Rapid interchange

If X is put equal to a distance of, say, 100 A, then t is about 10 sec, so that, due to Brownian motion, there is a very rapid interchange of molecules between the surface and the adjacent bulk region. 

As with AI(BH4)3 and related compounds (p. 230), solutions of AbMe6 show only one proton ninr signal at room temperature due to the rapid interchange of bridging and terminal Me groups at —75° this process is sufficiently slow for separate resonances to be observed. 

X-ray diffraction patterns from dendrimers tend to lack sharp features and are similar to those from amorphous linear polymers. This suggests a molecular arrangement in dendrimers that is fairly disordered. There is also the problem that these molecules can exist in a large number of energetically equivalent conformations and that in solution there can be rapid interchange between these conformations. This contributes to the overall amorphous structure of dendrimers. 

Organochromium compounds with various coordination modes of the ligand, namely CpCr(CO)2S2P(OPr )2 (bidentate), CpCr[S2P(OPr )2]2 (one bident-ate and one unidentate) and Cr[S2P(OlV)2]3 (bidentate) are formed in reactions of [CpCr(CO)3]2 with (Pr 0)2(S)PSSP(S)(0Pr )2. Variable temperature NMR shows rapid interchange between bi- and unidentate ligands in solutions of CpCr[S2P(OPr )2]2.61... 

Valuable information on conformational equilibrium can be obtained particularly by N.M.R. technique. When a molecule can exist in several conformations which rapidly interchange, then any proton which assumes all possible positions in a very short time, the n.m.r. spectrum would show only one peak. This happens in most open chain compounds and even in cyclohexanes where the interconversion is very rapid. But if the interconversion is slowed or prevented, either by cooling or due to the inherent structure in the molecule, the hydrogens of each conformer appear separately and so more than one peak would appear. For example by cooling cyclohexane to -110°C, two peaks appear, one due to equatorial and the other to the axial hydrogens. 

TIRF is an experimentally simple technique for selective excitation of fluorophores on or near a surface. It can be set up on a standard upright or inverted microscope, preferably but not necessarily with a laser source, or in a nonmicroscopic custom setup or commercial spectrofluorimeter. In a microscope, the TIRF setup is compatible and rapidly interchangeable with bright-field, dark-field, phase contrast, and epi-illumination and accommodates a wide variety of common microscope objectives without alteration. 

However, equatorial and axial fluorines rapidly interchange. The two out-of-plane equatorial fluorines bend outward at the same time as the top ar/a/fluorine bends downward and the bottom axial fluorine bends upward. 

The value of is dependent on the size of the population of molecules of component A in the stationary and eluent phases [1]. As the equilibrium is d3mamic, there is a continual, rapid interchange of molecules of component A between the two phases. The fraction of time, fm, that an average molecule of A spends in the mobile phase is given by ... 

Most of the information about structure in these systems is derived from the N—H signals. The spectra in Fig. 16 show a wide variation in chemical shift for the N protons which can be rationalized in relation to the over-all stereochemistry of the molecular ion. The band at 3.50 ppm in trans-[Coen2(NH3)2]3+ is attributed to both NH3 and CH2 protons and the band at 5.30 ppm to the symmetric NH2 groups (assuming there is a rapid interchange of conformers). The peak areas are consistent with this view and deuteration decreases the intensity of the NH3, —CH2—CH2— band and... 

The temperature-dependent 1H NMR spectrum of isopropyl cation 2 (prepared from isopropyl chloride in SbFs-SC ClF solution) demonstrated96 rapid interchange of two types of protons. Line-shape analysis showed the reaction to be intramolecular, with an activation energy barrier of lb iO kcalmoP1. Based on these observations, Saunders and Hagen96 suggested that the rearrangement involves n-propyl... 

While a demonstration of optical activity proves conclusively that a given aromatic hydrocarbon is not planar, the converse is not true. Failure to resolve a given compound into optical isomers may arise from a too rapid interchange of d- and Z-forms, or from use of an inappropriate resolution technique, instead of from the molecule being planar and hence inactive optically. 

Fig. 12. Predicted derivative spectrum for randomly oriented RCH2 radicals with rapid interchange between the relative locations of the methylene protons (Cochran et al., 1961).

The pressure dependence of the concentration of sorbed gas was consistent with the dual mode model while the relaxation data addressed itself to the validity of the assumptions made by the model. The assumption of rapid interchange was found to be valid for this system while the assumption of an immobile adsorbed phase could introduce a small error in the analysis It should be possible to reduce this error by more exact measurements of the concentration of sorbed gas as classical pressure experiments could... 

An axial proton becomes an equatorial proton and vice versa in the interconverting structures, and the spectrum consists of a single averaged peak. As the temperature is lowered, the peak broadens and at a sufficiently low temperature two peaks appear-one for the axial protons, one for the equatorial protons. In other words, at room temperature, the axial and equatorial protons are chemical-shift equivalent by rapid interchange. At very low temperatures, they are not chemical-shift equivalent in fact, in each frozen chair form, the protons of each CH2 group are diastereotopic... 

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