x ray diffraction

2.  X-Rays are high energy photons having a
wave length of 10-2 to several angstrom
 They are generated by the bombardment of a
metal anode with high voltage electrons
 The radiation is a result of ejection of inner
shell electrons from the target
atoms,followed by the descent of an outer
electrons to fill the vacant space and the
emission of a photon of sharply defined
energy

3.  For the most common anode materials the
ejected electron arise s from K shell
 The vacancy is filled in by descent of an
elecron from the L shell
 Kα radiation is produced
 If the descenting electrons come from the M
shell ,the emitted radiation Kβ has higher
energy and a shorter λ

4.  X –rays are used in chemical analysis in
several ways
 X –rays emitted by an excited element has a
wavelength charecteristics of that element
 This property is used in both quantitative and
qualitative analysis of element
 X-rays absorbed by an element also specific
to it
 The most useful analytical method involve the
diffraction of x- ray from the planes of a
crystel –the diffraction analysis

5.  When x –rays impinage on atoms, radiation is
scattered in various directions.
 When the radiation is reflected by successive
planes of atoms present in the crystal
 Bragg pointed out that the scattering of X-
rays could be considered as reflection from
successive planes of the crystel
 The reflection x-ray can take place only at
certain angles which are determined by the
wavelength of the X-rays and the distance
between the planes of the crystel

6.  At these angles ,the reflected rays are in
phase so that the scattered total intensity is
maximum
 The fundamental equation which gives a
simple relation between of the X- rays ,λ,the
interplanar distance in the crystel ,d ,and the
angle of reflection θ is known as Bragg’s
equation n = 2d Sin

7. BRAGG’s EQUATION
d




 The path difference between ray 1 and ray 2 = 2d
Sin
Ray 1
Ray 2

Deviation =
2

8.  The horizontal line represent parallel planes
in the crystalline structure, seperated from
one another by the distance d
 When a beam of x ray falls on the crystal at
an angle θ , some of the radiation will be
reflected from the first plane (upper plane ) at
the same angle θ , ABC and DEF represents
planes perpendicular to the incident and
reflected beams respectively

9.  OL and OM ,drawn parallel to ABC and DEF
respectively and perpendicular to the incident
and reflected beams
 All the incident rays are in phase ,but the
light wave represented by BL will have to
cover an additional path length LN+ NM than
the light wave AO
 If this pathlength is equal to λ or multiples of
λ ,both the reflected rays OD and ME will be
in phase and supplement to each other

10.  Thus the rays reflected by different layer
planes will be in phase with one another,only
if the difference in path length of the waves
reflected from the successive plane is equal
to an integral number of wavelengths
 The difference in path length δ=LN+NM
 δ= nλ (multiples of wave length)
 Nλ=LN +NM=2LN
 LN= d sin θ
 nλ=2 d sinθ

11.  Every macromolecule will have a definite set
of planes due to unique arrangement of
atoms within
 Atoms located exactly on the crystel planes
contribute maximally to the intensity of the
diffracted beam
 If the atom present half way between the
planes exert maximum destructive
interferance

12.  The scattering power of an atom for x- ray also
depends upon the number of the electrons it
posses
 The position of the diffracted beam from a
crystal depends on the size and shape of the
repetitive unit of the crystal and the wave length
of the incident beam
 Intensity of the diffracted beam depends on the
type of atoms in the fundamental repetitive unit
,the unit cell
 Scattered intensity from a given atom increase
directly with atomic number

13.  With a monochromatic x ray beam,only at a
limited number of angles diffraction of the
beam occur
 The actual angles are determined by the
wavelength of the x ray and the spacing
between the planes of the crystal

14.  Rotating crystal method
 A single crystal is being rotate about one of
its axes when a monochromatic x radiation is
incident on it
 The reflected beam lie as spots on the surface
of cones which are coaxial with the rotation
axis
 By rotating the crystal about various axes the
three unit cell dimensions are obtained

15.  The component of x ray spectrophotometer
for rotating crystal technique
 x ray generating equipment
 Collimator
 Monochromator
 Slit
 A turn table
 recorder

