Spectroscopy a laboratory method of analyzing matter using electromagnetic radiation
The electromagnetic spectrum
Radiation Scale of Absorption involves: Example of spectroscopy Gamma rays pm Nuclear reactions x-rays 0.1 nm Transitions of inner atomic electrons Photoelectron spectroscopy (PES) UV/vis nm Transitions of outer atomic electrons UV-Vis spectroscopy, Atomic Emission Spectroscopy, Colorimetry IR mm Molecular vibrations IR, FTIR, Raman microwave mm Molecular rotations Rotational spectroscopy Radar, radio waves cm >>m Oscillation of mobile or free electrons NMR
Some spectroscopic methods Used to elucidate structures of crystals and organic compounds NMR Nuclear Magnetic Resonance Often uses carbon-13 (organic chemistry) Like an MRI of the molecule IR Infra-red spectroscopy a type of absorption spectroscopy
Types of motion caused by IR radiation Stretching Symmetrical and assymetrical Scissoring Rocking Wagging Twisting
Different pieces of molecules absorb different IR wavelengths
The presence of IR absorption of specific wavelengths indicate that molecular piece is present in the molecule
Photoelectron Spectroscopy (PES)
Photoelectron Spectroscopy PES apparatus: iramis.cea.fr
Photoelectron Spectroscopy How it works: 1. Sample is exposed to EM radiation 2. Electrons jump out of sample and go through analyzer http://chemwiki.ucdavis.edu
Image source: Inna M Vishik http://www.stanford.edu/~ivishik/inna_vishik_files/page452.htm
X-ray or UV Source Kinetic Energy Analyzer 6.26 0.52 Binding Energy (MJ/mol) 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+
Relative Number of Electrons PES Data for Neon Photo Electron Spectra The bigger the peak, the more electrons Each peak represents the electrons in a single sublevel in the atom 0.1 1 10 100 Energy Electrons generally farther from the nucleus Energy to remove an electron binding energy a.k.a ionization energy often measured in ev (electron volts) 1 ev = 1.60217657 10-19 J Electrons generally closer to the nucleus
Relative Number of Electrons Hydrogen vs. Helium Photo Electron Spectra 0.1 1 10 Energy The helium peak is farther to the right (higher energy) thus more energy is needed to remove the 1s electrons in helium. They must be held more tightly because there is a higher effective nuclear charge. (Helium has 2 protons pulling on 1s but hydrogen only has 1) The helium peak is twice as tall because there are twice as many electrons in the 1s sublevel
Relative Number of Electrons Oxygen (1s 2 2s 2 2p 4 ) Photo Electron Spectra 0.1 1 10 100 Energy 4 electrons in 2p 2 electrons in 2s 2 electrons in 1s
Relative Number of Electrons Scandium (1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 ) Photo Electron Spectra 0.1 1 10 100 1000 Energy *Notice that it takes more energy to remove an electron from 3d than from 4s. This is because as electrons are added to 3d they shield 4s thus it s easier (takes less energy) to remove 4s electrons compared to 3d electrons. Remember when transition metals make positive ions - it s the s electrons that are lost first!
Relative Number of Electrons Ex1: Identify the element whose PES data is shown Sodium Photo Electron Spectra Why is one peak much Larger than the others? This peak represents 6 electrons in the 2p sublevel The other peaks represent only 1 or 2 electrons A 0.1 1 10 100 1000 Energy In which sublevel are the electrons Represented by peak A? 3s
Example 2: nitrogen oxygen #e - #e - energy energy The PES data above shows only the peak for the 1s electrons. Why is the peak for Nitrogen farther to the left? It takes less energy to remove a 1s electron from nitrogen because it has a lower effective nuclear charge (fewer protons) than oxygen
Relative Number of Electrons Ex3: Sketch the expected PES spectrum for Aluminum Photo Electron Spectra 0.1 1 10 100 1000 Energy
Colorimetry and uv/vis spectroscopy Many compounds absorb ultraviolet (UV) or visible (Vis.) light. When dissolved in water, the absorption of some of the frequencies (colors) of light causes them to transmit others, and the solutions are thus colored. For example aqueous Cu 2+ solutions often appear blue, because the Cu 2+ ion absorbs visible light in the 600 650nm range.
Co(II) ion Co 2+ solutions often appear red to our eyes, because the Co 2+ ion absorbs visible light in the 500 510nm range (blue-green to green) The perceived color is then red.
Choose the wavelength with maximum absorbance when analyzing a solution
The diagram below shows a beam of monochromatic radiation of radiant power P 0, directed at a sample solution. Absorption takes place and the beam of radiation leaving the sample has radiant power P.
The amount of radiation absorbed may be measured in a number of ways: Transmittance, T = P / P 0 % Transmittance, %T = 100 T Absorbance A = log 10 P 0 / P A = log 10 1 / T A = log 10 100 / %T A = 2 - log 10 %T
The last equation, A = 2 - log 10 %T, is worth remembering because it allows you to easily calculate absorbance from percentage transmittance data. The relationship between absorbance and transmittance is illustrated in the following diagram:
Co(II) ion The relationship between absorbance and transmittance
Beer s Law Now let us look at Beer s law - the equation representing the law is straightforward: A=abc Where A is absorbance (no units, since A = log 10 P 0 / P ) a is the molar absorbtivity with units of M -1 cm -1 b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained. We will express this measurement in centimeters. c is the concentration of the compound in solution, expressed in M
The reason why we prefer to express the law with this equation is because absorbance is directly proportional to the other parameters, as long as the law is obeyed. (We are not going to deal with deviations from the law.)
Beer s Law typical scenario 1. Measure the absorbance of a several samples of a solution of known concentrations 2. Plot A vs concentation (usually molarity) This is called a calibration curve Should be linear
3. Measure the absorbance of a solution of the same chemical, but with unknown concentration 4. Using the calibration curve, determine the molarity of the unknown solution
Spectrometry a laboratory method of analyzing matter using electromagnetic radiation.
Mass Spectrometry Determines the relative abundance of the different isotopes of an element Used to determine the average atomic mass of an element
a Mass Spectrograph for Ne
Portions adapted from http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm AP 2003 FRQ #5 Chemistry, Chang, 10 th edition APSI 2013 OU presentation; J. Beninga Wikipedia: IR spectroscopy gifs http://wwwchem.csustan.edu/tutorials/images/cychexol.gif http://orgchem.colorado.edu/spectroscopy/irtutor/images/etbenzat.gif