Factors Affecting Chemical Shift

11 Key excerpts on "Factors Affecting Chemical Shift"

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  Chemical and Biochemical Applications

  • Pierre Laszlo(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  P a r a m a g n e t i c E f f e c t s 90 D. D y n a m i c Effects 91 E. I s o t o p i c Effects o f N e i g h b o r i n g N u c l e i 93 F. H e a v y -A t o m Effects 9 4 G. C o r r e l a t i o n s b e t w e e n N u c l e a r S h i e l d i n g a n d O t h e r S p e c t r o s c o p i c O b s e r v a t i o n s 96 III. I n t e r m o l e c u l a r F a c t o r s 98 R e f e r e n c e s 100 I. Introduction In principle the factors that determine the measured chemical shifts of heavy nuclei are the same as those for light nuclei. In practice the relative importance of some of these factors changes on passing from lighter to heavier nuclei. A chemical shift is a shielding difference between two nuclei of the same species in different environments. The nuclei concerned may belong to the same or to different molecules. Nuclear shielding, from the applied magnetic N M R field, is due to the electrons and nuclei present in the vicinity of the nucleus in question. Hence a basic description of the vari-ous factors responsible for chemical shifts requires a knowledge of the electronic environment of the nucleus concerned. In spite of the many approximations involved, it is generally accepted that the most satisfactory current description of molecular electronic structure is that provided by molecular orbital (MO) theory (Pople and NMR OF NEWLY ACCESSIBLE NUCLEI, VOL. 1 79 Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-437101-9 80 G. A . W e b b Beveridge, 1970). The more sophisticated ab initio MO calculations are usually performed on small molecules containing light atoms. In general, less precise, semiempirical MO descriptions are available for larger mole-cules and for those containing nuclei up to and including the first transi-tion series of elements.

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  Spectroscopic Methods for Nanomaterials Characterization

  • Sabu Thomas, Raju Thomas, Ajesh K Zachariah, Raghvendra Kumar Mishra, Sabu Thomas, Raju Thomas, Ajesh K Zachariah, Raghvendra Kumar Mishra, Raghvendra Kumar(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  γ is the gyration ratio. Now the chemical shift can be expressed as:

  δ =

  σ R

  −

  σ S

  1 −

  σ

  R S

  ×

  10 6

  (13.48)

  13.7. Factors Affecting the Chemical Shift

  NMR-active nuclei are seldom found as isolated species and there exists very little information that can be gathered, as the shielding and deshielding influence of electrons in the neighborhood results in differences in the resonant frequencies. When the effective magnetic field becomes less than the applied magnetic field owing to the shielding by electrons in the neighborhood, the applied field requires an increase to achieve resonance. The converse is true when neighboring electrons shield the nucleus. In effect, shielding and deshielding result from different chemical environments, and resonance frequencies can be different on account of the surrounding electronic environment of the nuclei. Electronegative atoms present in molecules tend to draw the electron density toward them and deshield the nucleus. An increase in electronegativity of the surrounding groups will result in a decrease in the electron density, which leads to an increase in chemical shift due to the shielding of the nucleus.

  13.7.1. Effect of π-Electrons on Nuclear Magnetic Resonance Signal Generation

  The π-electrons are also found to produce magnetic fields, the same as the σ-electrons. The π-electrons can either shield or deshield the protons in the nuclei. These shielding and deshielding aspects refer to the direction of the π-electrons' magnetic lines of force with respect to the applied field. As the π-electrons deshield, the protons attach to the sp2 -hybridized carbons; hence, olefins and aromatic compounds resonate downfield from protons σ bonded to sp3 -hybridized carbons.

  13.7.1.1. Anisotropy

  Anisotropy is attributed to a molecule wherever one part of the molecule opposes the applied field and the other part strengthens the applied field. Chemical shifts are reliant on the orientation of neighboring bonds, in particular, the π bonds. Examples of nuclei showing chemical shifts due to π bonds are aromatics, alkenes, and alkynes. Hence anisotropic shifts are useful in identifying aromatics or extra conjugated structures present in the molecules (Fig. 13.16

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  Spin Resonance Spectroscopy

  Principles and applications

  • Chandran Karunakaran, CHANDRAN KARUNAKARAN(Authors)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)

  [4] .

