University:
Rutgers University Newark
Course:
CHEM 335 | Organic Chemistry
Academic year:

2024
Views:

498

Pages:

72
Author:

Mahitab S.

CHAPTER 13 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY ©2018 Cengage Learning. All Rights Reserved. Lecture Outline • Definitions in Nuclear Magnetic Resonance Spectroscopy • Interpretation of NMR Spectra • 1H NMR • 13C NMR ©2018 Cengage Learning. All Rights Reserved. Nuclear Magnetic Resonance (NMR) Spectroscopy • Spectroscopic technique that provides information about the number and types of atoms in a molecule, as well as connectivity, such as the number and types of: • Hydrogen atoms using 1H-NMR spectroscopy • Carbon atoms using 13C-NMR spectroscopy • Phosphorus atoms using 31P-NMR spectroscopy • Discovered by Felix Bloch and Edward Purcell in 1946 • Nobel Prize for Physics in 1952 •1H NMR is the most routine tool in organic chemistry •In almost all cases it is possible to identify molecules or determine the structure using a 1H NMR measurement ©2018 Cengage Learning. All Rights Reserved. Nuclear Spin States • An electron has a spin quantum number of 1/2 with allowed values of +1/2 and −1/2 • Moving charge has an associated magnetic field • In effect, an electron behaves as if it is a tiny bar magnet that has a magnetic moment and the same effect holds for certain atomic nuclei • Any atomic nucleus that has an odd mass number, an odd atomic number, or both has a spin and a resulting nuclear magnetic moment (important) • Allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus • A nucleus with spin quantum number I has 2I + 1 spin states ©2018 Cengage Learning. All Rights Reserved. Table 13.1 - Nuclear Spin Quantum Numbers and Allowed Nuclear Spin States ©2018 Cengage Learning. All Rights Reserved. Nuclear Magnetic Resonance • NMR signals are measured relative to the signal of the reference compound tetramethylsilane (TMS) • Signal from TMS becomes the reference point and is assigned a chemical shift of zero ppm (0.00 ppm) • Chemical shift (δ): Shift in ppm of an NMR signal from the signal of TMS (relative to TMS) • For a 1H-NMR spectrum, signals are reported by their shift from the signal of the 12 equivalent H atoms in TMS • For a 13C-NMR spectrum, signals are reported by their shift from the signal of the 4 equivalent carbon atoms in TMS ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following nuclei does not show magnetic behavior? (nuclear spin quantum number) 1. 2. 3. 4. 5. 1H 2H 12C 13C 17O • 32S, 16O, 12C ©2018 Cengage Learning. All Rights Reserved. NMR Spectrometer • Essentials elements of an NMR spectrometer are a powerful magnet, a radio-frequency generator, a radio-frequency detector, and a sample tube • The sample is dissolved in a solvent, most commonly CDCl3, DMSO-d6, or D2O, and placed in a tube, which is then suspended in the magnetic field and set spinning on its long axis (solvent has no 1H atoms to not interfere with 1H NMR recording) • By using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in less than 2 seconds: field is constant, short pulse (10-5 s) of radiofrequency irradiation, flips all spins, nuclei return to equilibrium and emit sinusoid, which decay w/time ©2018 Cengage Learning. All Rights Reserved. Figure 13.5 - 1H-NMR Spectrum of Methyl Acetate • Lower axis (δ, ppm, 0.00 = TMS) • Two signals • Baseline (flat, no noise) Downfield (less shielding) Upfield (more shielding) ©2018 Cengage Learning. All Rights Reserved. Shift of NMR Signal ©2018 Cengage Learning. All Rights Reserved. Equivalent Hydrogens • Hydrogens that have the same chemical environment give the same 1H NMR signal; non-equivalent hydrogens give different 1H NMR signals • Hydrogens