NMR Spectroscopy Dr. Joshua Osbourn Dept. of Chemistry, West - - PDF document

NMR Spectroscopy

• 1. Theory of NMR Spectroscopy
• 2. Spectrum Basics
• 3. Chemically Equivalent and Distinct Hydrogen
• 4. Chemical Shift
• 5. Integration (Peak Height)
• 6. Coupling (Splitting)
• 7. Complex Splitting
• 8. Special Features to Lookout For
• 9. Examples – 1H NMR
• 10. Carbon-13 NMR
• Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University

NMR Theory

NMR = Nuclear Magnetic Resonance Spectroscopy Spectroscopy is the interaction of matter with energy. NMR uses low energy radiation from the radio frequency (RF) region of the electromagnetic spectrum.

NMR Theory

Only certain nuclei are NMR active. Only nuclei containing odd mass numbers or odd atomic numbers will give NMR signals. These nuclei possess a property known as nuclear spin. The proton (1H) for example: 1 proton, 1 electron, 0 neutrons Nuclear Spin = +½ and –½ In the absence of any external stimuli, there is an equal probability of a proton being in either the a or the b state.

H H H H H H

Bext

H H H H H H

Bext α-state β-state RF Radiation

H H H H H H

Bext Relaxation

H H H H H H

Bext

+ NMR Signal

NMR Theory

NMR Theory

Every distinct proton in a molecule exists in a distinct chemical environment and thus requires a different RF frequency to cause excitation from the a to the b state. This results in each distinct proton providing a distinct signal in the NMR spectrum.

NMR Instrument

Spectrum Basics

2 4 6 8 10 12 PPM HO O H

Chemical Shift (d) Increasing ν

Chemically Equivalent/Distinct Hydrogen

Protons that exist in different chemical environments give rise to different NMR signals. Equivalent protons correspond to the same NMR signal. C H H H H

HO

Chemically Equivalent/Distinct Hydrogen

O H2 C H3C CH3 C H2 H2 C H3C CH3

Chemically Equivalent/Distinct Hydrogen

C H2 C H3C CH3 O C H2 C NH2 O CH H3C CH3

Chemically Equivalent/Distinct Hydrogen

H H H H3C

Chemically Equivalent/Distinct Hydrogen

Cl Cl Cl Cl

Chemically Equivalent/Distinct Hydrogen

Enantiotopic and Diastereotopic Protons The Chemical Equivalence Test (X Test): When the equivalency of two protons is in question, draw the compound twice. In one, replace one H with X. In the second, replace the other H with X.

• If the two are identical – the protons are identical.
• If the two are enantiomers – the protons are

enantiotopic, but are still equivalent and thus result in the same NMR signal.

• If the two are diastereomers – the protons are

diastereotopic and are not equivalent. These protons result in two different NMR signals.

Chemically Equivalent/Distinct Hydrogen

Enantiotopic and Diastereotopic Protons Identical Protons:

H H H

Chemically Equivalent/Distinct Hydrogen

Enantiotopic and Diastereotopic Protons Enantiotopic Protons:

H H

Chemically Equivalent/Distinct Hydrogen

Enantiotopic and Diastereotopic Protons Diastereotopic Protons:

H H Cl

Chemically Equivalent/Distinct Hydrogen

Enantiotopic and Diastereotopic Protons One Last Example:

OH

Chemical Shift

2 4 6 8 10 12 PPM HO O H

Downfield Deshielded Upfield Shielded Increasing Chemical Shift

Chemical Shift

A decrease in electron density around a nucleus deshields that nucleus and moves the signal downfield I CH3 Br CH3 Cl CH3 F CH3 H CH3 Increasing d

Chemical Shift

A decrease in electron density around a nucleus deshields that nucleus and moves the signal downfield

H CH3 Cl CH Cl Cl Cl CH2 Cl Cl CH3

Increasing d

Chemical Shift

Deshielding effects are diminished with distance.

