Ch 13: NMR Flashcards
What is NMR spectroscopy?
A powerful analytical technique used to determine molecular structure
Based on the absorption of radiofrequency radiation by atomic nuclei in a magnetic field.
What are the most common types of NMR spectroscopy?
- Proton NMR (¹H NMR)
- Carbon-13 NMR (¹³C NMR)
These types focus on hydrogen and carbon environments, respectively.
What is spectrometry?
The study of interactions btwn radiation/light and matter
Mass spectrometry
Provides molecular size and formula
Infrared spectrometry
Identifies functional groups present
NMR spectrometry
Provides structure determination by mapping carbon-hydrogen frame work
- Source of energy is radiowaves
- When mass # (protons+neutrons) is odd, nucleus rotates and produces magnetic moment
What creates a magnetic moment in atomic nuclei?
Nuclei with an odd number of protons/neutrons have a nuclear spin
This nuclear spin is responsible for the magnetic moment.
What happens to nuclei in an external magnetic field (B₀)?
Nuclei align either with or against the magnetic field, leading to α (along with magnetic field/lower energy) and β (against and higher energy requirement) spin states
α-state is lower energy, while β-state is higher energy.
What is a spin flip
The exact amount of RF radiation to change the energy state.
This allows us to see the structure in 1H-NMR
What causes resonance in NMR?
RF radiation flips nuclei from the lower-energy α-state to the higher-energy β-state
The energy required depends on shielding/deshielding effects.
What is the chemical shift (δ) in NMR?
Position on chart where nucleus absorbs aka position of NMR signals in parts per million (ppm)
Indicates the electronic environment of nuclei.
What is the formula associated with chemical shift (δ)?
What is electron shielding?
surrounding electrons shield nucleus from the full effect of the applied field
ie: CH3-CH2-CH2-Cl
- CH3 is most shielded because it is furthest from the negative charge of Cl (electron withdrawing group)
- CH2 closest to Cl is least shielded bc closest to Cl which is EN
Note: when both CH2 are compared the CH2 btwn the CH3 and CH2 would be most shielded. Comparison is relative
What are the common splitting patterns in NMR?
- Singlet (s) - no neighboring protons
- Doublet (d) - one adjacent proton
- Triplet (t) - two adjacent protons
- Quartet (q) - three adjacent protons
Indicates the number of neighboring protons affecting the signal.
What is TMS 0 point
- Tetramethyl Silane
- Represents most shielded proton and used as a comparison to gauge shielding
What effect does shielding have on NMR peaks?
Shielding shifts peaks upfield (lower ppm)
- closer to TMS 0 point (left)
Occurs in electron-dense environments.
What effect does deshielding have on NMR peaks?
Deshielding shifts peaks downfield (higher ppm/right side)
- Further from TMS 0 point
Occurs in electron-poor environments.
Chemical shift range for common protons
What are typical ¹H NMR shifts for alkanes?
0.9–2 ppm
Indicates the presence of hydrogen atoms in alkane structures.
What are typical ¹H NMR shifts for benzene?
6.5-8 ppm
What are typical ¹H NMR shifts for alkenes?
4.5-6ppm
What are typical ¹H NMR shifts for aldehyde?
9-10 ppm
What are typical ¹H NMR shifts for carboxylic acid?
10-12 ppm
What does ¹H NMR tell you?
- # of signal: diff. types of protons
- Position of signal: chemical shift
- Intensity of signal: proton count for each type
- Splitting of signal due to spin-spin coupling with neighbor protons
Number of signals from ¹H NMR
Determined by the # of chemically equivalent signals.
Rotational symmetry:
Yes: will produce the same signal Chemically equivalent - Homotopic protons
Planar/Axial symmetry:
Yes: Chemically equivalent - Enantiotopic protons
No: Not chemically equivalent - Diastereotopic protons
Homotopic protons
referring to CH2
Enantiotopic protons
referring to CH2
Diastereotopic protons
referring to CH2
What to look out for in chiral carbons and signalling?
Check for multiple chiral carbons to assess accurate number of signal
Intensity of signals in ¹H NMR
Refers to height of the peak
Could correspond to the actual number of protons (H) or could represent a multiple
What does integration in ¹H NMR represent?
Synonym for intensity. Area under peaks corresponds to the number of protons
Helps quantify the number of hydrogen atoms in different environments.
What is multiplicity in ¹H NMR?
Arises from spin-spin coupling with adjacent protons
Indicates the number of neighboring equivalent protons.
Splitting/spin-spin coupling in ¹H NMR
- Neighboring protons influence each other is not equivalent
- Each color represents the same energy field
- Must be within 3 bond
What does the n + 1 rule in ¹H NMR refer to?
Multiplicity pattern based on neighboring protons
n= neighboring proton
Where n is the number of neighboring equivalent protons.
What are the common splitting patterns in ¹H NMR?
