SLIDE 1 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
SLIDE 2 General Principle Sources of Chemical Shifts Summary
Outline
1
General Principle Influence of Electron Clouds
2
Sources of Chemical Shifts General Considerations Specific Interpretations
3
Summary
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 3 General Principle Sources of Chemical Shifts Summary Influence of Electron Clouds
Outline
1
General Principle Influence of Electron Clouds
2
Sources of Chemical Shifts General Considerations Specific Interpretations
3
Summary
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 4
Reminder: Energy Levels of a Spin-1/2
SLIDE 5 General Principle Sources of Chemical Shifts Summary Influence of Electron Clouds
The Induced Field
B0 B0 B0 B0 B0 B' B' B' B' B'
B′ = Bind = −σB0 Blocal = B0 + Bind = (1 − σ)B0 ω = γB0(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
SLIDE 6 General Principle Sources of Chemical Shifts Summary Influence of Electron Clouds
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 B0): δ = ν − νref νref = γB0(1 − σ) − γB0(1 − σref ) γB0(1 − σref ) = σref − σ 1 − σref 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 1H and 13C chemical shifts.
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 7
Shielding and Deshielding (1)
SLIDE 8
Shielding and Deshielding (2)
SLIDE 9
Shielding and Deshielding (3)
SLIDE 10
Shielding and Deshielding (4)
SLIDE 11
A Numeric Example
SLIDE 12 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
Outline
1
General Principle Influence of Electron Clouds
2
Sources of Chemical Shifts General Considerations Specific Interpretations
3
Summary
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 13 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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 = σdia + σpara with σdia > 0; σpara < 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
SLIDE 14 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 15 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 16 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
Outline
1
General Principle Influence of Electron Clouds
2
Sources of Chemical Shifts General Considerations Specific Interpretations
3
Summary
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 17
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.
SLIDE 18
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 B0, but have different magnitudes.
SLIDE 19 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 20 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 21 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
Effect of Hydrogen Bonds
Hydrogen bonds can lead to strong deshielding of the affected proton.
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 22 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 23 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
Functional Groups in Proteins
The difference in the electronic environment between the protonated and unprotonated forms can be used to determine the pKa of individual histidine side chains: δavg = δHA+[HA+] + δA[A] [HA+] + [A]
Lieven Buts Biophysical Chemistry: NMR Spectroscopy
SLIDE 24 General Principle Sources of Chemical Shifts Summary General Considerations Specific Interpretations
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
SLIDE 25 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
- ccupy 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
SLIDE 26 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
