is possible for some of them to be degenerate. For example, in a linear triatomic molecule there are four
normal modes of vibration since
3N – 5 = 4 for N = 3. These vibrational
modes are: (a) symmetric stretching
(breathing) vibrations; (b) antisymmetric stretching vibrations; and (c)
two bending vibrations, which are
degenerate.
N.T.P. See s.t.p.
n-type semiconductor See semiconductor.
nuclear magnetic resonance
(NMR) The absorption of electromagnetic radiation at a suitable precise
frequency by a nucleus with a
nonzero magnetic moment in an external magnetic Üeld. The phenomenon occurs if the nucleus has
nonzero *spin, in which case it behaves as a small magnet. In an external magnetic Üeld, the nucleus’s
magnetic moment vector precesses
about the Üeld direction but only certain orientations are allowed by
quantum rules. Thus, for hydrogen
(spin of ½) there are two possible
states in the presence of a Üeld, each
with a slightly different energy. Nuclear magnetic resonance is the absorption of radiation at a photon
energy equal to the difference between these levels, causing a transition from a lower to a higher energy
state. For practical purposes, the difference in energy levels is small and
the radiation is in the radiofrequency
region of the electromagnetic spectrum. It depends on the Üeld
strength.
NMR can be used for the accurate
determination of nuclear moments.
It can also be used in a sensitive form
of magnetometer to measure magnetic Üelds. In medicine, magnetic
resonance imaging (MRI) has been developed, in which images of tissue
are produced by magnetic-resonance
techniques.
The main application of NMR is as
a technique for chemical analysis
and structure determination, known
as NMR spectroscopy. It depends on
the fact that the electrons in a molecule shield the nucleus to some extent from the Üeld, causing different
atoms to absorb at slightly different
frequencies (or at slightly different
Üelds for a Üxed frequency). Such
effects are known as chemical shifts.
There are two methods of NMR spectroscopy. In continuous wave (CW)
NMR, the sample is subjected to a
strong Üeld, which can be varied in a
controlled way over a small region. It
is irradiated with radiation at a Üxed
frequency, and a detector monitors
the Üeld at the sample. As the Üeld
changes, absorption corresponding to
transitions occurs at certain values,
and this causes oscillations in the
Üeld, which induce a signal in the detector. Fourier transform (FT) NMR
uses a Üxed magnetic Üeld and
the sample is subjected to a highintensity pulse of radiation covering
a range of frequencies. The signal
produced is analysed mathematically
to give the NMR spectrum. The most
common nucleus studied is
1
H. For
instance, an NMR spectrum of
ethanol (CH 3 CH 2 OH) has three peaks
in the ratio 3:2:1, corresponding to
the three different hydrogen-atom
environments. The peaks also have a
Üne structure caused by interaction
between spins in the molecule. Other
nuclei can also be used for NMR spectroscopy (e.g.
13 C,
14 N,
19 F) although
these generally have lower magnetic
moment and natural abundance than
hydrogen. See also electron paramagnetic resonance; endor.
nuclear magneton See magneton.
nuclear Overhauser effect (NOE)
An effect in *nuclear magnetic reso379
nuclear Overhauser effect
n
www.AzShimi.ir www.AzShimi.com
normal modes of vibration since
3N – 5 = 4 for N = 3. These vibrational
modes are: (a) symmetric stretching
(breathing) vibrations; (b) antisymmetric stretching vibrations; and (c)
two bending vibrations, which are
degenerate.
N.T.P. See s.t.p.
n-type semiconductor See semiconductor.
nuclear magnetic resonance
(NMR) The absorption of electromagnetic radiation at a suitable precise
frequency by a nucleus with a
nonzero magnetic moment in an external magnetic Üeld. The phenomenon occurs if the nucleus has
nonzero *spin, in which case it behaves as a small magnet. In an external magnetic Üeld, the nucleus’s
magnetic moment vector precesses
about the Üeld direction but only certain orientations are allowed by
quantum rules. Thus, for hydrogen
(spin of ½) there are two possible
states in the presence of a Üeld, each
with a slightly different energy. Nuclear magnetic resonance is the absorption of radiation at a photon
energy equal to the difference between these levels, causing a transition from a lower to a higher energy
state. For practical purposes, the difference in energy levels is small and
the radiation is in the radiofrequency
region of the electromagnetic spectrum. It depends on the Üeld
strength.
NMR can be used for the accurate
determination of nuclear moments.
It can also be used in a sensitive form
of magnetometer to measure magnetic Üelds. In medicine, magnetic
resonance imaging (MRI) has been developed, in which images of tissue
are produced by magnetic-resonance
techniques.
The main application of NMR is as
a technique for chemical analysis
and structure determination, known
as NMR spectroscopy. It depends on
the fact that the electrons in a molecule shield the nucleus to some extent from the Üeld, causing different
atoms to absorb at slightly different
frequencies (or at slightly different
Üelds for a Üxed frequency). Such
effects are known as chemical shifts.
There are two methods of NMR spectroscopy. In continuous wave (CW)
NMR, the sample is subjected to a
strong Üeld, which can be varied in a
controlled way over a small region. It
is irradiated with radiation at a Üxed
frequency, and a detector monitors
the Üeld at the sample. As the Üeld
changes, absorption corresponding to
transitions occurs at certain values,
and this causes oscillations in the
Üeld, which induce a signal in the detector. Fourier transform (FT) NMR
uses a Üxed magnetic Üeld and
the sample is subjected to a highintensity pulse of radiation covering
a range of frequencies. The signal
produced is analysed mathematically
to give the NMR spectrum. The most
common nucleus studied is
1
H. For
instance, an NMR spectrum of
ethanol (CH 3 CH 2 OH) has three peaks
in the ratio 3:2:1, corresponding to
the three different hydrogen-atom
environments. The peaks also have a
Üne structure caused by interaction
between spins in the molecule. Other
nuclei can also be used for NMR spectroscopy (e.g.
13 C,
14 N,
19 F) although
these generally have lower magnetic
moment and natural abundance than
hydrogen. See also electron paramagnetic resonance; endor.
nuclear magneton See magneton.
nuclear Overhauser effect (NOE)
An effect in *nuclear magnetic reso379
nuclear Overhauser effect
n
www.AzShimi.ir www.AzShimi.com