- Singlet (s) - no neighboring protons
- Doublet (d) - one adjacent proton
- Triplet (t) - two adjacent protons
- Quartet (q) - three adjacent protons
Indicates the number of neighboring protons affecting the signal.
What is complex coupling and multiplicity in ¹H NMR?
Anything attached to an sp2 carbon will have complex coupling (double bond)
Multiplicity=(n1+1)(n2+1)
What does ¹³C NMR provide information about?
Carbon environments
Useful for determining carbon connectivity in organic molecules.
¹³C NMR
- Follows same rules as ¹H NMR regarding counting number of signals
- Chemical shift (shielding/deshielding): Larger chemical shift (0-200 ppm)
- Integration: different peak heights have no value as seen in ¹H NMR (proton count), still same number of protons.
- Coupling: Not observed
Why are carbon peaks in ¹³C NMR not split?
Due to proton decoupling
Simplifies the ¹³C NMR spectra.
What are typical ¹³C NMR shifts for alkenes?
0–200 ppm
Indicates the presence of sp² hybridized carbon atoms.
What is the role of NMR in structure determination?
Combining chemical shift, integration, and splitting patterns helps deduce molecular structures
Essential in organic chemistry, medicinal chemistry, and biochemistry.
What advanced NMR techniques are used for complex molecules?
- 2D NMR
- COSY
- HSQC
- HMBC
These techniques provide more detailed information about molecular structure.
What are key takeaways from NMR spectroscopy?
- ¹H NMR and ¹³C NMR provide complementary structural information
- Chemical shift, integration, and multiplicity are crucial in ¹H NMR
- Proton decoupling simplifies ¹³C NMR spectra
- NMR is invaluable for identifying functional groups, connectivity, and purity in organic compounds
Highlights the importance of NMR in analytical chemistry.
What principle does NMR Spectroscopy rely on?
Certain atomic nuclei possess spin and generate a magnetic field.
What happens to atomic nuclei when placed in an external magnetic field?
They align either with or against the field, creating energy differences that can be detected upon RF excitation.
What does the chemical shift (δ) in ¹H NMR measure?
The electronic environment of hydrogen atoms.
What does multiplicity in NMR indicate?
The number of adjacent hydrogen atoms through spin-spin coupling.
What is the difference between decoupled and coupled ¹³C NMR spectra?
Decoupled spectra simplify by eliminating splitting, while coupled spectra show interactions with adjacent protons.
What causes coupling in NMR Spectroscopy?
Spin-spin interactions between non-equivalent nuclei.
What is J-Coupling in NMR?
It quantifies the interaction between nuclear spins, measured in Hz.
What is a doublet in NMR?
A peak splits into two due to one adjacent proton.
What is a triplet in NMR?
A peak splits into three due to two adjacent protons.
What is a quartet in NMR?
A peak splits into four due to three adjacent protons.
What is a multiplet in NMR?
Complex splitting occurs due to multiple neighboring protons.
What is second-order coupling in NMR?
Occurs when nuclei have similar chemical shifts, leading to more complex patterns.
Where is long-range coupling typically observed?
Between protons separated by more than three bonds.
What are some applications of NMR Spectroscopy?
- Structural Determination
- Conformational Analysis
- Quantitative Analysis
True or False: NMR spectroscopy can only be used for qualitative analysis.
False
Fill in the blank: NMR spectroscopy offers unparalleled insight into molecular structures through the principles of _______.
[chemical shifts, integration, and coupling interactions]
What is the typical chemical shift range for aromatic protons in a ¹H NMR spectrum?
6.5–8 ppm
How does electronegativity affect the chemical shift of a proton?
Electronegative atoms deshield nearby protons, shifting their signal downfield (higher ppm)
What functional group typically appears around 9–10 ppm in a ¹H NMR spectrum?
Aldehydes (-CHO)
Why do carboxylic acid (-COOH) protons appear at a very high chemical shift (~10-12 ppm)?
Strong hydrogen bonding causes extreme deshielding
Where do aliphatic protons (-CH₃, -CH₂, -CH) typically appear in a ¹H NMR spectrum?
0.9–2 ppm
What is the expected splitting pattern for a CH₃CH₂- group in ¹H NMR?
The CH₃ protons appear as a triplet, and the CH₂ protons appear as a quartet due to coupling
What does a doublet of doublets (dd) suggest in an NMR spectrum?
A proton is coupled to two different non-equivalent protons
What is the difference between geminal and vicinal coupling?
Geminal (J ≈ 0–3 Hz): Coupling between two protons on the same carbon. Vicinal (J ≈ 6–12 Hz): Coupling between two protons on adjacent carbons
How does deuterium (²H) affect proton NMR spectra?
Deuterium does not couple with ¹H and appears invisible in proton NMR
What is the typical J-coupling constant for cis-alkene protons (-CH=CH-)?
6–12 Hz
What is the typical J-coupling constant for trans-alkene protons (-CH=CH-)?
12–18 Hz
What is the typical J-value for an ortho-coupled proton pair in benzene?