16.  The source of x ray is an x ray tube which is
a large vacuum tube containing a heated
cathode (emit the electrons ) and an anode
(target )
 Electrons emitted by the cathode are
accelerated through a high voltage field
between cathode and the target
 The electrons strike the target and transfer
their KE to the atoms of the target material
,which emits x rays

17.  The wave length of the x ray is decided by the
target element and voltage
 An increase in the tube voltage results in an
increase in the total energy emitted and the
rays produced will be of shorter λ
 The target metal kept is usually copper or
molybdenum
 The impinging electron keep the target
hotand there fore it is cooled by running
water

18.  The radiation from an x ray tube is collimated
either by a series of closely spaced parallel
metal plates or by bundle of tubes 0.5mm or
lesser diameter
 The monochromators employed usually are
crystals of sodium chloride ,quartz or heavy
metal fatty acid
 A monochromatic radiation is obtaining by
reflecting x –rays from a crystal placed at a
specific angle

19.  For shorter wavelengths ,crystals having small d –
spacing are used
 Crystals having larger spacing between the
planes yield x rays of higher wavelengths
 The ample ,if a crystal is used ,is generally
affixed to a thin glass capillary which in turn is
fastened to a brasspin
 This assembly is mounted on a turn table which
is rotated gradually to change the glancing angle
of the x ray incident on a crystal
 The rays reflected from the crystal fall on a
detector

20.  The detector used may be a photographic
plate or a diffractometer
 The photographic method are used for
preliminary crystal studies intended to reveal
the unit cell parameters and symmetry

21.  X rays like visible light ,activate silver halides
for reduction and thus the position of the
diffracted beams are located in photographic
plate used as a detector
 X –ray films usually have a heavier coating of
silver halide than ordinary photographic
emulsions
 When the reflected rays fall on the
photographic plates ,dark areas are produced
where the reflections are maximum

22.  In the powder method , the crystal is
replaced by the powder of the sample , which
contains a larger number of very small
randomly oriented crystals
 Taken in a test tube and a continous cone of
refracted rays are produced
 The x ray diffraction assembley is a circular
in shape

23.  the film is in the form of a circular arc flush
against the inner circumferance of the
assembly and has a diameter of
57.31/114.6/143.2 mm
 In this case ,lighted areas in the form of arcs
or lines at different distance from the incidant
beam are produced
 Once the angle θ has been calculated and λ
known . the interplanar spacing d is
calculated using standard tables

24.  The process of determining a crystal structure
can be divided in to two parts-experimental and
computational
 The first involve choosing a crystal , determing
its lattice geometry and symmetry and measuring
the relative intensities of a larger number of
diffracted rays
 The second transforms the crude data to a
representation of the electric distribution in the
crystal from which the desired information about
the molecule can be deduced

25.  The unit cell dimensions are determined from
the spacings between the adjacent intensity
spots in the diffraction image
 The closer spacing between atoms in the
crystal ,the farther the intensity peak
positions in the diffraction image

26.  The intensities of the diffracted beams
depend on the position of the atoms in the
unit cell
 The spot closer to the center of the pattern
result from x rays scattered at smaller angle
from the crystal and provide information on
the grosser aspects of the molecule ,i.e the
long spacing within the molecule
 The spot closer to the periphery of the
photograph represents the closely spaced
aspects of the molecules within the crystal

27.  X ray diffraction gives a clear information on
the molecular architecture of complex
organic structures
 The gross molecular connectivity and the
relative stereochemistry of the molecules are
obtained
 The position of carbon and heavier atoms can
be found with a precision of about 0.003Å

28.  The location of hydrogen atom is less precise
,but the occurance of hydrogen bonding and
H-H interactions on conformation and
packing are easily followed
 The dimension of the unit cell help in the
identification of substances ,determination of
the molecular weights ,molecular shapes,
bond lengths and bond angles

29.  X-ray diffraction was the main tool in the
structure determination of the many
macromolecules such as structural proteins
,enzymes and nucleic acid