138 ChemActivity 16 Spectroscopy Model 2: Infrared Spectroscopy Most bonds of in

Question

138 ChemActivity 16 Spectroscopy Model 2: Infrared Spectroscopy Most bonds of interest to organic chemists are excited by (and absorb) light in the infrared (IR) region of the electromagnetic spectrum (frequencies near 10' cycles per second) Measurements are made using an infrared spectrometer (see Figure 2). Figure 2: Diagram of Infrared Spectrometer certain vis absorbed by sample light with all IR vs infrared souurce Detector prints a graph showing which vs never made it to the detector Sample Frequency (v) By tradition, IR spectra are printed upside-down, so the peaks look like valleys. Another thing to look out for is that the x axis is marked in wavenumbers (cm) instead of frequency (Hz). Fortunately, cm are proportional to Hz. This means wavenumbers are just another system for measuring frequency Critical Thinking Questions 7. What happened to the frequencies of light that never made it to the detector? 8. Light hitting the detector in Figure 2 is missing light of frequency A, but is not missing light of frequency B (see CTQs 5 & 6). What can you conclude about the structure of the molecule in the sample tube? That is, what structural features does the molecule likely have and not have?

Explanation / Answer

Infrared spectroscopy is bases on the molecular vibrations. When the infrared light beams conicide on the sample, functional groups or the compunds absorb perticular amount of energy to vibrate. This happens at resonate energy. Frequency that doesn't made it to the detectore relates with the energy absorbed by the functional groups to vibrate or in the othe hand we can say it as an oscillation. This vibration can also be a stretching.

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