Coherent transfer

In this section we discuss the coherent motion of excitons through an assembly or aggregate of N molecules. This is applicable when h/ J is much smaller than the dissipative relaxation times. 

Suppose that the operator creates an exciton on the mth molecule. Thus, it corresponds to exciting the molecule from its ground state to the state EX)to with an excitation energy A. The Hamiltonian that describes the coherent motion of the exciton through the aggregate is 

Eor simplicity we will assume that only one exciton is excited in the aggregate. Then the basis states are single-particle basis states of the form, 

1 Dimers We discuss dimers for two particular geometrical arrangements, namely coUinear and parallel, as shown in Fig. 9.2. In the coUinear arrangement the exciton transfer integral, J = —2J (R), whereas in the parallel arrangement, J = where is defined in eqn (9.12). In both cases there are bonding, 

The transition dipole moment from the ground state to the excited states of 

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The key for optimally extracting infonnation from these higher order Raman experiments is to use two time dimensions. This is completely analogous to standard two-dimensional NMR [136] or two-dimensional 4WM echoes. As in NMR, tire extra dimension gives infonnation on coherence transfer and the coupling between Raman modes (as opposed to spins in NMR). 

Plenary 7 7. P M Champion et al, e-mail address champ neu.edu (TRRRS). Femtosecond impulsive preparation and timing of ground and excited state Raman coherences in heme proteins. Discovery of coherence transfer along a de-ligation coordinate. See above for fiirther connnent. 

Braunschweiler L and Ernst R R 1983 Coherence transfer by isotropic mixing application to proton correlation spectroscopy J. Magn. Reson. 53 521 -8... 

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H/), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

Two-dimensional spectroscopy has two broad classes of experiments (a) 2D /-resolved spectra (Mtlller et ai, 1975 Aue et ai, 1976), in which no coherence transfer or mixing process normally occurs, and chemical shift and coupling constant frequencies are spread along two different axes. 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

Five different types of peaks can occur in 3D spectra. These are illustrated in an ABC spin system, in which the Larmor frequencies of the three nuclei are v,, Vg, and v, -, and the coherence transfers are associated with two different mixing processes, M, and Mj ... 

Increasing the number of dimensions from two to three may result in a reduction in the signal/noise (S/N) ratio. This may be due either to the distribution of the intensity of the multiplet lines over three dimensions or to some of the coherence transfer steps being inefficient, resulting in weak 3D cross-peaks. 

The pulse sequences for HMQC-COSY and HMQC-NOESY experiments are presented in Fig. 6.10. The 3D HMQC-COSY spectrum of a N labeled tripeptide is shown in Fig. 6.11. Since the coherence transfer involved in this experiment is N([Pg.362]

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

Figure 7.24 (a) Coherence transfer echos formed after a dephasing period of... 

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

Coherence transfer The transfer of coherence between two different types of nuclei. 

However, careful kinetic measurements on related systems showed the invalidity of wire-type behavior [41]. Furthermore, Sen and coworkers [42] recently showed that the appearance of rapid, long-distance charge transfer for metallointercalators may be an artifact caused by the formation of aggregates. Currently, there are no data that clearly support the existence of a coherent transfer process in DNA over a distance greater than one or two base pairs [43, 44]. 

In strongly coupled systems, it is not possible to eliminate chemical shifts by refocusing nor is it possible to describe the evolution in terms of an effective Hamiltonian.44 A 90° or a 180° pulse leads to coherence transfer between various transitions, and a multitude of new effective precession frequencies may appear in the F1 dimension. A detailed analysis shows artefacts resulting of strong coupling induced by the 180° pulse applied on the H channel can be efficiently removed by applying a LPJF before acquisition.42 Likewise, artefacts present in HMBC with a terminal LPJF are suppressed by an LPJF in the beginning of the sequence as in conventional HMBC. 

In a heteronuclear two-spin system the build-up of AP magnetization for coherence transfer during a delay A is determined by the amplitude function... 

If the conditions for Forster transfer are not applicable, then the theory must be extended. There is recently experimental evidence that coherent energy transfer participates in photosynthesis [74, 75], In this case, the participating molecules are very close together. The excited state of the donor does not completely relax to the Boltzmann distribution before the energy can be shared with the acceptor, and the transfer can no longer be described by a Forster mechanism. We will not discuss this case. There has been active discussion of coherent transfer and very strong interactions in the literature for a longer time [69], and references can be found in some more recent papers [70-72, 76, 77],... 

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. 

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