pchemII.lecture27.Spectroscopy

pchemII.lecture27.Spectroscopy - Page 1 of 13 27....

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Page 1 of 13 Scott Kirkby Last revised: 17 March 2008 27. Spectroscopy Suggested Reading: Chapter 15 of the text. Introduction: Spectroscopy is the most powerful tool available to the scientist for probing the microscopic world of atoms and molecules. Using this tool, we can determine molecular structure in great detail with an extraordinary level of accuracy. Molecular events that occur on widely differing time scales ranging from days to picoseconds and most recently, even femtoseconds, can be monitored and studied in remarkable depth. Modern spectroscopy permits the transition states of chemical reactions to be probed and examined. Taken together, quantum mechanics and spectroscopy provide a powerful tool - a skeleton key - that allows us to unlock the innermost secrets of molecular dynamics and structure. Types of Spectroscopy All types of measurements in science are difference measurements. So it is with spectroscopy measurements, which determine the spacings between discrete energy states, not the absolute values of the energies of the states themselves. Because of this feature, it is common practice to characterize
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Physcal Chemistry I Lecture # 27 Page 2 of 13 Scott Kirkby Last revised: 17 March 2008 different types of spectroscopy in terms of the magnitudes of the energy-level spacings that are generally observed. The spacings in turn are usually given in terms of the wavelength λ or wave number , using the relationship (27-1) where c is the speed of light in a vacuum. The table below lists the approximate wavelengths and energies often associated with different types of spectroscopy. The two general types of spectroscopy are also classified by whether the experimentalist is measuring the energy required to excite the system from a low-lying energy state to a state higher in energy or whether he or she is measuring energy emitted when a system in an excited state undergoes a Type Approximate Energy Spacing (kJmol -1 ) Approximate Wavelength (cm) Nuclear magnetic resonance (NMR) 10 -6 10 4 Electron spin resonance (ESR) 10 -3 10 Rotational 10 -1 0.1 (microwave) Vibrational 10 10 -3 (infrared) Electronic 10 3 -10 4 10 -5 -10 -6 (ultraviolet- visible) Nuclear 10 8 10 -10 ν Eh ν = hc λ ----- = ν =
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Physcal Chemistry I Lecture # 27 Page 3 of 13 Scott Kirkby Last revised: 17 March 2008 transition to a state with lower energy. These two types are called absorption and emission spectroscopy, respectively. Transition Probabilities, Selection Rules and Bandwidths Spectroscopic absorption measurements are usually executed by placing and ensemble of molecules in the path of a beam of electromagnetic radiation. Such radiation is characterized by a sinusoidal oscillation of electric and magnetic field vectors perpendicular to the direction of propagation, as shown in the figure below.
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This note was uploaded on 04/19/2008 for the course CHEM PCHEM taught by Professor Kirkby during the Spring '08 term at East Tennessee State University.

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pchemII.lecture27.Spectroscopy - Page 1 of 13 27....

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