Chemical Shift nmr

8 Key excerpts on "Chemical Shift nmr"

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  eBook - ePub

  Spectroscopy for Materials Characterization

  • Simonpietro Agnello, Simonpietro Agnello(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  ref are the resonant frequencies of the sample and the reference, respectively. The total frequency range of the chemical shifts of a certain nucleus depends both on the applied magnetic field and on the isotope to be studied. The NMR spectra are represented with decreasing frequencies toward the right with zero coinciding with the resonance of the reference. If one nucleus is more shielded than another, its signal will be shifted to lower frequencies (or higher fields). The observation of an NMR spectrum of a specific molecule depends precisely on this chemical shielding property.

  The importance of the chemical shift lies in the fact that it reveals changes in the chemical and physical surroundings of a molecule. The first advantage of the chemical shift is that the nuclei exhibit specific resonances which depend on their chemical nature. In addition to the characteristic frequencies of certain groups in IR or Raman spectroscopy, similar functional groups will have similar chemical shifts.

  The spins can also interact with each other directly. If this interaction occurs through space, then it is called a direct dipole–dipole interaction. This interaction is important as it provides structural information since the intensity of the dipolar interactions depends on the internuclear distance.

  Let us consider the general model of an isolated pair of spin ½ nuclei, which we will call μ l and μ 2 , interacting through their own dipoles.

  The dipole associated with μ l precesses around B 0 at its Larmor frequency, thus generating a static component along the direction of the field and a rotating component in the plane perpendicular to the direction of B 0 . The static component of μ l produces a small static field at the μ 2 dipole site. The intensity of this last local magnetic field B loc depends on the relative positions of the two spins and their orientations with respect to B 0 . If a sample containing this isolated pair of nuclei is subjected to the static magnetic field B 0 , the result will be that each nucleus will experience an effective magnetic field B eff

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  eBook - ePub

  Essential Concepts in MRI

  Physics, Instrumentation, Spectroscopy and Imaging

  • Yang Xia(Author)
  • 2022(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  1 ] could be due to structural symmetry, rapid rotation, or simply by coincidence.

  If another group of nuclei has very different chemical shifts, the letters at the end of the alphabet are commonly used to name them, such as X, Y. (It may also be used as an approximation for nuclei of the same species where the coupling is much less than the chemical shift differences.) When there are more than two well-separated groups of nuclei, a middle letter of the alphabet can be used, for example, AMX implies the existence of three different nuclei with three different magnetic environments. Other nomenclatures of the spin system in NMR spectroscopy also exist [1–3 ].

  8.2 PEAK SHIFT – THE EFFECT OF CHEMICAL SHIFT

  In Chapter 4.2 , the equation for chemical shift was given as

  f = γ

  B 0

  ( 1 − σ ) / 2 π

  , where σ is the shielding constant and f is the temporal/linear frequency in hertz; and chemical shift is discussed in this book with the assumption of diamagnetic materials. Practically, chemical shift of a resonance is compared to that of a reference, commonly tetramethylsilane (TMS) in 1 H and 13 C spectroscopy. In a typical plot of an NMR spectrum, the horizontal axis is the chemical shift in ppm and the vertical axis is the signal amplitude. The chemical shift of the nucleus is commonly quoted in the δ scale, given by

    (8.1)

  where the 106 factor converts the chemical shift from the frequency ratio to ppm [cf. Eq. (4.10)].

  In the early days of NMR experiments, the electric current was slowly increased in the coil of an electromagnet to increase the external magnetic field

  B 0

  [cf. Eq. (5.1)]. If a resonance occurred in the sample, an absorption peak would show up on the cathode-ray screen of an analog oscilloscope, which was recorded on paper by a chart recorder. If there was no resonance after a reasonable amount of time, one increased the electric current to produce a higher field. The left-hand side of the recording paper therefore represented the effect of the lower magnetic field and was called “downfield.” The right-hand side of the recording paper contained the result of the higher magnetic field and was called “upfield.” A larger shielding constant, σ , requires a higher field to reach the resonance; so a peak with a large σ would appear on the right side of the recording paper. Since

  ω = γ

  B 0

  ( 1 − σ )

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  eBook - ePub

  Essentials of Chemical Biology

  Structure and Dynamics of Biological Macromolecules

  • Andrew D. Miller, Julian A. Tanner(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  spin–spin coupling. These topics will be covered very briefly in turn.

