Spin recoupling

Finally, we should also mention that assisted spin recoupling is by no means restricted to 1H, 13C, and 15N spin species but may also be used to facilitate H and 2H to low- spin transfers. This may be exploited in extensively deuterated samples using rf fields on JH and 2H which are matched to a rotary resonance condition to facilitate spin-diffusion between 13C spins in a longitudinal mixing process. This was recently demonstrated by van Rossum and coworkers in terms of the doublenucleus enhanced recoupling (DONER) experiment [143]. 

An entirely different description emerges for the two 1,3-dipolar cycloaddition reactions that we have studied [3,4]. For such systems, the bond breaking and bond formation involves instead the shifts of well-identifiable orbital pairs, rather than any spin recouplings. Such heterolytic mechanisms, that do not pass through an aromatic structure, now seem to be a likely outcome of studies on other gas-phase concerted 1,3-dipolar cycloaddition reactions. 

For example, the quality of a large-scale Cl calculation also depends on the choice of expansion CSFs, but this choice, which is dictated by the need to calculate quantitatively the dynamical electron correlation energy, is much different from that of a typical modern MCSCF calculation, which is more concerned with near-degeneracy, avoided-crossing and spin-recoupling effects. 

Another important concept which emerges from this example is that of spin recoupling. The ground state of N(2p ) is Sy, in which the spins of the three valence electrons are coupled to give the maximum resultant, S =. As the H atoms approach, these spins uncouple from each other and form three pair bonds. In many cases, this kind of recoupling has been found to occur over a very short range of internuclear distances (0.5-1 bohr), and is frequently associated with a maximum in the potential surface (see Section III). 

The CASVB representations of the CASSCF wave functions for these model /ra A -regular polyenes indicate that excitation from the Ag ground state to the second root of the same symmetry is mainly a matter of spin recoupling, with somewhat less significant changes to the orbitals. 

Scheme 5 The spin recoupling scheme in the intermediates of the reaction...

Heteronuclear and homonuclear dipolar recoupling under magic-angle spinning... 

Recently, numerous studies reported the application of homonuclear and heteronuclear selective recoupling schemes on uniformly labelled ligand interacting with membrane receptors. The polarization exchange curves were fitted with the two-spin model and showed that it is possible to determine intemuclear distances up to 4.5 A.118... 

Spin-Diffusion and Third-Spin-Assisted Recoupling. 25... 

Average or effective Hamiltonian theory, as introduced to NMR spectroscopy by Waugh and coworkers [55] in the late 1960s, has in all respects been the most important design tool for development of dipolar recoupling experiments (and many other important experiments). In a very simple and transparent manner, this method facilitates delineation of the impact of advanced rf irradiation schemes on the internal nuclear spin Hamiltonians. This impact is evaluated in an ordered fashion, enabling direct focus on the most important terms and, in the refinement process, the less dominant albeit still important terms in a prioritized manner. 

Floquet theory [60] represents a powerful method to analyze and describe NMR experiments including, in the present context, methods for dipolar recoupling. In particular the bimodal variants are promising in the sense that they may handle perturbations/interactions involving multiple time periodicities, as for example induced by combined sample spinning and rf irradiation in the case of dipolar... 

We note that DRAMA has also formed the basis for refined variants such as the chemical-shift-compensated MELODRAMA [85], windowless DRAWS [29], and the DRAMA-XY4 [86] experiments, finding important applications for example for measurement of 13C-13C distances involving carbonyl spins. We should also note the existence of many other important pulsed homonuclear recoupling experiments such as the widely used Radio Frequency Driven Dipolar Recoupling (RFDR) [24] and Back-to-Back (BaBa) [87] experiments. 

Vega, and coworkers as a sequence of well-placed ideal (i.e., infinitely strong) 7r-pulses serving to reintroduce the zero-quantum (ZQ) part of the homonuclear dipole-dipole coupling operator in a homonuclear two-spin system. The coherent averaging due to MAS is in the toggling frame of the n-pulses partially disrupted by a differential chemical shift term and thereby recoupling takes place. 

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