Introduction to Spectroscopy
Illustrations from
R. T. Morrison and R. N. Boyd,
Organic Chemistry
NMR Chemical Shift
• Chemical Shift ~ measure of local electron density
•  = (  - ref )/ref * 106 ppm
Peak Integrals
• 3.8 : 2.9 : 8.8 = 1.31 : 1 : 3.03 = 3.9 : 3 : 9
• If formula is known C11H16 :
• 16 H/15.5 units = 1.03 H per unit
Correlation Charts
13C
Spin-Spin Splitting
• Multiplicity : singlet, doublet, triplet, quartet, pentet
• Determined by number of near neighbors
• N-Equivalent neighbors: multiplicity = N + 1
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Branched Chains have Characteristic Patterns
Quartet-triplet = ethyl
Septet-doublet = gem-dimethyl
(geminal – attached to same atom)
Inequivalent Neighbors
• N-Neighbors: multiplicity = 2N
Spin Systems
• Mutually coupled (or isolated equivalent spins)
• Two spin systems
Unknown
• Double bonds and rings: CcHhOoNn
• Dbr = (2c-h+n+2)/2
(2*8-10+2)/2 = 4
Effect of Field Strength
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Chemical Shift the same in ppm
Multiplets have same spacing in Hz
60 MHz: 10 ppm = 600 Hz
1 Hz = 0.0166 ppm
500 MHz: 10 ppm = 5000 Hz
1 Hz = 0.002 ppm
500 MHz
• Multiplets simplified (more first order)
– Better definition in aromatic region
• More sensitive (less sample or less time)
Infrared Spectroscopy
%T
• Molecular Vibrations
• Characteristic of functional groups
• Chain Branching
Aldehydes and Ketones
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Aldehyde RCHO 1725 cm-1
Aromatic ArCHO 1700 cm-1
Conjugated
-C=C-CHO 1685 cm-1
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Ketone R2CO 1710 cm-1
Aromatic ArCOR 1690 cm-1
-C=C-C=O 1675 cm-1
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Cyclobutanones 1780 cm-1
Cyclopentanones 1740 cm-1
Alcohols
Carboxylic Acids
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Aliphatic R-COOH 1700-1725 cm-1 Conjugated -C=C-COOH 1680-1700 cm-1
Aromatic Ar-COOH 1680-1700 cm-1
Carboxylic Acid Derivatives
• Esters R-COOR
C=O 1714-1740 cm-1
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two bands C-O-R 1050-1300 cm-1
Ethers
Phenols
• -CH2-CH2-CH2-CH2- rock 720 cm -1