NMR Spectroscopy Notes

Chapter 13: Proton and Carbon NMR Spectroscopy

Molecular Spectroscopy

• Molecular spectroscopy studies the interaction between substances and electromagnetic radiations.
• Spectroscopic techniques measure energy absorption/emission by a substance, correlating it with molecular structure.
• Electromagnetic radiations have a dual wave-particle nature; energy (photons) relates to frequency/wavelength/wavenumber.
• $c = \omega \lambda$ where $\omega$ is wavenumber and $\lambda$ is wavelength.
• Molecules undergo vibrational, electronic, or nuclear transitions, corresponding to different electromagnetic spectrum regions.
• Common spectroscopic techniques for determining organic compound structures:
  • Nuclear Magnetic Resonance Spectroscopy (NMR):
    • Low energy radio waves absorbed during transitions between nuclear spin states.
    • Provides information on carbon and hydrogen atom connectivity.
  • Infrared Spectroscopy (IR):
    • Identifies functional groups in organic compounds.
    • Energy associated with molecular vibrations falls in the IR region.
  • Ultraviolet Spectroscopy (UV-vis):
    • UV and visible regions involved in electronic transitions.
    • Gives information about molecules with conjugated pi bonds.
  • Mass Spectrometry (MS or Mass Spec):
    • Provides molar mass and composition by analyzing fragments of radical ions.

Nuclear Magnetic Resonance (NMR) Spectroscopy

• NMR detects local magnetic fields around atomic nuclei to determine molecular structure.
• Nuclei with odd mass numbers (e.g., $^{1}$H, $^{13}$C, $^{31}$P) have nuclear spin quantum states that can be studied.
• All nuclei with an odd mass number ($^1$H, $^{13}$C) have spin angular momentum because they have an unpaired proton.
• Two common NMR types:
  • $^{1}$H NMR: Determines the type and number of hydrogen atoms in a molecule.
  • $^{13}$C NMR: Determines the types of carbon atoms in a molecule.

Proton NMR ($^1$H NMR)

• Atomic nuclei have spinning charged particles (protons) that generate a magnetic field.
• Nuclear magnetic spins are randomly oriented.
• In an external magnetic field, nuclear magnetic fields align with (lower energy) or against (higher energy) the external field.
  • (↑) α spin state: Protons align with the external magnetic field (lower energy).
  • (↓) β spin state: Protons align against the external magnetic field (higher energy).
• Energy difference (ΔE) between α and β spin states depends on the applied magnetic field.
• Stronger magnetic field → larger energy difference.
• ΔE corresponds to low-frequency radio waves.
• Radio wave frequency causes spin to flip; nuclei are in resonance with the applied field, producing an NMR signal.
• Stronger magnetic field → higher frequency needed for resonance.

Characteristic Features of $^1$H NMR for Structural Information

1. Number of signals
2. Position of signals (Chemical Shift)
3. Integration or intensity of signals
4. Splitting of signals (Multiplicity)

1. Number of Signals

• Nuclei of different types resonate at different frequencies based on their chemical and electronic environment.
• Chemically equivalent protons (protons in the same molecular environment) resonate at the same frequency and give the same signal.
• Magnetically equivalent protons experience identical influence when placed in an external magnetic field; they have the same absorption frequency.
• Protons within a molecule are magnetically equivalent if they experience identical influence when placed in an external magnetic field.
• To determine magnetic equivalence, replace protons with another atom (e.g., Br, Cl, D).
  • Homotopic protons: Replacement forms the same compound – magnetically equivalent.
  • Enantiotopic protons: Replacement forms enantiomers – magnetically equivalent.
  • Diastereotopic protons: Replacement forms diastereomers – nonequivalent.
• Protons attached to symmetry-related atoms are magnetically equivalent due to the presence of a plane, axis, or center of symmetry.

2. Position of Signals: Chemical Shift

• Signal position in the NMR spectrum indicates the type of proton responsible for the signal.
• NMR spectrum: plot of peak intensity against chemical shift (δ), measured in parts per million (ppm).
• Chemical shift (δ): measure of signal position relative to tetramethylsilane (TMS) reference signal at 0 ppm.
• Electronic environment around a nucleus affects its chemical shift.
• Electrons create a small magnetic field that opposes the applied magnetic field ($B_0$), shielding the proton.
• Different types of protons are shielded to different extents.
• Greater electron density → more shielding → upfield (farther right).
• Less electron density → less shielding → downfield (higher ppm).
• Inductive Effect: Chemical shift of a C-H bond increases with increasing alkyl substitution:
  • $RCH_2-H$: δ ~ 0.9 ppm
  • $R_2CH-H$: δ ~ 1.3 ppm
  • $R_3C-H$: δ ~ 1.7 ppm
• Electronegative atoms/electron-withdrawing groups deshield protons, giving signals at higher ppm; effect diminishes with distance.
• More electron-withdrawing groups → proton signal shifts farther downfield.
• Ring current effect (magnetic anisotropy): magnetic field created by π electrons or rings.
• π electrons in alkenes, aromatic systems, and other compounds tend to deshield the nuclei of adjacent protons through ring currents, i.e., protons feel larger effective magnetic field. For example, the aryl hydrogen of benzene is de-shielded by ring currents δ = 6.5 – 8 ppm.
• In a magnetic field, the p electrons of C≡C are induced to circulate but, in this case, the induced magnetic field opposes the applied magnetic field. Thus, protons feel a weaker magnetic field. Thus, a lower frequency is needed for resonance. The nucleus is shielded, and absorption is up-field, approximately at 2.5 ppm.
• In benzene derivatives the presence of electron donating group shields and electron withdrawing groups deshields the protons of the ring by inductive effect or resonance effect.