  The following factors affect the chemical shift values of protons: 1. Inductive effects by electronegative groups 2. Magnetic anisotropy 3. The ring current effect in cyclic π-systems 4. H-bonding 5. Mesomeric effect 6. Hybridization

  Figure 2.1  Diamagnetic shielding of nucleus by electron.

  2.3.1.1. Electronegativity

  As described above, the electrons around the nucleus create a magnetic field that opposes the applied field. This reduces the magnetic field experienced at the nucleus. Because the induced field opposes the applied field, the electrons are said to be diamagnetic and the effect on the nucleus is referred to as diamagnetic shielding . Because the field experienced by the nucleus affects the energy difference between the different spin states, the frequency (and hence the chemical shift δ) will change depending on the electron density around the nucleus. Electronegative groups decrease the electron density around the nucleus, and there is less shielding (i.e., deshielding ) so the chemical shift value increases.

  The applied magnetic field, B0 , induces circulations in the electron cloud surrounding the nucleus such that its magnetic moment μ, opposed to B0 , is produced (Lenz's law). Nuclei in a region of high electron density are more shielded from the applied field than those in regions of lower electron density. If inductive effects present in a molecule reduce the electron density in the hydrogen 1s orbital, deshielding (shift to higher frequencies) is observed. For example, the increasing electronegativity from left to right in Table 2.2 decreases the shielding effect and hence the increase in chemical shifts [4]

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  Basic 1H- and 13C-NMR Spectroscopy

  • Metin Balci(Author)
  • 2005(Publication Date)
  • Elsevier Science

    (Publisher)

  3

  Chemical Shift

  Publisher Summary

  This chapter discusses the chemical shift occurring in nuclei and begins with a discussion on local magnetic fields around a nucleus. The behavior of electrons in an atom when applied to external magnetic field induces circulations in the electron cloud surrounding the nucleus causing the electrons to generate their own magnetic fields. Chemical shifts are measured with reference to the protons of proposition of a resonance signal in a NMR spectrum. The preparation of the sample for the measurements of NMR spectra is as important as the recording of the spectra. The chapter explains the choice of a suitable solvent for the determination of the NMR spectra that largely depends on the solubility of the selected solvent of the compound to be studied. The physical properties of solvents that are important for NMR spectroscopy is presented in a tale and the factors influencing the chemical shift are discussed. The magnetic anisotropy of carbon–carbon and carbon–hydrogen bonds and their effect on chemical shifts are illustrated using figures. Exercises are provided at the end to propose a plausible structure for the given H-NMR spectra.

  3.1 LOCAL MAGNETIC FIELDS AROUND A NUCLEUS

  In the previous chapter we explained that the following criterion has to be met in order to bring a nucleus into resonance:

  v = γ

  H 0

  2 π

  (13)

  (13)

  As we have seen before, the gyromagnetic ratio, γ, is a constant characteristic of a particular nucleus. The proton has a definitive magnitude for the gyromagnetic ratio (see Table 2.1 ). The different chemical environment does not have any effect on the gyromagnetic ratio. Consequently, only a single proton peak is to be expected from protons of all kinds in a given magnetic field, in accordance with the basic NMR equation (eq. 13 ). In this case, the nuclear magnetic resonance spectroscopy would allow us to provide the information about the type of nucleus, the elemental composition (C, N, O, etc.), which are characterized by their individual resonance frequencies. Then, proton NMR spectroscopy would provide information whether a proton is present in a compound or not. However, there are much simpler techniques for determining the presence of protons, such as elemental analysis. The fact that all protons do not have a single resonance frequency according to eq. 13 was the main factor leading to the development of NMR spectroscopy. Since different protons have different resonance frequencies, we have to try to understand this phenomenon in relation to eq. 13

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  Nuclear Magnetic Resonance Spectroscopy

