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Nuclear Magnetic Resonance Spectrometry

Nuclear Magnetic Resonance Spectrometry. Nuclear Magnetic Resonance Spectrometry. Principles Proton NMR Spectra Other Nuclei Advanced Techniques. Principles (1). Nuclear Magnetic Resonance Spectrometry Developed in the 1960’s

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Nuclear Magnetic Resonance Spectrometry

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  1. Nuclear Magnetic Resonance Spectrometry

  2. Nuclear Magnetic Resonance Spectrometry • Principles • Proton NMR Spectra • Other Nuclei • Advanced Techniques

  3. Principles (1) Nuclear Magnetic Resonance Spectrometry • Developed in the 1960’s • The study of the absorption of radio waves by a sample in a magnetic field. • Measures transitions in nuclear spin. • Samples (1-30mg) are dissolved in a solvent not containing the type of nucleus of interest

  4. Principles (2) • Atomic nuclei with an odd number of either protons or neutrons (e.g. 1H, 13C, 31P) have a property called nuclear spin. • Spin induces a local magnetic field around the nucleus to arise -the nucleus behaves like a small magnet. • In an external magnetic field, the nucleus may align with or against the magnetic field. Alignment against the field is a higher energy state than alignment with the field.

  5. Principles (3) • Irradiation with the correct frequency (ν) of radio waves causes the nucleus to “spin-flip” from ground-state (aligned with the field, called α) to the higher energy β spin state (against the applied field). • This absorbs energy (ΔE) and the nucleus is said to be in resonance. • The energy is then released in a variety of complex processes, and the nucleus can relax back to the ground state.

  6. Principles (4) ν for any particular nucleus is proportional to ΔE ΔE depends on atomic identity and local chemical environment ΔE is also proportional to the strength of the applied field (H0) • H0 ranges from 14,100 to 176,250Gauss for modern spectrometers - operating frequencies (ν) normally 60 to 750MHz (based on the resonance of protons in (CH3)4Si or TMS) • Increasing field strength gives increased resolution & better spectra. • Modern high-field (>270MHz) machines use a superconducting magnet and complex hardware and computer software to collate and process the data. • Powerful computers allow more complex data analysis and programming of “pulse” sequences

  7. Principles (5) • All nuclei are surrounded by electrons, which produce their own magnetic field • Electrons shield the nucleus from the applied field • Shielding varies according to the local chemical environment, because this affects gives rise to variation in electron density and distribution. • Increased electron density gives increased shielding and vice versa For any particular nucleus ν field strength felt by nucleus Beff Beff= Applied field (B0) - local electronic field (Blocal) .

  8. Principles (6) • ν of chemically different nuclei are different, but not as different as νof different atomic types • Can do NMR spectrometry of one type of atomic nuclei at a time. • by using a defined range of frequencies suitable for the type of nuclei to be observed (e.g. the protons in a molecule) a spectrum can be recorded. • Spectrum consists of distinct signals for each type of chemical environment

  9. Proton NMR Spectra • Chemical Shift • Integration • Multiplicity • Coupling Constants

  10. Chemical Shift (1) • ν is measured in Hz and varies with spectrometer frequency • The position of a signal from a particular proton is therefore measured relative to TMS • This is known as chemical shift (δ) in parts per million (ppm). δ = Distance from TMS signal (Hz) in ppm Spectrometer frequency (MHz) • 1H NMR spectra are normally of the range 0-10ppm (gives a 900 Hz range on a 90MHz machine).

  11. Neighbouring electronegative atoms Deshielded nucleus Increased ν needed for resonance Downfield shift in signal (increased ppm) Chemical Shift (2) Electronegative Neighbour Atoms: e.g. O, F 1H δ for RCH3 (1-2ppm)< RCH2Br (3-4ppm)< RCH2OH (3.5-4.5ppm) ALSO: Because hydrogen is more electropositive than carbon, increasing substitution (branching) also gives a downfield shift. i.e 1H d for RCH3 < RR’CH2 < RR’R”CH Both effects decrease rapidly with distance.

  12. π -bonds have high electron density Asymmetric magnetic fields Anisotropy Downfield shift for adjacent protons Chemical Shift (3) Unsaturated systems: i.e. Alkenes, alkynes Typical Benzene C-H is at 7-8.5ppm Aldehyde (RCHO) protons are observed the furthest downfield (>9.5ppm) as they have an electronegative atom and a double bond. Amines and Alcohols: RNH2, ROH Protons attached directly to nitrogen or oxygen give broad, variable position signals, because they become involved in hydrogen-bonding which affects their electron density.

