Nuclear Magnetic Resonance (NMR) Spectroscopy, Chapter 7

Chapter 7 NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY NMR is concerned with change in the direction of spin orientation of Nuclei of atoms rather than outer electrons absorption of by resonant radiofrequency radiation of 4 to 900 MHz. NMR the most powerful tools for elucidating the structure of chemical species; by gives an information/Identifies about the number and types of nucleus atoms (H, C,…) and their relative number in a molecule. Quantum Description of NMR: The maximum number of spin components or values for a particular nucleus is its spin quantum number I. The nucleus will then have 2I + 1 discrete states. The four nuclei that have been of greatest use are 1H, 13C, 19F, and 31P. 1 7.1. Basic principle of NMR Nuclear Spin States: Any atomic nucleus that has an odd mass and/or an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment.  The allowed nuclear spin states are determined by the spin quantum number, I , for these nuclei is ½. Thus, each nucleus has two spin states corresponding to I = + ½ and I = - ½. A spinning, charged nucleus creates a magnetic field. A spinning nucleus such as 1H behaves as a spinning charge and generates a magnetic field. It can be likened to a bar magnet. When it is placed in an externally applied field it can align with, or against, the field. The energy difference between the two states (DE) depends on the applied field. • • • • • • POSSESS SPIN DON’ T POSSESS SPIN 1 H 1 12 C 6 2 H 1 13 C 6 19 F 9 31 P 15 2 Two variables characterize NMR: 1.Applied magnetic field B0, measured in tesla (T), and 2.the frequency ν of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz=106 Hz). A nucleus is in resonance when it absorbs RF radiation and spin flips to higher energy state. Common types of NMR spectroscopy 1H-NMR and 13C-NMR:: determine the type and number of H and C atoms respectively in a molecule; The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency. When lowenergy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1H and 13C. g-rays x-rays UV VIS IR 10-10 10-8 m-wave radio 10-6 10-4 10-2 wavelength (cm) 100 102 3 7.2. Component and Instrumentation of NMR spectrometers Components of an NMR spectrometer are a 1. powerful magnet that supply range of magnetic fields variable by sweep generator. 2. a radio-frequency generator 3. a radio-frequency receiver 4. a radio-frequency detector and 5. a sample holder Instrumentation of NMR spectrometers 4 Sample and Solvent  The sample (5-50 mg) is dissolved in a solvent (~0.5 mL), most commonly CDCl3 or D2O, and placed in a sample tube (5 mm diam.) which is then suspended in the magnetic field and set spinning.  Ideal solvent: no proton, inexpensive, low boiling, non-polar & inert. Deuterated solvents are used to eliminate 1H signals from the solvent. Like: CCl4, CDCl3, CS2 Reference compound It is customary to measure the resonance frequency (signal) of individual nuclei relative to the resonance frequency (signal) of a reference compound; now universally accepted is tetramethylsilane (TMS).  TMS is added to the sample (as reference). Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.  Organic protons absorb downfield (to the left) of the TMS signal. 5 TETRAMETHYLSILANE –TMS           It PROVIDES THE REFERENCE SIGNAL non-toxic liquid - SAFE TO USE inert - DOESN’T REACT WITH COMPOUND BEING ANALYSED has a low boiling point - CAN BE DISTILLED OFF AND USED AGAIN all the hydrogen atoms are chemically equivalent - PRODUCES A SINGLE PEAK twelve hydrogens so it produces an intense peak - DON’T NEED TO USE MUCH signal is outside the range shown by most protons - WON’T OBSCURE MAIN SIGNALS given the chemical shift of δ = 0 the position of all other signals is measured relative to TMS. The molecule contains four methyl groups attached to a silicon atom in a tetrahedral arrangement. All the hydrogen atoms are chemically equivalent. H.U 6 Types of NMR Spectra There are several types of NMR spectra, which depend upon the kind of instrument used, the type of nucleus involved, the physical state of the sample, the environment of the analyte nucleus, and the purpose of the data collection. Most NMR spectra can be categorized as either wide line or high resolution. • Wide-Line spectra: Wide-line spectra are those in which the bandwidth of the source of the lines is large enough so that the fine structure due to chemical environment is hidden. A single peak is associated with each species. 7 High-Resolution Spectra: Most NMR spectra are high resolution and are collected by instruments capable of differentiating between very small frequency differences of 0.01 ppm or less. In the lower-resolution spectrum for the protons in ethanol, three peaks are observed arising from absorption by the CH3, CH2, and OH protons. But in the higher resolution spectrum, two of the three peaks can be resolved into additional peaks. 