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Physics: Post your doubts here!

MemoryMatrix 21

MemoryMatrix 21

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faizay warsi said:
can someone provide the short notes on Xray n MRI .....? :(
Click to expand...
X-Ray crystallography
Tells us how the atoms are arranged in a crystal of the compound by determining the
positions of all atomic nuclei/atoms except that of hydrogen atom. Hydrogen atoms in a
structure cannot be detected by this technique because there is insufficient electrons / electron
density / electron cloud around the H atom. structures of complex molecules such as
enzymes, derived from X-ray crystallography can help explain their biochemical behaviour
such as their 3D shape and hence their active sites. It is also called X-ray diffraction.
It uses a solid crystal or powder as its sample.
Principle;
The X-rays are diffracted by the electron clouds around the individual atoms in the structure.
Interference patterns relate to the position and electron density of atoms
 
MemoryMatrix 21

MemoryMatrix 21

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faizay warsi said:
can someone provide the short notes on Xray n MRI .....? :(
Click to expand...

Nuclear Magnetic Resonance (NMR) spectroscopy
Tells us;
The number of hydrogen atoms in each chemical environment in a molecule (from the
integration trace and the chemical shift (δ) values), and
The number of their nearest neighbours (from the splitting patterns).
If a particular type of proton has n nearest neighbours, its peak is split into (n+1) lines.
When a spinning 1H nucleus is put in an external magnetic field the nuclei may either align
itself with the external field or against (i.e. opposed to) the external field thus;
In the absence of an applied field, the spin states of a given nucleus are of equal energy,
however, the spin states in an external magnetic field are no longer of equivalent energy and
the two spin states occupy two different energy levels;
The phenomenon of NMR occurs when nuclei aligned with the applied field absorb energy
and change their spin orientation with respect to the field.
Quiz: Explain with reference to energy states how 1H NMR can supply information about
the structure of molecules.
The sample is dissolved in a solvent that contains no protons, e.g.CCl4 or heavy water, D2O,
in which all H atoms are deuterium isotope 2H. A drop of tetramethylsilane (TMS), (CH3)4Si,
is then added to act as an internal reference.
N/B;
 TMS is used as a references because it is volatile, inert and produces a strong singlet peak at
a higher field than most 1H absorptions in organic molecules, so its peak does not interfere
with the other peaks.
 The TMS peak is always ignored when determining the structures of compounds, no matter
how high it is!
An NMR spectrum consists of a graph of absorbance against frequency. The frequency scale
is measured in units of chemical shift (symbol δ). The chemical shift of a proton is the
difference between its absorption frequency and that of TMS, measured in part per million
 
MemoryMatrix 21

MemoryMatrix 21

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503
MissBellum said:
I dont understand why they multiplied (6.02*10^23) into the equation, would someone care to explain? in oct/nov 2013/41 Q2)c)(i)
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u may b asked to find the *number* of moles ..... so we hv to ultiply with this avagadro
constant (6.02*10^23)
as, by definition : mole is the amount containing a no. of particles equal to avagadro constant
 
Last edited:
salvatore

salvatore

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sweetjinnah said:
Q7c) N/No = 1/10
N/No = e^(-decayconstant * t)
putting the values nd solve for t = 3.11*10^4 sec convert into hrs dividing by 3600 = 8.63 hrs.
Click to expand...
Hello,
I didn't understand how you solved this part of the question. How did you get N/No = 1/10?
I'll be really grateful if you could explain the whole solution in detail.
Jazakallah
 
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