Excitation, polarization transfer

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

As it concerns the band in the UV region (at 315 nm in the present case), Benesi and Hildebrand [5] assigned this absorption to a charge-transfer transition, where the phenyl ring acts as an electron donor (D) and the iodine as an electron acceptor. The interaction can be described in resonance terms as D-I2 <-> D+I2", the band being assigned to the transition from the ground non polar state to the excited polar state. 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

The fluorescence excitation polarization of the monomer is almost 1/7 regardless of the excitation wavelength. A value of 1/7 is typical when both the absorption and the emission oscillators are degenerate and polarized in the same plane. Since the dimer is regarded as a weakly coupled, three-dimensional, double-oscillator, energy transfer between the dimer partners will randomize the excitation between the two porphyrin planes oriented in a tilt angle. In fact, the observed polarization of the dimer is less than 1/7. 

On the other hand when more than two polarization transfer steps are combined (ID analogs of 4D experiments) one can make a decision, without affecting the end result, as to whether the second, the third or both these steps should be selective. In order to avoid possible losses of magnetization during the selective pulses, due to either relaxation and/or nonperfect excitation profiles, it is usually possible to make one of these steps nonselective. 

The NOESY and TOCSY polarization transfers can also be arranged so that two NOESY steps are interrupted by one TOCSY transfer. This is useful for situations when a proton which is intended as a starting point for a ID TOCSY-NOESY experiment cannot be selectively excited, nevertheless it has a NOE contact to an isolated proton. The ID NOESY-TOCSY-NOESY sequence [72] (fig. 4(b)) is obtained by appending another NOESY step to the ID NOESY-TOCSY pulse sequence of fig. 1(c). The last NOESY step can be either selective or nonselective depending whether a selective 180° pulse is applied after the nonselective 90° pulse at the end of the TOCSY transfer. 

The selective excitation of the proton signal can be achieved through a heteronuclear spin, to which the proton is bonded, by the HMQC or HSQC type of heteronuclear polarization transfer. Many versions of the ID HMQC-TOCSY or HSQC-TOCSY have been proposed. The selective exeitation of the desired heteronucleus can be accomplished by using a selective pulse on the heteronuclear signal [28, 42, 55], or by using either a proton or a C CSSF [52-54]. 

The ID TOCSY module has been used in many pseudo-3D experiments (or alternatively referred to as ID analogues of 3D experiments in the literature) such as ID TOCSY-NOESY or ID TOCSY-ROESY experiments. The TOCSY part of these experiments are similar to that of a regular ID TOCSY where a selective excitation of a desired signal is followed by a MLEV17-type isotropic mixing. The second polarization transfer (NOESY or ROESY) step can either be non-selective [29, 59-61] or selective [62-65]. 

The imaging of conversion within the fixed bed was achieved by using a distortionless enhancement by polarization transfer (DEPT) spectroscopy pulse sequence integrated into an imaging sequence, as shown in Fig. 44. In theory, a signal enhancement of up to a factor of 4 (/hZ/c 7i is the gyromagnetic ratio of nucleus i) can be achieved with DEPT. In this dual resonance experiment, initial excitation is on the H channel. Consequently, the repetition time for the DEPT experiment is constrained by Tih (< T lc) where Tn is the Ty relaxation time of... 

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