Studio_11_IR_FINAL_F2006 - Chem 25 Studio#11 NAME_Infrared...

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Chem 25: Studio #11____ __Infrared Spectroscopy NAME:___________________________ STUDIO:___________________________ INFRARED SPECTROSCOPY: A USE FOR A QUANTIZED PROCESS Introduction You have seen the spectrum of electromagnetic radiation in class. The spectrum is continuous but is conventionally separated into regions that have a historical connotation and in part define the way the radiation interacts with matter. Figure 1. Spectrum of electromagnetic radiation highlighting the relationship between UV, visible and IR regions. The relatively narrow region of visible light ( λ = 400 nm to 800 nm) borders on the ultraviolet region (higher frequency, shorter wavelength) and the infrared region (lower frequency, longer wavelength). The consequence of the interaction of infrared radiation with molecules is the subject of this studio. Almost any compound with covalent bonds absorbs frequencies of electromagnetic radiation in the infrared region, and the absorption of this radiation is quantized. Radiation in the IR range causes vibrational motions of the covalent bonds in the molecule; precisely the right energy of radiation is needed to cause the motion – that’s what quantization means. Hence, the absorption of infrared radiation from a beam of incident IR radiation by a molecule containing covalent bonds removes certain frequencies of radiation from the beam. The plot of the transmittance (how much light comes through, where 100% is all the light – the sample is transparent) of this altered beam versus the frequency of the radiation generates an IR spectrum. Where the radiation has not been absorbed, the transmittance is near 100%, meaning the sample 1
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does not have any bonds that absorb light of that energy. Where radiation of a specific wavelength and frequency has been removed from the spectrum due to absorption by the molecules through which it has passed, the transmission drops significantly. This minimum in the spectrum is called a peak. (Admittedly, this is a bit odd – a minimum is a peak? Don’t fight it; it’s just the way chemists talk.) Why is this worth bothering with? Every type of bond (carbon-carbon single bond, carbon-hydrogen bond, carbon-oxygen single bond, carbon-oxygen double bond, almost any covalent bond you can think of) has a different natural frequency of vibration. Furthermore, two of the same type of bond in different molecules have slightly different vibrational frequencies because their environments differ slightly. As a consequence, every molecule produces a unique plot of transmittance vs. energy – an IR spectrum. The IR spectrum of a molecule is like a set of fingerprints is for a human being: unique, and hence an identifying characteristic. This could all be horrifically complicated but some generalizing concepts come to the
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This note was uploaded on 02/27/2008 for the course CHEM 025 taught by Professor X during the Fall '06 term at Lehigh University .

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Studio_11_IR_FINAL_F2006 - Chem 25 Studio#11 NAME_Infrared...

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