NMR Spectroscopy Notes

NMR Spectrometer

• The NMR spectrometer uses several components:
  • Radio frequency transmitter
  • Magnet with poles
  • Sweep coils
  • Sweep generator
  • Spinning sample tube
  • Radio frequency receiver and amplifier
  • Shimming components
  • Control console and recorder

Sensitivity of NMR Experiments

• Sensitivity in NMR is quantified by the signal-to-noise ratio (S/N).
• Several factors influence S/N:
  • N: Number of spins (concentration)
  • $\gamma$: Gyromagnetic ratio of detected nuclei
  • n: Number of scans
  • B: External magnetic field
• Formula: $S/N \propto N \gamma^3 B^3 \sqrt{n}$
• An example comparing a 750 MHz spectrometer with a 300 MHz spectrometer, illustrating the improvement in sensitivity with a higher magnetic field:
  • $(\frac{750}{300})^\frac{3}{2} = c \frac{N<em>1}{N</em>2}$

NMR Sensitivity: Magnetic Field Strength

• Increasing magnet strength (B0) is a major way to increase sensitivity.
• Approximate costs:
  • ~$2,00,000
  • ~$4,500,000
• Formula: $S/N \propto N \gamma^3 B^3 \sqrt{n}$

NMR Sensitivity: Concentration

• Increasing the concentration of the sample is a common approach to increase sensitivity.
• Example comparing two different concentrations on signal to noise ratio.
• Formula: $S/N \propto N \gamma^3 B^3 \sqrt{n}$

NMR Sensitivity: Gyromagnetic Ratio and Natural Abundance

• The gyromagnetic ratio ($\gamma$) and the natural abundance of the isotope significantly affect NMR sensitivity.
• (γH/γC)3 for 13C is 64x
• (γH/γN)3 for 15N is 1000x
• 1H is approximately 64 times as sensitive as 13C and 1000 times as sensitive as 15N.
• Considering the natural abundance:
  • 13C has a natural abundance of 1.1%.
  • 15N has a natural abundance of 0.37%.
  • Relative sensitivity increases to ~6,400x and ~2.7x105x, respectively.
• 1H NMR spectra of caffeine (8 scans ~12 secs)
• 13C NMR spectra of caffeine (8 scans ~12 secs)
• 13C NMR spectra of caffeine (10,000 scans ~4.2 hours)
• Formula: $S/N \propto N \gamma^3 B^3 \sqrt{n}$

NMR Sensitivity: Number of Scans

• Increasing the number of scans (NS) or signal averaging is a common approach to increase sensitivity and signal-to-noise ratio (S/N).
• $S/N \approx \sqrt{n}$
• $S/N \propto N \gamma^3 B^3 \sqrt{n}$
• The S/N increases with the square root of the number of scans.
• Comparison of number of Scans and S/N:
  • n = 1, S/N = 1
  • n = 8, S/N = 2.83
  • n = 16, S/N = 4
  • n = 80, S/N = 8.94
  • n = 800, S/N = 28.3
• Experimental Time = Number of Scans x Acquisition Time, so it takes significantly longer to acquire the spectrum as the number of scans increase.

Solvent Selection in NMR

• The ideal solvent should:
  • Contain no protons
  • Be inert
  • Have a low boiling point
  • Be inexpensive
• Deuterated solvents are necessary for modern NMR instruments:
  • Deuterium signal is used to lock or stabilize the B0 field of the magnet.
  • Instruments with a deuterium channel constantly monitor and adjust (locks) the B0 field to the frequency of the deuterated solvent.
  • The deuterium signal is also used to shim the B0 field, ensuring the homogeneity of the field is precise at the center of the sample.
• Commonly used deuterated solvents: CDCl3, C6D6.

NMR Sample Preparation

• Shows a picture of NMR Sample Preparation and a 500MHZ/52 MM setup.

