Instrumental Chemistry Prof. Zvi C. Koren Topics Spectrometric Techniques: UV/Vis (ultraviolet/visible) Spectrometry – electronic transitions FTIR (Fourier Transform Infrared) Spectrometry – molecular vibrations NMR (Nuclear Magnetic resonance) Spectrometry – nuclear spins MS (Mass Spectrometry) AA (Atomic Absorption) Spectrometry Chromatographic Methods: GC-MS (Gas Chromatography – Mass Spectrometry) HPLC-PDA (High-Performance Liquid Chromatography – Photodiode Array detection) GPC (Gel Permeation Chromatography) Thermal Analysis: DSC (Differential Scanning Calorimetry) DMTA (Dynamic Mechanical Thermal Analysis) Others: TOC (Total Organic Carbon) Analyzer Spectroscopy = Spectrometry = Spectrophotometry The study of the interaction between light and matter UV/Vis Spectroscopy: Absorption of light leads to a transition of an electron from one level to the next. Wave Theory of Light l A The Electromagnetic Spectrum Spectral Regions and Molecular Transitions Region Wavelengths () Transitions X-ray 10–2 – 10 nm K and L electrons Far UV 10 – 200 nm Mid-shell e’s Near UV 200 – 400 nm Valence e’s Visible 400 – 800 nm Valence e’s Near Infrared, NIR 0.8 – 2.5 m Molecular vibrations IR 2.5 – 50 m Molecular vibrations Far IR 50 – 300 m Molecular rotations Microwave 0.3 mm – 0.5 m Molecular rotations Radiowave 0.5 – 300 m NMR, Nuclear Magnetic Resonance Visible Light R O Y G. B I V Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange:585 - 620 nm Red: 620 - 780 nm Color Wheel: Complementary Colors Absorbed and Reflected Colors: “What you see is NOT what you got.” Complementary colors are diametrically opposite each other: color absorbed vs. Color observed Note: Green is unique in that it can be created by absorption close to 400 nm as well as absorption near 800 nm. UV/Vis Spectrometer: Dual Beam (multiple l‘s) (prism) mono- chromatic single l Transmittance vs. Absorbance Transmittance: T = I/Io < 1 (sometimes expressed as a percent, %, and not as a fraction) Absorbance: A = log10(Io/I) = –log10T If no absorption of light has occurred: A = 0 T = 1 Beer’s Law (or Beer-Lambert Law) Absorbance: A = a·b·c (but often written as A = bc) a or = molar absorptivity (or extinction coefficient), l-dependent units of M–1cm–1 b = light path through the cell (cuvet), typically 1 cm c = concentration of the solute in the solution A = f(l), and of the material of course A is linear with c, ideally For strongly absorbing compounds: > 10,000. For weakly absorbing compounds: = 10 – 100. Typical UV/Vis Spectrum Note the Experimental Conditions: c = 0.142 M (in 95 % ethanol), Homework Assignment 1: b = 1.0 cm (Email the answers to me) 1. What is the color of the solution? Explain. 2. What are the values of 395 and 255? Use the appropriate units. Show the calculation. Do the numbers make sense? 3. What is the name of the compound? (Use the naming tool from the ChemSketch program from the free site www.acdlabs.com.) Why Are Some Organic Molecules Colored? extensively conjugated pi-electrons.