Coherence Transfer Processes Research Articles

Relaxation-allowed transfer of coherence in NMR between spins which are not scalar coupled

In nuclear magnetic resonance spectroscopy in an isotropic phase it is shown that coherence can be transferred with a single rf pulse between two spins which possess no mutual scalar coupling. Such anomalous coherence transfer processes are associated with multiexponential transverse relaxation that can arise, amongst other mechanisms, from cross-correlation between two time-dependent dipolar interactions.

A diagrammatic representation of both in-phase and antiphase coherence transfer processes with a simple application

A diagrammatic representation of both in-phase and antiphase coherence transfer processes with a simple application

Interference effects on relaxation in three-level systems: Breakdown of the rate equation description

The stochastic Liouville equation and exact quantum mechanical simulations have been used to test the validity of rate law predictions for the vibrational energy relaxation in a three-level system. The model system consists of a low lying ground state which is radiatively coupled to only one of three closely spaced excited states. All three excited states are strongly coupled to baths of known statistical properties (either Gaussian or Poisson). We follow the energy exchange between the excited states after the light is turned off. Two different types of bath correlation have been considered: (A) The couplings between the different levels are the same and (B) the couplings are uncorrelated with each other. We find that relaxation differs for these two cases, in contradication to the prediction of the simple rate law description. We also present the first quantitative study of the interference between different pathways that can occur in such a three-level system. We find that the interference has a considerable effect on the decay of the middle level for the same bath (case A), even in the Markovian limit, while for independent baths (case B) interference becomes important only in the non-Markovian limit. These results are entirely different from the prediction of the conventional rate laws. We also discuss the effect of the excitation on the subsequent relaxation and the coherence transfer processes that lead to an oscillatory decay in the non-Markovian limit.

Assignments and structural information via relayed coherence transfer spectroscopy

Investigations of molecular structures by nuclear magnetic resonance have, for the most part, relied on interpreting spectral parameters such as chemical shifts and spin couplings. A more direct approach can be based on the determination of extended coupling networks. When the coupling networks include nuclei which are in no way coupled to one another but do share a common coupling partner then new information is gained which can aid in both structural determination and in the assignment of resonances (1, 2). An example of this approach is the correlation of the chemical shifts of carbon13 nuclei with not only the directly coupled neighbor protons but to those remote protons coupled to the neighbor protons. Adjacent carbon sites in a molecule can be identified since their coupling networks overlap. The experiment might be thought of as an extension of two-dimensional chemical shift correlation spectroscopy (3, 4) which uses a heteronuclear spy to report on those protons which are directly coupled to the heteronucleus (5, 6). In the experiment presented below the heteronuclear spy reports not only on the directly coupled protons but those protons coupled to the directly coupled protons. The procedure will be referred to as relayed coherence transfer as it is based on relaying the spectral information about the remote nuclei through a neighbor to the heteronucleus. The basic idea which allows the observation of the remote protons via the heteronucleus is the merging of two, or more, distinct coherence transfer processes. In the example presented here the first coherence transfer process is between protons, which is then followed by a heteronuclear transfer. A two-coherence transfer experiment is most easily illustrated by considering the case of an AMX spin system with the heteronuclear coupling JAX vanishing and with JMx being much larger than JAM as is typically the case for proton-proton and proton-carbon13 couplings. The pulse sequence used to do the coherence transfers is shown in Fig. 1. The first two proton pulses, separated by a time t,, transfer coherence from A to M spins, among other pathways, as occurs in a homonuclear correlation experiment (7, 8). Once the magnetization has been passed from the remote A spin to the neighbor M spin it can then be relayed to the heteronucleus. The heteronuclear transfer occurs with the proton and carbon-13 pulses at the end of the time t, + fu. The time r, + tH is chosen so that the multiplets which are out of phase with

Separation and suppression of coherent transfer effects in two-dimensional NOE and chemical exchange spectroscopy

A new method for the separation of coherent and incoherent magnetization transfer in two-dimensional (2D) NOE and chemical exchange spectroscopy is described. The new experiment differs from previous 2D exchange experiments, in that the mixing time τ m is incremented systematically together with the evolution time t 1. This permits the distinction of the different orders of multiple-quantum coherence during the mixing time and allows the separation of coherent and incoherent transfer processes. The resulting clarification of chemical exchange and 2D NOE spectra is illustrated with experiments on small molecules and on a globular protein, the basic pancreatic trypsin inhibitor.

Coherence transfer processes in vibrational relaxation of polyatomic molecules in condensed media

The vibrational relaxation of a polyatomic molecule in a condensed host is studied by a consideration of two molecular vibrations. Relaxation processes, intermode coupling terms and vibrational frequency fluctuation contributions are retained. Population decay ( T 1), dephasing ( T 2), and coherence transfer rates are evaluated through second order in the limit where the host bath dynamics are rapid compared to these molecular timescales. The rates are expressed in terms of temperature and frequency dependent bath correlation functions. For the special case of a three level system (the ground state and ones where one of the two vibrational modes is excited) the important effects of anharmonicity are incorporated. It is shown that certain coherence transfer terms involve zero frequency bath correlation functions, so they should be larger than the high frequency ones which obey modified energy gap laws. A discussion is presented of the types of interactions which may contribute to these coherence transfer processes.