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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
• 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
• 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
• Molecular Vibrations• Characteristic of functional groups• Chain Branching
%T
Aldehydes and Ketones
• Aldehyde RCHO 1725 cm-1 Ketone R2CO 1710 cm-1 Cyclobutanones 1780 cm-1
• Aromatic ArCHO 1700 cm-1 Aromatic ArCOR 1690 cm-1 Cyclopentanones 1740 cm-1
• Conjugated• -C=C-CHO 1685 cm-1 -C=C-C=O 1675 cm-1 • | | |
Alcohols
Carboxylic Acids
• 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
• two bands C-O-R 1050-1300 cm-1
Ethers
Phenols
• -CH2-CH2-CH2-CH2- rock 720 cm -1