IR Spectrum of Complex Molecules • There are many possible vibrational modes giving rise to complicated spectra with many peaks. • IR spectra are mainly used to identify unknown compounds • Peak positions can demonstrate what functional groups are present in the molecule. Each functional group gives rise to an absorption peak The region of IR spectrum generally ranges from 4000-600 cm‒1and the infrared spectrum can be broadly divided into three regions as given below. i) The region spanning the high frequency range from 3600 to 1200 cm‒1is called the functional group region and includes the stretching vibrations or group frequencies of common functional groups. Infrared (IR) Spectroscopy 18 Lecture Supplement: Infrared Spectroscopy Characteristic IR Stretching Frequencies Functional Group Bond Stretching, cm-1 Intensity Zone 1: 3700 - 3200 cm-1 alcohol O-H 3650 - 3200 variable; usually broad alkyne ≡ C-H ~3300 strong amine, amide N-H 3500 - 3300 medium, often broad Zone 2: 3200 - 2700 cm-1 2 The Origins of the Infrared Spectrum 2 3 Spectral Interpretation by Application of Vibrational Group Frequencies 6 3.1 The Hydrocarbon Species and Molecular Backbone 6 3.2 Simple Functional Groups 9 3.3 The Carbonyl Group 12 3.4 Other Functional Groups Associated with Heteroatoms 13 3.5 Simple Inorganics 14 4 The Practical Situation Theory The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The far-infrared, approximately 400-10 cm-1(1000-30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The IR Spectrum Table is a chart for use during infrared spectroscopy. The table lists IR spectroscopy frequency ranges, appearance of the vibration and absorptions for functional groups. There are two tables grouped by frequency range and compound class. IR Spectrum Table by Frequency Range An IR spectrum is a plot of the percentage of light transmitted (%T) versus the frequency in wavenumbers (ṽ, cm-1). Figure 2 shows the IR spectrum of ethanol (CH 3CH 2OH). Notice the typical IR frequency scale ranging from 400 to 4000 cm-1. When observing an IR spectrum, it is useful to visualize an imaginary line at 1500 cm-1. Which functional groups had the strongest Raman peaks? Which had the weakest? Remember that Raman intensity is directly proportional to the integration time. 2. Suggest a reason that some functional groups presented more strongly in the Raman spectrum than did others. 3. What functional groups were present in the unknown sample? To generate the IR spectrum, different frequencies of infrared light are passed through a sample, and the transmittance of light at each frequency is measured. The transmittance is then plotted versus the frequency of the light (which is presented in the somewhat unusual units of cm -1). Different functional groups produce bond absorptions at •IR alone cannot determine a structure. •Functional groups are usually indicated. •The absence of a signal is definite proof that the functional group is absent. •Correspondence with a known sample's IR spectrum confirms the identity of the compound. Final Notes • By Mike Bradley 12.21.2015. FTIR Basic Organic Functional Group Reference Chart (PDF) Functional groups are structural units within organic compounds defined by specific atom and bond arrangements. Infrare