are equivalent if either : • H atoms are bonded to the same sp3 hybridized carbon atom, and that carbon atom can rotate freely at room temperature = average on NMR scale (for example, –CH3 or –CH2–) • H atoms are related by symmetry, namely a plane or point of symmetry in a molecule ©2018 Cengage Learning. All Rights Reserved. Equivalent Hydrogens • Molecule with one set of equivalent hydrogens gives one NMR signal as each H atom is in the same environment (examples) • Molecule with two or more sets of equivalent hydrogens gives a different NMR signal for each set ©2018 Cengage Learning. All Rights Reserved. Example 13.3 - Structure Prediction I • Each compound gives only one signal in its 1H-NMR spectrum • Propose a structural formula for each compound a. C2H6O b. C3H6Cl2 c. C6H12 d. C4H6 • Solution ©2018 Cengage Learning. All Rights Reserved. Signal Areas • Modern NMR spectrometers electronically integrate and record the relative area under each signal • Relative areas of signals are proportional to the number of equivalent hydrogens giving rise to each signal • Example - 1H-NMR spectrum of tert-butyl acetate Vertical rise of the line of integration is proportional to the number of equivalent H atoms ©2018 Cengage Learning. All Rights Reserved. Example 13.4 - Structural Prediction II • Following is a 1H-NMR spectrum for a compound of molecular formula C9H10O2 • From the integration, calculate the number of hydrogens giving rise to each signal ©2018 Cengage Learning. All Rights Reserved. Example 13.4 - Solution • Total vertical rise in the line of integration is 88 chart divisions and corresponds to 10 hydrogens • From these numbers, one can calculate that 44/88×10, or 5, of the hydrogens give rise to the signal at δ 7.34 • By similar calculations, the signals at δ 5.08 and 2.06 correspond to two hydrogens and three hydrogens, respectively ©2018 Cengage Learning. All Rights Reserved. Example • The spectrum shown could represent the molecule in the illustration 1. 2. True False ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following molecules best fits the following NMR spectrum? 1. 2. 4. 3. 5. ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following molecules best fits the following NMR spectrum? 1. 2. 4. 3. 5. ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following molecules best fits the following NMR spectrum? 1. 2. 3. 4. ©2018 Cengage Learning. All Rights Reserved. 5. Example • Which of the following molecules best fits the following NMR spectrum? 1. 2. 4. 3. 5. ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following molecules best fits the following NMR spectrum? 1. 2. 4. 3. 5. ©2018 Cengage Learning. All Rights Reserved. Figure 13.8 - Average Values of Chemical Shifts of Representative Types of Hydrogens (important) Chemical shift gives information about the type of hydrogens giving rise to that signal CH3 bonded to sp3 carbon: δ = 0.8-1.0 ppm CH3 bonded to carbonyl carbon: δ = 2.1-2.3 ppm CH3 bonded to oxygen: δ = 3.7-3.9 ppm ©2018 Cengage Learning. All Rights Reserved. Example 13.5 - Predicting 1H-NMR Spectra I • Following are structural formulas for two constitutional isomers with the molecular formula C6H12O2 1. Predict the number of signals in the 1H-NMR spectrum of each isomer 2. Predict the ratio of areas of the signals in each spectrum 3. Show how you can distinguish between these isomers on the basis of chemical shift ©2018 Cengage Learning. All Rights Reserved. Example 13.5 - Solution a. The 1H-NMR spectrum of each consists of two signals b. Ratio of areas of signals - 9:3, or 3:1 c. One can distinguish between constitutional isomers by the chemical shift of the single —CH3 group • • Hydrogens of CH3O