NMR Spectra Analysis

• This section details the analysis of an NMR spectrum.
• Spectrum for ethylacetate.
• Key parameters:
  • Experiment = zg.ppg
  • Pulse length = 22.00 usec
  • Recycle delay = 3.00 sec
  • Number of acquisitions (NA) = 8
  • Points (PTS1d) = 8192
  • F1 = 90.019463 MHZ
  • SW1 = 1470.60 Hz
  • AT1 = 5.57 sec

1. Hz per Pt 1stD = 0.18 Hz
2. 01 = 495.40
3. LB1 = 0.00 Hz
4. TP A = -75.50
5. B
6. -76.30

• Peaks at:
  • 4.159 ppm
  • 4.079 ppm
  • 2.032 ppm
• Spectrum includes signals for H3C and CH3 groups.

NMR: Spectrum Concepts

• Key concepts in NMR spectrum analysis:
  • Shielding and deshielding
  • Chemical Shift (ppm)
  • Number of Signals
  • Integral of Signals
  • Coupling Constant (Hz)
  • Understanding Magnetic Anisotropy

Chemical Shift

• Explains why hydrogen atoms have different frequencies.
• $\nu = \frac{\gamma}{2 \pi} B_0$
  • $B_0$ is 300, 400, or 600 MHz.

Chemical Shift: Shielding

• Nuclei are surrounded by electrons, which generate a magnetic field that alters the B0 field in the microenvironment around the nucleus; this is called SHIELDING of the nucleus.
• Shielding (σ) varies with chemical environments.
• Electrons move to create an opposing field.
• 1 T = 10 kG
• For Naked Nuclei: $\nu = \frac{\gamma}{2 \pi} B_0$
• Effective magnetic field: $B<em>{eff} = B</em>0 - \sigma$

Chemical Shift: Shielding (Continued)

• At a constant applied magnetic field (B0), different protons experience different effective magnetic fields (Beff).
• Electronic shielding σ is normally positive.
• The shielding constant (σ) is closely related to the degree of electron density surrounding the nucleus.
• Nuclei in regions of high electron density are shielded.
• Conversely, nuclei in low electron density are deshielded.
• Variation of the resonance frequency with shielding has been termed the Chemical Shift.
• Example calculation:
  • $\nu_{eff} = 299.999994 \ MHz$
  • $\nu = \frac{\gamma}{2 \pi} B_{eff} = 300 \ MHz$
  • $\Delta \nu = 6.0 \ Hz$
  • $\Delta \nu = \frac{\gamma}{2 \pi} \sigma B_0$

Chemical Shift: ppm (parts per million)

• The basic frequency of the pulse (B1) is not the same as the sample resonance.
• The observed FID (Free Induction Decay) is the difference of two frequencies: the B1 (radiofrequency, known) and the frequency emitted by excited nuclei.
• These differences in resonance frequency are very small (Hz) compared with B1 (MHz).
• The unit of chemical shift is ppm (parts per million).
• No attempt is made to measure the exact resonance frequency of any proton.
• Different machines have different real chemical shifts (ppm).
• A standard reference is used: tetramethylsilane (TMS, Me4Si).
• The chemical shift δ expresses the amount by which a proton resonance is shifted from TMS, in parts per million, of the spectrometer’s basic operating frequency.
• It is a field-independent measure.
• Variation of the resonance frequency with shielding has been termed the Chemical Shift.
• Formulas:
  • $FID: \nu<em>{real} = \nu</em>{sample} - \nu_{pulse}$
  • $\Delta \delta = \delta<em>{sample} - \delta</em>{TMS} = \frac{\nu<em>{sample} - \nu</em>{TMS}}{\nu_{pulse}}$
  • $\delta = \frac{\Delta \nu}{\nu_{pulse}} = ppm$
  • $\delta = \frac{\nu_{real}}{xMHz} = \frac{Hz}{MHz}$
  • Example: $\delta = \frac{162 Hz}{60 MHz} = 2.70 ppm$

