Protonation separation

Since one proton separation will hardly occur at pH 2.2, extraction ratios of Fe " with compounds 1-7 are quite low. Yet compound 8, which carries oxime groups, was able to extract a considerable amount of Fe- at pH 2.2. 

Now lei us turn to the problem of how the composition of a nucleus affects its stability. The forces that exist between the particles in the nucleus are very large. The most familiar of ihe intranuclear forces is the coulomb force of repulsion which the protons must exert on one another. In order to appreciate the magnitude of this repulsive force, let us compare the force between two protons when they are separated by 10 8 cm, as they are in the hydrogen molecule, with the force between two protons separated by 10-18 cm, as they are in a helium nucleus. In the first case we have... 

A long-range cross-ring coupling (0.7 Hz) is also found between the protons separated by five bonds in tliienothiophene isomers 2 and 3, in selenopheno[3,2-6]thiophene (15), and selenopheno[2,3-c]thiophene (16). 

In 2-substituted formyl thienothiophene 1 or 2 there is a slight long-range interaction (/cho-s Hz) between protons separated by seven bonds. With 3-formyl thienothiophene 1 or 2 no long-range coupling is observed between the proton of the formyl group and those of the thiophene system. A similar effect is also observed in 2-methyl-thieno[3,2-6]thiophene (35) and 3-methylthieno[3,2-b]thiophene (27). 

As shown in Fig. 7.2, at a shorter proton-proton separation (/ <16 a.u. or <8 A), the electron in the Is state in the vicinity of one proton has an appreciable probability of tunneling to the Is state in the vicinity of another proton. The tunneling matrix element can be evaluated using the perturbation theory we presented in Chapter 2. A schematic of this problem is shown in Fig. 7.3. By defining a pair of one-center potentials, Ul and Ur, we define the right-hand-.side states and the left-hand-side states. Because the potential Ul is different from the potential of a free proton, Uro, the wavefunction i ii, and the energy level Eo are different from the Is state of a free hydrogen atom. (The same is true for Ur and We will come back to the effect of such a distortion later in this section. 

As shown in Fig. 7.1, as the proton-proton separation becomes even smaller, the picture of the resonance becomes obscured, and the proton-proton repulsion is no longer screened by the electron. Slater (1963) showed that the Morse function can match the exact potential curve for the hydrogen molecular ion very precisely ... 

AB system. Two protons constitute the simplest example of a strongly coupled system when the difference in their frequencies of resonance is comparable to their coupling constant J. This system gives rise to a total of four peaks, two for each proton, separated by J Hz. However, the intensities are not equal. The chemical shifts, which can no longer be measured from the spectrum, have to be calculated using the relations shown in Fig. 9.21. 

The calculated values for the proton-proton separation, Rey the binding energy, Es, and the optical transition energy, AE, are given in Table I. 

In the present case, four states of each spin were taken into account. The procedure is dependent upon three additional parameters, a, 3 and A, which represents an energy needed to excite the intruder configuration and must be added to the eigenvalues of the = 3 configuration. The parameter A can be expressed in terms of the two-proton separation energies [HEY85]... 

Protons separated by more than three bonds usually do not produce observable spin-spin splitting. Occasionally, such long-range coupling does occur, but these cases are unusual. For now, we consider only nonequivalent protons on adjacent carbon atoms (or closer) to be magnetically coupled. 

Energy transfer How light energy is captured, transmitted from an initial absorbing molecule through a series of intermediate molecules to a site at which that energy is captured in the form of electron-proton separation across a membrane. 

EXAMPLE 15.1 At what orientation is the dipolar coupling at its maximum Its minimum Exactly zero What is the maximum splitting for two protons separated by 1 A (10 8 cm) ... 

Even before the common use of Fourier transform NMR, it was known that the homonuclear nuclear Overhauser enhancement (NOE) could be used to identify pairs of protons separated by a few angstroms in space.2 It was also clear, however, that many such distances and accurate assignments to specific protons would be needed to define a solution structure fully. The advent of two-dimensional NMR was a major step toward providing the necessary data,3-4 but the introduction of the two-dimensional NOE measurement (NOESY)5-6 truly revolutionized the ability both to assign spectral peaks and to acquire large quantities of distance information. 

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