are more deshielded (appear farther downfield) than the hydrogens of CH3C=O See Figure 13.8 for approximate values for each chemical shift and Figure 13.7 for the spectrum of tert-butyl acetate (1) ©2018 Cengage Learning. All Rights Reserved. Chemical Shift • Depends primarily on the following factors: • Electronegativity of nearby atoms • Hybridization of adjacent atoms • Magnetic induction within an adjacent pi bond ©2018 Cengage Learning. All Rights Reserved. Electronegativity of Nearby Atoms • The greater the electronegativity, the greater the chemical shift • Presence of an electronegative atom or group reduces electron density and shielding on atoms bonded to it, which deshields nearby nuclei and causes nearby nuclei to resonate farther downfield. Note that this effect falls off with distance (2 atoms away = 10%). ©2018 Cengage Learning. All Rights Reserved. Table 13.3 - Effect of Hybridization on Chemical Shift • H bonded to sp3 carbon: δ = 0.8-1.7 ppm • H bonded to sp2 carbon: δ = 4.6-5.7 ppm (deshielding because s-bonding orbital in sp2 is more s in character than in sp3, 33% vs. 25%, i.e. more electronegative C atom) However, H bonded to sp carbon: δ = 2.0-3.0 ppm (explained by relative electronegativity of sp C atoms and magnetic induction from π bonds) ©2018 Cengage Learning. All Rights Reserved. Example • 1H NMR will allow one to distinguish between the following two molecules: 1. 2. True False ©2018 Cengage Learning. All Rights Reserved. Signal Splitting and the (n + 1) Rule ©2018 Cengage Learning. All Rights Reserved. Figure 13.12 - 1H-NMR Spectrum of 1,1Dichloroethane (Signal Splitting) • Two sets of H • 3:1 ratio • More downfield (less shielding) ©2018 Cengage Learning. All Rights Reserved. Example 13.6 - Predicting 1H-NMR Spectra II • Predict the number of signals and the splitting pattern of each signal in the 1H-NMR spectrum of a, b, c • solution ©2018 Cengage Learning. All Rights Reserved. Example 13.6 – Solution • The sets of equivalent hydrogens in each molecule are labeled a, b, and c • Molecule (a) has three sets of equivalent hydrogens • Its 1H-NMR spectrum shows a singlet, a quartet, and a triplet in the ratio 3:2:3 • Molecule (b) has two sets of equivalent hydrogens • Its 1H-NMR spectrum shows a triplet and a quartet in the ratio 3:2 • Molecule (c) has three sets of equivalent hydrogens • Its 1H-NMR spectrum shows a singlet, a septet, and a doublet in the ratio 3:1:6 ©2018 Cengage Learning. All Rights Reserved. Origins of Signal Splitting ©2018 Cengage Learning. All Rights Reserved. Figure 13.13 - Vicinal H Atoms and the Spin-Spin Coupling That Occurs between Them • Vicinal hydrogens: H atoms on two C atoms that are bonded to each other • Coupling between vicinal hydrogens is referred to as vicinal coupling • H atoms with more than three bonds between them generally do not exhibit noticeable coupling ©2018 Cengage Learning. All Rights Reserved. Figure 13.15 - Quartet-Triplet 1H-NMR Signals of 3-Pentanone Showing the Original Trace and a Scale Expansion (Example) ©2018 Cengage Learning. All Rights Reserved. Table 13.4 - Approximate Values of J for Compounds Containing Alkyl and Alkenyl Groups (important) ©2018 Cengage Learning. All Rights Reserved. Figure 13.26 - 300 MHz 1H-NMR Spectrum of 1-Chloro-3Iodopropane – Example • The central CH2 (c) has the possibility of splitting into 3×3 = 9 peaks (a triplet of triplets) • Only 4 + 1 = 5 peaks are distinguishable as the values of Jab and Jbc are similar ©2018 Cengage Learning. All Rights Reserved. Figure 13.27 - 300MHz 1H-NMR Spectrum of 1Chloropropane – Example ©2018 Cengage Learning. All Rights Reserved. Fast Exchange • Hydrogen atoms bonded to oxygen or nitrogen atoms