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Chapter 14 Notes.docx

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University of Toronto Scarborough
Wanda/ Lana

14.1 An Introduction to NMR Spectroscopy  NMR Spectroscopy is used to study the structure of organic compounds o Identify carbon-hydrogen framework  ONLY nuclei with an odd number of protons or odd number of neutrons or both have a spin property o Because protons were the first nuclei studied, the acronym “NMR” is generally assumed to mean H NMR (proton magnetic resonance)  In presence of an applied magnetic field, the magnetic moments align either with or against the applied magnetic field o α-spin state – aligned with the applied magnetic field = low energy o β-spin state – aligned against the applied magnetic field = high energy  More nuclei are in the α-spin state o Differences in populations forms the basis of NMR Spectroscopy  Energy difference (∆E) between α- and β-spin states depends on strength of applied magnetic field (0 )  Sample is subjected to radio frequency (rf) radiation whose energy corresponds to the ∆E between the α- and β-spin states, then nuclei in the α-spin state are promoted to the β-spin state (called “flipping”) o When nuclei flip they generate signals whose frequency depends on the ∆E between the α- and β-spin states  NMR spectrometer detects these signals  Generates NMR Spectrum o Plot of signal frequency vs. Intensity o *NOTE: Right-hand side of NMR spectrum is the low-frequency side and left-hand side is the high-frequency side  Nuclei are said to be in “resonance” with the rf radiation  Hence “Nuclear Magnetic Resonance” or NMR   The magnetic field is proportional to the operating frequency o Which is directly proportional to ∆E 14.3 Shielding Causes Different Hydrogens to Show Signals at Different Frequencies  Hydrogen atoms within an organic compound do not experience the same magnetic field o Nucleus is embedded within electron cloud  Partially shields from applied magnetic field  Effective magnetic field – Amount of magnetic field that the nuclei actually “sense” o o The larger the magnetic field sensed by a proton, the higher is the frequency of the signal  Increased  Electron-rich environment = more shielded = low frequency  Electron-poor environment = less shielded = high frequency 1 14.4 The Number of Signals in an H NMR Spectrum  Protons in same environment are chemically equivalent protons 1 o Each set of chemically equivalent protons in a compound gives rise to a signal in the H NMR spectrum of that compound  Number of sets of chemically equivalent protons a compound has = the number of signals in its H NMR spectrum  If bonds are prevented from freely rotating (eg. Compounds with double bonds) two protons on same carbon may not be equivalent 14.5 The Chemical Shift Tells How Far the Signal Is from the Reference Signal  Inert reference compound is added to sample tube containing the compound whose NMR spectrum is to be taken o Tetramethylsaline (TMS) is commonly used  Highly volatile, easy to evaporate  TMS protons are more electron-rich than most other protons  Si is less electronegative than carbon o More shielded = lower frequency  Chemical shift – position at which a signal occurs in the NMR spectrum o Measure of how far the signal is from the signal for the reference compound o Scale measured with the δ scale (in ppm)  TMS defines the zero position on the scale  Has low frequency due to electropositive Si atom  Most proton chemical shifts are between 0 and 12 ppm  To measure chemical shift: o Measure distance from TMS peak in hertz and dividing by the operating frequency in megahertz ( ) o ( ) ( ))  The greater the value of the chemical shift (δ), the higher the frequency  Advantage of the δ scale is that the chemical shift of a given nucleus is independent of the operating frequency of the NMR spectrometer 14.6 The Relative Positions of H NMR Signals  Protons in electron-poor environments show signals at higher frequencies o  Electron withdrawal causes NMR signals to appear at higher frequencies (at larger δ values) o Depends on proximity to electron withdrawing group and electronegativity of electron withdrawing group 14.7 The Characteristic Values of Chemical Shifts  Methine – sp carbon bonded to 3 carbons  Methylene – sp carbon bonded to 2 carbons 3  Methyl – sp carbon bonded to 1 carbon  (Highest frequency) Methine protons > Methylene protons > Methyl protons (Lowest frequency) o Methine proton are more deshielded  Have more carbons which decrease electron density through inductive electron withdrawal  Deshielded = higher frequency 14.9 The Integration of NMR Signals Reveals the Relative Number of Protons Causing each Signal  Area under each signal is proportional to the number of protons giving rise to the signal  An H NMR spectrometer is equipped with a computer that calculates the integrals of signals  Integration tells us the relative number of protons that give rise to each signal, not the absolute number 14.10 The Splitting of the Signals is Described by the N + 1 Rules  Shape of a signal may differ by the number of peaks o Multiplicity – the number of peaks in a signal  Can be described by the N + 1 rule  Singlet = 1 peak  Doublet = 2 peaks  Quartet = 4 peaks 1  An H NMR signal is split into N + 1 peaks o N is the number of equivalent protons bonded to adjacent carbons  NOTE: By “equivalent protons” we mean the number of protons bonded to an adjacent carbon are equivalent to each other, but not equivalent to the proton giving rise to the signal  Splitting is always mutual o If the a protons split the b protons, then the b protons must split the a protons  The a and b protons, in this case, are coupled protons  Coupled protons split each other’s signal  Coupled protons are bonded to adjacent carbons  Equivalent protons do not split each other’s signal  Spin-spin coupling – the splitting of a signal because of different kinds of protons that are close enough for their magnetic fields to influence one another  The relative intensities of the peaks reflect the number of ways the neighbouring protons can be aligned relative to the applied magnetic field o eg. A quartet has relative peak intensities 1:3:3:1  Relative peak intensities conform to Pascal’s triangle  Usually non-equivalent protons split each other’s signal only if they are on adjacent carbons o A “through-bond” effect, not a “through-space” effect  Rarely observed if the protons are more than three sigma bonds away  Exception: If separated by more than three sigma bonds but one of the bonds is a double or triple bond, a small splitting is sometimes observed o Called “long-range coupling” Table 14.2 Multiplicity of the Signal and Relative Intensities of the Peaks in the Signal Number of equivalent protons Multiplicity of the signal Relative peak intensities causing splitting 0 singlet 1 1 doublet 1 : 1 2 triplet 1 : 2 : 1 3 quartet 1 : 3 : 3 : 1
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