  13. Chemical Shift (4) Chemical Equivalence: Nuclei in identical environments have the same chemical shifts and therefore give only one signal. • Atoms are said to be chemically equivalent if mentally substituting one of them would give identical results as substituting another. • Equivalence can be identified using symmetry: • Plane of symmetry: Atoms that are reflections of each other through a plane of symmetry are equivalent. • Rotational symmetry: Atoms that can be interchanged by rotation about a chemical bond are equivalent (e.g. methyl protons) provided that bond is able to rotate freely.

  14. 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (5) Typical Chemical Shifts (1H) CH3C=C CH2Cl CH=0 CONH, NH2 HC=C CCH2C CH3Si CH2Br COOH, Phenol OH Aromatic CH3N- CH3C CH02 CH3O, CH2O CH3C=O Low field High field δ/ppm

  15. Integration • The greater the number of equivalent nuclei giving rise to a particular signal, the larger that signal. • By integrating the area under a particular signal we can discern relative numbers of atoms with that particular chemical shift. e.g. An ethyl group CH3CH2R will give two proton signals for the two groups of protons with an integration ratio of 3:2.

  16. Multiplicity (1) • Nuclei that are close, but are not equivalent, affect each other’s local magnetic fields. • This leads to splitting of signals (spin-spin splitting) • The neighbours’ signal will also be split by the original proton, by the same amount. Rule of thumb: Signals from nuclei with n non-equivalent neighbours are split into n+1 signals.

  17. Multiplicity (2) Proton with one non-equivalent neighbour • The neighbour may be in one of two states: α or β. • This affects Beff for the proton, giving two values for ν • normally half the molecules in a sample will have the neighbour in state α and half in state β • This gives two equal height signals (a doublet).

  18. Multiplicity (3) Proton with two non-equivalent neighbours • The neighbours may be in one of four states: αα or αβ, βα or ββ. • As αβ and βαproduce the same effect on Beff there are three values for ν • This gives three signals (a triplet) in height ratio 1:2:1.

  19. J AB J AC Multiplicity (4) Exceptions to n+1 rule • More than one set of neighbours must be treated sequentially: leading to complex splitting patterns • Protons which are exchanging rapidly in solution e.g. amine or alcohol protons may not couple All processes which occur faster than about once every half second are “seen” by NMR as averaged. e.g. conformation flipping, rotation of methyl groups

  20. Coupling Constants • The distance between split is known as a coupling constant (J ) which is measured in Hz • These are independent of the external field – hence a high field magnet gives better resolution Saturated compounds (alkanes) • For coupling to occur the protons must be within three bonds or less distance from each other. • J is about 7Hz Unsaturated compounds (alkenes, alkynes): • They must be within four bonds or less distance • CH2 same carbon J~2Hz • CH=CH2cisJ~7-11Hz; CH=CH2transJ~12-18Hz • CH2-CH=CH2 allylic J~2Hz Aromatic Jortho ~ 6-9Hz, Jmeta~1-3Hz, Jpara~0-1Hz

  21. Other Nuclei Carbon NMR • Principles as for proton NMR but ΔE is 4x less • 12C (~99.5% natural abundance) does not have a nuclear spin, and so cannot be studied by NMR. • 13C has spin (same as 1H) but is only of 1.11% abundance • It requires much longer time to acquire a 13C spectrum • because two 13C atoms are almost never adjacent 13C-13C spin coupling is not observed. • 13C-1H coupling can be observed, but usually a technique called decoupling is used to simplify the spectrum by removing splitting. • δ for 13C NMR spectra are affected by the same constraints as for 1H NMR, although the range is of the order 1-200ppm (as opposed to 1-10ppm).

  22. Advanced Techniques A few of the more commonly used methods are outlined below: • Solvent suppression: pre-irradiates overly large signals to remove them from the spectrum • Spin decoupling: Simplifies the spectrum by removing splitting of signals – using pre-irradiation of the whole proton region • Correlation spectroscopy (COSY): uses a complex pulse pattern to produce a 3-D spectrum mapping coupling interactions. • Nuclear Overhauser Effect Spectroscopy (NOESY): Used to map through space (rather than along bonding) interactions. Can provide information about spatial arrangement of a molecule.

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