Nuclear Magnetic Resonance Spectrometry

Introduction to Nuclear Magnetic Resonance Spectrometry

Nuclear magnetic resonance (NMR) is a form of absorption spectrometry akin to IR and UV spectrometry.
Under appropriate condition in a magnetic field, a sample can absorb electromagnetic radiation in the radio frequency range (rf) region at frequencies governed by the characteristics of the sample.
A plot of the frequencies of the absorption peaks verses peak intensities constitute NMR spectrum.

Basic Theory

• All nuclei carry a charge
• In some nuclei, this charge “spins” on the nuclei axis (see. fig. 1)
• The circulation of nuclear charges
  generate a magnetic dipole along the axis
• The angular momentum of the spinning charge can be described in terms of its quantum spin number I
• These numbers have values 0, 1/2, 1, 3/2 and so on (Note that I = 0 denotes no
  spin).
• Nuclei that have spin number of the
  value 1/2 usually have spherical charge distribution.
• The spectra of such nuclei (i.e.
  1H, 3H, 13C, 15N, 19F, 31P) can be readily obtained.

 

• In quantum mechanical terms, the spin number I determines the number of orientations a nucleus may assume in an external uniform magnetic field in accordance with the formula (2I +1).
• In NMR spectrometry the interest is with nuclei whose spin number I is 1/2.
• For such nuclei, in external magnetic field, there are two energy levels
• A slight excess of nuclei population in the lower energy state (Nα > Nβ) is observed
• Nα equals the population of nuclei in lower energy level
• Nβ the population at higher energy level) in accordance with Boltzman distribution.

• The energy of excitation is given as: ΔE = (hγ/2π) Bo
• Once two energy levels for the proton have been established,
• It is possible to introduce energy in the form of radio frequency (ν1) to effect a transition between these energy levels in a stationary magnetic field of a given strength Bo.
• The fundamental NMR equation connecting the applied radio frequency ν1 with the magnetic field strength is stated thus:

ν1 = (γ/2π) Bo

 since ΔE = hν

Proton Nuclear Magnetic Resonance Spectrometry (HNMR)

• Proton is one of the nuclei with spin number I= ½
•  As such it can assume two energy levels when placed in external magnetic field of uniform strength.
• Proton is ubiquitous in organic compounds and this makes HNMR a versatile and the most widely used spectrometry in structure elucidation of organic compounds.

Solvent selection in HNMR

• The ideal solvent for the measurement of HNMR should contain no proton, be inert, have low boiling and be inexpensive.
• Deuterated solvents are frequently used in NMR measurements.
• Deuterium has spin number I = 1 and as such cannot presses in magnetic field.
• Besides, deuterated solvents are necessary for modern NMR
  instruments because they depend on deuterium signal to lock or
  stabilize B0 field of the magnet.
• Examples of deuterated solvents include:
  – deuterated water (D2O),
  – deuterated chloroform (CDCl3), deuterated methanol (CD3OD),
  – deuterated DMSO (DMSO-d6) etc.
• Deuterated chloroform is used when circumstances permit – in fact most of the time. The small sharp proton
  peak at δ 7.26 from CHCl3 impurity present rarely interferes seriously.
• Traces of ferromagnetic impurities cause severe broadening of absorption peaks.
• Common sources are rust particles from tap water, steel wool, Raney nickel and particles from metal spatula and fittings.
• Traces of common laboratory solvents can be annoying.
• Other offenders are greases and plasticizers (phthalate in particular).

Chemical Shift in NMR Spectroscopy

• Going by the basic NMR equation
  stated below,
  – ν1 = (γ/2π) Bo
• only a single proton peak should be expected from the interaction of radio frequency energy and a strong magnetic field on all protons.
• Fortunately, the situation is not
  that so simple.
• A proton in a molecule is shielded
  to a very small extent by its
  electron cloud.
• The density of this electron cloud
  varies with the “chemical
  environment”.
• These variations gives rise to
  differences in chemical shift
  positions.

• The basic NMR equation for all protons is thus modified for an
  assemblage of equivalent protons in a molecule

νeff = (γ/2π) Bo(1-σ)

• Where σ is the shielding constant whose value is proportional to the
  degree of shielding by its electron cloud.
• Electrons under the influence of magnetic field circulate and in
  circulating generate their own magnetic field opposing the applied
  field, hence the shielding effect.
• “Chemical shift” of a proton is thus defined as the difference in
  absorption positions of that particular proton from the absorption position of a reference proton
• Protons in “different” chemical environment thus have different
  chemical shifts.
• Conversely, protons in the “same” chemical environment have the
  same chemical shift.
• The most generally useful reference compound is tetramethylsilane (TMS) – Si(CH3)4
• This compound gives a single, intense, sharp absorption peak and its protons are more “shielded” than almost all
  organic protons.
• By convention, the TMS reference peak is placed at the right hand edge of spectrum and designated zero on either Hz or δ scale (Figure 4).
• Positive Hz or δ numbers increases to the left of TMS, negative numbers increases to the right.
• The term “shielded” means towards the right (“upfield”); “deshielded” means towards the left (downfield).

Factors affecting chemical shifts

A. Inductive effect

• The degree of shielding of a proton in a carbon atom will depend on the inductive effect of other groups attached to the carbon.
• Groups (i.e. O, N, Cl etc) that cause negative inductive effect (electron withdrawing) will definitely cause more deshielding of the proton in question.
• Thus by intuition, protons in  CH3CH2OH will exhibit different chemical shifts.
• The two methylene protons will be more deshielded than the three methyl protons since the influence of inductive effect of the OH group will be more on the α carbon than on the β carbon.
• The concept of electronegativity of substituents near the proton in question is thus, a dependable guide (at least up to a point) to a chemical shift.

B. Diamagnetic anisotropy

• The influence of the π electrons of a bond can cause shielding or deshielding of a particular proton in question.
• In the case of acetylene
• The molecule is linear and the triple bond is symmetrical about the axis.
• If this axis is alliend with the applied magnetic field, the π electrons of the bond can circulate at right angles to the applied field
• Thus inducing their own magnetic field opposing the applied field.
• Since the proton lie along the magnetic axis, the magnetic lines of force induced by circulating electrons act to shield the protons and the NMR peaks if found further to the right (δ 1.80) than electronegativity would predict.
• This effect depends on diamagnetic anisotropy, meaning that shielding or deshielding depends on the orientation of the molecule with respect to the applied magnetic field
• Similar argument as above can thus be adduced to rationalize the unexpected deshielded position of aldehyde proton.
• In this case, the effect of the applied magnetic field is greatest along the transverse axis of C=O bond.
• The geometry is thus that the aldehyde proton, which lies in front of the page is in deshielded portion of the induced magnetic field.
• The same argument can also be used to account for at least part of the rather deshielding of alkene protons (δ 5.25).
• The so called ring “ring current effect” is another example of diamagnetic anisotropy and accounts for the large deshielding of benzene protons

References

1. Chemical Shift. Wikipedia. Accessed August 8, 2021
2. Nuclear Magnetic Resonance Spectrometry. Accessed August 8, 2021