7.3. Interpretation of NMR Spectra NMR spectra provide information about the structure of organic molecules from the ... A. number of different signals in the spectrum B. position of the signals (chemical shift) C. intensity of the signals D. splitting pattern of the signals 7.3.1. Information from 1H-NMR spectra: A. Number of signals: How many different types of hydrogen's in the molecule. Number of Signal: CH3 H3C C CH3 Br one CH3CH2CH2-Br three CH3CH2-Br two CH3CHCH3 Cl two 9 B. Position of signals (chemical shift) (σ) : The resonant frequency of a certain atom convenience, primary secondary tertiary aromatic allyl benzyl chlorides bromides iodides alcohols alcohols is called chemical shift. what types of hydrogens? For the chemical shift is expressed as follows: 0.9 ppm 1.3 n(sample) _n(reference) 6 (ppm) = 10 • 1.5 spectrometer frequency 6-8.5 = 106 . Shift in frequency from TMS (Hz) 1.7 Frequency of spectrometer (Hz) 2.2-3 3-4 H-C-Cl 2.5-4 H-C-Br 2-4 H-C-I a a b CH3 3.4-4 H-C-O CH3CH2-Br H3C C CH3 1-5.5 H-Oa a Br a b c CH3CH2CH2-Br a b a CH3CHCH3 Cl 10 Factors Affecting Chemical Shift Other factors are Acidic Hydrogens: Some of the least-shielded protons are those attached to carboxylic acids. These protons have their resonances at 10 to 12 ppm. Both resonance and the electronegativity effect of oxygen withdraw electrons from the acid proton. Hydrogen Bonding and Exchangeable Hydrogens Protons that can exhibit hydrogen bonding (e.g., hydroxyl or amino protons) exhibit extremely variable absorption positions over a wide range. The more H.U deshielded a proton becomes. 11 hydrogen bonding that takes place, the more 11 C. Integration (relative areas under each signal): to answer how many hydrogen's of each type.  the area under a signal is proportional to the number of hydrogen atoms  an integration device scans the area under the peaks  lines on the spectrum show the relative abundance of each hydrogen type (old method)  computers now display the information as numerical data  By measuring the distances between the integration lines one can work out the simple ratio between the various types of hydrogen. a CH3 H3C C CH3 a a Br a a 9H b c CH3CH2CH2-Br a 3H b 2H c 2H a b CH3CH2-Br a b 3H 2H a b a CH3CHCH3 Cl a b 6H 1H 12 Conti--- Measure how much each integration line rises as it goes of a set ofsignals. Compare the relative values and work out the simple ratio between them. In the above spectrum the rises are in the ratio...1:2:3 The signal due to the hydroxyl (OH) hydrogen is a singlet ... there is no splitting H’s on OH groups do not couple with adjacent hydrogen atoms D. Splitting pattern: how many neighboring hydrogen's? Peaks are often split into multiple peaks due to magnetic interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting. The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule”. The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle Pascal pattern: 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 singlet doublet triplet quartet pentet hextet heptet 13 Note: n must be equivalent neighboring hydrogens to give rise to a Pascal splitting pattern. If the neighbors are not equivalent, then you will see a complex pattern (aka complex multiplet). a CH3 H3C C CH3 a a Br a CH3CH2-Br a 3 H triplet b 2 H quartet a 9 H singlet a b c a b a CH3CHCH3 Cl CH3CH2CH2-Br a 3 H triplet b 2 H complex c 2 H triplet 2-bromobutane b a 6 H doublet b 1 H septet b d c a CH3CHCH2CH3 Br a triplet 3H c complex 2H b doublet 3H d complex 1H Marye M. 14 ethyl bromide 1-bromopropane isopropyl chloride a b CH3CH2-Br a triplet 3H b quartet 2H a b c CH3CH2CH2-Br a triplet 3H b complex 2H c triplet 3H a b a CH3CHCH3 Cl a doublet 6H b septet 1H H.U 15 7.3.2. Information from 13C – NMR spectra: After hydrogen, the most useful atom providing information is carbon-13. Natural carbon contains about 1% of this isotope so the instruments for its detection need to be sensitive and spectra will take longer to record and Provides a count of the different types of environments of carbon atoms in a molecule are measured in ppm (d) from the carbons of TMS 13C resonances are 0 to 220 ppm downfield from TMS Chemical shift affected by electronegativity of nearby atoms O, N, halogen decrease electron density and shielding (“deshield”), moving signal downfield. sp3 C signal is at  0 to 9; sp2 C:  110 to 220 C(=O) at the low field,  160 to 220  • Recall that splitting occurs when two NMR active nuclei—like two protons—are close to each other. Because of the low natural abundance of 13C nuclei (1.1%), the chance of two 13C nuclei being bonded to each other is very small (0.01%), and so no carbon-carbon splitting is observed.16 •A 13C NMR signal can also be split by nearby protons. This 1H-13C splitting is usually eliminated from the spectrum by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13C NMR spectrum appears as a singlet. •The two features of a 13C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals. 13C NMR—Number of Signals • The number of signals in a 13C spectrum gives the number of different types of carbon atoms in a molecule. • Because 13C NMR signals are not split, the number of signals equals the number of lines in the 13C spectrum. • In contrast to the 1H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13C NMR signals are not integrated. 17 17 13C NMR—Position of Signals • In contrast to the small range of chemical shifts in 1H NMR (1-10 ppm usually), 13C NMR absorptions occur over a much broader range (0-220 ppm). • The chemical shifts of carbon atoms in 13C NMR depend on the same effects as the chemical shifts of protons in 1H NMR. 18 1H and 13C NMR compared:  both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons)  both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.)  the signal for the NMR of a 13 C nucleus is 10-4 times weaker than the signal for a hydrogen nucleus  a signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and  at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)  13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei 19 C13-NMR: coupled to proton OH O HC C singlet CH3 CH2 HO doublet C13-NMR : proton decoupling OH O HC C HO CH2 CH3 quartet Triplet