can exchange faster with each other than the time it takes to acquire a 1H-NMR spectrum (OH, NH, CO2H) • Important affected functional groups - Carboxylic acids, alcohols, amines, and amides • Consequences • Signals for exchanging H atoms are generally broad singlets that do not take part in splitting with other signals • Signal will disappear if D2O or a deuterated alcohol is added to the sample • H atoms will be replaced with D atoms, which are 1HNMR silent (D2O can be used to identify fast exchange) ©2018 Cengage Learning. All Rights Reserved. Stereochemistry and Topicity • Stereochemistry can affect chemical shift • Depending on the symmetry of a molecule, otherwise equivalent hydrogens may be: • Homotopic • Enantiotopic • Diastereotopic • Simplest way to visualize topicity (i.e. class above) • Substitute an atom or groups of atoms by an isotope • Decide whether the resulting compound would be (a) the same or (b) different from its mirror image or whether (c) diastereomers are possible ©2018 Cengage Learning. All Rights Reserved. Stereochemistry and Topicity - Homotopic Groups • Atoms or groups on an atom that give an achiral molecule when one of the groups is replaced by an isotope • Have identical chemical shifts under all conditions ©2018 Cengage Learning. All Rights Reserved. Stereochemistry and Topicity - Enantiotopic Groups • Atoms or groups on an atom that give a chiral center when one of the groups is replaced by an isotope • Have identical chemical shifts in achiral environments and different chemical shifts in chiral environments ©2018 Cengage Learning. All Rights Reserved. Stereochemistry and Topicity - Diastereotopic Groups • Atoms or groups on an atom that are bonded to an atom that is bonded to two nonidentical groups, one of which contains a chiral center • When one of the atoms or groups is replaced by an isotope, a new chiral center is created and a set of diastereomers results • Example - Hydrogens of the CH2 group of 2-butanol are diastereotopic • Have different chemical shifts under all conditions ©2018 Cengage Learning. All Rights Reserved. Stereochemistry and Topicity - Diastereotopic Groups • Methyl groups on carbon-3 of 3-methyl-2-butanol are diastereotopic • If a H atom on one of the methyl groups on carbon-3 is substituted with a deuterium, a new chiral center is created • Because there is already one chiral center, diastereomers are now possible • Diastereotopic hydrogens have different chemical shifts under all conditions ©2018 Cengage Learning. All Rights Reserved. Figure 13.28 - 1H-NMR Spectrum of 3-Methyl-2Butanol (500 MHz) • Methyl groups on carbon-3 (c) are diastereotopic and nonequivalent • Appear as two doublets ©2018 Cengage Learning. All Rights Reserved. Example 13.7 - Stereochemistry and Chemical Shift • Indicate whether the highlighted hydrogens in the following compounds are homotopic, enantiotopic, or diastereotopic ©2018 Cengage Learning. All Rights Reserved. Example 13.7 - Solution a. Diastereotopic (near a chiral center) • These hydrogens will have different chemical shifts b. Enantiotopic • These hydrogens will have the same chemical shift except in chiral environments c. Enantiotopic ©2018 Cengage Learning. All Rights Reserved. Example • What is the relationship between Ha and Hb in the following compound? 1. 2. 3. 4. 5. Chemically unrelated Homotopic Enantiotopic Diastereotopic None of these ©2018 Cengage Learning. All Rights Reserved. Example • What is the relationship between Ha and Hb in the following compound? 1. 2. 3. 4. 5. Chemically unrelated Homotopic Enantiotopic Diastereotopic None of these ©2018 Cengage Learning. All Rights Reserved. 