  5.2.4.1 Chemical shift

  Chemical shift may be thought of as the manner in which the resonance frequencies, ν L , of nuclei that are part of molecular structures vary in a systematic and reproducible way in response to local chemical environment. According to Equation (5.9) , changes in external magnetic field strength experienced by a given nucleus must have a direct affect on the energy difference between nuclear spin states leading to a proportionate change in resonance frequency, ν L . Chemical shift arises because the strength of the effective external magnetic field experienced by any nucleus in a molecular structure appears to vary in response to local movements in neighbouring electron density. In other words, local electronic effects have direct and reproducible effects on ν L values. Electronic effects are both shielding and deshielding in character. Shielding effects are generated by the tendency of an external magnetic field to induce electron density to ‘circulate’ in such a way as to create a local magnetic field in opposition to the applied field. The effective external magnetic field experienced by any such nucleus is then modulated according to

  (5.10)

  where B eff, z is the effective field experienced by the nucleus and shielding is characterised by the shielding parameter, σ N , also known as the chemical shift tensor. Deshielding arises out of hetero-atom σ -bond inductive effects and π -bond ring current effects. The former effect reduces local electron density around a given nucleus, hence increasing the effective field and hence ν L . The latter effect creates local magnetic fields that cooperate with the applied field to increase the effective external magnetic field experienced by nuclei and hence their ν L values (Figure 5.6 ). Together, shielding and deshielding effects are primarily responsible for ensuring that ν L values vary as a direct consequence of local chemical environment and are therefore a direct indication of the nature of this chemical environment. In order to ensure that variation in ν L values as a function of local chemical environment is standardised between NMR experiments and NMR spectrometers, the δ chemical shift scale was introduced. This scale is defined by Equation (5.11)

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  eBook - ePub

  Practical Approaches to Biological Inorganic Chemistry

  • Robert R. Crichton, Ricardo O. Louro(Authors)
  • 2019(Publication Date)
  • Elsevier

    (Publisher)

  In these conditions, a different chemical environment gives rise to a characteristic nuclear shielding. Therefore there are very typical spectral regions where the signals of nuclei with a particular chemical nature can be found.

  As seen from the Larmor equation, the resonance frequency is dependent on the static field strength, and therefore the frequency difference between two signals increases with field strength. On the other hand, the chemical shift (δ ) is defined as the ratio between the nuclear shielding and the static magnetic field and is reported in parts per million. The very useful outcome in this definition of chemical shift is that signals measured in spectrometers with different field strength can be matched because they will have the same chemical shift (in ppm). Nonetheless, the spectral dispersion is larger in the experiment performed at the strongest field. In practice, chemical shifts are measured as the difference in parts per million between the signal of interest and the signal of a reference substance added to the sample. Different compounds are used as reference depending on the solvent, applications, and nuclei being observed. In water or deuterated water, the methyl signal of tetramethylsilane (TMS) is typically used as a marker of 0 ppm for protons.

  δ =

  ν sample

  −

  ν ref

  ν ref

  . 10

  6

  (5.15)

  (5.15)

  In this way, a positive δ indicates that the frequency of resonance of the nucleus under observation is higher than that of the reference. In the old CW spectrometers where the spectrum was obtained by varying the intensity of the field, observation of such signal required that the field was lower relatively to that required to observe the reference. Therein lies the still often used designation of “low field” for signals with positive chemical shift.

  The phenomenon of chemical shift has a very important operational consequence in modern FT spectrometers with fixed B 0 . The frequency of the B 1

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  eBook - ePub

  Data Processing Handbook for Complex Biological Data Sources

  • Gauri Misra(Author)
  • 2019(Publication Date)
  • Academic Press

    (Publisher)

  50] .

  The sensitivity refers to the distinction between the background noise and a real resonance signal. Often these two characteristics may be associated because when there is a loss of resolution (peak broadening) there is associated decrease in sensitivity (peak height).

  In NMR measurements, the electron shielding is the primary effect observable that happens by the movement of the electrons in the orbit of nuclei. In this way, the value of the field effectively applied to a given nucleus will be different of B0 . The electron shielding shifts the position line in the spectrum, often to the right side, strengthening the effect. The high field of the spectrum, the values of the chemical shift are smaller, usually <3 ppm, when considering hydrogen frequency. At the same time, the value of frequency will also be lower. On the other hand, when the electron density is lower, the strength of the field B 0 will be more effective. In this case, the nucleus is deshielded. Under such conditions, it is observed that higher values of the resonance frequency, higher values of chemical shifts in the low field region are usually located to the left side of the spectrum [51] .

  The inductive effect may be the simplest way to understand the mechanism of this electron density displacement [45] . The stable state of the partial polarization of the bond between any two atoms represents this change in the local electron density. The force of the inductive effect is a factor of the distance between the two nuclei covalently bound and the difference in the value of electronegativity between them. It is also important to note that the electron density is also related to the presence of π resonance (double and triple bonds) and the bond geometry. In all cases, the practical result of this phenomenon is the asymmetric distribution of charges in the polarized bond. When the electronegative atom is the primary source of asymmetry in the bond (electron attractor), it is termed as negative induction [52]

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  eBook - ePub

  Polymer Characterization

  Physical Techniques, 2nd Edition

  • Dan Campbell, Richard A. Pethrick, Jim R. White(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  6 (expressed in ppm), which is independent of the operating conditions.

