PHYS CHEM BIOC II
PHYS CHEM BIOC II CHEM 453
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This 3 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 453 at University of Washington taught by Staff in Fall. Since its upload, it has received 10 views. For similar materials see /class/192614/chem-453-university-of-washington in Chemistry at University of Washington.
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Date Created: 09/09/15
University at Washington a Department or Chemisu39y einistry 453 a Spring Qumer 2003 Lecture 26 05 0208 Text reading Chap 10 p 530538 541554 UV Spectroscopy Micmxcapic Dexcn39pu39an W m w n UV hght The rnteraetron leading to absorptaon oflightis e1eetromagnetae in origin The oseruatrng eteetronre state we say that a transrtaon has oeeurred thrs is absorptron net Enev v energy levels In any speetroseopy rt is neeessary to rnduee atransrtron between two energy that oeeurs Let us eonsrder a system eontatnrng two energy levels 5 and E state Fm t w H r number ofelectxons mm 1 According to Boltzmann39 drstrrbutron new quot2 If a system wrth two energy levels is exposedto afrequency E e5 V then the energy absorbedis proporhonat to the difference 1 en If gtgt as is the ease for opheat speetroseopy then essentaany all the molecules will be m therr rnereasrng the average numbernz and deereasrng why the same amount If instead AE m kT as is the case for example for NMR spectroscopy then n1 m I12 and the net absorption of energy will be very small because the rate of upward transitions is equal to the rate of downward transitions For this reason NMR spectroscopists use magnets of increasing strength to separate energy level more and increase the sensitivity of the experiment If very intense monochromatic laser light is used this is very actually much easier to do with radio frequency eg in NMR then the population can be momentarily inverted so that n2 gt MI A nonequilibrium state can be generated this way where the population is higher for the state of higher energy and therefore violates thermodynamics but beware we are not at equilibrium so it is alright Under these conditions we can have stimulated emission ie the radiation field induces a transition down to a lower energy state ie ground state Polarization circular dichroism An important biological application of optical spectroscopy is called circular dichroism CD and takes advantage of the differential absorption of circularly polarized light by chiral molecules such as proteins and nucleic acids These molecules and their structures e g ochelices or B form DNA are not equal to their mirror images As a consequence they absorb differently right or left circularly polarized light CD is a very sensitive probe of helical structures and is used to measure the helical content of proteins and nucleic acids for example as a function of temperature or a denaturant Optical rotation dispersion 0RD is the rotation of linearly polarized light by a chiral sample while CD is the difference in absorption between right and left circularly polarized light CD is most often used to characterize the structure of biological molecules such as proteins or nucleic acids A simple expression for CD is given by AA A L AR Where each of the absorbances represent the absorption of left and right circularly polarized light respectively CD unlike absorption can be either positive or negative Furthermore CD can only occur where the sample absorbs radiation in the first place The equivalent of extinction coefficient is called molar CD Ag 8L 8R w lC Lifetime of excited states and linewidth What happens to the energy after it is absorbed to restore thermodynamic equilibrium Boltzmann distribution It can be dissipated into heat increased vibrations of the atoms within the molecule or it can be reemitted back either directly or by going through other states of lower energy or it can perform chemical reaction However energy is conserved as always Heat dissipation occurs very fast ps lifetime for uorescence decay is much 1ongertn uu msr H r for d s not nst The ume eonstant forretum to the gr und state rs ayery rm ortant parameter othe s 0 P 12 o ag re speetrum beeause they de ne the hape ofapeak whreh ean be understood yery raprd1y usmg Hersenberg39 s uneertatnty pnnerp e 1 h A EA gt 27 In thrs eontext At rs the me oyerwhreh we obserye the energy othe exerted state L L Therefore L d L energy othe state to apreersron hmrted by Hersen erg39s n pl meehanrsm rs p nm e Ihrs otten refenedto as homogeneous broadenrng and ddffers for ddfferent types of 0 tr 0 I h Tu W ddstmgmsh eaeh othe proton for example m the NM speetrum ofa protern Addmonal meehanrsms determrne the apparenthnewrdth of opuea1 speetra The rstrs complex than between e1eetronre states In the gas phase auyely L hm m where measurements on bro1ogrea1 molecules are made Lhmgs are mueh more eompheated sum Molecules m soluuon eolhde eontrnuously wrth water andr ns for example and there are rnteraeuons that 1eadto shrthng othe energy levels and therr perturbauon Instead of a wellrresolved s ofvxbromc transrtrons are le wrth atypreal broad absorptron speetrum that represents the envelope of all yrbronre transruons End ofLeeture 26
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