Hz vs ppm

• The chemical shift in ppm is independent of the strength of the applied magnetic field.
• Formula: $\nu = \delta * x \ Hz$
• Example: for a 400 MHz machine with δ = 2.0 ppm, $\nu = 2.0 * 400 \ Hz = 800 \ Hz$
• Formulas:
  • $\delta = \frac{\nu<em>{Hz}}{x</em>{MHz}}$
  • $\nu = \delta<em>{ppm} * x</em>{MHz} = \delta * 10^{-6} * x * 10^6 Hz = \delta * x \ Hz$
  • $\delta = \frac{\nu<em>{sample} - \nu</em>{TMS}}{xMHz}$

Chemical Shift unit: ppm vs Hz

• $\nu = \delta * x \ Hz$
• The same chemical shift (e.g., 2.032 ppm) in ppm will have a different frequency under different magnetic fields.
• Stronger magnetic field, higher chemical shift in Hertz.
• Examples:
  • B0 = 90 MHz: $\nu = 2.032 * 90 \ Hz = 182.88 \ Hz$
  • B0 = 400 MHz: $\nu = 2.032 * 400 \ Hz = 812.8 \ Hz$

Effect of Changing Field Strength

• The strongest Magnetic field available should be used to spread out the chemical shifts.
• $\nu = \delta * x \ Hz$

Range of Chemical Shifts (δ)

• Different nuclei have different shift ranges, depending on γ.
• Nucleus Shift Range (ppm) Reference
  • 1H -30 to 20 (commonly 0-12) (CH3)4Si
  • 13C -100 to 200 (commonly 0-220) (CH3)4Si
  • 19F -200 to 200 CFCl3
  • 31P -100 to 250 H3PO4
• $\Delta \nu = \frac{\gamma}{2 \pi} \sigma B_0$

Downfield and Upfield

• Downfield: Deshielded nuclei. Higher frequency. Larger effective magnetic field.
• Upfield: Shielded nuclei. Lower frequency. Smaller effective magnetic field.

Protons in a molecule

• More shielded protons absorb at a higher field (upfield).
• Less shielded protons absorb at a lower field (downfield).
• Examples include H-C-O and H-O-C-H.

Proton NMR Chemical Shifts of Common Functional Groups & Factors

• Electronegativity
• Anisotropy (Ring Currents)
• Electron Delocalization
• H‐Bonding

Proton NMR Chemical Shifts of Common Functional Groups

• Ranges of chemical shifts for different types of protons:
  • OH : 12 - 1
  • Y-C-H: F, etc 4 - 2
  • C=O -C-H: 10 - 9
  • C-H: 5 - 0
• Regions:
  • Deshielded, High Frequency, High Chemical Shift, Downfield
  • Shielded, low Frequency, Low Chemical Shift, Upfield
  • H-Bonding
  • anisotropy
  • Electronegativity

Factors on Chemical Shift: Electron density

• Electronegativity
• Anisotropy (π-bond)
• Electron Delocalization
• H-Bonding

Deshielding by Electronegative Element

• Chlorine “deshields” the proton by taking valence electron density away from carbon, which in turn takes more density from hydrogen.
• “highly shielded” protons appear at upfield (lower δ).
• “deshielded“ protons appear at downfield (higher δ).
• deshielding moves proton resonance to lower field and higher δ.
• $C^{\delta+} - H^{\delta-} Cl^{\delta-}$

Deshielding by Electronegative Element examples

• The effect increases with greater numbers of electronegative atoms:
  • CHCl3: 7.27 ppm
  • CH2Cl2: 5.30 ppm
  • CH3Cl: 3.05 ppm
• The effect decreases with increasing distance from the electronegative atom:
  • −CH2−Br: 3.30 ppm
  • −CH2−CH2−Br: 1.69 ppm
  • − CH2−CH2−CH2−Br: 1.25 ppm
• The effect completely vanishes at the fourth bond from the electronegative atom.

