13.12 DEPT 13C NMR Spectroscopy
3 min read•may 7, 2024
13C NMR spectroscopy is a powerful tool for analyzing carbon atoms in molecules. It differentiates carbons based on attached protons, helping identify CH, CH2, CH3, and . This technique enhances our understanding of molecular structure.
Chemical shift analysis in 13C NMR provides crucial insights into carbon environments. Factors like , , and affect shifts. Combining DEPT and chemical shift data allows for accurate structure determination, a key skill in organic chemistry.
DEPT 13C NMR Spectroscopy
Interpretation of DEPT NMR spectra
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- DEPT () differentiates between different types of carbon atoms in a molecule based on the number of attached protons
- spectrum shows only CH carbons () as positive peaks, does not show CH2, CH3, or quaternary carbons (carbon atoms with no attached protons)
- spectrum shows CH and CH3 carbons as positive peaks, CH2 carbons as negative peaks, does not show quaternary carbons
- Quaternary carbons do not appear in either DEPT-90 or DEPT-135 spectra because they lack attached protons, making them distinguishable from other carbon types
- DEPT experiments utilize specific to manipulate states and enhance signal intensity
Analysis of 13C NMR chemical shifts
- 13C NMR chemical shift values are influenced by the electronic environment of the carbon atom, providing insights into the structural features of the molecule
- Factors affecting chemical shift include:
- Hybridization of the carbon atom
- ###[sp](https://www.fiveableKeyTerm:sp)^3_0### carbons have lower (0-90 ppm) (alkanes, alcohols)
- carbons have higher chemical shifts (100-200 ppm) (alkenes, aromatics)
- carbons have the highest chemical shifts (70-100 ppm for alkynes, 100-150 ppm for nitriles)
- Electronegativity of neighboring atoms
- Electronegative atoms (O, N, halogens) the carbon, causing a higher chemical shift (alcohols, amines, alkyl halides)
- Conjugation and effects
- Conjugated systems and aromatic rings cause higher chemical shifts due to increased electron delocalization (benzene, conjugated dienes)
-
- Bulky substituents can shield the carbon, causing a lower chemical shift (tert-butyl groups)
- Hybridization of the carbon atom
- contributes to the observed chemical shifts, especially in molecules with asymmetric electronic environments
Structure determination from NMR data
- 13C NMR spectrum shows all carbon atoms in the molecule as , regardless of the number of attached protons, providing information about the total number of unique carbon atoms
- Combining information from broadband decoupled, DEPT-90, and DEPT-135 spectra allows for the determination of the number of CH, CH2, CH3, and quaternary carbons in the molecule
- Compare the number of peaks in each spectrum to determine the number of each carbon type
- Use chemical shift values to identify the structural environment of each carbon atom (hybridization, neighboring atoms, conjugation)
- Consider the molecular formula and to propose possible structures
- Eliminate structures that do not match the NMR data
- Example: For a compound with a molecular formula of and the following NMR data:
- Broadband decoupled: 4 signals
- DEPT-90: 1 signal
- DEPT-135: 2 positive signals, 1 negative signal
- The structure must contain 1 CH, 1 CH2, 1 CH3, and 1 quaternary carbon (possible structures: butanone, isobutyraldehyde)
Advanced NMR Concepts
- is the fundamental principle behind NMR spectroscopy, involving the interaction between nuclear spins and external magnetic fields
- techniques are used to convert time-domain NMR signals into frequency-domain spectra, enabling rapid data acquisition and improved sensitivity
- of nuclear spins influence signal intensity and resolution in NMR experiments, providing information about molecular dynamics and interactions
Key Terms to Review (29)
Broadband Decoupled: Broadband decoupled refers to a technique used in 13C NMR spectroscopy where the 1H signals are continuously irradiated during the acquisition of the 13C spectrum. This decoupling process simplifies the 13C spectrum by collapsing the multiplet patterns into singlets, making the interpretation of the spectrum more straightforward.
Chemical Shift Anisotropy: Chemical shift anisotropy (CSA) is a phenomenon in nuclear magnetic resonance (NMR) spectroscopy where the observed chemical shift of a nucleus depends on the orientation of the molecule with respect to the external magnetic field. This effect is particularly significant in solid-state NMR and is a useful tool for studying the local environment and dynamics of molecules.
Chemical Shifts: Chemical shifts refer to the slight variations in the resonance frequencies of nuclear spins in a magnetic field, which provide valuable information about the chemical environment of atoms in a molecule. This term is particularly important in the context of 13C NMR Spectroscopy: Signal Averaging and FT–NMR, as well as DEPT 13C NMR Spectroscopy, as it allows for the identification and characterization of different carbon environments within a compound.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Degree of unsaturation: The degree of unsaturation in a molecule indicates the total number of pi bonds and rings present. It provides insight into the molecule's complexity by revealing how many double bonds, triple bonds, or rings it contains.
Degree of Unsaturation: The degree of unsaturation refers to the number of carbon-carbon double bonds and/or carbon-carbon triple bonds present in a molecule. It provides information about the level of saturation and the potential for chemical reactivity of a compound.
DEPT: DEPT (Distortionless Enhancement by Polarization Transfer) is a 13C NMR spectroscopic technique that enhances the signal intensity of carbon-13 nuclei by transferring polarization from the more abundant and sensitive hydrogen-1 nuclei. This method is particularly useful in the analysis of carboxylic acid derivatives, as it can provide information about the types of carbon environments present in the molecule.
DEPT-135: DEPT-135 is a type of 13C NMR spectroscopy technique that allows for the differentiation of carbon atoms based on their degree of protonation. It provides valuable information about the structure and connectivity of organic compounds by selectively enhancing the signals of methine (CH), methylene (CH2), and quaternary (C) carbons.