  An Introduction to Principles, Applications, and Experimental Methods

  • Joseph B. Lambert, Eugene P. Mazzola, Clark D. Ridge(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  75 3 The Chemical Shift 3.1 Factors That Influence Proton Shifts Interpreting the location of a resonance in the 1 H spectrum in terms of molecular struc-ture requires understanding several contributing factors. Chemical shifts vary according to structure because nuclei experience different degrees of shielding by magnetic fields produced by their surrounding electrons. In the absence of shielding, nuclei experi-ence a field with the value of B 0 , but when shielded by their surrounding electrons, the field experienced by a nucleus becomes B 0 (1 − 𝜎 ), in which the quantity 𝜎 is called the shielding (Figure 3.1a,b). Because the magnetic field ( − B 0 ⋅ 𝜎 ) induced by the elec-trons opposes the static B 0 field, the effect is said to be diamagnetic (represented by the symbol 𝜎 d ). 3.1.1 Local Fields Shielding by the electrons that surround the resonating nuclei is said to arise from local fields , which may be assessed by considering electron density. For the proton, the elec-tronic effects of physical organic chemistry (electronegativity and conjugation) conve-niently describe the role of structure vis-à-vis electron density. In this way, both the atom to which the proton is attached and more distant atoms can modulate the electron density at the proton and hence alter the shielding effect. The effects of electronegativity, usually called polar or inductive effects, are mani-fested in the following fashion. An attached or nearby electron-withdrawing group such as —OH or —CN decreases the electron density and hence the diamagnetic shielding, with the result that the resonance of the attached proton moves toward the left of the chart (to a higher frequency, or downfield; Figure 1.9). By contrast, an electron-donating atom or group increases the diamagnetic shielding and moves the resonance toward the right of the chart (to a lower frequency, or upfield).

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  Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry

  • L. M. Jackman, S. Sternhell, D. H. R. Barton, W. Doering(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  PART 2 THEORY OF CHEMICAL EFFECTS IN NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY CHAPTER 2-1 TIME-DEPENDENT EFFECTS IN NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY, THE nature of all n.m.r. spectra depends on the rates of various processes such as inter- and intra-molecular motions and chemical exchange. As these processes can influence the values of spectral parameters, such as line width, chemical shifts and spin-spin coupling constants, it is important that the underlying principles should be fully understood before we proceed with a detailed treatment of the relation between n.m.r. spectra and chemical con-stitution. In particular we will see that the spectra of some molecules can change radically if the rates of certain processes are changed by altering the conditions of observation, e.g. temperature or pH of solution. We have already considered one process, namely intermolecular inter-actions with neighbouring dipoles (p. 8), the effect of which is time-dependent. Here, we noted that in solids a particular nucleus experiences a field from neighbouring magnetic nuclei and the magnitude of this field depends on the spin orientations of these nuclei. Furthermore, we observed that this effect was absent in the liquid state because of rapid random motion of molecules. What we really meant was that in solids the magnetic environ-ment of a particular nucleus exists for a period which is long compared with the time of survival of its environment in the liquid state. In the latter situa-tion the nucleus experiences an average environment (in this example the average contribution is, for all practical purposes, zero). What we need to know now is how rapidly the environment must change in order for its effect to be equal to the average over all instantaneous values. It will be convenient at this stage to consider an example in which the environment can have only two values rather than the general case where many values may be possible.

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  Applied NMR Spectroscopy for Chemists and Life Scientists

  • Oliver Zerbe, Simon Jurt(Authors)
  • 2013(Publication Date)
  • Wiley-VCH

    (Publisher)

  π cloud of aromatic systems. Such anisotropies can dramatically change the appearance of proton spectra. They usually dramatically increase the dispersion of proton spectra. However, in order to give substantial effects, the protons that are influenced must be sterically fixed relative to the anisotropic group. This is, for example, the case for folded proteins that do adopt a unique structure. The observed shifts can be either shielding or deshielding depending on the exact position of the proton.

  These effects can be dramatic. Aliphatic protons that are fixed in space above the plane of an aromatic ring can be shifted to values below 0 ppm, and those in the plane can be shifted to values higher than 10 ppm. A number of examples are presented in Section 3.2.2.

  In a single or triple bond the magnetic susceptibility is similar in the two directions orthogonal to the bond, and the CSA can be computed as

  (7.5)

  where θ is the angle with respect to the bond axis, r the distance to the center, and χ the magnetic susceptibilities parallel and perpendicular to the bond axis. As can be seen from the formula, the effect depends strongly on the distance and orientation. Anisotropy effects play a particularly large role in proton NMR, and they are described in more detail in Section 3.2.2.

