470
M. Rashid et al.
high-resolution magnetic resonance (NMR) spectroscopy by Swiss scientists Richard
Robert Ernsts for which he was awarded the 1991 Noble Prize in Chemistry [2b].
Ernst’s work not only made NMR spectroscopy basic and essential tool in structural
determination of compounds in chemistry and also extends its applications in Physics,
biology and medical diagnosis [3].
Aim of this chapter is to describe theory of nuclear magnetic resonance (the NMRphenomenon), instrumentation, interpretation of basic NMR techniques—proton
(
1 H) and carbon (
13 C) spectra of organic/inorganic compounds; brief introduction of
DEPT, fluorine (
19 F), phosphorus (
31 P) NMR techniques and detailed applications
of NMR technique in various fields.
Analyzing the NMR spectrum of organic compounds enable us to obtain three
kinds of information about the molecule:
1. Differences in magnetic properties of nuclei present in the molecule i.e. types of
nuclei and their position within that molecule.
2. Different kinds of atomic environment in the molecule and nature of neighboring
atoms.
3. Number of atoms present into each of the specific environments.
The phenomenon of NMR was first observed on the simplest nucleus i.e. proton
by Bloch and Purcell and hence most appropriate way to learn basics and theory
of NMR interpretation is to study proton-NMR (
1 H-NMR). Much of the theory
and instrumentation relevant to proton-NMR is also applicable to other NMR-active
nuclei—
13 C,
31 P,
19 F,
15 N,
17 O etc. [1, 4–7].
2 Proton-NMR ( 1 H-NMR) Spectroscopy
2.1 Theory of Proton NMR-Spectroscopy
Nuclear Spin. The nuclei of isotopes of some elements have mechanical spin around
the axis just like child’s top and magnetic field and thus behave as tiny bar magnets.
Magnetic Field and Precessional Motion. The protons being a tiny bar magnets can
be influenced by external magnetic field and oriented themselves into two possible
orientations—either aligned with the field (the lower energy state) or opposed to the
field (the higher energy state) (Fig. 1).
The secondary effect of strong magnetic field is that, it exerts a torque on the
spinning nucleus. This causes the nuclear spin to perform ‘waltz like motion’ in
which the spinning axis of the nucleus moves around the axis of an applied external
field called ‘precessional motion’. The orientation of motion can either be aligned
with the field (low energy) or opposed to the field (high energy) (Fig. 2).
The frequency of spinning motion of nucleus does not change but frequency of
precessional motion change. The precessional frequency υ is directly proportional
to the strength of external magnetic field B 0 that is,
M. Rashid et al.
high-resolution magnetic resonance (NMR) spectroscopy by Swiss scientists Richard
Robert Ernsts for which he was awarded the 1991 Noble Prize in Chemistry [2b].
Ernst’s work not only made NMR spectroscopy basic and essential tool in structural
determination of compounds in chemistry and also extends its applications in Physics,
biology and medical diagnosis [3].
Aim of this chapter is to describe theory of nuclear magnetic resonance (the NMRphenomenon), instrumentation, interpretation of basic NMR techniques—proton
(
1 H) and carbon (
13 C) spectra of organic/inorganic compounds; brief introduction of
DEPT, fluorine (
19 F), phosphorus (
31 P) NMR techniques and detailed applications
of NMR technique in various fields.
Analyzing the NMR spectrum of organic compounds enable us to obtain three
kinds of information about the molecule:
1. Differences in magnetic properties of nuclei present in the molecule i.e. types of
nuclei and their position within that molecule.
2. Different kinds of atomic environment in the molecule and nature of neighboring
atoms.
3. Number of atoms present into each of the specific environments.
The phenomenon of NMR was first observed on the simplest nucleus i.e. proton
by Bloch and Purcell and hence most appropriate way to learn basics and theory
of NMR interpretation is to study proton-NMR (
1 H-NMR). Much of the theory
and instrumentation relevant to proton-NMR is also applicable to other NMR-active
nuclei—
13 C,
31 P,
19 F,
15 N,
17 O etc. [1, 4–7].
2 Proton-NMR ( 1 H-NMR) Spectroscopy
2.1 Theory of Proton NMR-Spectroscopy
Nuclear Spin. The nuclei of isotopes of some elements have mechanical spin around
the axis just like child’s top and magnetic field and thus behave as tiny bar magnets.
Magnetic Field and Precessional Motion. The protons being a tiny bar magnets can
be influenced by external magnetic field and oriented themselves into two possible
orientations—either aligned with the field (the lower energy state) or opposed to the
field (the higher energy state) (Fig. 1).
The secondary effect of strong magnetic field is that, it exerts a torque on the
spinning nucleus. This causes the nuclear spin to perform ‘waltz like motion’ in
which the spinning axis of the nucleus moves around the axis of an applied external
field called ‘precessional motion’. The orientation of motion can either be aligned
with the field (low energy) or opposed to the field (high energy) (Fig. 2).
The frequency of spinning motion of nucleus does not change but frequency of
precessional motion change. The precessional frequency υ is directly proportional
to the strength of external magnetic field B 0 that is,