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