  7.1.1.5 The Electric Field Effect

  Strongly polar groups create intramolecular electric fields. This has the effect of distorting the electron density in the rest of the molecule and will hence influence the chemical shifts. It can act to increase or decrease the chemical shifts of adjacent nuclei.

  7.1.1.6 Hydrogen Bonds

  Hydrogen bonds decrease the electron density at the proton site involved and hence lead to a high-frequency shift. The effect is especially pronounced for symmetric hydrogen bonds (those in which the distance from the proton to both acceptors is equal). Protons that are hydrogen bonded are often recognized from their chemical shift. Their shift is less temperature, concentration, and solvent dependent. Protons that are part of hydrogen bonds exchange much more slowly with labile solvent deuterons and can therefore be differentiated from others. This is used in protein NMR experiments to identify β sheets or α

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  Optical, Electric and Magnetic Properties of Molecules

  A Review of the Work of A.D. Buckingham

  • D.C. Clary, B.J. Orr(Authors)
  • 1997(Publication Date)
  • Elsevier Science

    (Publisher)

  11 NMR chemical shifts: theory and experiment Cynthia Jameson, A review of reprinted paper [B54]: ; Medium effects in proton magnetic resonance. I. Gases W.T. Raynes, A.D. Buckingham and H.J. Bernstein J. Chem. Phys., 1962, 36, 3481–3488 Nuclear magnetic resonance spectroscopy is a powerful technique that is used very widely in the characterization of systems ranging from simple molecules in low-density gases to molecules in biological systems, whole tissues or even whole animals, as well as materials complex and heterogeneous, such as polymer blends and catalysts. The NMR parameter which permits the dispersion of nuclear resonance frequencies into separate signals at separations proportional to the strength of the applied magnetic field is the nuclear magnetic shielding. The difference between nuclear shielding values in two different nuclear sites is called the NMR chemical shift. The extreme sensitivity of the nuclear shielding to the electronic environment gives rise to the dispersion of resonances; for example the 13 C nuclei of the alpha carbons of the various alanine residues in a protein all have different resonance frequencies each of which is different from the free amino acid. In a series of papers [B40, B41, B54], Buckingham introduced the idea of additive contributions to NMR chemical shifts arising from molecular interactions with solvent molecules. The individual chemical-shift contributions identified are as follows: bulk susceptibility σ b, magnetic anisotropy σ a, electric-field effects σ E, and van der Waals σ W. These papers constitute the framework on which nearly all attempts at the interpretation of the relation of the proton chemical shifts in proteins to the secondary structure are based. In the recent past, the powerful multidimensional NMR methods of determining protein structure in solution made no use at all of the chemical shift information which is a natural byproduct of the resonance frequency assignment step

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  Soil Sampling, Preparation, and Analysis

  • Kim H. Tan(Author)
  • 2005(Publication Date)
  • CRC Press

    (Publisher)

  Relaxation time can be relatively long (in the order of hours) in NMR analysis of solids or viscous liquids. This would usually result in the development of broad-line spectra. Relaxation time is, however, considerably faster (1 to 20 seconds) with liquids and gases, and the much shorter relaxation times generally yield sharper narrow-line spectra.

  20.3 Chemical Shift

  An NMR spectrum is usually presented with and interpreted by units, called chemical shifts, δ. Each absorption signal in the spectrum is assigned a chemical shift value. It is defined as some unitless number, but its value is commonly expressed in terms of ppm (part per million).

  Slightly different versions of explanations have been given for what exactly a chemical shift is. Pfeffer and Gerasimowicz (1989) indicate that each nucleus has a third localized magnetic field B0 σ, produced by the electronic currents in the atoms, in which σ is called the shielding constant. Without the shield-effect all the nuclei would absorb at the same frequency. The presence of σ is then the reason for the different nuclei in the atoms of the sample to absorb energy at slightly different resonance (frequency) positions. The value of these resonances compared to that of a standard is called a chemical shift and is formulated as follows: in which δ = chemical shift in ppm units, vs = resonance frequency of sample, and vr = resonance frequency of the reference or standard.

  δ =

  v s

  −

  v r

  v r

  ×

  10 6

  (20.2)

  The chemical shift can be positive or negative in signs. A positive δ value suggests the presence of a greater degree of shield-effect in the sample.

