3IR- Empirical Comparisons Identifying functional groups in organic molecules
4Infrared Spectroscopy Region of infrared that is most useful lies between mm ( cm-1)depends on transitions between vibrational energy statesStretching: higher energy / higher wave number (cm-1)Bending: lower energy / lower wave number (cm-1)2
5A 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 distancebetween 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.
6The energy (IR frequency/ wave number) and the intensity of the absorption band also depends on the concentration of solution from Beer’s law, A = bc.It is easier to stretch an O–H bond if it is hydrogenbonded
7Functions & Infrared Spectra Analyzing Structure:Functions & Infrared SpectraThe 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 notdetected.
8Structural/Functional Components Figure: UNCaption: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.
19Structural Components & Functional Differences: The nitrogen of an amide is less electronegative than the oxygen of an ester.Therefore the amide has a longer (weaker) C=O bond ( cm-1) and the ester ( cm-1) is shorter (stronger).
20Infrared Absorption Frequencies Structural unit Frequency, cm-1Stretching vibrations (carbonyl groups)Aldehydes and ketonesCarboxylic acidsAcid anhydrides andEstersAmides8