Gareth A. Morris

An improved method for heteronuclear chemical shift correlation by two-dimensional NMR

Since the use of two-dimensional NMR for correlating the spectra of coupled heteronuclei was first proposed in 1977 (I), a considerable number of papers have appeared describing different techniques for heteronuclear correlation (2-8). The most widely used method (5, 9-12) produces a two-dimensional spectrum in which one signal appears for each directly bonded carbon-hydrogen pair in a molecule. The fi domain of the spectrum is governed by the proton resonance frequencies and thef, by the carbon-13, so that the correlation signal for a given CH, group appears at frequency coordinates which are just the proton chemical shift in fi, and the carbon-13 chemical shift in fi. Heteronuclear couplings are absent from both frequency domains, although proton-proton scalar coupling structure remains in fi. Such experiments are relatively simple to perform, and show good sensitivity; data for a typical shift correlation two-dimensional spectrum may be obtained in about 10 times the time required to produce a good protondecoupled carbon13 spectrum. A number of chemical applications of such shift correlation experiments have been published (9-ZZ), all using the basic pulse sequence of Ref. (5). This sequence generates proton-decoupled carbon-13 free-induction decays which are modulated as a function of r1 by the frequency difference between the proton chemical shift and the proton transmitter frequency. Since the signals are amplitude modulated with respect to t,, it is not possible to distinguish between positive and negative fi frequencies (13), so that to avoid ambiguity it is necessary to position the proton transmitter either to low field or to high field of the proton spectrum. This is unfortunate in that much of the proton transmitter power is thus wasted, which can lead to severe heating problems with ionic samples if the same proton frequency is used for pulses and for decoupling, as is usually the case. A second problem is that twice as many t1 samples are needed to digitize thef, domain of the spectrum if the proton transmitter lies to one side of the proton spectrum. Both from the point of view of optimum decoupling and from that of minimizing data storage needs and complexity of data processing, it would be desirable to be able to place the proton transmitter in the middle of the proton spectrum. This communication suggests the use of phase cycling of the proton and carbon-13 transmitter pulses, similar to that recently introduced in homonuclear correlation experiments (14-16). This converts amplitude modulation as a function oft, into phase modulation, allowing positive and negativef, frequencies to be distinguished. Thus in the usual experiment (5), examination off2 spectra for successive r1 values would show the amplitudes of individual signals oscillating while their phases remained fixed. The proposed modification would lead tof, spectra with signals oscillating between absorption and dispersion mode while

Correlation of proton chemical shifts by two-dimensional Fourier transform NMR

The first two-dimensional Fourier transform NMR experiment of Jeener (1) has proved to be of considerable historical importance in the development of twodimensional NMR spectroscopy (2,3) but in its original form (a 90”-t,-90”~t, sequence) it has been surprisingly little used. On the other hand, the corresponding heteronuclear chemical shift correlation experiments (4) have been quite popular (S-8). Recently Nagayama et at. (9) have proposed a modification of the Jeener experiment in which acquisition is delayed by a further period t1 so that it starts at the peak of the spin echo. They have demonstrated that this “spin-echo correlated spectroscopy” technique (SECSY) can be extremely useful in the assignment of proton spectra of fairly large molecules, since it identifies the resonances connected by a scalar spin-spin coupling. The purpose of the present communication is to show that the original Jeener experiment can have some advantages in this application. For the proton spectroscopy of large molecules, the refocusing effect which occurs at time 2t, is not usually an important consideration since the linewidths are largely determined by T2 relaxation; sensitivity can thus be improved by starting acquisition immediately after the second 90” pulse. The size of the data matrix can be reduced by the equivalent of quadrature phase detection in the F1 dimension. It is also possible that the form of the two-dimensional spectrum obtained in the Jeener experiment is better suited to the task of sorting out the complicated correlation networks appropriate to large molecules. A detailed theoretical analysis of the Jeener experiment has been given by Aue ef al. (3), showing that in general the spectrum S(F1,F2) contains three kinds of resonance response: axial, diagonal, and cross peaks. Axial peaks appear along the F2 axis and arise from transverse nuclear magnetization created by the second 90” pulse from longitudinal magnetization; they are weak when t1 % T,. Consider the simple example of a first-order AX spin system with chemical shift difference 6 and coupling constant J. The second 90” pulse (called a mixing pulse) interchanges transverse nuclear magnetization between the four resonance frequencies. If the resulting frequency change is only zero or ?J Hz, the corresponding responses lie on or near the diagonal F1 = Ft. With the convention that all responses of a two-dimensional spin multiplet constitute a “peak,” these are diagonal peaks. The frequency change may, however, be of the order of 6, and then there are responses far away from the diagonal; these “cross peaks” are of particular interest because they correlate the shifts of groups of ‘spincoupled nuclei. One of the factors governing the efficiency of magnetization transfer from A to X is a term sin (27rJf,) sin (2mJtZ), which implies that both time dimensions should extend to at least 1/(4J). Another way of visualizing this requirement notes that the cross peaks in the frequency-domain spectrum consist of two pairs of

A simple pulse sequence for selective excitation in Fourier transform NMR

A pulse-repetition rate set to q = 2np radians, i.e., Dfn = n/t, where q = degrees, n = an integer, f = frequency, and t = sec, provides, for a very narrow band of frequencies near the q = 2np conditions, a magnetization vector in which the steps caused by pulses outweigh those caused by precession and which induces a max. NMR signal which is close to the pure absorption-mode condition. Selective excitation by this simple pulse sequence was illustrated by added multiplet subspectra in the Fourier transform 13C NMR of Me2C:CHNMe2. [on SciFinder (R)]

