NMR and Chemistry: An introduction to the Fourier transform-multinuclear era

NMR and chemistry : an introduction to the Fourier transform-multinuclear era
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Additional structural and chemical information may be obtained by performing double-quantum NMR experiments for pairs of spins or quadrupolar nuclei such as 2 H. Furthermore, nuclear magnetic resonance is one of the techniques that has been used to design quantum automata, and also build elementary quantum computers.

In the first few decades of nuclear magnetic resonance, spectrometers used a technique known as continuous-wave CW spectroscopy, where the transverse spin magnetization generated by a weak oscillating magnetic field is recorded as a function of the oscillation frequency or static field strength B 0.

Although NMR spectra could be, and have been, obtained using a fixed constant magnetic field and sweeping the frequency of the oscillating magnetic field, it was more convenient to use a fixed frequency source and vary the current and hence magnetic field in an electromagnet to observe the resonant absorption signals. This is the origin of the counterintuitive, but still common, "high field" and "low field" terminology for low frequency and high frequency regions, respectively, of the NMR spectrum.

One radio coil operated continuously, sweeping through a range of frequencies, while another orthogonal coil, designed not to receive radiation from the transmitter, received signals from nuclei that reoriented in solution. CW spectroscopy is inefficient in comparison with Fourier analysis techniques see below since it probes the NMR response at individual frequencies or field strengths in succession. Since the NMR signal is intrinsically weak, the observed spectrum suffers from a poor signal-to-noise ratio.

Solid State NMR Spectroscopy and Phase Transitions in Minerals

This can be mitigated by signal averaging, i. While the NMR signal is the same in each scan and so adds linearly, the random noise adds more slowly — proportional to the square root of the number of spectra see random walk. Hence the overall signal-to-noise ratio increases as the square-root of the number of spectra measured. Early attempts to acquire the NMR spectrum more efficiently than simple CW methods involved illuminating the target simultaneously with more than one frequency. A revolution in NMR occurred when short radio-frequency pulses began to be used, with a frequency centered at the middle of the NMR spectrum.

Nuclear magnetic resonance

In simple terms, a short pulse of a given "carrier" frequency "contains" a range of frequencies centered about the carrier frequency , with the range of excitation bandwidth being inversely proportional to the pulse duration, i. Applying such a pulse to a set of nuclear spins simultaneously excites all the single-quantum NMR transitions. In terms of the net magnetization vector, this corresponds to tilting the magnetization vector away from its equilibrium position aligned along the external magnetic field. The out-of-equilibrium magnetization vector then precesses about the external magnetic field vector at the NMR frequency of the spins.

This oscillating magnetization vector induces a voltage in a nearby pickup coil, creating an electrical signal oscillating at the NMR frequency. This signal is known as the free induction decay FID , and it contains the sum of the NMR responses from all the excited spins. NMR frequency this time-domain signal intensity vs. Fourier methods can be applied to many types of spectroscopy. See the full article on Fourier transform spectroscopy.

Richard R. The use of pulses of different durations, frequencies, or shapes in specifically designed patterns or pulse sequences allows the NMR spectroscopist to extract many different types of information about the molecules in the sample. In multi-dimensional nuclear magnetic resonance spectroscopy, there are at least two pulses and, as the experiment is repeated, the pulse timings are systematically varied and the oscillations of the spin system are probed point by point in the time domain.

Multidimensional Fourier transformation of the multidimensional time signal yields the multidimensional spectrum. In two-dimensional nuclear magnetic resonance there will be one systematically varied time period in the sequence of pulses, which will modulate the intensity or phase of the detected signals.

The remaining "dimension" is always provided by the directly detected signal. There are many such experiments. In some, fixed time intervals allow among other things magnetization transfer between nuclei and, therefore, the detection of the kinds of nuclear-nuclear interactions that allowed for the magnetization transfer. Interactions that can be detected are usually classified into two kinds. There are through-bond and through-space interactions, the latter being a consequence of dipolar couplings in solid-state NMR and of the nuclear Overhauser effect in solution NMR.

This technique complements X-ray crystallography in that it is frequently applicable to molecules in an amorphous or liquid-crystalline state, whereas crystallography, as the name implies, is performed on molecules in a crystalline phase. In electronically conductive materials, the Knight shift of the resonance frequency can provide information on the mobile charge carriers. Though nuclear magnetic resonance is used to study the structure of solids, extensive atomic-level structural detail is more challenging to obtain in the solid state.

Due to broadening by chemical shift anisotropy CSA and dipolar couplings to other nuclear spins, without special techniques such as MAS or dipolar decoupling by RF pulses, the observed spectrum is often only a broad Gaussian band for non-quadrupolar spins in a solid. Professor Raymond Andrew at the University of Nottingham in the UK pioneered the development of high-resolution solid-state nuclear magnetic resonance. He was the first to report the introduction of the MAS magic angle sample spinning; MASS technique that allowed him to achieve spectral resolution in solids sufficient to distinguish between chemical groups with either different chemical shifts or distinct Knight shifts.

