Infrared Spectroscopy Near Infrared: 770 to 2500 nm Near Infrared: 770 to 2500 nm 12,900 to 4000 cm -1 Mid Infrared: 2500 to 50,000 nm (2.5 to 50  m) Mid Infrared: 2500 to 50,000 nm (2.5 to 50  m) 4000 to 200 cm -1 Far Infrared: 50 to 1000  m Far Infrared: 50 to 1000  m 200 to 10 cm -1

Infrared Spectroscopy: Vibrational Modes Ingle and Crouch, Spectrochemical Analysis

Pretsch/Buhlmann/Affolter/ Badertscher, Structure Determination of Organic Compounds Group Frequencies Estimate band location:

Are you getting the concept? Estimate the stretching vibrational frequency for a carbonyl group with a force constant, k, of 12 N/cm. If a C=S bond had the same force constant, where would its stretching band appear in the infrared absorption spectrum? Recall: 1 amu = x kg 1N = 1 kg*m*s -2 Atomic masses C → amu O → amu S → amu

Infrared Spectroscopy Near Infrared: 770 to 2500 nm Near Infrared: 770 to 2500 nm 12,900 to 4000 cm -1 * Overtones * Combination tones * Useful for quantitative measurements Mid Infrared: 2500 to 50,000 nm (2.5 to 50 um) Mid Infrared: 2500 to 50,000 nm (2.5 to 50 um) 4000 to 200 cm -1 * Fundamental vibrations * Fingerprint region 1300 to 400 cm -1 (characteristic for molecule as a whole) (characteristic for molecule as a whole) Far Infrared: 2.5 to 1000 um Far Infrared: 2.5 to 1000 um 200 to 10 cm -1 * Fundamental vibrations of bonds with heavy atoms (useful, e.g., for organometallics) atoms (useful, e.g., for organometallics)

Example of an Overtone Wagging vibration at 920 cm -1. Wagging vibration at 920 cm -1. Overtone at approximately 2 x 920 cm -1 = 1840 cm -1. Overtone at approximately 2 x 920 cm -1 = 1840 cm -1.

Fermi Resonance N.B. Colthup et al., Introduction to Infrared and Raman Spectroscopy, Academic Press, Boston, 1990.

Example of a Fermi Resonance Stretching vibration of C-C=(O) at 875 cm -1. Stretching vibration of C-C=(O) at 875 cm -1. Overtone at approximately 2 x 875 cm -1 = 1750 cm -1 Overtone at approximately 2 x 875 cm -1 = 1750 cm -1 coincides with C=O stretch coincides with C=O stretch

Light Source: Globar Silicon Carbide Rod (5mm diameter, 50 mm long) Heated electrically to 1300 – 1500 K Positive temperature coefficient of resistance Electrical contact must be water cooled to prevent arcing Ingle and Crouch, Spectrochemical Analysis

Sample Preparation for IR Spectroscopy Ingle and Crouch, Spectrochemical Analysis

Liquid Samples: Cell Thickness Ingle and Crouch, Spectrochemical Analysis

Window and Cell Materials Ingle and Crouch, Spectrochemical Analysis

Solvents Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

Suspension Media for Solid Samples Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

Interferences Pretsch/Buhlmann/Affolter/ Badertscher, Structure Determination of Organic Compounds