Proton NMR Chemical Shifts of Common Functional Groups (slide 2)

• Common Functional Groups Examples

Unexpected Shielding

• H-C-OR
• | C=C
  H 5.5 ppm
• H-C=CR.
• 2.0 ppm

Diamagnetic Anisotropic effect DUE TO THE PRESENCE OF -BONDS

• The presence of a nearby pi bond or pi system greatly affects the chemical shift.
• Induced magnetic fields due to the  - electrons have greatest effect.

Diamagnetic Anisotropy

• Shielding and deshielding depend on the orientation of the molecule with respect to the applied magnetic field.
• Electron density of chemical bonds could be higher in one direction than another: anisotropic
• Nuclei in close proximity of these bonds could be affected
• Most pronounced in delocalised π‐ systems
• For example H-C≡C-H Shielding of alkyne protons Shielded δ = 2.5 ppm shielded

Diamagnetic Anisotropy: Aldehyde δ = 9-10 ppm

• Shielding and deshielding depend on the orientation of the molecule with respect to the applied magnetic field.
• Deshielding of aldehyde protons
• The effect of B0 is greatest along the transverse axis of the C=O.

Diamagnetic Anisotropy: Aldehyde spectrum

• Displays a spectrum of benzaldehyde, showing a peak at 9.984 ppm for the aldehyde proton.

Diamagnetic Anisotropy: Alkene δ = 5-6 ppm

• The induced field reinforces the external field (deshielding).
• Shows the anisotropic effect in alkenes.

Diamagnetic Anisotropy: Alkene examples

• Examples:
  • H/δ = 5-6 ppm
  • 8 = 2.2 ppm
  • C
  • 8 = 2.2 ppm H

Diamagnetic Anisotropy: Alkene values

• Shows chemical shift values of miscellaneous Alkenes

Diamagnetic Anisotropy: Ketone δ = 2-2.5 ppm

• Examples for methyl, methylene groups.

Diamagnetic Anisotropy: Ketone explanation

• The magnetic anisotropy of C=O has a strongly deshielding (+δ) region in the plane of the carbonyl group.

Diamagnetic Anisotropy: π-bond shielding

• Chemical shifts values and deshielding.

Proton NMR Chemical Shifts of Common Functional Groups (slide 3)

• Chemical shifts values and deshielding.

Diamagnetic Anisotropy: Aromatic

• Protons on the benzene ring are deshielded.
• Benzene ring contains delocalized -electrons
• Deshielded δ = 7-8 ppm.

Diamagnetic Anisotropy: Aromatic example

• Displays a spectrum of styrene, showing signals in the aromatic region (7-8 ppm) and alkene region (5-6 ppm).

Examples of Ring-Current Effect

• Shows multiple examples of ring-current effects on chemical shifts in various aromatic compounds, with specific chemical shift values listed for different protons.

Reinforced double Diamagnetic Anisotropy

• All the ring protons of acetophenone are deshielded because of the ring current effect.
• The ortho protons are further deshielded.

Reinforced double Diamagnetic Anisotropy - Spectra

• Spectra of 4'-Methylacetophenone
• Spectra of 4-Methoxybenzaldehyde illustrating reinforced double diamagnetic anisotropy.

Reinforced double Diamagnetic Anisotropy- Spectra values

• Spectra values of 4'-Methylacetophenone

How Do Ring Currents Affect 1H NMR Chemical Shifts?

• Abstract explaining the downfield ¹H of benzene is not due to deshielding ring current effects.

How Do Ring Currents Affect 1H NMR Chemical Shifts? continued

• Conventional explanations of proton NMR chemical shifts need fundamental revisions: arene hydrogens are NOT deshielded by ring current effects.

Proton NMR Chemical Shifts of Common Functional Groups (slide 4)

• Review of chemical shifts values.