13C-NMR Spectroscopy • Almost all 13C atoms in a molecule have only magnetically inactive 12C next to them (1.1%) • 13C—13C signal splitting is not normally observed • According to the (n + 1) rule, a 13C signal is split by the hydrogens bonded to it (e.g. 13C for CH3 as quartet) • There is often significant overlap among signals and splitting patterns can be difficult to determine because coupling 13C– H constants of 100 and 250 Hz are common • The most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode • In a hydrogen-decoupled mode, all 13C signals appear as singlets ©2018 Cengage Learning. All Rights Reserved. Figure 13.29 - Hydrogen-Decoupled 13C-NMR Spectrum of 1-Bromobutane ©2018 Cengage Learning. All Rights Reserved. Figure 13.30 - 13C-NMR Chemical Shifts of Representative Groups (important) • Wider range of chemical shifts than 1H NMR (0-220 ppm vs. 1-10 ppm) • Very uncommon to find non-equivalent C atoms with identical chemical shifts • Characteristic regions: C=O, C(sp2), C(sp3) • Because of long relaxation of 13C nuclei integration cannot be used ©2018 Cengage Learning. All Rights Reserved. Example 13.8 - Predicting 13C-NMR Spectra • Predict the number of signals in a proton-decoupled 13C-NMR spectrum of each compound ©2018 Cengage Learning. All Rights Reserved. Example 13.8 - Solution a. Methyl acetate: three signals (δ 171.37, 51.53, and 20.63) b. 2-Pentanone: five signals (δ 208.85, 45.68, 29.79, 17.35, and 13.68) c. 3-Pentanone: three signals (δ 211.97, 35.45, and 7.92) • 13C NMR gives information about (1) the number of signals in the proton-decoupled spectrum, (2) the chemical shifts of each signal • The chemical shifts of the carbonyl carbons are quite distinctive and in these examples occur at δ 171.37, 208.85, and 211.97 (160-220 ppm) ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following is true of 13C-NMR spectra? 1. 2. 3. 4. 5. The number of carbon atoms in a molecule can be ascertained The number of hydrogen atoms in a molecule can be ascertained Certain functional groups can be deduced from the locations of the peaks Both the number of carbon atoms and the number of hydrogen atoms in a molecule can be ascertained All of these ©2018 Cengage Learning. All Rights Reserved. Example • How many signals will appear in the 13C-NMR spectrum of the following molecule? 1. 2. 3. 4. 5. 1 2 3 4 5 ©2018 Cengage Learning. All Rights Reserved. Example • How many signals will appear in the 13C-NMR spectrum of the following molecule? 1. 2. 3. 4. 5. 3 4 5 6 7 ©2018 Cengage Learning. All Rights Reserved. Example • The painkiller Demerol has the following structure • How many peaks would you expect to see in the 13C-NMR spectrum of this substance? 1. 10 2. 11 3. 12 4. 14 5. 15 ©2018 Cengage Learning. All Rights Reserved. Interpreting NMR Spectra (important) • Alkanes • 1H-NMR chemical shifts fall within the range of δ 0.8–1.7 • 13C-NMR chemical shifts fall within the considerably wider range of δ 10–60 • Alkenes • 1H-NMR signals appear in the range δ 4.6–5.7 • Coupling constants are generally larger for trans vinylic hydrogens (11–18 Hz) when compared with cis vinylic hydrogens (5–10 Hz) • Signal of each vinylic hydrogen in vinyl acetate is predicted to be a doublet of doublets • 13C-NMR signals for sp2 hybridized carbons appear in the range δ 100–150 ppm, which is considerably downfield from sp3 hybridized carbons ©2018 Cengage Learning. All Rights Reserved. Figure 13.31 - 1H-NMR Spectrum of Vinyl Acetate δ 2.12 CH3 (s) δ 4.58 and δ 4.90 vinylic CH δ 7.30 internal olefinic CH shifted downfield because of the adjacent OCOMe (typically 5.0-5.7) • Olefinic CH are distinguished by J couplings (trans 11-18 Hz, cis 5-10 Hz, geminal 0-5 Hz) • a and b as doublets instead of the