disposition of substituents in aminohalogenocyclophosphazenes. Four criteria are utilized for this purpose (a) the number of proton environments, (b) the value of 3J (P—H), (c) the relative chemical shifts (geminal versus nongeminal and cis versus trans), and (d) the presence or absence of "virtual coupling (see detailed discussion in the following). 

Answer There are four distinct proton environments. These are axial or equatorial terminal sites on the wingtip BH2 groups, terminal sites on the hinge BH group and bridging sites. 

A large enhancement of H2 from H3 would not have been expected for conformation a because H2 is much closer to Hy. So there must also be a large population of conformation b. We cannot easily quantify the relative populations from this NOE difference spectrum because H2 and H4 are in such different proton environments ( 6.5.3). Equilibration between the two conformations is obviously fast on the chemical shift timescale. The appropriate experiment here is separate irradiation of H2 and H4 and measurement of the relative enhancements at H3 and H . This experiment is illustrated below, and the resulting spectra are shown on the next page. 

This is one of several reactions of this type in which an organic negative radical-ion and its parent molecule react in the presence of an alkali metal. It is found, rather interestingly, that the rate coefficients depend on the nature of the metal. To account for this, it has been postulated that the metal is involved in a bridging role in the activated complex, e.g., dipy.. K" ". . dipy for the case of 2,2 -dipy-ridyl (dipy) A more extreme case of this association between the radical-ion and the ion of the alkali metal used to form it occurs in the reaction of benzophenone with its negative ion. The spectrum of (benzophenone)" in dme has many hyper-fine lines caused by the interaction of the free electron with the and, when the metal is sodium, the Na nuclei. When benzophenone is added, the structure, due to the proton interaction, disappears and only the lines associated with the sodium interaction remain. To account for this, it has been suggested that the odd electron moves rapidly over all the proton positions too fast for the lines characteristic of the electron in the different proton environments to be seen), but relatively slowly from one sodium nucleus to another. Seen another way, this means that the transfer of an electron from molecule to molecule is associated with the transfer of the cation . ... 

Non-stoichiometric proton environments in crystalline and glassy materials. 

The interplanar distance in unsubstituted HBC is 0.342 nm,164 and thus the aromatic protons of one layer will experience the ring currents of the extended -electron systems of adjacent layers. In HBC—C12, three different aromatic proton environments can thus be identified with respect to the degree to which the proton experiences the ring current of the adjacent layers. The unshaded circles represent protons that lie neither above nor below the 71 orbitals of an adjacent layer, and therefore correspond to the least shielded resonance (highest ppm). Fully shaded and hatched circles then represent protons that lie over or below an inner and outer part, respectively, of an adjacent ring system the fully shaded protons would be expected to be the most shielded. Assigning the unshaded, hatched, and fully shaded protons to the A, B, and C resonances in Figure 23b, respectively, the observed presence of... 

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