Chapter 13 - Nuclear Magnetic Resonance Spectroscopy

  • How can we know how many signals to expect in a compound's 1H-NMR spectrum given its structural formula?
    • The solution is that equivalent hydrogen produce the same 1H-NMR signal, but nonequivalent hydrogens produce various 1H-NMR signals.
    • The chemical environment of equivalent hydrogens is the same.
    • If one of the following two criteria exists, H atoms are comparable (in the same chemical environment): They are bound to the same sp3 hybridized carbon atom, which may freely spin at ambient temperature.
  • Because of the quick bond rotation, the H atoms linked to the same carbon atom experience the same chemical environment and are hence equal.
    • For example, ^^on a freely spinning —CH3 group, all three H atoms are equal, and both H.^^
    • ^^They are linked by symmetry, specifically a molecule's plane or point of symmetry.^^
  • In the 2-chloropropane molecule, for example, all six methyl group H atoms are equal.
    • The methyl groups rotate freely and are connected by a plane of symmetry, as depicted (as shown in the image attached).
    • The replacement test is a simple approach to assess whether two hydrogen atoms are comparable.
    • Replace each of the hydrogen atoms in question with a test atom in your thoughts (e.g., chlorine).
  • The image attached shows that If the substitution produces the same molecule in each case, then the initial hydrogen atoms are equal.

   

  • Structure of 2-chloropropane showing the plane of symmetry responsible for making the two methyl groups (and therefore the six methyl group H atoms) equivalent.
  • There are four sets of comparable hydrogens in 2-methyl butane.
    • This molecule has nine main hydrogens: three sets of three and six sets of six.
  • ^^To demonstrate the existence of two sets, consider that replacing any hydrogen in the set of three with chlorine yields 1-chloro-3-methylbutane.^^
    • Any hydrogen in the set of six can be replaced with chlorine to produce 1-chloro-2-methylbutane.
    • Furthermore, the molecule has two comparable secondary hydrogens and one tertiary hydrogen.
  • You should be able to understand right away that counting the amount of signals in a compound's 1H-NMR spectrum can provide vital information about its molecular structure.
  • Consider the two constitutional isomers of the chemical formula C2H4Cl2.
    • In its 1H-NMR spectrum, the molecule 1,2-dichloroethane exhibits one set of equivalent hydrogens and one signal.
  • %%In its 1H-NMR spectrum, its constitutional isomer 1,1-dichloroethane contains two sets of equivalent hydrogens and two signals.%%
    • As a result, merely counting signals allows you to differentiate between these two molecules.
  • The chemical shift of a certain kind of hydrogen is mostly determined by the amount of shielding it receives. Shielding is determined by three factors:
    • (1) the electronegativity of surrounding atoms,
    • (2) hybridization of adjacent atoms, and
    • (3) magnetic induction inside an adjacent p bond. Let us take each of these things one at a time.
  • %%An electronegative substituent's impact fades fast with distance.%%
  • The impact of an electronegative substituent two atoms distant is only around 10% of that of an electronegative substituent on the nearby atom.
    • An electronegative substituent three atoms distant has essentially little impact.
  • Electronegativity and chemical shift are connected in the following way: the presence of an electronegative atom or group decreases electron density on atoms bound to it, resulting in less shielding.
    • This action deflects neighboring nuclei, causing them to resonate further downfield (i.e., with a larger chemical shift).
    • Ring current: refers to An applied magnetic field causes the p electrons of an aromatic ring to circulate giving rise to the so-called ring current and an associated magnetic field that opposes the applied field in the middle of the ring but reinforces the applied field on the outside of the ring.
  • The vicinal atom Hb, whose nuclear spin may be aligned with or against an applied magnetic field in a 1H-NMR spectrometer, influences nuclear spin and hence the chemical shift of the atom labeled Ha in the image attached above.
  • Because of spin-spin coupling, aligning the Hb nuclear spin with the applied magnetic field results in a slightly different chemical shift of the signal for Ha than aligning the Hb nuclear spin against the applied magnetic field.
    • A comparable amount of molecules in a population of molecules in a sample will have each spin alignment for Hb.
    • Any single molecule produces a single Ha signal, but the spectrum of the overall sample contains both.
  • The image attached shows ​​a coupling that arises when Hb is split by two different nonequivalent H atoms Ha and Hc%%.
    • This analysis assumes that there is no other coupling in the molecule and that Jab ? Jbc .%%

   

  • So far, we've focused on spin-spin coupling with only one additional set of non-equivalent H atoms.
    • ^^In molecules that do not have quick bond rotation, however, more complicated circumstances frequently emerge in which the nuclei of a set of H atoms are connected to the nuclei of more than one set of nonequivalent H atoms.^^
    • %%In these cases, the connection between nearby non-equivalent sets of H atom nuclei combines to produce more complicated signal splitting patterns.%%
  • In many circumstances, using a tree diagram might assist you to comprehend splitting.
    • The various couplings are applied successively in a tree diagram.