F H2 C C H2 CH3

Chemical Shift

Attached electron withdrawing groups deshield a proton.

H3C H2 C CH3 O CH3 O H3C

Chemical Shift

Increasing the # of alkyl groups deshields a proton.

H CH3 H3C CH CH3 CH3 H3C C H CH3 H2C H H CH3

Increasing d

Chemical Shift

The chemical shift of protons attached to sp2 hybridized carbon atoms are unusually high.

H H H H 7.2 ppm 5.4 ppm 2.5 ppm 0.9 ppm sp2 sp2 sp sp3

This is due to a phenomenon known as magnetic anisotropy where the loosely held p electrons move in a circular path in the presence of a magnetic field. This induced magnetic field reinforces the magnetic field applied by the instrument resulting in a larger deshielding effect.

Chemical Shift

The chemical shift of protons attached to sp2 hybridized carbon atoms are unusually high.

H H H H 7.2 ppm 5.4 ppm 2.5 ppm 0.9 ppm sp2 sp2 sp sp3

This is due to a phenomenon known as magnetic anisotropy where the loosely held p electrons move in a circular path in the presence of a magnetic field. This induced magnetic field reinforces the magnetic field applied by the instrument resulting in a larger deshielding effect.

Chemical Shift

The chemical shift of protons attached to sp2 hybridized carbon atoms are unusually high.

H H H

Chemical Shift

General Regions in a 1H NMR Spectrum

2 4 6 8 10 12 PPM O OH O H H H H X X = O, N, Halogen H saturated vinyl aromatic aldehyde carboxylic acid Some Basic Chemical Shift Regions: Z H

Z= O,C

Integration (Peak Height)

The area under the signal is proportional to the number of protons that signal corresponds to.

Integration (Peak Height)

1 2 3 4 PPM

4H 6H

1 2 3 PPM

2H 3H 3H

O

Integration (Peak Height)

The area under an NMR signal is proportional to the number of protons that signal corresponds to.

1 2 3 4 5 6 7 PPM

2H 1H

O C H2 Cl Cl H O CH3

3H

Integration (Peak Height)

1 2 3 4 5 6 7 PPM

2H 1H

O C H2 Cl Cl H O CH3

3H Case 1: Integration values accurately reflect the number of H.

Integration (Peak Height)

1 2 3 PPM CH3 H3C CH3 H3C O 1H 3H C6H12O

Case 2: Whole number integrations values that do not integrate for enough total H.

Integration (Peak Height)

Case 3: One or more integration values are less than 1.

1 2 3 4 5 PPM C4H8O 0.153 0.230 0.230

Integration (Peak Height)

Case 4: Integration values appear as very large numbers

1 2 3 PPM

2950 1475 4430 8840

O

C6H12O

Integration (Peak Height)

Case 5: The height of integral symbols printed on the spectrum reflect the area under the curve.

1 2 3 4 PPM

C3H7Cl

Integration (Peak Height)

1 2 3 4 PPM

C3H7Cl 40 mm 40 mm 60 mm Peak Ratio – 1 : 1 : 1.5 2 : 2 : 3 x 2 Case 5: The height of integral symbols printed on the spectrum reflect the area under the curve.

Integration (Peak Height)

1 2 3 4 PPM

Br

In some cases the integrals may all be connected together.

NMR Spectroscopy

• 1. Theory of NMR Spectroscopy
• 2. Spectrum Basics
• 3. Chemically Equivalent and Distinct Hydrogen
• 4. Chemical Shift
• 5. Integration (Peak Height)
• 6. Coupling (Splitting)
• 7. Complex Splitting
• 8. Special Features to Lookout For
• 9. Examples – 1H NMR
• 10. Carbon-13 NMR
• Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University

Coupling (Splitting)

The signal corresponding to a particular proton will split due to the protons on adjacent atoms.