In amorphous materials, residual line broadening remains since each segment is in a slightly different environment, therefore exhibiting a slightly different NMR frequency. Dipolar and J-couplings to nearby 1 H nuclei are usually removed by radio-frequency pulses applied at the 1 H frequency during signal detection.

The concept of cross polarization developed by Sven Hartmann and Erwin Hahn was utilized in transferring magnetization from protons to less sensitive nuclei by M. Gibby, Alex Pines and John S. Then, Jake Schaefer and Ed Stejskal demonstrated the powerful use of cross polarization under MAS conditions CP-MAS and proton decoupling, which is now routinely employed to measure high resolution spectra of low-abundance and low-sensitivity nuclei, such as carbon, silicon, or nitrogen, in solids.

Significant further signal enhancement can be achieved by dynamic nuclear polarization from unpaired electrons to the nuclei, usually at temperatures near K. Because the intensity of nuclear magnetic resonance signals and, hence, the sensitivity of the technique depends on the strength of the magnetic field the technique has also advanced over the decades with the development of more powerful magnets.

Advances made in audio-visual technology have also improved the signal-generation and processing capabilities of newer instruments.

As noted above, the sensitivity of nuclear magnetic resonance signals is also dependent on the presence of a magnetically susceptible nuclide and, therefore, either on the natural abundance of such nuclides or on the ability of the experimentalist to artificially enrich the molecules, under study, with such nuclides. The most abundant naturally occurring isotopes of hydrogen and phosphorus for example are both magnetically susceptible and readily useful for nuclear magnetic resonance spectroscopy.

In contrast, carbon and nitrogen have useful isotopes but which occur only in very low natural abundance. Other limitations on sensitivity arise from the quantum-mechanical nature of the phenomenon.

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For quantum states separated by energy equivalent to radio frequencies, thermal energy from the environment causes the populations of the states to be close to equal. Since incoming radiation is equally likely to cause stimulated emission a transition from the upper to the lower state as absorption, the NMR effect depends on an excess of nuclei in the lower states. Several factors can reduce sensitivity, including:.

Many isotopes of chemical elements can be used for NMR analysis. Other nuclei usually used in the studies of their complexes and chemical bonding, or to detect presence of the element :. NMR is extensively used in medicine in the form of magnetic resonance imaging. NMR is used industrially mainly for routine analysis of chemicals. The technique is also used, for example, to measure the ratio between water and fat in foods, monitor the flow of corrosive fluids in pipes, or to study molecular structures such as catalysts.

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The application of nuclear magnetic resonance best known to the general public is magnetic resonance imaging for medical diagnosis and magnetic resonance microscopy in research settings. Biochemical information can also be obtained from living tissue e. These spectroscopic studies are possible because nuclei are surrounded by orbiting electrons, which are charged particles that generate small, local magnetic fields that add to or subtract from the external magnetic field, and so will partially shield the nuclei.

The amount of shielding depends on the exact local environment. For example, a hydrogen bonded to an oxygen will be shielded differently from a hydrogen bonded to a carbon atom.

William Cox: An Intuitive Introduction to the Fourier Transform and FFT

In addition, two hydrogen nuclei can interact via a process known as spin-spin coupling , if they are on the same molecule, which will split the lines of the spectra in a recognizable way. As one of the two major spectroscopic techniques used in metabolomics , NMR is used to generate metabolic fingerprints from biological fluids to obtain information about disease states or toxic insults. By studying the peaks of nuclear magnetic resonance spectra, chemists can determine the structure of many compounds.

It can be a very selective technique, distinguishing among many atoms within a molecule or collection of molecules of the same type but which differ only in terms of their local chemical environment. NMR spectroscopy is used to unambiguously identify known and novel compounds, and as such, is usually required by scientific journals for identity confirmation of synthesized new compounds. A chemist can determine the identity of a compound by comparing the observed nuclear precession frequencies to known frequencies. Further structural data can be elucidated by observing spin-spin coupling , a process by which the precession frequency of a nucleus can be influenced by the spin orientation of a chemically bonded nucleus.

At low enough temperatures, a distinction can be made between the axial and equatorial hydrogens in cyclohexane.

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An example of nuclear magnetic resonance being used in the determination of a structure is that of buckminsterfullerene often called "buckyballs", composition C This now famous form of carbon has 60 carbon atoms forming a sphere. The carbon atoms are all in identical environments and so should see the same internal H field.

NMR Spin Coupling

Unfortunately, buckminsterfullerene contains no hydrogen and so 13 C nuclear magnetic resonance has to be used. However, in the spectrum was obtained by R. Taylor and co-workers at the University of Sussex and was found to contain a single peak, confirming the unusual structure of buckminsterfullerene.