Basics of NMR Spectroscopy B o N S Magnet B 1 Frequency - - PowerPoint PPT Presentation

Basics of NMR Spectroscopy

N S

Bo B1 Detector

Frequency Generator Recorder

Magnet

Electromagnetic Spectrum

Electromagnetic Spectrum

Electronic

Electromagnetic Spectrum

Electronic Vibration

Electromagnetic Spectrum

Electronic Vibration Rotation

Electromagnetic Spectrum

Electronic Vibration Spin resonance Rotation

Certain nuclei absorb radiofrequencies (electromagnetic radiation) when they are placed in a magnetic field. Essential criterion: spin number (I)  0.

• Even atomic mass & number

 I = 0 (12C, 16O)

• Even atomic mass & odd number

 I = whole integer (14N, 2H, 10B)

• Odd atomic mass

 I = half integer (1H, 13C, 15N, 31P) Angular momentum = [I(I+1)]1/2 h/2 Z-component of angular momentum = m h/2 m = I, (I - 1), (I - 2), … , -I For 1H: m = 1/2, -1/2

Nuclear Magnetic Resonance (NMR)

Certain nuclei absorb radiofrequencies (electromagnetic radiation) when they are placed in a magnetic field. Essential criterion: spin number (I)  0.

• Even atomic mass & number

 I = 0 (12C, 16O)

• Even atomic mass & odd number

 I = whole integer (14N, 2H, 10B)

• Odd atomic mass

 I = half integer (1H, 13C, 15N, 31P) Angular momentum = [I(I+1)]1/2 h/2 Z-component of angular momentum = m h/2 m = I, (I - 1), (I - 2), … , -I For 1H: m = 1/2, -1/2

Nuclear Magnetic Resonance (NMR)

Certain nuclei absorb radiofrequencies (electromagnetic radiation) when they are placed in a magnetic field. Essential criterion: spin number (I)  0.

• Even atomic mass & number

 I = 0 (12C, 16O)

• Even atomic mass & odd number

 I = whole integer (14N, 2H, 10B)

• Odd atomic mass

 I = half integer (1H, 13C, 15N, 31P) Angular momentum = [I(I+1)]1/2 h/2 Z-component of angular momentum = m h/2 m = I, (I - 1), (I - 2), … , -I For 1H: m = 1/2, -1/2

Nuclear Magnetic Resonance (NMR)

Certain nuclei absorb radiofrequencies (electromagnetic radiation) when they are placed in a magnetic field. Essential criterion: spin number (I)  0.

• Even atomic mass & number

 I = 0 (12C, 16O)

• Even atomic mass & odd number

 I = whole integer (14N, 2H, 10B)

• Odd atomic mass

 I = half integer (1H, 13C, 15N, 31P) Angular momentum = [I(I+1)]1/2 h/2 Z-component of angular momentum = m h/2 m = I, (I - 1), (I - 2), … , -I For 1H: m = 1/2, -1/2

Nuclear Magnetic Resonance (NMR)

Certain nuclei absorb radiofrequencies (electromagnetic radiation) when they are placed in a magnetic field. Essential criterion: spin number (I)  0.

• Even atomic mass & number

 I = 0 (12C, 16O)

• Even atomic mass & odd number

 I = whole integer (14N, 2H, 10B)

• Odd atomic mass

 I = half integer (1H, 13C, 15N, 31P) Angular momentum = [I(I+1)]1/2 h/2 Z-component of angular momentum = m h/2 m = I, (I - 1), (I - 2), … , -I For 1H: m = 1/2, -1/2

Nuclear Magnetic Resonance (NMR)

The effect of magnetic fields on nuclei

For a steady magnetic field B0, E = -m B0 (m = Magnetic moment = g I ) g = Magnetogyric ratio; għ = gl mN ) mN = eħ/2mp = Nuclear magneton = 5.051 x 10-27 JT-1 gl = Nuclear g factor (Range = -6 to +6), Ĥ = - g B0 î Considering the field to be along the z-direction, mz = gIz = g m ħ; E = - mz B0 = - g m ħ B0 Different spin states have different energies in the presence of a magnetic field

The effect of magnetic fields on nuclei

For a steady magnetic field B0, E = -m B0 (m = Magnetic moment = g I ) g = Magnetogyric ratio; għ = gl mN ) mN = eħ/2mp = Nuclear magneton = 5.051 x 10-27 JT-1 gl = Nuclear g factor (Range = -6 to +6), Ĥ = - g B0 î Considering the field to be along the z-direction, mz = gIz = g m ħ; E = - mz B0 = - g m ħ B0 Different spin states have different energies in the presence of a magnetic field

