is possible for some of them to be de-
generate. For example, in a linear tri-
atomic molecule there are four
normal modes of vibration since
3 N– 5 = 4 for N = 3. These vibrational
modes are: (a) symmetric stretching
(breathing) vibrations; (b) antisym-
metric stretching vibrations; and (c)
two bending vibrations, which are
degenerate.
N.T.P. See s.t.p.
n-type semiconductor See semi-
conductor.
nuclear magnetic resonance
(NMR)The absorption of electromag-
netic radiation at a suitable precise
frequency by a nucleus with a
nonzero magnetic moment in an ex-
ternal magneticÜeld. The phenome-
non occurs if the nucleus has
nonzero *spin, in which case it be-
haves as a small magnet. In an exter-
nal magneticÜeld, the nucleus’s
magnetic moment vector precesses
about theÜeld direction but only cer-
tain orientations are allowed by
quantum rules. Thus, for hydrogen
(spin of ½) there are two possible
states in the presence of aÜeld, each
with a slightly different energy. Nu-
clear magnetic resonance is the ab-
sorption of radiation at a photon
energy equal to the difference be-
tween these levels, causing a transi-
tion from a lower to a higher energy
state. For practical purposes, the dif-
ference in energy levels is small and
the radiation is in the radiofrequency
region of the electromagnetic spec-
trum. It depends on theÜeld
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 mag-
neticÜelds. In medicine, magnetic
resonance imaging (MRI) has been de-
veloped, 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 mol-
ecule shield the nucleus to some ex-
tent from theÜeld, causing different
atoms to absorb at slightly different
frequencies (or at slightly different
Üelds for aÜxed frequency). Such
effects are known as chemical shifts.
There are two methods of NMR spec-
troscopy. In continuous wave (CW)
NMR, the sample is subjected to a
strongÜeld, which can be varied in a
controlled way over a small region. It
is irradiated with radiation at aÜxed
frequency, and a detector monitors
theÜeld at the sample. As theÜeld
changes, absorption corresponding to
transitions occurs at certain values,
and this causes oscillations in the
Üeld, which induce a signal in the de-
tector. Fourier transform (FT) NMR
uses aÜxed magneticÜeld and
the sample is subjected to a high-
intensity 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 spec-
troscopy (e.g.^13 C,^14 N,^19 F) although
these generally have lower magnetic
moment and natural abundance than
hydrogen. See also electron paramag-
netic resonance; endor.nuclear magnetonSee magneton.
nuclear Overhauser effect (NOE)
An effect in *nuclear magnetic reso-379 nuclear Overhauser effect
n