Mechanism dipolar

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

With a suitable combination of electron-deficient fluoroalkene and electron-rich addend, cycloaddition can proceed by a dipolar mechanism involving zwitte-rion intermediates Like Us isomer, l,2-bis(trifluoromethyl)-l,2-dicyanoethylene [85], l,l-bis(trifluoromethyl)-2,2-dicyanoethylene forms cyclobutanes by an ionic mechanism [104, 105, 106] (equations 39 and 40)... 

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

Nuclear Overhauser effect (nOe) The change in intensity in the signal of one nucleus when another nucleus lying spatially close to it is irradiated, with the two nuclei relaxing each other via the dipolar mechanism. Nuclear Overhauser effect correlation spectroscopy (NOESY) A 2D... 

Two factors contribute to r K. One is the ratio of the magnetogryric ratios of the two different spins, and the other depends on relaxation mechanisms. Provided that the relaxation mechanism is purely dipole-dipole, other relaxation mechanisms affect spin I, then 4> may approach zero. Assuming that the dipolar mechanism is operational (no quadrupolar nuclei with I > 1/2 are present), r has the value ys/ 2y and is regarded as rimax. In the homonuclear case we have r max = 1/ 2. Usually one chooses nuclei where ys > y/ to ensure that the NOE is significant. For observation of 13C for instance, if the protons in the molecule are double irradiated, the ratio is 1.99 and 1 + r max equals approximately 3. To repeat a statement made above, proton broad-band irradiation enhances the intensity of the 13C nucleus, which otherwise has very low receptivity. 

Cp2Zr -induced alkene stereoisomerization via partially non-concerted dipolar mechanism. 

Dicyclopropylallene reacts with olefins activated by ester groups at 200 °C to yield the adducts 20 predominantly (Table 12.1). In contrast, the reactions with methylenemalononitrile occurred at room temperature with selective production of the adduct 19 [18], A dipolar mechanism is assumed for the latter reaction. 

Although the mechanism of stereoinversion is not yet clear, the - - mechanism widely accepted for the stereoisomerization of allylpalladium derivatives cannot operate in this case, since the concerted - - mechanism should be accompanied by a stereoinversion of the second double bond, which has not been observed. The observed inversion of only the bromine-bearing C=C bond may involve a dipolar mechanism shown in Scheme 397. This mechanism needs to be experimentally supported, however. 

It has been stated above that for a series of isostructural lanthanide complexes the paramagnetic shifts of analogous nuclei should be proportional to the parameter A, if a purely dipolar mechanism operates. The paramagnetic shift is usually taken as the difference between the chemical shift of the nucleus in the paramagnetic complex under investigation, and the... 

Differing 7j values for CH3, CH2, and CH carbon nuclei within a molecule can arise not only by methyl rotation or anisotropic molecular motion, but also from the segmental mobility of partial structures, even when the dipolar mechanism predominates. Thus the spin-lattice relaxation times of methylene carbon atoms in long alkane chains pass through a minimum at the middle of the chain. In the presence of heavy nonassociating... 

B) Complexation of the alkene to the metal followed by its insertion between the metal-oxygen bond according to an intramolecular 1,3-dipolar mechanism, forming a five-membered pseudocyclic peroxometallacycle which decomposes to give the epoxide and the metal alkoxide.121,162,193,634... 

Nuclear relaxation of different nuclei other than the proton can also be used to determine the rotational correlation time. The longitudinal relaxation of the 170 is governed by quadrupolar [68] and dipolar mechanisms [69], 1 /Tlq and 1 /Tld, respectively, both dependant on rotation ... 

The second approach is the use of the dynamic nuclear polarization (DNP) detection principle. Dorn and co-workers have pioneered the application of this technique [9,10], Whereas the NOE enhancement of 13C nuclei in the conventional 13C H recording is dependent upon the 7h/7c ratio (NOE = Th/ Tc = 2 1), the DNP enhancement relates to the ye/yuc ratio (2640 1). In an electron-nucleus spinsystem, the electron-electron transitions are saturated by microwave irradiation and magnetization transfer from electron to nucleus (Overhauser effect) occurs via a scalar and/or dipolar mechanism. The DNP enhancement, A, is described by the following equation ... 

The energy transfer can also proceed through an alternative mechanism, which can occur between molecular entities separated by distances considerably greater than the sum of their van der Waals radii (eg in molecular monolayers). The electron shift to less energy in AB and the increase in energy of electron in Q are coupled by their electric fields. It is described in terms of an interaction between the transition dipole moments (a dipolar mechanism, called Forster excitation transfer, Figure 4.3). 

In early years of NMR, extensive studies of molecular dynamics were carried out using relaxation time measurements for spin 1/2 nuclei (mainly for 1H, 13C and 31P). However, difficulties associated with assignment of dipolar mechanisms and proper analysis of many-body dipole-dipole interactions for spin 1/2 nuclei have restricted their widespread application. Relaxation behaviour in the case of nuclei with spin greater than 1/2 on the other hand is mainly determined by the quadrupolar interaction and since the quadrupolar interaction is effectively a single nucleus property, few structural assumptions are required to analyse the relaxation behaviour. 

Dipolar mechanism [of energy transfer) Same as Forster excitation transfer. See also energy transfer. 

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