Coupling (Splitting)

The most common coupling that is observed is between protons on adjacent carbon atoms. Simple coupling follows the n+1 rule. n = # of protons on the adjacent carbon.

Cl CH3 H H

H H Cl Cl H H H H

Coupling (Splitting)

3 4 5 6 7 PPM

Br Cl H H Cl H

Coupling (Splitting)

Many names describing the same phenomenon….

• Splitting or Spin-Spin-Splitting – refers to a proton

signal being split due to neighboring protons.

• Coupling – refers to one type of proton being

“coupled” to neighboring proton(s). This results in splitting of the signal.

• Multiplicity – this refers to the type of splitting
• bserved. If the signal is split into two peaks, the

multiplicity is said to be a doublet.

Coupling (Splitting)

What gives rise to signal splitting?

H H H H H

Coupling (Splitting)

Predicting connectivity using signal splitting

R Ethyl

R Isopropyl

R tert-Butyl 2H 3H 1H 6H 9H

Coupling (Splitting)

Common Features:

• 1. The most common coupling occurs from non-

equivalent proton that are separated by 3-bonds.

Br Cl H H Cl H

H O Cl H H

H Br Br H H H

Coupling (Splitting)

Common Features:

• 2. Coupling will be observed between protons

separated by two bonds if the two protons are chemically distinct.

H H Cl

Coupling (Splitting)

Common Features:

• 3. Coupling between protons separated by 4 or more

bonds is not generally observed.

O H H H Cl H H

Coupling (Splitting)

Common Features:

• 4. Coupling is usually not observed through oxygen

and nitrogen.

O H Cl H H

N H H CH3 H H

Alcohol and amine proton NMR signals usually show up as a singlet.

Coupling (Splitting)

Common Features:

• 5. Chemically equivalent protons do not couple!

Cl Cl H H H H

H3C Cl

Coupling (Splitting)

Example:

HO O

1 2 3 4 5 PPM

The Coupling Constant

For every split signal in an NMR spectrum, a coupling constant can be calculated. Essentially, a coupling constant is a measure of the interaction between coupled protons. Coupling constants are also known as J-values and reported in Hz.

The Coupling Constant

1 2 3 4 PPM

J = 8.1 Hz J = 8.1 Hz

Cl CH3 H H

Calculating the Coupling Constant

Subtract the chemical shift values of two adjacent peaks in a split signal and multiple that value by the

• perating frequency of the NMR spectrometer.

J = 8.1 Hz

Cl CH3 H H

3.482 ppm 3.455 ppm 3.428 ppm 3.401 ppm

Spectrum recorded on 300 MHz spectrometer

Typical Coupling Constants

Structure J (Hz) Structure J (Hz) Structure J (Hz) 6 – 8 12 – 15** 12 – 18 5 – 7 6 – 9 7 – 12 2 – 12* 1 – 3 0.5 – 3 0.5 – 3 0 – 1 3 – 11*

H3C R H H H3C R H3C H R1 R2 H H

H O H H

H H

H H

H H H H

H H H H H H

H H H

*Depends on the H-C-H dihedral angle. **Must be diastereotopic

NMR Spectroscopy

• 1. Theory of NMR Spectroscopy
• 2. Spectrum Basics
• 3. Chemically Equivalent and Distinct Hydrogen
• 4. Chemical Shift
• 5. Integration (Peak Height)
• 6. Coupling (Splitting)
• 7. Complex Splitting
• 8. Special Features to Lookout For
• 9. Examples – 1H NMR
• 10. Carbon-13 NMR
• Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University

Complex Splitting

Sometimes coupling is more complex than n+1. This occurs when a proton is coupled to two or more non-equivalent protons.

O H H O H

Complex Splitting

Splitting Tree

O H H O H

Complex Splitting

Sometimes coupling is more complex than n+1. This occurs when a proton is coupled to two or more non-equivalent protons.

3 4 5 6 7 PPM

Cl H H O H H

Complex Splitting

Sometimes coupling is more complex than n+1.

