2. • NMR spectroscopy involves the characteristic absorption of energy by certain
spinning nuclei in a strong magnetic field, when irradiated by a second and
weaker field perpendicular to it, permits the identification of atomic
configurations in molecules.
Introduction
Definition
NMR SPECTROSCOPY
Nuclear magnetic resonance spectroscopy: commonly referred to as NMR, is a
technique which exploits the magnetic properties of certain nuclei to study
physical, chemical, and biological properties of matter
3. Absorption occurs when these nuclei undergo transitions from one
alignment in the applied field to another one.
The amount of energy required to cause a particular nucleus realign
depends on factors as field strength, electronic configuration
around the nucleus, anisotropy, type of molecule and
intermolecular interaction.
4. CHEMICAL SHIFT
This secondary field either opposes or
reinforces the applied field, therefore
applied magnetic field has to change
slightly to bring the nucleus into
resonance, this variations in applied
field and effective field experienced
by proton is explained as chemical
shift.
The value of shift depends upon
chemical environment of the proton
(i.e. aliphatic, aromatic, primary,
secondary, tertiary, halogenated)
Diamagnetic shielding
5. • The chemical environment around the proton is decided by the
electrons present in bond close to the proton and electrons
present in neighbouring bonds.
• These electrons induce their own magnetic field which may
reinforce or oppose the applied magnetic field. In order to
occur NMR, the proton has to flip its spin from lower energy
level to higher energy level (i.e. it has to change its spin ↑ to↓).
6. • If induced magnetic field opposes the applied magnetic field, the
stronger magnetic field required to flip the proton. The proton is
then said to be shielded and the absorption spectra is shifted to
upfield. Electron releasing substances (e.g. Alkyl groups) shields
protons.
7. If induced magnetic field
reinforces the applied magnetic
field, the small external field is
required to flip the spin of
proton. Such proton is said to be
deshielded and the absorption
spectra is shifted towards
downfield. Electron
withdrawing substances (e.g.
halogens) deshields protons
Shielding (+),
Deshielding (-)
8. Differently situated proton in a molecule may be
magnetically non-equivalent and come successively in to
resonance as the field is changed by the sweep coils. These
differences in positions of resonance are called chemical
shifts.
9. Measurement of chemical shift
• Chemical shift is measured in frequency unit “Herz”.
Most of routine instruments operate at 60,80,100 MHz.
More sophisticated instrument operates as high as
600MHz.
• The chemical shift recorded in the Hz may vary with the
spectrometer. To avoid this complication the chemical
shift value are expressed in terms of delta and tau scale.
which are independent of field strength Chemical shift in
delta scale are expressed in parts per million (ppm).
10.
11.
12. FACTORS INFLUENCING CHEMICAL SHIFT
Electro negativity and Inductive Effect
Hydrogen Bonding
Effect of Magnetic Anisotropy – Space Effect
Van der Waal’s de-shielding
