1 Infrared Spectroscopy and global warming

Infrared spectroscopy Certain bonds in a molecule absorb infra-red radiation at characteristic frequencies causing the covalent bonds to vibrate Complicated spectra can be obtained than provide information about the types of bonds present in a molecule BELOW 1500 cm-1 – “Fingerprinting” Complicated and contains many signals – picking out functional group signals difficult. This part of the spectrum is unique for every compound, and so can be used as a “fingerprint”. A computer will compare the IR spectra against a database of known pure compounds to identify the compound ABOVE 1500 cm-1 – “Functional group identification” Use an IR absorption table provided in exam to deduce presence or absence of particular bonds or functional groups e.g. C=O 1640 – 1750 cm-1 O-H (acid) 2500- 3300 cm-1 H3C CH2 CH2 C O H H3C C O OH N Goalby chemrevise.org 1 2000 1500 Absorption or trough in between 1640-1750 cm-1 range indicates presence of C=O bond Always quote the wave number range from the data sheet Absorption or trough in between 2500-3300 cm-1 range indicates presence of O-H bond in an acid 4.2.4 Modern Analytical Techniques Bond Wavenumber C-O 1000-1300 C=O 1640-1750 C-H 2850 -3100 O-H Carboxylic acids 2500-3300 Very broad N-H 3200-3500 O-H Acohols, phenols 3200- 3550 broad N Goalby chemrevise.org 2 •Carbon dioxide (CO2 ), methane (CH4 ) and water vapour (H2O) are all greenhouse gases. (They trap the Earth’s radiated infra red energy in the atmosphere). •Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane. The Earth is thought to be getting warmer, and many scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere. Concentrations of Carbon dioxide in the atmosphere have risen significantly in recent years due to increasing burning of fossil fuels. Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming. The ‘Greenhouse Effect’ Infrared radiation is absorbed by C=O, O–H and C–H bonds in H2O, CO2 and CH4 . These absorptions contribute to global warming The ‘Greenhouse Effect’ of a given gas is dependent both on its atmospheric concentration and its ability to absorb infrared radiation and also its residence time. (Time it stays in atmosphere) Modern breathalysers measure ethanol in the breath by analysis using infrared spectroscopy Infrared spectroscopy can be used to monitor gases causing air pollution (e.g. CO and NO from car emissions)use spectra to identify particular functional groups limited to data presented in wavenumber form e.g. an alcohol from an absorption peak of the O–H bond, O-H absorptions tend to be broad