Characteristic Chemical Shift

• $CH_3$: 0.8-1 ppm
• $RCH_2R$: 1.2-1.4
• $R_2CH(R)$: 1.4-1.7
• Allylic/Benzylic $CH_2$ or next to O: 1.5-2.5 ppm
• Terminal Alkyne: 2.5 ppm
• $R-CH=CH_2$: ~4.5-6.5 ppm
• $R-CH=CH-R$ and derivatives: 5-6 ppm
• Benzene derivatives: 6.5-8.5 ppm
• $R-O-R$: 3.3-3.8 ppm
• $R-X$ (X=Br, Cl): 3.3 - 4.3
• O-H, N-H: 0.5-5 ppm (broad signals)
• Aldehyde: 9-10 ppm
• Carboxylic Acid: 11-13 ppm

3. Integration/Intensity of Signals

• Integration: total area under the signal (signal intensity).
• Area under each signal is proportional to the number of equivalent protons producing that signal.
• Spectrometer calculates the integrated relative area of each signal.
• Integration numbers do not generally correspond to the exact or absolute number of protons.

4. Splitting of Signals

• Proton resonances are influenced through bonds by local fields generated by protons attached to neighboring carbons. This is called spin-spin splitting.
• Splitting provides information on connectivity.
• Peaks split into multiple peaks due to magnetic interactions between nonequivalent protons on carbons separated by three bonds (vicinal coupling), two bonds (geminal coupling), or sometimes four bonds.
• J coupling is represented as $^n$$J_{AB}$, where n is the number of intervening bonds, and A and B identify the two coupled spins.

Coupled Protons

• 1,1,2-trichloroethane: Ha and Hb are coupled protons and split each other’s signals (mutual splitting).
• Hb peak splits into two peaks (doublet) due to neighboring proton (Ha) spin.
  • Ha α spin adds to the field felt by Hb.
  • Ha β spin weakens the field felt by Hb.
• When parallel, Ha feels a stronger field (applied field + Hb); when anti-parallel, Ha feels a weaker field.
• The external magnetic field either deshields or shields Ha based on the alignment of Hb spin.

Multiplicity

• Resonance signal splitting due to J-coupling is referred to as multiplicity.
• The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5 = quintet).
• Number of peaks is given by the (n + 1) rule, where n is the number of equivalent H’s on adjacent carbons.
• Relative intensities follow Pascal’s Triangle.

Coupling Constant (J)

• Distance between two adjacent peaks of a split signal.
• Magnitude measures how strongly coupled protons influence each other.
• Dependent on the number/types of bonds connecting the coupled protons and their geometric relationship.
• Measured in hertz (Hz): $J (Hz) = Δ ppm x instrument frequency$
• Δ ppm is the difference in ppm of two peaks for a given proton.
• The instrument frequency is determined by the strength of the magnet.
• J value varies with instrument field strength; stronger field strength gives larger J values and better resolution.
• In acyclic systems, J for vicinal coupling between similar types of H’s is generally ~ 7 Hz.
• (n+1) rule and Pascal triangle apply.
• Alkenes
  • {J{ab}} (trans) > {J{ac}} (cis) > {J_{bc}} (geminal)
• Ha appears as a doublet of doublets (d,d)
• No of lines for Ha = $(n<em>b + 1)(n</em>c + 1)$

Rapid Exchange Protons (O-H and N-H)

• Protons on O and N can exchange due to intermolecular H bonding.
• Rapid exchange of OH protons in alcohols often prevents coupling with hydrogens on adjacent carbons; these protons are often decoupled.
• $^1$H NMR of O-H or N-H generally appears as a broad singlet.

Long-Range Coupling

• Occurs when protons are separated by more than 3 bonds, and one bond is a double or triple bond.
• Four-bond coupling ($^4$J coupling) is observed in π systems like terminal alkynes, allylic systems, and aromatic compounds.

13C NMR

• The number of signals indicates the number of different types of carbons in the compound.
• $^{13}$C NMR signals: resonance frequencies of different sets of equivalent carbons.
• Lower sensitivity than $^{1}$H NMR, due to the low natural abundance of $^{13}$C (~1.1%).
• $^{13}$C coupling has much larger J values (~100 Hz) compared to $^{1}$H J values.
• $^{13}$C NMR absorption range: δ 0.0 to δ 220 ppm (TMS at 0.0 ppm).
• Wide range, each carbon gives a well-separated signal.
• $^{1}$H – $^{13}$C splitting is usually eliminated using an instrumental technique (proton decoupling), so every peak in a $^{13}$C NMR spectrum is a singlet.

13C Chemical Shifts

• Alkene/Aromatic: 100-160 ppm
• Alkyne: 65-95 ppm
• C-O (alcohol, ether): 50-90 ppm
• Ester, Amide: 170-185 ppm
• Aldehyde or Ketone: 190-220 ppm
• Examples of $^{13}C$ and $^{1}H$ NMR spectra for various compounds such as 3-chloropentane, 3,3-dichloropentane, 1-butene, and 4-methoxybutan-2-one are provided with their corresponding peak assignments.

Degree or Sites of Unsaturation / Index of Hydrogen Deficiency

• The total number of cyclic rings and π bonds in a compound.
• One π bond or one cyclic ring equals one degree of unsaturation.
• For the molecular formula $C<em>nH</em>mX<em>pNp</em>qO_r$:
  • Degree of unsaturation = $\frac{2n + 2 - m - p + q}{2}$
• Degree of unsaturation can have 2 π bonds, one ring and one π bond, or two rings.