Infrared Spectroscopy Energy Absorption and Vibration IR electromagnetic radiation is just less energetic than visible light This energy is sufficient to cause excitation of vibrational energy levels Wavelength (l) = 2.5-16.7 x 10-6 m n = wavenumbers. Larger n = higher energy Excitation depends on atomic mass and how tightly they are bound Hooke’s Law for 2 masses connected by a spring C—H Bond: Reduced Mass = (12+1)/(12x1) = 13/12 = 1.08 C—C Bond: Reduced Mass = (12+12)/(12x12) = 24/144 = 0.167 k = constant f = force constant = bond strength m-term = reduced mass

Many possible absorptions per molecule exist: stretching, bending,… Vibrational modes leading to IR absorptions:

Using IR in Organic Chemistry Functional Groups have characteristic IR absorptions Fingerprint Region (600-1500 cm-1) is unique for every molecule and lets us match an unknown with a known spectrum

Regions of the Infrared Spectrum 4000-2500 cm-1 N-H, C-H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm-1 CºC and C º N (stretching) 2000-1500 cm-1 double bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1 Below 1500 cm-1 “fingerprint” region

12.8 Infrared Spectra of Some Common Functional Groups Alkanes, Alkenes, Alkynes C-H, C-C, C=C, C º C have characteristic peaks absence helps rule out C=C or C º C

IR of Alkenes Alkene C—H absorbs at higher energy than alkanes because the force constant is stronger than alkanes (sp2 hybridization) Substitution pattern of alkenes give characteristic absorptions Terminal alkenes give 910, 990 cm-1 Geminal disubstituted gives 890 cm-1 trans disubstituted gives 970 cm-1

IR: Aromatic Compounds Weak C–H stretch at 3030 cm1 Weak absorptions 1660 - 2000 cm1 range Medium-intensity absorptions 1450 to 1600 cm1

IR: Alcohols and Amines O–H 3400 to 3650 cm1 Usually broad and intense N–H 3300 to 3500 cm1 Sharper and less intense than an O–H

IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cm1 Exact absorption characteristic of type of carbonyl compound 1730 cm1 in saturated aldehydes 1705 cm1 in aldehydes next to double bond or aromatic ring

1715 cm1 in six-membered ring and acyclic ketones C=O in Ketones 1715 cm1 in six-membered ring and acyclic ketones 1750 cm1 in 5-membered ring ketones 1690 cm1 in ketones next to a double bond or an aromatic ring 1735 cm1 in saturated esters 1715 cm1 in esters next to aromatic ring or a double bond C=O in Esters