The effect of magnetic fields on nuclei

For a steady magnetic field B0, E = -m B0 (m = Magnetic moment = g I ) g = Magnetogyric ratio; għ = gl mN ) mN = eħ/2mp = Nuclear magneton = 5.051 x 10-27 JT-1 gl = Nuclear g factor (Range = -6 to +6), Ĥ = - g B0 î Considering the field to be along the z-direction, mz = gIz = g m ħ; E = - mz B0 = - g m ħ B0 Different spin states have different energies in the presence of a magnetic field

Nuclear Magnetic Resonance

Spin ½ nucleus (1H, 13C)

Nuclear Magnetic Resonance

Spin ½ nucleus (1H, 13C)

E = E-E= ½ għB0 – (- ½ għB0) = għB0

Nuclear Magnetic Resonance

Spin ½ nucleus (1H, 13C)

E = E-E= ½ għB0 – (- ½ għB0) = għB0

= hL

i.e. L = g B0 / 2

Nuclear Magnetic Resonance

Spin ½ nucleus (1H, 13C)

E = E-E= ½ għB0 – (- ½ għB0) = għB0

Resonance: The energy of the EM radiation

matches the energy gap B0 = 12T, L = 500 MHz for protons

= hL

i.e. L = g B0 / 2

Nuclear Magnetic Resonance

Spin ½ nucleus (1H, 13C)

E = E-E= ½ għB0 – (- ½ għB0) = għB0

Resonance: The energy of the EM radiation

matches the energy gap B0 = 12T, L = 500 MHz for protons

= hL

i.e. L = g B0 / 2

L : precessional frequency

Chemical shift

The chemical environment alters the effective magnetic field on the nuclei Beff = Bo( 1 - s ) s = magnetic shielding of the nucleus. Factors that affect it include neighboring atoms, aromatic groups, etc., the polarization of the bonds to the observed nuclei

Chemical shift

The chemical environment alters the effective magnetic field on the nuclei Beff = Bo( 1 - s ) s = magnetic shielding of the nucleus. Factors that affect it include neighboring atoms, aromatic groups, etc., the polarization of the bonds to the observed nuclei

2π γB σ) (1 2π γB ν

eff L

• 



Chemical shift

The chemical environment alters the effective magnetic field on the nuclei Beff = Bo( 1 - s ) s = magnetic shielding of the nucleus. Factors that affect it include neighboring atoms, aromatic groups, etc., the polarization of the bonds to the observed nuclei

2π γB σ) (1 2π γB ν

eff L

• 



1H/ 13C nuclei in different environments

resonate at different frequencies

Chemical shift

The chemical environment alters the effective magnetic field on the nuclei Beff = Bo( 1 - s ) s = magnetic shielding of the nucleus. Factors that affect it include neighboring atoms, aromatic groups, etc., the polarization of the bonds to the observed nuclei L

low field high field Intensity  Population

HO-CH2-CH3

2π γB σ) (1 2π γB ν

eff L

• 



1H/ 13C nuclei in different environments

resonate at different frequencies

The d scale

• The frequency of resonance is field-dependent

The d scale

• The frequency of resonance is field-dependent
• A relative scale, is a less ambiguous representation of the signal of a

particular nucleus.

The d scale

• The frequency of resonance is field-dependent
• A relative scale, is a less ambiguous representation of the signal of a

particular nucleus.

The d scale

• The frequency of resonance is field-dependent
• A relative scale, is a less ambiguous representation of the signal of a

particular nucleus.

d is field-independent d  (-s)

The d scale

• The frequency of resonance is field-dependent
• A relative scale, is a less ambiguous representation of the signal of a

particular nucleus.

d is field-independent d  (-s)

Reference: Tetramethyl silane (TMS) soluble in most organic solvents, inert, volatile, and has 12 equivalent 1Hs and 4 equivalent 13Cs Other references: residual solvent peak, dioxane for 13C, H3PO4 for 31P

H 3 C Si CH3 CH3 CH3

Characteristic Chemical shifts: 1H Resonances

Characteristic Chemical shifts: 13C resonances

Ring currents

Ring currents

Shielded

Ring currents

Shielded Deshielded

Ring currents

Shielded Deshielded

Ring currents

Shielded Deshielded Shielded

A representative spectrum: Ethanol

A representative spectrum: Ethanol

Three groups of lines = Three kinds of protons

A representative spectrum: Ethanol

Three groups of lines = Three kinds of protons Areas: Relative intensities = Abundance

A representative spectrum: Ethanol

Three groups of lines = Three kinds of protons Areas: Relative intensities = Abundance

What is the significance

• f the multiplicity of the

lines?

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra E = J . I1 . I2

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra E = J . I1 . I2 Coupling Constant J

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra E = J . I1 . I2 Interacting spins Coupling Constant J

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra E = J . I1 . I2 Interacting spins Coupling Constant J

The fine structure: Spin-spin Coupling

Br CH3

Small alteration in the magnetic field experienced by a nucleus due to other magnetic nuclei ► Fine structure in the spectra E = J . I1 . I2 Interacting spins Coupling Constant J