NMR

What is the molarity of water?

55.5 mol

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Give examples of where NMR is used (4)

Structure determination, metabolomics/mixture analysis (e.g. identify metabolites in a cell), MRI (e.g. cardiac infarct) and small molecules

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What type of nucleus has a nuclear spin?

A nucleus with an odd atomic number or an odd mass number

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The spinning charged nucleus generates what?

A magnetic field (formation of a magnetic moment) - spinning charged nuclei precess around magnetic field

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What causes moments to align?

External magnetic field (B0) - more magnetic used leads to more alignment

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Are there more parallel or anti-parallel moments?

More parallel - produces net magnetisation vector

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What are the two types of states (align or oppose external field)?

Alpha and beta (E=hv). A photon with the right amount of energy can be absorbed and cause spinning proton to flip

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The energy difference between the two spin states depends on what?

The strength of the magnetic field (see graph)

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Which elements have a nuclear spin (6)?

1H, 13C, 15N, 17O, 19F, 31

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What are is the relative sensitivity for the following: 1H, 2H, 13C, 14N, 15N, 17O, 19F, 31P?

1.00, 9.65x10-3, 1.59x10-2, 1.01x10-3, 1.04x10-3, 2.91x10-2, 0.83, 6.63x10-2

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What are is the natural abundance for the following: 1H, 2H, 13C, 14N, 15N, 17O, 19F, 31P?

99.985, 0.015, 1.10, 99.63, 0.37, 0.037, 100, 100

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Describe features of NMR hardware

Apparatus involves sample being placed in superconducting coil surrounded by liquid He and liquid N (prevent evaporation of He). Magnetic field is applied and results are displayed on the computer

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Outline the principles of NMR

B0 leads to rotation (cone shape) and the 3D signal is generated. Converted to 2D. Sharp peaks are due to slow relaxation of sample and broad peaks due to fast relaxation of sample (time-frequency domain)

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What happens if all protons absorbed the same amount of energy in a given magnetic field?

Not much information could be obtained

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What shields protons from the external field?

Electrons - circulating electrons create an induced magnetic field that opposes the magnetic field. Depending on the chemical environment, protons in a molecule are shielded by different amounts (e.g. due to electronegativity)

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Which are the four aspects to consider when looking at proton shielding?

Number (different kinds of protons are present). Location (shows how shielded or deshielded the proton is). Intensity (shows the number of protons of that type). Splitting (the number of protons on adjacent atoms)

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What is used as a reference to the sample in the NMR spectrum?

Tetramethyl silane (TMS) or trimethylsilyl propanoic acid - silicon is less electronegative than carbon, TMS protons are highly shielded (most upfield), signal defined as zero, organic protons downfield (left) of TMS signal

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How do you calculate the chemical shift?

Shift in Hz/Spectrometer frequency in MHz (a particular proton in a given molecule will always come at the same chemical shift, constant value)

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Which are the units for chemical shift?

ppm

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What is the spectrometer frequency independent of?

The instrument used

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What are the two types of spin-spin coupling?

Scalar coupling (weak 1-20 Hz, through bond via electron spins, not scaled with field). Dipolar coupling (large 1-20 kHz, through space, related to orientation of internuclear vector wrt field, not observed in solution due to rapid motions)

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Describe features of J-coupling

Non-equivalent protons on adjacent Cs have magnetic fields that align with or oppose external field. This magnetic coupling causes the proton to absorb slightly downfield when B0 is reinforced and slightly upfield when B0 is opposed. Signal split

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Do equivalent protons split each other?

No - protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent Cs normally will couple. Protons separated by four or more bonds will not couple

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Describe features of coupling constants

Distance between peaks of multiplet, measured in Hz, not dependent on strength of the external field, multiplets with the same coupling constants may come from adjacent groups of protons that split each other

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Describe features of spin-spin decoupling by exchange

OH and NH groups not seen. Due to rapid exchange between solvent molecules in solution. Coupling seen in ultra pure alcohols. Exchange happens so quickly, C-H group sees many Hs on OH during time spectrum is determined (spin=0)

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Describe features of complex splitting (e.g. styrene)

Signals may be split by adjacent protons, different from each other, with different coupling constants. Ha split by adjacent H trans to it (J = 17 Hz) and an adjacent H cis to ti (J = 11 Hz)

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What type of splitting would you expect to see for the Hs in styrene?

Ha (double doublet, J = 17 Hz, J = 11 Hz). Hb (double doublet, J = 17Hz, J = 1.4 Hz). Hc (double doublet, J = 11 Hz, J = 1.4 Hz)

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Describe the protons on a 1,4 disubstituted benzene ring

Protons 2 and 6 or protons 3 and 5 are chemical equivalent but not magnetic equivalent

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What is the J coupling constant for ortho coupling?

7-8 Hz

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What is the J coupling constant for meta coupling?

1-2 Hz

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What is the J coupling constant for para coupling?

0-1 Hz

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Describe features of C-13

C-12 has no magnetic spin. C-13 has magnetic spin but only 1.1% of C in a sample. Gyromagnetic ratio of C-13 is 1/4 of that of H-1. Signals are weak (lost in noise). Hundreds of spectra are taken, averaged

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What is DEPT?

Distortionless Enhancement by Polarisation Transfer. Used to fully separate C signals. C (0). CH (+). CH2 (+, 0, -). CH3 (+, 0, +)

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What is 2D NMR?

Preparation, evolution, mixing, detection. Give data plotted in a space defined by two frequency axes rather than one

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What is COSY?

Correlation Spectroscopy. Maps out correlations between protons that are coupled (up to 3 bonds). Provides clear representation of which groups are connected and easier to interpret than splittings observed in 1D spectra (large spin systems)

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What is NOESY?

Nuclear Overhauser Effect Spectroscopy. Correlates nuclei through space (5 A). Useful to determine structure. Cross peak intensities relate to internuclear distance (structural models)

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What is TOCSY?

Total Correlation Spectroscopy. Correlation between all protons in spin system (not just directly coupled). Shows proton spin networks which are interrupted

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What is HSQC?

Heteronuclear Single Quantum Correlation. Allows correlations between protons and C-13 for small molecule research or N-15 for proteins

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