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NMR Spectroscopy. Part II. Signals of NMR. Free Induction Decay (FID). FID represents the time-domain response of the spin system following application of an radio-frequency pulse.

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NMR Spectroscopy

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Nmr spectroscopy l.jpg

NMR Spectroscopy

Part II. Signals of NMR


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Free Induction Decay (FID)

  • FID represents the time-domain response of the spin system following application of an radio-frequency pulse.

  • With one magnetization at w0, receiver coil would see exponentially decaying signal. This decay is due to relaxation.


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Fourier Transform

The Fourier transform relates the time-domainf(t) data with the frequency-domainf(w) data.


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Fourier Transform


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Fourier Transform


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NMR line shape

Lorentzian line

Aamplitude

Whalf-line width


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Resolution

  • Definition

    For signals in frequency domain it is the deviation of the peak line-shape from standard Lorentzian peak. For time domain signal, it is the deviation of FID from exponential decay. Resolution of NMR peaks is represented by the half-height width in Hz.


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Resolution


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Resolution-digital resolution


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Resolution

  • Measurement

    half-height width:

    10~15% solution of 0-dichlorobenzene (ODCB) in acetone

    Line-shape:

    Chloroform in acetone


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Resolution

  • Factors affect resolution

    Relaxation process of the observed nucleus

    Stability of B0(shimming and deuterium locking)

    Probe (sample coil should be very close to the sample)

    Sample properties and its conditions


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Sensitivity

  • Definition

    signal to noise-ratio

    A : height of the chosen peak

    Npp :peak to peak noise


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Sensitivity

  • Measurement

    1H0.1% ethyl benzene in deuterochloroform

    13CASTM, mixture of 60% by volume deuterobenzene and dioxan or 10% ethyl benzene in chloroform

    31P1% trimehylphosphite in deuterobenzene

    15N90% dimethylformamide in deutero-dimethyl-sulphoxide

    19F0.1% trifluoroethanol in deuteroacetone

    2H, 17Otap water


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Sensitivity

  • Factors affect sensitivity

    Probe: tuning, matching, size

    Dynamic range and ADC resolution

    Solubility of the sample in the chosen solvent


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Spectral Parameters

  • Chemical Shift

    Caused by the magnetic shielding of the nuclei by their surroundings. d-values give the position of the signal relative to a reference compound signal.

  • Spin-spin Coupling

    The interaction between neighboring nuclear dipoles leads to a fine structure. The strength of this interaction is defined as spin-spin coupling constant J.

  • Intensity of the signal


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Chemical Shift

  • Origin of chemical shift

  • sshielding constant

  • Chemically non-equivalent nuclei are shielded to different extents and give separate resonance signals in the spectrum


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Chemical Shift


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Chemical Shift

  • d – scale or abscissa scale


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Chemical Shift

Shielding s

CH3Br < CH2Br2 < CH3Br < TMS

90 MHz spectrum


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Abscissa Scale


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Chemical Shift

  • d is dimensionless expressed as the relative shift in parts per million ( ppm ).

  • d is independent of the magnetic field

  • d of proton0 ~ 13 ppm

    d of carbon-130 ~ 220 ppm

    d of F-190 ~ 800 ppm

    d of P-310 ~ 300 ppm


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Chemical Shift

  • Charge density

  • Neighboring group

    Anisotropy

    Ring current

    Electric field effect

    Intermolecular interaction (H-bonding & solvent)


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Chemical Shift –anisotropy of neighboring group

c susceptibility

r distance to the dipole’s center

Differential shielding of HA and HB in the dipolar field of a magnetically anisotropic neighboring group


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Chemical Shift –anisotropy of neighboring group

d~2.88

d~9-10


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  • Electronegative groups are "deshielding" and tend to move NMR signals from neighboring protons further "downfield" (to higher ppm values).

  • Protons on oxygen or nitrogen have highly variable chemical shifts which are sensitive to concentration, solvent, temperature, etc.

  • The -system of alkenes, aromatic compounds and carbonyls strongly deshield attached protons and move them "downfield" to higher ppm values.


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  • Electronegative groups are "deshielding" and tend to move NMR signals from attached carbons further "downfield" (to higher ppm values).

  • The -system of alkenes, aromatic compounds and carbonyls strongly deshield C nuclei and move them "downfield" to higher ppm values.

  • Carbonyl carbons are strongly deshielded and occur at very high ppm values. Within this group, carboxylic acids and esters tend to have the smaller values, while ketones and aldehydes have values 200.


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Ring Current

  • The ring current is induced form the delocalized p electron in a magnetic field and generates an additional magnetic field. In the center of the arene ring this induced field in in the opposite direction t the external magnetic field.


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Ring Current -- example


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Spin-spin coupling


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Spin-spin coupling


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AX system


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AX2 system


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Spin-spin coupling


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AX3 system


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Multiplicity Rule

Multiplicity M (number of lines in a multiplet)

M = 2n I +1

n equivalent neighbor nuclei

I spin number

For I= ½

M = n + 1


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ExampleAX4 system

I=1; n=3

AX4


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Order of Spectrum

Zero order spectrum

only singlet

First order spectrum

Dn >> J

Higher order spectrum

Dn ~ J


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AMX system


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Spin-spin coupling

  • Hybridization of the atoms

  • Bond angles and torsional angles

  • Bond lengths

  • Neighboring p-bond

  • Effects of neighboring electron lone-pairs

  • Substituent effect


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JH-H and Chemical Structure

  • Geminal couplings 2J (usually <0)

    H-C-H bond angle

    hybridization of the carbon atom

    substituents


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Geminal couplings 2Jbond angle


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Geminal couplings 2J

Effect of Neighboring p-electrons

Substituent Effects


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Vicinal couplings 3JH-H

  • Torsional or dihedral angles

  • Substituents

  • HC-CH distance

  • H-C-C bond angle


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Vicinal couplings 3JH-Hdihedral angles

  • Karplus curves


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Chemical Shift of amino acid

http://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htm


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Chemical Shift Prediction

Automated Protein Chemical Shift Prediction

http://www.bmrb.wisc.edu:8999/shifty.html

BMRB NMR-STAR Atom Table Generator for Amino Acid Chemical Shift Assignments

http://www.bmrb.wisc.edu/elec_dep/gen_aa.html


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http://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htm


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Example 1


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