General Principle Sources of Chemical Shifts Summary Biophysical Chemistry: NMR Spectroscopy The Chemical Shift Lieven Buts Vrije Universiteit Brussel 28th October 2011 Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle Sources of Chemical Shifts Summary Outline General Principle 1 Influence of Electron Clouds Sources of Chemical Shifts 2 General Considerations Specific Interpretations Summary 3 Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle Sources of Chemical Shifts Influence of Electron Clouds Summary Outline General Principle 1 Influence of Electron Clouds Sources of Chemical Shifts 2 General Considerations Specific Interpretations Summary 3 Lieven Buts Biophysical Chemistry: NMR Spectroscopy
Reminder: Energy Levels of a Spin-1/2
General Principle Sources of Chemical Shifts Influence of Electron Clouds Summary The Induced Field B 0 B 0 B 0 B 0 B 0 B ′ = B ind = − σ B 0 B local = B 0 + B ind = ( 1 − σ ) B 0 B' B' B' B' B' ω = γ B 0 ( 1 − σ ) The external magnetic field induces currents in the electron cloud around the nucleus. These currents in turn generate a magnetic field, proportionate to the external field and slightly weakening or enhancing it at the site of the nucleus. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle Sources of Chemical Shifts Influence of Electron Clouds Summary The Chemical Shift The frequency difference is expressed as a relative difference with respect to a standard, eliminating the influence of the external field and resulting in a characteristic chemical shift value (independent of B 0 ): = γ B 0 ( 1 − σ ) − γ B 0 ( 1 − σ ref ) δ = ν − ν ref = σ ref − σ γ B 0 ( 1 − σ ref ) ν ref 1 − σ ref Since the numeric value of δ is in practice never more than 10 − 4 , it is generally expressed in terms of "parts per million" ( ppm ). Tetramethyl silane (TMS) and 2,2-dimethyl 2-silapentane 5-sulphonic acid (DSS) are the most commonly used reference compounds for 1 H and 13 C chemical shifts. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
Shielding and Deshielding (1)
Shielding and Deshielding (2)
Shielding and Deshielding (3)
Shielding and Deshielding (4)
A Numeric Example
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Outline General Principle 1 Influence of Electron Clouds Sources of Chemical Shifts 2 General Considerations Specific Interpretations Summary 3 Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary General Description The shielding factor σ can in general be divided into a diamagnetic contribution (resulting from electron currents within molecular orbitals), and a paramagnetic contribution (due to movements of electrons between different orbitals): σ = σ dia + σ para with σ dia > 0 ; σ para < 0 Both terms can be further subdivided into components that can be rationalised in terms of molecular structure with different degrees of success. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Molecular Symmetry In symmetric molecules many (or all) hydrogen atoms will have the same electronic environment, and thus end up with the same chemical shift. Simple symmetry arguments can sometimes go a long way in the analysis of spectra. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Quantitative Integration of Intensities In simple cases the relative area under each peak can be used to "count" the atoms in each distinct chemical environment. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Outline General Principle 1 Influence of Electron Clouds Sources of Chemical Shifts 2 General Considerations Specific Interpretations Summary 3 Lieven Buts Biophysical Chemistry: NMR Spectroscopy
Specific Interpretations In some cases, such as the methyl halides (below) and the substituted aromatics (right), the observed chemical shifts can be explained using elementary chemical principles.
Neighbouring Groups This schematic molecule has an elongated cloud of electrons (blue), and two sites A and B at different fixed positions relative to this cloud. µ // and µ ⊥ are the externally induced dipoles in the electron cloud for the two orientations shown, and will in general both be aligned with the external field B 0 , but have different magnitudes.
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Effect of a Triple Bond ( µ ⊥ > 0 (mainly a paramagnetic effect) and µ // ≈ 0 , which means µ // − µ ⊥ < 0 ) Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Effct of an Aromatic Ring ( µ ⊥ ≈ 0 , while µ // < 0 (mainly a diamagnetic effect), which again means µ // − µ ⊥ < 0 ) Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Effect of Hydrogen Bonds Hydrogen bonds can lead to strong deshielding of the affected proton. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Protein Spectra Although the hundreds or thousands of protons in a protein molecule all have unique environments and hence chemical shifts, the limited range means that the spectrum is still very crowded, even in ideal conditions: A few signals can be studied in isolation, but the vast majority is lost in a forest of overlapping signals. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Functional Groups in Proteins The difference in the electronic environment between the protonated and unprotonated forms can be used to determine the pK a of individual histidine side chains: δ avg = δ HA + [ HA + ] + δ A [ A ] [ HA + ] + [ A ] Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle General Considerations Sources of Chemical Shifts Specific Interpretations Summary Special Cases Atoms and radical with unpaired electrons give rise to strong paramagnetic effects, leading to extreme negative chemical shifts. (The gyromagnetic ratio of the electron is 660 times larger than even that of the proton.) Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle Sources of Chemical Shifts Summary Summary (1) The resonance frequency of an isolated nucleus in an external magnetic field is determined only by the gyromagnetic ratio of the nucleus and the strength of the field. In an atom or molecule, however, the nucleus is surrounded by a cloud of electrons that proportionately modifies the effective magnetic field at the site of the nucleus. Because different nuclei of the same type can occupy different chemical (and therefore electronic) environments, they can resonate at slightly different frequencies and be distinguished. The most convenient quantitative measure for this effect of the local chemical environment is the chemical shift δ , which is characteristic of a given chemical environment and independent of the external field strength. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
General Principle Sources of Chemical Shifts Summary Summary (2) With a suffciently detailed description of the behaviour of the electrons in a molecule, chemical shifts can in principle be calculated. Even when a detailed interpretation is not feasible, chemical shifts are still extremely valuable for the identification of atoms in molecules and as markers for processes. Lieven Buts Biophysical Chemistry: NMR Spectroscopy
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