13: Structure Determination - Nuclear Magnetic Resonance Spectroscopy
- Page ID
- 448671
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- fulfillall of the detailed objectives listed under each individual section.
- solve road-map problems which may require the interpretation of 1H NMR spectra in addition to other spectral data.
- define, and use in context, the key terms introduced.
In Chapter 12, you learned how an organic chemist could use two spectroscopic techniques, mass spectroscopy and infrared spectroscopy, to assist in determining the structure of an unknown compound. This chapter introduces a third technique, nuclear magnetic resonance (NMR). The two most common forms of NMR spectroscopy, 1H NMR and 13C NMR, will be discussed, the former in much more detail than the latter. Nuclear magnetic resonance spectroscopy is a very powerful tool, particularly when used in combination with other spectroscopic techniques.
- 13.0: Why This Chapter?
- The chapter on Nuclear Magnetic Resonance (NMR) Spectroscopy emphasizes its importance in determining the structure of organic compounds. It introduces how NMR provides insights into molecular structure, dynamics, and environment. By analyzing the interactions between nuclear spins and magnetic fields, students will learn how to interpret NMR spectra, which is crucial for understanding molecular composition and function in organic chemistry.
- 13.1: Nuclear Magnetic Resonance Spectroscopy
- Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful analytical technique used to determine the structure of organic molecules. It operates on the principle that certain nuclei resonate in a magnetic field, providing information about the environment of atoms in a compound. Key aspects include chemical shifts, which reveal the electronic environment of nuclei, and spin-spin coupling, which indicates interactions between neighboring nuclei.
- 13.2: The Nature of NMR Absorptions
- Nuclear Magnetic Resonance (NMR) absorptions are influenced by the local electronic environment of nuclei, specifically protons and carbons. Factors affecting these absorptions include electronegativity, hybridization, and steric effects, which shift chemical shifts and determine peak positions. The resulting spectra provide valuable insights into molecular structure, such as the number of neighboring protons and functional groups. Understanding these principles is essential for interpreting NMR
- 13.3: Chemical Shifts
- Chemical shifts in NMR spectroscopy reflect the electronic environment surrounding nuclei, mainly protons. The presence of electronegative atoms and hybridization affects these shifts, resulting in varying resonance frequencies. Chemical shifts are reported in parts per million (ppm) on the δ scale, facilitating comparisons across different compounds. This metric is crucial for identifying functional groups and understanding molecular structure.
- 13.4: Chemical Shifts in ¹H NMR Spectroscopy
- In H-NMR H-NMR spectroscopy, chemical shifts provide insights into the electronic environment of hydrogen atoms in a molecule. Factors like electronegativity and hybridization influence these shifts, allowing for the differentiation of hydrogen atoms in various chemical settings. The 𝛿 δ scale, measured in parts per million (ppm), is used to represent these shifts, which help identify functional groups and elucidate molecular structure.
- 13.5: Integration of ¹H NMR Absorptions- Proton Counting
- In H-NMR H-NMR spectroscopy, integration refers to measuring the area under absorption peaks, indicating the relative number of protons contributing to each peak. This allows chemists to determine the number of hydrogen atoms in specific environments within a molecule. The integration values can provide insights into the molecular structure and help identify functional groups. Accurate integration is essential for interpreting complex spectra effectively.
- 13.6: Spin-Spin Splitting in ¹H NMR Spectra
- Spin-spin splitting in H-NMR H-NMR spectroscopy occurs due to interactions between neighboring hydrogen atoms, leading to the splitting of NMR signals into multiple peaks. This phenomenon is quantified using the n+1 rule, where n is the number of adjacent protons. The resulting splitting pattern provides insight into the molecular structure, revealing the number of neighboring protons and helping to identify functional groups.
- 13.7: ¹H NMR Spectroscopy and Proton Equivalence
- This section explains how proton equivalence in molecules influences NMR signals. It describes how equivalent protons produce the same signal, simplifying the spectrum, while nonequivalent protons yield distinct signals. Proton equivalence is determined by symmetry and chemical environments, which help distinguish different hydrogen atoms in a molecule.
- 13.8: More Complex Spin-Spin Splitting Patterns
- Complex spin-spin splitting patterns in H-NMR H-NMR spectroscopy arise from interactions between non-equivalent protons on adjacent carbons. This results in multiplet patterns rather than simple doublets or triplets. The number of peaks and their arrangement depend on the number of neighboring protons and their configuration. Understanding these patterns helps in deducing the structure of organic molecules.
- 13.9: Uses of ¹H NMR Spectroscopy
- H-NMR spectroscopy is widely used in organic chemistry for structure elucidation, confirming compound identities, and studying molecular interactions. It helps determine molecular structure by providing information on hydrogen environments, functional groups, and the connectivity of atoms. Additionally, H-NMR H-NMR is essential in industries such as pharmaceuticals for drug development and quality control. Its applications extend to analyzing complex mixtures and studying dynamic processes.
- 13.10: ¹³C NMR Spectroscopy - Signal Averaging and FT-NMR
- C-NMR spectroscopy utilizes signal averaging and Fourier transform (FT) techniques to enhance the detection of carbon nuclei in organic compounds. This method provides detailed insights into molecular structure, allowing chemists to identify carbon environments and functional groups more effectively. FT-NMR significantly improves sensitivity and resolution, making it a powerful tool for structural determination in organic chemistry.
- 13.11: Characteristics of ¹³C NMR Spectroscopy
- C-NMR spectroscopy is valuable for identifying carbon atoms in various environments, with each signal representing a unique carbon. While it is less sensitive than H-NMR, it provides crucial structural details, functional groups, and stereochemistry insights. C-NMR's ability to analyze complex mixtures enhances its significance in organic chemistry.
- 13.12: DEPT ¹³C NMR Spectroscopy
- DEPT (Distortionless Enhancement by Polarization Transfer) C-NMR spectroscopy is a technique that enhances the detection of carbon atoms in organic molecules, allowing chemists to distinguish between CH, CH2, and CH3 groups based on the number of attached hydrogen atoms. This method provides clear insights into carbon environments and helps elucidate molecular structure. It is particularly useful for simplifying complex spectra, thus facilitating the analysis of organic compounds.
- 13.13: Uses of ¹³C NMR Spectroscopy
- C-NMR spectroscopy is widely used for structural elucidation in organic chemistry, allowing chemists to determine the carbon framework of molecules. It helps identify functional groups, study complex mixtures, and verify the purity of compounds. This technique can also assist in determining stereochemistry and identifying structural isomers. Its non-destructive nature makes it ideal for analyzing sensitive samples.
- 13.14: Chemistry Matters—Magnetic Resonance Imaging (MRI)
- Magnetic Resonance Imaging (MRI) utilizes principles of nuclear magnetic resonance to create detailed images of the body's internal structures. It is non-invasive and provides high-resolution images, making it invaluable in medical diagnostics. MRI primarily focuses on hydrogen nuclei in water, which is abundant in the body, allowing for the visualization of soft tissues. This technique has revolutionized medical imaging, aiding in the diagnosis of various conditions, including tumors and brain
- 13.16: Summary
- Nuclear Magnetic Resonance (NMR) spectroscopy is essential for determining molecular structures in organic chemistry. Key aspects include chemical shifts, integration of peaks, and spin-spin splitting, which provide insights into molecular environment and connectivity. H-NMR and C-NMR techniques allow detailed analysis of hydrogen and carbon environments, respectively. These methods are widely applied in research and industry for characterizing compounds and their behavior in different contexts.