  In contrast, Willard et al. (1974) believe that the shielding factor, σ, is the nondimensional constant, which may be either positive or negative. They assume that electron clouds are providing the shield, and each group of nuclei is supposed to be shielded differently by these electron clouds. The denser or thicker the clouds, the higher the magnetic field that must be applied to penetrate the shield for producing detectable resonance signals from the nuclei. The assertion is made that the use of NMR spectrometers with different field strengths would yield different resonance signals, making comparisons very difficult. They feel that it is necessary to have one common magnetic field-independent expression for the assignment of the resonance peaks. The magnitude of the resonance peak of the sample against that of a standard, in field-independent units, is also taken as the chemical shift, but its formulation is somewhat different from equation (20.2). Willard and coworkers use v1 (operating frequency of the spectrometer) as the denominator instead of vr

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  LC-NMR

  Expanding the Limits of Structure Elucidation

  • Nina C. Gonnella(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  1 H NMR, p-orbitals are not involved in bonding hydrogen atoms to other nuclei, which is why only a small chemical shift range (0–20 ppm) is observed.

  Some typical chemical shift ranges for 1 H and 13 C are given in Figure 2.9(a) and (b) , respectively.

  FIGURE  2.9   (a) Chemical shift ranges for 1 H nuclei in organic molecules. (b) Chemical shift ranges for 13 C nuclei in organic molecules.

  2.5 Spin Coupling

  Spin-spin coupling is another property that plays a critical role in structure elucidation. Consider proton-proton interactions. When signals for single protons appear as multiple lines, this is due to 1 H-1 H coupling, also known as spin-spin splitting or J -coupling. The spin-spin splitting arises as a result of inter-nuclear magnetic influences.

  As mentioned previously, protons may be viewed as tiny magnets that can be oriented with or against the external magnetic field. For a molecule which contains a proton (HA ) attached to a carbon that is attached to another carbon containing a proton (HB ), HA will feel the presence of the magnetic field of HB . When the field created by HB reinforces the magnetic field of the NMR instrument (B 0 ), HA experiences a slightly stronger field, but when the field created by HB opposes B 0 , HA experiences a slightly weaker field. The same situation occurs for HB relative to magnetic influences from HA . The result is two resonance lines for HA and two resonance lines for HB

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  Introduction to Quantum Mechanics

  • S.M. Blinder(Author)
  • 2020(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 18: Nuclear magnetic resonance

  Abstract

  Nuclear magnetic resonance (NMR) involves the interaction of nuclei with magnetic moments with radiofrequency radiation. This can serve as a sensitive probe of the electronic environment of these nuclei. Chemical shifts and spin-spin interactions provide much insight into chemical bonding, to supplement spectroscopic and other measurement techniques. The technology of NMR is now highly developed, using pulse techniques and Fourier transforms. Two-dimensional NMR can be used to study the structure of proteins and other biomolecules. An offshoot of NMR is magnetic resonance imaging (MRI), which has proven to be an invaluable noninvasive medical diagnostic technique.

  Keywords

  Nuclear magnetic resonance (NMR); magnetic nuclei; chemical shift; spin-spin coupling; relaxation processes; pulse techniques; two-dimensional NMR; magnetic resonance imaging

  Nuclear magnetic resonance (NMR) is a versatile and highly-sophisticated spectroscopic technique which has been applied to a growing number of diverse applications in science, technology and medicine. We will consider, for the most part, magnetic resonance involving 1 H and 13 C nuclei.

  18.1 Magnetic properties of nuclei

  In all our previous work, it has been sufficient to treat nuclei as structureless point particles characterized fully by their mass and electric charge. On a more fundamental level, as was discussed in Chap. 1 , nuclei are actually composite particles made of nucleons (protons and neutrons), which are themselves made of quarks. The additional properties of nuclei which will now become relevant are their spin angular momenta and magnetic moments. Recall that electrons possess an intrinsic or spin angular momentum s which can have just two possible projections along an arbitrary direction in space, namely

  ±

  1 2

  ħ

  . Since ħ is the fundamental quantum unit of angular momentum, the electron is classified as a particle of spin one-half. The electron's spin state is described by the quantum numbers

  s =

  1 2

  and

  m s

  = ±

  1 2

  . A circulating electric charge produces a magnetic moment μ proportional to the angular momentum J

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