DEPT-90: DEPT-90 (Distortionless Enhancement by Polarization Transfer with a 90-degree pulse) is a specific type of 13C NMR spectroscopy technique that provides information about the number of hydrogen atoms attached to each carbon atom in a molecule. This method is particularly useful for determining the carbon atom environment and connectivity within organic compounds.
DEPT-NMR: DEPT-NMR (Distortionless Enhancement by Polarization Transfer Nuclear Magnetic Resonance) is a specialized NMR spectroscopy technique used to differentiate between carbon atoms in organic compounds based on the number of hydrogen atoms attached to them. It provides detailed information about the carbon framework of molecules by selectively highlighting CH, CH2, and CH3 groups.
Deshield: Deshielding refers to the reduction or removal of the shielding effect experienced by a nucleus in a molecule, resulting in a change in the chemical shift observed in NMR spectroscopy. This term is particularly relevant in the context of 13C NMR spectroscopy and DEPT 13C NMR spectroscopy.
Distortionless Enhancement by Polarization Transfer: Distortionless Enhancement by Polarization Transfer (DEPT) is a 13C NMR spectroscopy technique that enhances the signal intensity of carbon-13 nuclei without distorting the spectrum. It achieves this by transferring polarization from protons to carbons, resulting in a more sensitive and informative 13C NMR analysis.
Electronegativity: Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. It is a fundamental concept in understanding the nature and strength of chemical bonds, as well as predicting the polarity and reactivity of molecules.
Electronegativity (EN): Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it is part of a compound. The higher the electronegativity value, the more strongly an atom can pull electrons towards itself.
Fourier Transform: The Fourier transform is a mathematical operation that decomposes a function or signal into its constituent frequencies. It is a fundamental tool in the analysis and interpretation of nuclear magnetic resonance (NMR) spectroscopy, as it allows the conversion of time-domain signals into frequency-domain spectra.
Hybridization: Hybridization is a fundamental concept in chemistry that describes the process of mixing atomic orbitals to form new hybrid orbitals, which are used to explain the geometry and bonding patterns of molecules. This term is closely related to the development of chemical bonding theory, valence bond theory, and molecular orbital theory, as well as the structure and properties of various organic compounds.
Magnetic Resonance: Magnetic resonance is a phenomenon in which nuclei in a strong magnetic field absorb and re-emit electromagnetic radiation at a specific frequency. This property is fundamental to the techniques of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), which are widely used in chemistry, physics, and medicine to study the structure and dynamics of molecules.
Methine: A methine is a carbon atom that is bonded to three other atoms, typically hydrogen and/or other carbon atoms. It is a key structural feature in organic chemistry that plays a crucial role in the interpretation of 1H NMR and 13C NMR spectra.
Nuclear magnetic resonance (NMR) spectroscopy: Nuclear Magnetic Resonance (NMR) Spectroscopy is an analytical technique used in organic chemistry to determine the structure of molecules by observing the interaction of atomic nuclei with magnetic fields. It provides detailed information about the arrangement of atoms and chemical environment in a molecule.
Nuclear Spin: Nuclear spin is a fundamental property of atomic nuclei that arises from the angular momentum of protons and neutrons within the nucleus. This intrinsic spin of the nucleus is a critical concept in understanding various spectroscopic techniques, including NMR spectroscopy.
Pulse Sequences: Pulse sequences are the precise timing and ordering of radio frequency (RF) pulses and signal acquisition used in nuclear magnetic resonance (NMR) spectroscopy to selectively excite and detect specific nuclear spins. They are a fundamental aspect of NMR experiments, allowing for the manipulation and observation of spin systems to extract valuable structural and dynamic information about molecules.
Quaternary Carbons: Quaternary carbons are carbon atoms that are bonded to four other carbon atoms, with no hydrogen atoms directly attached. They represent the highest degree of carbon substitution and are a key structural feature in organic chemistry.
Relaxation Times: Relaxation times refer to the time constants that describe the return of nuclear spins to their equilibrium state after being perturbed in nuclear magnetic resonance (NMR) spectroscopy. These time constants are crucial in understanding the behavior of nuclear spins and how they contribute to the observed NMR signals, particularly in the context of 13C NMR spectroscopy using the DEPT (Distortionless Enhancement by Polarization Transfer) technique.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Singlets: In the context of 13.12 DEPT 13C NMR Spectroscopy, singlets refer to the signal patterns observed in the 13C NMR spectrum for carbon atoms that have no attached hydrogen atoms. These singlet signals indicate the presence of quaternary carbon centers in the molecule.
Sp: The sp hybridization is a type of atomic orbital hybridization that occurs when an atom has one s orbital and one p orbital that combine to form two equivalent sp hybrid orbitals. This hybridization is particularly relevant in the context of 13.12 DEPT 13C NMR Spectroscopy, as it influences the chemical shifts and multiplicities observed in the 13C NMR spectra of organic compounds.
Sp^2: The sp^2 hybridization is a type of atomic orbital hybridization that occurs when one s orbital and two p orbitals of an atom combine to form three equivalent sp^2 hybrid orbitals. This hybridization is commonly observed in organic chemistry, particularly in the context of 13.12 DEPT 13C NMR Spectroscopy.
Sp^3: The sp^3 hybridization is a type of orbital hybridization in which an atom's s orbital and three p orbitals combine to form four equivalent hybrid orbitals. This is the characteristic hybridization state of carbon atoms in alkanes and other saturated organic compounds.
Steric Effects: Steric effects refer to the influence of the spatial arrangement and size of atoms or functional groups on the chemical and physical properties of a molecule. These spatial factors can impact reactivity, stability, and spectroscopic characteristics of chemical compounds.