Selective excitation in Fourier transform nuclear magnetic resonance

Abstract The applications of frequency-selective excitation methods in Fourier transform NMR are discussed, and a simple technique is described for selective excitation of a narrow frequency region of a high-resolution NMR spectrum in a Fourier transform spectrometer. A regular sequence of identical radiofrequency pulses of small flip angle exerts a strong cumulative effect on magnetizations close to resonance with the transmitter frequency or one of a set of equally spaced sidebands separated by the pulse repetition rate. All other magnetizations precess through an incomplete number of full rotations between pulses, and are caught by successive pulses at an ever changing phase of their precession, which destroys the cumulative effect. The motion of the various nuclear magnetization vectors may be described pictorially according to the Bloch equations, neglecting relaxation during the pulse sequence. A general theory is presented for selective or “tailored” excitation by an arbitrary modulation of the radiofrequency transmitter signal. It confirms earlier conclusions that the frequency-domain excitation spectrum corresponds to the Fourier transform of the transmitter modulation pattern, provided that the NMR response remains linear. The excitation spectra calculated for the selective pulse sequence by these two alternative approaches show good agreement within their respective limitations. A number of practical applications of selective excitation are explored, including solvent peak suppression, the detection of partial spectra from individual chemical sites, selective studies of relaxation and slow chemical exchange, and hole-burning or localized saturation.

Pure Shift 1H NMR: A Resolution of the Resolution Problem?

Suppressing multiplet structure in 1H??NMR spectra offers a large improvement in spectral resolution (see picture), equivalent to the use of a spectrometer in the GHz range. Such ldquopure shiftrdquo techniques are readily extended to multidimensional methods, for example DOSY.

Development of a geography-referenced regional exposure assessment tool for European rivers - great-er contribution to great-er #1

Abstract The objective of the GREAT-ER project is to develop and validate a powerful and accurate aquatic chemical exposure prediction tool for use within the EU environmental risk assessment schemes. Current techniques to estimate regional predicted environmental concentrations (PECs) use a generic multimedia ‘unit world’ approach and do not account for spatial and temporal variability in landscape characteristics, river flows and/or chemical emissions. Hence, the results are merely applicable on a generic screening level since these models do not offer a realistic prediction of actual steady-state background concentrations. A software system will be developed to calculate the distribution of predicted environmental concentrations (PECs) of down-the-drain chemicals in European surface waters on both a river and catchment area level. Data on dissolved oxygen, biological oxygen demand and ammonia will also be used to assess water quality and to provide data for calibration and verification. The system will use a Geographic Information System (GIS) for data storage and visualization, combined with simple mathematical models for the prediction of chemical fate. Hydrological databases and models will be used to determine river flows. This refined exposure assessment tool should significantly enhance the accuracy of current local and regional exposure estimation methods. The new exposure assessment methodology will integrate specific environmental information and be worked out in a geographically-referenced framework, ultimately on a pan-European scale. The initial data collection, collation and model application will be applied to two pilot study areas, representative of different hydrological and climatological situations in Europe. A blueprint of the methodology will be developed and applied to these pilot study areas, which will allow refining, optimization and verification of the system.The ultimate objective is to implement GREAT-ER for the entire European Union.This work will be performed in the second phase of the project, after the initial three years which are limited to the development of the methodology and verification in the pilot study areas.

Pulse sequences for high‐resolution diffusion‐ordered spectroscopy (HR‐DOSY)

Six pulse sequences are described, all based on the stimulated echo, for use in high resolution diffusion‐ordered spectroscopy (HR‐DOSY). HR‐DOSY requires spectra with clean baselines, pure phases and lineshapes that are independent of field gradient pulse amplitude. Lineshape problems arising from the static field perturbations caused by field gradient pulses and phase errors caused by zero quantum coherence in strongly coupled spin systems are discussed, and the performance of the six sequences is compared. Pulse sequences which use balanced pairs of antiphase field gradient pulses show significant advantages.

Ultrahigh-Resolution NMR Spectroscopy

All psyched up: A flexible and general pure shift experiment (PSYCHE) has been developed that offers superior sensitivity, spectral purity, and tolerance of strong coupling over existing methods for broadband homonuclear decoupling. The partial spectra of estradiol in [D6]DMSO obtained by normal 1H NMR spectroscopy and PSYCHE are shown for comparison.

Simultaneously Enhancing Spectral Resolution and Sensitivity in Heteronuclear Correlation NMR Spectroscopy

BIRDs eye view: Adding periodic BIRD J-refocusing (BIRD=bilinear rotation decoupling) to data acquisition in an HSQC experiment causes broadband homonuclear decoupling, giving a single signal for each proton chemical shift. This pure shift method improves both resolution and signal-to-noise ratio, without the need for special data processing.

NMR spectra of some simple spin systems studied by two-dimensional Fourier transformation of spin echoes

Phase-modulated spin echoes were generated by a 90-180 Deg pulse sequence applied to some strongly coupled proton spin systems (AB, ABX, and ABC), and the modulation was analyzed by 2-dimensional Fourier transformation. Projection of the resulting 2-dimensional spectra onto the F1 axis leads to a J spectrum, whereas projection onto the F2 axis gives the conventional NMR spectrum. Anal. solns. are presented which correctly predict the shape of the AB and ABX J-spectra, and an iterative numerical anal. were used to fit the ABC case. Projections onto the F1-axis are predicted to have exact symmetry about F1 = 0, and this is borne out by expt., whereas the spin multiplets of the conventional high-resoln. spectrum in general lack symmetry. Certain peaks in the 2-dimensional spectra are predicted to have neg. intensities, although no spin population inversion is involved. Exptl. spectra from AB, ABX, and ABC spin systems illustrate these effects, and strong lines of neg. intensity are detected unequivocally. [on SciFinder (R)]