expected dd (small J) ©2018 Cengage Learning. All Rights Reserved. Interpreting NMR Spectra • Alcohols • Chemical shift of a hydroxyl hydrogen in a 1H-NMR spectrum is variable and depends on the purity of the sample, the solvent, the concentration, and the temperature • Often appears in the range δ 3.0–4.0, but may be as low as δ 0.5 (typically, broad signal) • Hydrogens on the carbon bearing the —OH group are deshielded by the electron-withdrawing inductive effect of the oxygen atom and their signals appear in the range δ 3.4–4.0 ©2018 Cengage Learning. All Rights Reserved. Figure 13.32 - 1H-NMR Spectrum of 1-Propanol ©2018 Cengage Learning. All Rights Reserved. Interpreting NMR Spectra • Ethers • A distinctive feature in the 1H-NMR spectra of ethers is the chemical shift of hydrogens, δ 3.3–4.0, on the carbons bonded to the ether oxygen • Range corresponds to a downfield shift of approximately 2.4 units compared with their normal position in alkanes • Aldehydes and ketones • Signal for aldehyde hydrogens appears between δ 9.5 to 10.1 in the 1H-NMR spectrum • Η atoms on α-carbons of aldehydes and ketones appear around δ 2.2 to 2.6 • In 13C-NMR, carbonyl carbons have characteristic positions between δ 180 to 215 ©2018 Cengage Learning. All Rights Reserved. Interpreting NMR Spectra • Carboxylic acids and esters • Signals for hydrogens on the α-carbon to a carboxyl group in acids and esters appear in a 1H-NMR spectrum in the range δ 2.0 to 2.6 (the same as in aldehydes and ketones) • Hydrogens α to an ester oxygen are strongly deshielded and resonate between δ 3.7 and 4.7 (range for CHnOH) • Hydrogen of a carboxyl group gives a very distinctive signal in the range δ 10 to 13 (typically, broad signal) • 13C resonance of the carboxyl carbon in acids and esters appears in the range δ 165 to 185 (upfield vs. RCHO, RCOR’) ©2018 Cengage Learning. All Rights Reserved. Interpreting NMR Spectra • Amines • In the 1H-NMR spectrum, amine hydrogens appear from δ 0.5 to 5.0 depending on experimental conditions due to hydrogen bonding • Amine hydrogens generally appear as broad singlets • Carbons bonded to nitrogen appear in the 13C-NMR spectrum approximately 20 ppm higher than in alkanes of comparable structure, but about 20 ppm below carbons attached to oxygen in ethers or alcohols (between alkanes and OR) ©2018 Cengage Learning. All Rights Reserved. Figure 13.33 - 1H-NMR Spectrum of 2-Methylpropanoic Acid (Isobutyric Acid) Assignment (vs. identification of an unknown compound) • No. of signals • Integration • Pattern of chemical shifts (where they are = which functional group • Signal splitting ©2018 Cengage Learning. All Rights Reserved. Example • Chemically equivalent nuclei always show a single absorption 1. 2. True False ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following molecules best fits the following 13C-NMR data? • 13C-NMR data: 20, 22, 32, 44, and 67 ppm 1. 2. 3. 4. ©2018 Cengage Learning. All Rights Reserved. Example • Order the following protons from lowest to highest chemical shift value 1. 2. 3. 4. 5. Ha < Hc < Hb < Hd Ha < Hc < Hd < Hb Hc < Ha < Hd < Hb Hc < Ha < Hb < Hd Hc < Hd < Ha < Hb ©2018 Cengage Learning. All Rights Reserved. Spectral Problem 1 • Molecular formula C5H10O 1. 2. 3. 4. 5. Molecular formula (vs. alkane C5H12 = ring or π bond) No. of signals (2) Integration (3:2 = 6H and 4H) Pattern of chemical shifts (alkyl, next to C=O) Signal splitting (t and q) ©2018 Cengage Learning. All Rights Reserved. Spectral Problem 2 • Molecular formula C7H14O 1. 2. 3. 4. 5. Molecular formula No. of signals (3) Integration (9:3:2) Pattern of shifts (sp3 and next to C=O) Splitting pattern (3 x s) ©2018 Cengage Learning. All Rights Reserved.