  In practice, measurements are made of the frequencies at which the individual peaks occur and the chemical shift is then expressed by

  δ =

  v sample

  −

  v TMS

  v 0

  where v 0 is the operating frequency of the spectrometer or alternatively, the frequency of the proton for which δ = 0.

  Protons in organic molecules, including polymers, are usually less well screened than in TMS and the chemical shifts lie in the range 0–10 ppm. By comparison 13 C chemical shifts extend over a range of 250 ppm with respect to TMS and this has some advantages in structural analysis. The chemical shift ranges for different chemical groupings have been tabulated so that many assignments may be made by reference to model compounds. Table 6.2 contains typical values. It is the convention to record spectra such that the TMS line is to the right hand side with the descreened nuclei to the left, i.e the resonance frequency increasing from right (high field) to left (low field).

  The shielding due to the surrounding electrons is found to be anisotropic with the chemical shift, depending on the orientation of the chemical bonds with respect to the applied magnetic field. In liquids, the anisotropy is averaged to zero but will contribute to the spectral line shape in solid specimens and hence is important in the study of polymers.

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  eBook - ePub

  Characterization of Solid Materials and Heterogeneous Catalysts

  From Structure to Surface Reactivity

  • Michel Che, Jacques C. Vedrine, Michel Che, Jacques C. Vedrine(Authors)
  • 2012(Publication Date)
  • Wiley-VCH

    (Publisher)

  What is often overlooked is the power of NMR not just to yield information on chemical structure but also to probe sorbate–sorbent interactions and molecular diffusion processes within porous materials – all of which play an important role in the performance of these materials as catalysts. At larger length scales, MRI offers the potential to study how the macroscopic flow fields inside the reactor and the resulting reactant–catalyst contacting patterns further influence catalytic performance. Figure 8.1 shows a schematic of a typical NMR setup with the key components indicated [13]. Figure 8.1 Schematic of a modern NMR spectrometer. ADC: analog-to-digital converter [13]. 8.2.1 Basic Principles of NMR A general Hamiltonian, H, describing the interactions experienced by a nucleus of spin I may be expressed as the sum of contributions from the Zeeman (H Z), dipolar (H D), quadrupolar (H Q), chemical shift (H CS), and indirect electron coupled (H J) interactions: (8.1) The precise energy level splitting is therefore sensitive to a number of nuclear spin interactions which are themselves determined by the physical and chemical properties of the nuclear spin system. Thus, by careful design of the NMR experiment, such that we probe a specific contribution to the overall nuclear interaction, much can be learned about the physical and chemical structure and also the dynamics of the sample under study. Only a phenomenological description of the various interactions is given here, in particular the origin of the interactions is noted so as to appreciate the different characteristics of the spin system that can be probed

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  eBook - ePub

  Compendium of Biomedical Instrumentation

  • Raghbir Singh Khandpur(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  341 Spectrometer, NMR

  Purpose

  Nuclear magnetic resonance (NMR ) spectrometer is an analytical instrument for non‐destructive mapping of molecular structures and understanding how molecules function and relate to each other. It is recognized as one of the most powerful techniques for analysis in medicine and biology. The NMR spectroscopy technique is vital for studying cell metabolism, because of the fact that metabolism is based on interactions between molecules and cells. NMR applications include analysis of cells, tissues, and biological fluids. The importance of this technique is reflected in the efforts that have been made to extend its applicability to smaller and smaller sample sizes.

  Principle

  The study of absorption of radio frequency (RF ) radiation by certain atomic nuclei in a magnetic field is called NMR. For a particular nucleus, an NMR absorption spectrum may consist of one to several groups of absorption lines in the RF region of the electromagnetic spectrum. They indicate the chemical nature of the nucleus and the spatial positions of neighbouring nuclei. NMR spectroscopy uses RF radiation to induce transitions between different nuclear spin states of samples in a magnetic field. The technique can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.

  The principle behind NMR spectroscopy is based on the observation that many atomic nuclei spin about an axis and generate their own magnetic field, or magnetic moment that has both magnitude and direction. In body tissue or any other specimen, the magnetic moments of the nuclei making up the tissue are randomly aligned and have zero net magnetization (M = 0). When the same material is placed in a magnetic field B 0 , some of the randomly oriented nuclei experience an external magnetic torque, which tends to align the individual parallel or anti‐parallel magnetic moments to the direction of an applied magnetic field. There is a slight excess of nuclei aligned parallel with the magnetic field and this gives the tissue a net magnetic moment M 0 . The difference in energy between the two spin states increases with increasing strength of B 0

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