7–10 Hz
Why does an -OH proton often appear as a singlet?
Rapid exchange with other hydroxyl protons disrupts splitting
What is long-range (four-bond) coupling, and when does it occur?
Coupling across four bonds (J ≈ 1–3 Hz), often seen in benzene rings or conjugated systems
What are the two common types of NMR spectroscopy used to characterize organic structure?
1H NMR and 13C NMR
1H NMR determines the type and number of H atoms, while 13C NMR determines the type of carbon atoms.
What is the source of energy in NMR?
Radio waves
Radio waves have long wavelengths, low energy, and frequency.
How do charged particles like protons create a magnetic field?
By spinning on their axis
This creates a tiny bar magnet effect.
What happens to the orientation of nuclear spins in an applied magnetic field?
They align with or against the magnetic field B0.
What is the energy difference between the two states of a proton in a magnetic field?
Very small (<0.1 cal).
What occurs when an external energy source matches the energy difference (ΔE) between two nuclear spin states?
Energy is absorbed, causing the nucleus to ‘spin flip’.
What two variables characterize NMR?
Applied magnetic field B0 and frequency ν of radiation used for resonance.
How is the strength of the applied magnetic field B0 measured?
In tesla (T).
What does the frequency needed for resonance depend on?
The strength of the applied magnetic field.
What is Free Induction Decay in NMR?
The process where protons in different environments absorb at slightly different frequencies.
Which nuclei give rise to NMR signals?
Nuclei containing odd mass numbers or odd atomic numbers.
What do NMR absorptions generally appear as?
Sharp peaks.
What does ‘upfield’ and ‘downfield’ mean in NMR?
‘Upfield’ means to the right, ‘downfield’ means to the left.
What is the reference peak at 0 ppm in NMR?
Tetramethylsilane (TMS).
What does the chemical shift of the x-axis indicate in a 1H NMR spectrum?
The position of an NMR signal, measured in ppm.
What four features of a 1H NMR spectrum provide information about a compound’s structure?
- Number of signals
- Position of signals
- Intensity of signals
- Splitting of signals
What are the types of protons in NMR?
- Homotopic
- Enantiotopic
- Diastereotopic
How do you determine equivalent protons in cycloalkanes and alkenes?
Draw all bonds to hydrogen.
What effect does shielding have on proton absorption in NMR?
Shielded protons absorb upfield (smaller ppm).
What happens to a deshielded nucleus in NMR?
It absorbs downfield (larger ppm).
What phenomenon causes the chemical shift of a C—H bond to increase?
Increasing alkyl substitution.
What is the impact of π electrons in benzene on NMR?
They create a ring current that reinforces the applied magnetic field, causing deshielding.
How is the area under an NMR signal related to protons?
It is proportional to the number of absorbing protons.
What does spin-spin coupling result in?
Signal splitting.
What is the coupling constant in NMR?
The frequency difference between two peaks of a doublet, measured in Hz.
What is the n + 1 rule in NMR?
A set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks.
What is complex coupling in NMR?
Observed when two sets of adjacent protons are different from each other.
What is unique about 13C NMR spectra in terms of splitting?
Lack of splitting due to low natural abundance of 13C.
Under usual conditions, how does an OH proton behave in NMR?
It does not split the NMR signal of adjacent protons.
What happens to the NMR spectrum of cyclohexane due to ring flipping?
It shows a single signal for the average environment.
What is the appearance of the 1H NMR spectrum for benzene?
A single peak at 7.27 ppm.
What is a characteristic of monosubstituted benzenes in NMR?
They contain five deshielded protons that are no longer equivalent.
What is the natural abundance percentage of 13C nuclei?
1.1%
This low abundance affects the likelihood of 13C nuclei being bonded together.
What is the chance of two 13C nuclei being bonded to each other?
0.01%
This low probability explains the absence of carbon-carbon splitting in 13C NMR.
What type of splitting occurs in NMR when two active nuclei are close?
Splitting occurs with two NMR active nuclei, like two protons.
How is 1H-13C splitting usually eliminated from a 13C NMR spectrum?
By using an instrumental technique that decouples proton-carbon interactions.
What appearance does each peak in a 13C NMR spectrum have due to decoupling?
Each peak appears as a singlet.
What are the two key features of a 13C NMR spectrum that provide structural information?
- Number of signals observed
- Chemical shifts of those signals
What does the number of signals in a 13C spectrum indicate?
The number of different types of carbon atoms in a molecule.
Why are the 13C NMR signals not split?
Due to the low natural abundance of 13C nuclei.
In a 13C NMR spectrum, how does the number of signals relate to the number of lines?
The number of signals equals the number of lines in the 13C spectrum.
Is peak intensity in 13C NMR proportional to the number of absorbing carbons?
No, peak intensity is not proportional to the number of absorbing carbons.
Are 13C NMR signals integrated?
No, 13C NMR signals are not integrated.