Cl H H O H H

Complex Splitting

Sometimes coupling is more complex than n+1.

Cl H H O H H

Overlapping Signals

Protons that are chemically distinct, yet have similar chemical environments can potentially overlap creating a multiplet.

1 2 3 PPM I

2H 3H 2H 6H

NMR Spectroscopy

• 1. Theory of NMR Spectroscopy
• 2. Spectrum Basics
• 3. Chemically Equivalent and Distinct Hydrogen
• 4. Chemical Shift
• 5. Integration (Peak Height)
• 6. Coupling (Splitting)
• 7. Complex Splitting
• 8. Special Features to Lookout For
• 9. Examples – 1H NMR
• 10. Carbon-13 NMR
• Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University

NMR Solvents

Deuterated solvents are typically used in NMR spectroscopy.

• Deuterated solvents are ones in which all of the

hydrogen in the molecule have been replaced by deuterium.

• Deuterium absorptions are not detected in the

range that is observed for 1H NMR.

CDCl3 D3C O CD3 D2O D3C OD D3C S O CD3 chloroform-d acetone-d6 deuterium oxide methanol-d4 DMSO-d6

TMS – Internal Standard

Oftentimes the NMR solvent is spiked with 0.1% trimethylsilane (TMS) which serves as an internal standard.

H3C Si CH3 CH3 CH3

D2O Shake

Shaking an alcohol with D2O results in rapid exchange

• f the OH proton with deuterium, which eliminates the

–OH resonance.

1 2 3 4 PPM 1 2 3 4 PPM

HO D2O Shake DO

Coupling with –OH Proton

When a sample containing an –OH group is very pure and completely dry, coupling through the alcohol will be observed. Typically alcohols are contaminated with a small amount of water, acid, or base which collapses the OH signal into a singlet.

8 7 6 5 4 3 2 1 ppm OH Ph MeO Ph 2.66 H

NMR Spectroscopy

• 1. Theory of NMR Spectroscopy
• 2. Spectrum Basics
• 3. Chemically Equivalent and Distinct Hydrogen
• 4. Chemical Shift
• 5. Integration (Peak Height)
• 6. Coupling (Splitting)
• 7. Complex Splitting
• 8. Special Features to Lookout For
• 9. Examples – 1H NMR
• 10. Carbon-13 NMR
• Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University

Example 1 – C7H8O

1 2 3 4 5 6 7 8 PPM 5 2 1

Example 2 – C8H9BrO

1 2 3 4 5 6 7 8 PPM 2 2 2 3

Example 3 – C5H10O2

2 4 6 8 10 PPM t, 1H d, 2H sep, 1H d, 6H

Example 4 – C3H9N

1 2 3 PPM 2H 2H 2H 3H

Example 5 – C10H12O2

1 2 3 4 5 6 7 8 9 PPM 2H 1H 2H 2H 2H 3H

IR Data: 1720, 1610, 1505, 1210, 1010 cm-1

Example 6 – C3H7BrO

1 2 3 4 5 6 PPM 2H 2H 3H

Example 7 – C9H20

1 2 3 PPM 2H 3H

Example 8 – C7H14O2

2 4 6 8 10 12 PPM s, 1H t, 2H t, 2H s, 9H

Example 9 – C6H12O

1 2 3 4 PPM s, 6H t, 2H quin, 2H t, 2H

Example 10 – C6H10O2

2 4 6 8 10 12 PPM s, 1H t, 1H d, 2H s, 3H s, 3H

IR Data: 3100 (broad, strong); 1730, 1650 cm-1

A Terminal Alkene

1 2 3 4 5 6 PPM

H

4.98 4.98 4.38 4.38

OH

3.05 3.05 1.35 1.35 1.77 1.77

H

4.82 4.82

An Internal Alkene

1.63 1.63 3.97 3.97

Br H

5.77 5.77

H

5.68 5.68

1 2 3 4 5 6 PPM