IR is a good ____ technique and a bad _____ technique
Good qualitative . Bad quantitative
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The biggest use for IR spec
to be stamped as BP- must pass 2 tests - IR is one of these tests
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How much energy doe it require compared to UVis and NMR?
Less than UV-Vis more than NMR
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Priniciple
based on molecular vibrations- covalent bonds are not rigid - they are flexible like a spring
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Energy of vibration is dependent on 2 things
1. type of bond 2. whats on the ends of the bonds
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If we apply different amounts of energy
we can make bonds vibrate in different ways - ie. bending stretchin - each mode of vibration has a specific amount of energy associated with it
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RHS of IR spectra
Fingerprint region - no other molecule has same fingerprint - used to ID - BP test compare to database
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Intensity of a absorption is dependent on
dipole moment of that bond
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C=O
1700cm - 1
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NH2
3400cm-1
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OH
3500cm-1
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CH
3000cm-1
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Instrumentation 2 types
1. Dispersive 2. Fourier transfromation
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Dispersive
monochrom- selects each WL in turn - when bond absorbs energy to enter diff mode of vibration - less energy reaches detector - drop in transmission - peak
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Fourier transformation
uses michelson interference. MOVING MIRRORS. does not select each WL in turn . all at once. beam splitter - half to fixed mirror, half to moving mirror. Bouces back and splits again . If WL paths are diff lengths - overlap and come back stronger...
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...
then when started. If come back and same length they dont overlap and the cancel each other out.
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Why should water be minimized from the sample
water has dipole and will show up on the spectra
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KBr disc
sample ground down 2mg with 300mg KBr pressure applied to form v thin disc- then placed in holder for IR to pass thru. If too much sample it will absorb all IR
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Liquid film
Small drops of sample sandwiched between KBr or NaCl windows to produce thin film
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Nujol Mull (liquid paraffin)
semi solids grounf into paste with Nujol, and then sanwiched between KBr or NaCl. Does not req hydrolic press, good for semi solids as arent pressed well into a disc.
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problems with Nujol
organic - so will contribute to the spectra. so need to use as little as possible
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Solution
sample dissolved in organic solvent and placed in a fixed path length KBr or NaCl cell, again will contribute to its own peaks
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Diffuse reflectance
mixed with KBr if needed and scattered IR measured from surface - different fro KBr disc as no grinding/pressure - good for polymorphs as disc process would destroy
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Attenuated total reflectance
placed on zinc selenide/ diamond crystal and IR measured . Good for polymorph, creams, gels
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Other application
BP TEST . GC-IR (gas chromatography IR) . Spectral subtraction
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Other cards in this set
Card 2
Front
The biggest use for IR spec
Back
to be stamped as BP- must pass 2 tests - IR is one of these tests
Card 3
Front
How much energy doe it require compared to UVis and NMR?
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