Intermediate States in Coherence Transfer

We can consider the state — 2IZSZ as an intermediate state in coherence transfer, just as we did in the analysis of populations (SPT) above. Often in INEPT-based experiments, this intermediate state is used as a way of cleaning up other coherences that are not desired. A pulsed field gradient (PFG, Chapter 8) is a way of temporarily messing up the shims, and this will destroy any magnetization that is in the x-y plane. The intermediate state —2IZSZ is not affected, however, because there is no net magnetization in the x-y plane. After... 

The remaining two are the longitudinal spin order, which results when the macroscopic z magnetization of one nucleus (e.g., H) is opposite depending on the microscopic z magnetization (a or P) state of the other nucleus (e.g., 13C), and the identity (1) operator, which simply represents the vast majority of spins that cancel each other out and play no role in NMR experiments. Longitudinal spin order can be viewed as an intermediate state in coherence transfer 2 US- 21-S- 2I-S.r = 2SX z. Like z magnetization, it is not affected... 

To understand the pulse sequence, we will try to get an overview of what is happening and then look at some simplified product operator analysis. Consider first the CH case in the DEPT-90 experiment. Ignoring the 180° pulses, the DEPT-90 sequence can be viewed as an INEPT sequence in which the coherence transfer is split up into two steps (Fig. 7.41) the two 90° pulses are no longer simultaneous and between them we have an intermediate state in coherence transfer multiple-quantum coherence (ZQC and DQC). 

Suppose that we are talking about a double-quantum transition in which both the proton and carbon change from the a state to the p state. This transition is thus from the aH c state to the PuPc state ol l lc two-spin, four-state system. This transition corresponds to DQC. Likewise, if the proton flips from ft to a while the carbon simultaneously flips from a to P, we have a zero-quantum transition (P ac to a Pc) because the total number of spins in the excited (ft) state has not changed. This transition corresponds to ZQC. What can we say about these mysterious coherences In Section 7.10, we encountered ZQC and DQC as intermediate states in coherence transfer, created with pulses from antiphase SQC ... 

The MQC intermediate state in coherence ( INEPT ) transfer can also be used to clean up the spectrum. In this case, we can apply a double-quantum filter (using either gradients or a phase cycle) to kill all coherences at the intermediate step that are not DQC. We will see the usefulness of this technique in the DQF (double-quantum filtered) COSY experiment (Chapter 10). As with the spoiler gradient applied to the 2IZSZ intermediate state, a doublequantum filter destroys any unwanted magnetization, leaving only DQC that can then be carried on to observable antiphase magnetization in the second step of INEPT transfer. 

The factor of 4 reflects the change from observing XH to observing 13C, a change in our standard of comparison for magnitude). The net effect of these two steps is to convert antiphase proton SQC into antiphase carbon SQC, an overall coherence transfer with ZQC/DQC as an intermediate state. In Section 7.11 the product operator representations of pure ZQC and DQC were introduced, and we saw that pure DQC rotates in the x-y plane just like SQC, but at a frequency that is the sum of the frequencies of the two spins involved ... 

A different approach to destroying the 12C-bound artifact is to use the gradient in a simpler way—as a spoiler that just kills all of the magnetization in the x -y plane while our desired signal is stored briefly on the z axis. To do this we go back to our discussion of intermediate states in INEPT coherence transfer (Section 7.10) and recall that instead of using simultaneous 90° pulses on lH and 13C to effect coherence transfer, we can start with the H 90° pulse and then, after a short delay, complete the INEPT transfer with the 13C 90° pulse ... 

Friis and coworkers [80] considered an intermediate mechanism, in which an electron first transfers to the molecule and the molecule begins to relax towards the reduced state. However, before it is fully relaxed to the reduced state, an electron transfer from the molecule to the second electrode occurs when the temporarily occupied level passes the Fermi level of the second electrode. They called this process coherent two-step electron transfer. More recently, Kuznetsov and Ulstrup [81] have developed a systematic theory for the sequential two-step process. The electron transfers from one electrode to the molecule and reduces the molecule, and then transfers to the second electrode and reoxidizes the molecule, so the process is reviewed as a cycle of consecutive molecular reduction and reoxidation. A particular interesting feature in the theory is that each reduction-reoxidation cycle is composed of a large number of individual electron transfer events between the molecule and both the tip and the substrate. This significantly enhances the tunneling current compared to a single electron transfer. 

Alternatively, according to Kwart and George,28 the available experimental data are coherent with a hydrogen transfer by way of a cyclic five-membered transition state. A mechanism as in Equation below would be consistent both with a manganese ester intermediate and with the five-membered transition state suggested by Kwart and George. 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

With a coherent stimulated Raman process (STIRAP) (see Sect. 7.3), nearly the whole initial population Ni may be transferred into the final level If) [1048]. Here no exact resonance with the intermediate excited level ) is wanted in order to avoid transfer losses by spontaneous emission from level ). The population transfer can be explained by an adiabatic passage between dressed states (that is, states of the molecule plus the radiation field) [1049, 1050]. 

Apart from the most electropositive metals, most other metals extracted through molten salt routes are recovered as solids these include many important refractory and other transition metals, the lanthanides, and some actinides. Particularly interesting problems arise in the electrowinning of the refractory metals. Attempts to deposit these metals in a coherent, massive form of theoretical density usually meet with a number of difficulties. Deposits may be dendritic, for example, if electrodeposition proceeds under mass transfer control, or they may be powdery and nonadherent if secondary reactions, such as alkali metal deposition, followed by backreaction with the solute, occurs. Moreover, powdery deposits may also arise if low oxidation states, formed as intermediates during the reduction process, disproportionate in the metal-melt interphase. Charge-transfer-controlled electrodeposition or coupled chemical steps appear to be a prerequisite for obtaining dense, coherent, and adherent deposits. Such deposits have been obtained... 

---