Unexpected Deshielding

• Cyclohexane
• values of axial vs equatorial protons

σ-Bond Diamagnetic Anisotropy

• In general, an axial protons are more shielded and resonate at lower frequency than equatorial protons, typically by about 0.5 ppm.
• shielded =0.1-0.7 ppm

Electron delocalization

• There are 'activating' and 'deactivating' groups that can redistribute electron densities through delocalized orbitals.

Hetero-aromatic compounds

• Explain the difference in the chemical shift of a furan

Heteroaromatic compounds

• Explain the difference in the chemical shift of a pyridine

Unexpected Activated Proton

• Ethanol
• CH3CH2OH Spectrum

Hydrogen Bonding

• Hydrogen bonds deshield a proton signal
• The position of OH and NH signals are unpredictable, as they are dependent on the amount of hydrogen bonding 10-12ppm (Why?)

Hydrogen Bonding Spectra Example

• Hydrogen Bonding Spectra with specific chemical shift values labeled.

Hydrogen Bonding Example

• Hydrogen Bonding Values

Proton NMR Chemical Shifts of Common Functional Groups(slide 5)

• An overview of proton NMR chemical shifts and functional groups.

Chemical Shift of Common Functional Groups

• Overview of Common Functional Groups

Estimation of Chemical Shift

• Appreciation of the concepts of
  • Electronegativity (Inductive Effects)
  • Electron Delocalization (Resonance Effects)
  • Diamagnetic anisotropy
• Permits both rationalization and prediction of Chemical Shift.
• Software: Chemdraw i) Draw the structure, ii) select it, iii) click iv) Click

CHEMDRAW Estimation of Chemical Shift Protocol

• Node Shift Base + Inc. Comment (ppm rel. to TMS)

Estimation of Chemical Shift—Shoolery’s Rule for CH2

• δ = 0.23 + σY + σZ
• For Example: PhCH2Br δ = 0.23 + 1.83 + 2.33 = 4.41 Found δ 4.43
• For Example: HCH2Br δ = 0.23 + 0.34 + 2.33 = 2.90 Found δ 2.69

Estimation of Chemical Shift—Shoolery’s Rule for CH

• δ = 2.5 + σX + σY + σZ
• For Example: MeCH(OEt)2 δ = 2.5 + 0 + 1.14 + 1.14 = 4.78 Found δ 4.72
• Two substituents are EWG, only one alkyl.

Estimation of Chemical Shift—Friedrich-Runkle Relationship

• δ = δ(Me2CHZ) + xy
• Substitution of methyl plus correction factors If two substituents are alkyl groups, then correction is needed!!

Estimation of Chemical Shift—Friedrich-Runkle Relationship example

• δ = δ(Me2CHZ) + xy So, xy = 0.4 for vinyl δ = 2.89 + 0.4 = 3.29 Found: δ = 3.44 Table B.2b δ = 2.89

APPENDIX C CHEMICAL SHIFTS IN ALICYCLIC AND HETEROCYCLIC RINGS

• Tables and values for chemical shifts in alicyclic and heterocyclic rings.

APPENDIX D AND Estimation of Chemical Shift— Tobey-Pascual-Meier-Simon

• δ = 5.25 + Zgem + Zcis+ Ztrans

Estimation of Chemical Shift— Tobey-Pascual-Meier-Simon example

• Trans: δ(HA) = 5.25 + 0.44 + 0.97 + 0 = 6.66 δ(HB) = 5.25 + 1.03 + (-0.26)+ 0 = 6.02
• Cis: δ(HA) = 5.25 + 0.44 + 0 +1.21 = 6.90 δ(HB) = 5.25 + 1.03 + 0 + (-0.29) = 5.99
• Observed: δ 6.87 & 6.03
• Calculated:

Try to assign the signals

• Example for assignments.

Try to assign the signals (slide 2)

• Example for assignments for CH=CHCHO

Try to assign the signals (slide 3)

• 10 9 8 7 6 5 4 3 210 6.00
• Нь C=C C-OCH3
• +ab 5 6.28 5.72