Active spins

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

It is interesting to note that 12C nucleus is not magnetically active (spin quantum number I = 0),... 

PatxiR) 9- ( )> defined through active spin-orbital indices as / X (r)> to calculate the triply excited contribution Eq. (79), to the denominator One can easily extend the above MMCC(2,3)/CI approx-... 

Any NMR experiment consists of a series of pulses and delays. Pulses are applied to perturb the thermal equilibrium of an ensemble of magnetically active spins and to force the spins to speak in a controlled and synchronised way. The evolution of these spins conversations , i.e. the evolution of coherences to be more precise, occurs in the intermediate delays and is manipulated by the pulses in the course of the pulse sequence. The spins response at the end of the pulse sequence is detected in a final detection or acquisition period. The general scheme for any pulse experiment is shown below ... 

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

NMR spectroscopy is essentially based on energy absorption by atomic nuclei with active spin (nonzero value of quantum number spin I) in the presence of a static magnetic field. This energy absorption is... 

The 12C nucleus is not magnetically active (spin number, /, is zero), but the 13C nucleus, like the XH nucleus, has a spin number of 1/2. However, since the natural abundance of I3C is only 1.1% that of I2C and its sensitivity is only about 1.6% that of 1H, the overall sensitivity of 13C compared with H is about l/5700.f... 

Incubate cells with transfection complex for 24 h. Test cell viability using 0.4% Trypan Blue to stain cells and count viable cells at seeding, day of transfection, and just prior to harvest. Harvest cells and assay for reporter gene activity Spin down cells and lyse cells to assay for luciferase activity. Figures 1 and 2 illustrate typical transfection results, observed visually as lacZ stained cells (Fig. 1) or Relative Light Units (RLUs) when luciferase activity is measured (Fig. 2). 

In principle, all of the experiments mentioned above would be applicable for the other two NMR active spin-1 nuclei 6Li and 14N. Solid-state 6Li NMR have so far been utilized in materials research39-42 and even exchange experiments on 6Li enriched Li2Si04 have been performed.42... 

As an example, we describe one of many similar experiments devised for assigning resonances in proteins, a subject that we take up in more detail in Chapter 13. This particular experiment is designed to correlate the frequencies within the H—15N—13C=0 portion of a peptide group and is appropriately called simply HNCO. The basic pulse sequence for HNCO is shown in Fig. 12.16. To simplify the notation, instead of I, S, and T, we identify the active spins as H, N, and C, and use K to denote the spin of ar-13C. In a peptide chain, one a carbon is bonded and spin coupled to the nitrogen and another a carbon is bonded and coupled to the carbonyl carbon atom. With recombinant DNA methods, the protein is uniformly and highly enriched in both 13C and 15N, so all of these spins need be considered. 

Hollow membrane fibers are required for many medical application, e.g. for disposable dialysis. Such fibers are made by usmg an appropriate fiber spinning technique with a special inlet in the center of the spinneret through which the fiber core forming medium (liquid or gas) is injected. The membrane material may be made by melt-spinning, chemical activated spinning or phase separation. The thin wall (15-500 xm thickness) acts as a semi-permeable membrane. Commonly, such fibers are made of cellulose-based membrane materials such as cellulose nitrate, or polyacrylonitrile, polymethylmethacrylate, polyamide and polypropylene (van Stone, 1985). 

Since the effects of a perturbation on the relaxation rate are mechanism-specific, clarity about the active spin relaxation mechanism is essential. While there is no doubt about the overwhelming dominance of quadrupolar relaxation for Na, Rb, and Rb,... 

Willker and Leibfritz (1992a) introduced an extension of the E.COSY principle that yields additional flexibility. In addition to coherence transfer between the active spins i and j, polarization of spin p, which is passive during is transferred to a spin q, which plays the role of the passive spin during t - Hence, in general, the E.COSY triad is opened up. The two- and three-dimensional JHH-TOCSY experiments for the determination of coupling constants of Willker and Leibfritz (1992a) use a combination of homonuclear TOCSY transfer and two BIRD (bilinear rotation decou-... 

In the product operator representation of multiple quantum coherences it is usual to distinguish between active and passive spins. Active spins contribute transverse operators, such as Ix, I and 7+, to the product passive spins contribute only z-operators, Iz. In a sense the spins contributing transverse operators are "involved" in the coherence, while those contributing z-operators are simply spectators. 

Multiple quantum coherence between spins i and j does not evolve under the influence of the coupling between the two active spins, i and j. 

Each NMR-active spin 4 nucleus in an external magnetic field, HOI acts as a magnetic dipole which aligns with H0 in specific states. For protons, carbons and other spin 4 nuclei, two states exist parallel or anti parallel to HQ. Since each... 

Moreover, once the cluster Ansatz is introduced (for an option of directly solving Bloch equations without invoking the cluster Ansatz, see Ref. [200]), it is essential that the so-called complete model space (CMS), spanned by configurations involving all possible occupancies of valence or active (spin) orbitals, be used, lest the desirable property of size-extensivity be violated. This requirement, however, leads not only to highly dimensional (and thus computationally demanding) model spaces, but, most importantly, to the occurrence of the so-called intruder states. 

Solid-state NMR techniques are nowadays indispensable for the study of a wide variety of crystalline and amorphous systems. In general, a Hamiltonian describing any NMR-active spin system can be expressed as the sum of isotropic and anisotropic terms. The anisotropic part of a Hamiltonian, which is averaged to zero in isotropic fluids, contains valuable information about the spatial orientation of the NMR-active species. Although the first Co NMR work can be traced back to the 1950s, relatively few solid-state Co NMR data have been reported compared with the large body of literature concerning other quadrupolar nuclei such as Na or Al. The main reasons... 

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