Infrared Radiation: Molecular Vibrations 1

Electromagnetic Radiation: Light, Energy, Heat IR Tutor: http://chemistry.beloit.edu/Warming/pages/infrared.html

What do the sun’s energy, a molecule’s shape and a TV remote have to do with greenhouse gases? http://chemistry.beloit.edu/Stars/EMSpectrum/index.html

Infrared Absorbance

Infrared Absorbance TV Remote

Infrared Absorbance Sample TV Remote

Infrared Absorbance & Molecular Vibrations for CCl4 Stretching & Bending

Infrared Spectroscopy 1

Infrared Absorbance

IR- Empirical Comparisons Identifying functional groups in organic molecules

Infrared Spectroscopy Region of infrared that is most useful lies between 2.5-16 mm (4000-625 cm-1) depends on transitions between vibrational energy states Stretching: higher energy / higher wave number (cm-1) Bending: lower energy / lower wave number (cm-1) 2

A bond must have a dipole or an induced dipole in order to have an absorbance in the IR spectrum. When the bond stretches, the increasing distance between the atoms increases the dipole moment. Therefore, the greater the dipole, the more intense the absorption. (i.e., The greater the molar extinction coefficient () in Beer’s law, A = bc.

Functions & Infrared Spectra Analyzing Structure: Functions & Infrared Spectra The molecular formula is a critical piece of information, which limits the functional possibilities. The presence & absence of absorption bands must be considered in identifying a possible structure in IR spectroscopy. Empiricism is critical to successful identification. NOTE: Bonds which lack dipole moments are not detected.

Structural/Functional Components Figure: 12-14-04UN Caption: The IR spectrum has distinct regions. The left of the spectrum shows the C-H, O-H, and N-H stretches. The triple bonds absorb around 2200 cm-1 followed by the double bonds to the right at around 1700 cm-1. The region below 1400 cm-1 is called the fingerprint region.

An Infrared Spectrum The peaks are quantized absorption bands corresponding to molecular stretching and bending vibrations

The functional group stretching region The fingerprint region

Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 First examine the absorption bands in the vicinity of 4000-3000 cm–1 8

Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200 8

Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (carbonyl groups) Aldehydes and ketones 1710-1750 Carboxylic acids 1700-1725 Acid anhydrides 1800-1850 and 1740-1790 Esters 1730-1750 Amides 1680-1700 8

Cyclic aliphatic ketone

Mono substituted aromatic methyl ketone

Mono substituted aromatic ketone

Aliphatic ester I

Aliphatic ester II

Aliphatic ester III

Mono substituted aromatic ester

Mono substituted aromatic conjugated ester

Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp2 C—O 1200 sp3 C—O 1025-1200 8

Dihexyl Ether ~1100 cm-1 1025-1200 cm-1 8

Carboxylic Acid ~1200 cm-1

Distinctive Stretch of C–H Bond in an Aldehyde (the “waggle” vibration)

Aliphatic aldehyde

Mono-substituted aromatic aldehyde

Mono substituted aromatic ester

Para di-substituted aromatic ether & aldehyde

Infrared Spectroscopy Common Functional Groups Handout 1

Acetate Esters: Smell / Pheromones http://chemconnections.org/COT/COT-chemcomm-eg.html Pheromone Synthesis http://chemconnections.org/organic/chem226/Labs/acetate%20syn/Z-selective%20Insect%20Pheromones%20syn.pdf http://www.learner.org/resources/series61.html [20:40-23:51]

Infrared Spectroscopy Common Functional Groups Questions 1

2,4,6-cycloheptaheptatrien-1-one Question #1 C7H6O Identify the compound from the IR above. benzyl alcohol 2,4,6-cycloheptaheptatrien-1-one acetophenone benzaldehyde phenylacetic acid

2,4,5-trimethylbenzaldehyde Question #2 C10H12O Identify the compound from the IR above. 2,4,5-trimethylbenzaldehyde p-allylanisole 2-allyl-4methylphenol 1-phenyl-2-butanone

Question #3 Identify the compound from the IR above. cyclopropanone C3H4O Identify the compound from the IR above. cyclopropanone propynol acrylaldehyde propenoic acid

Question #4 Identify the compound from the IR above. methylbenzoate C8H8O2 Identify the compound from the IR above. methylbenzoate o-hydroxyacetophenone o-toluic acid p-anisaldehyde (p-methoxybenzaldehyde)

Question #5 Identify the compound from the IR above. benzylformate C8H8O2 Identify the compound from the IR above. benzylformate o-hydroxyacetophenone 2-methoxytropone p-toluic acid o-anisaldehyde (p-methoxybenzaldehyde)