0000002473 00000 n Phys. 3. 0000033368 00000 n The experiment showed that finding the wavelength of maximum absorbance of a conjugated Count the number of bonds in the conjugated pi-system for each dye. It will focus on the expansion of wavefunctions in basis sets of other functions. stream subscribers only). HTMo0vnY?;vkZpH=z(YYPD:*?oi6A6oR0%J*e,uD|.j'NZ1NlD'Z{ ]^>IlrcN" YGHI4Hr4&y Ls43)i3 ef=:(+K)U\2%=+MQ5~P4zihve1\^v'M( 7v.OFz`\71+k(MmM]u36vU[q%pZ%qJf{7*`#u& sqJbY=rB1+C7R'rKyM^]L*HVZ4qcKs. Our observation suggests that PIB is predictive only when it is applied to a homologous series, wherein all compounds in the series have similar functional groups and similar chemical properties, with the only difference here being an increase in carbon number. Equations (6 - 9) Nguyen Quoc Trung. 4 0 obj Image 3. \[E = n^{2} \frac{h^{2}}{8mL^{2}}\] \[n=1,2,3 \] The concentration of each dye solution was adjusted so that each peak maximum absorbance reading fell in between 0.4 and 0.8 absorbance units. Correct the wavelength scale using the calibrations given in the class handout. /Transparency PNG Marine Scientific Research Guidelines 2021, CLRC Review - This document is provided by the Law Reform Commission of Papua New Guinea on, Volume 2 Attorney Generals Law Journal 2020, Books 3955 0 - This lecture notes was researched through google search at the University of, Diplomacy and International Relations THT, Jomo Kenyatta University of Agriculture and Technology, Kwame Nkrumah University of Science and Technology, L.N.Gumilyov Eurasian National University, Applied Heat Transfer (MEP 460), Differential & Integral Calculus (MAT 111), ACCA(Association Of Chartered Certified Accountants), Atatrk nkilaplar ve nkilap Tarihi II (ATA 102), Avar Kamps,Makine Mhendislii (46000), Power distribution and utilization (EE-312), Kasap SM Ch01 - Solution Manual for chapter 1, BBM 216 Intermidiate Microeconomic Module, 367807296 Gender Studies by Amanullah Gondal NOA Book, [ Peugeot] Manual de taller Peugeot 407 2004, Sample/practice exam 2020, questions and answers, Public Goods Vs Private Goods - Difference and Comparison - The Investors Book, Solutions manual for probability and statistics for engineers and scientists 9th edition by walpole, 114 Sayfa Ramazan Yetkin Kpss Tarih Ders Notlar - Pdf, Applied M I Lnote - Lecture notes All chapters, Assignment 1. 0000001473 00000 n Thus the percentage difference was approximately 40%. All of these systems contain long conjugated systems that can be approximated at some level to being linear and therefore can be modelled using the quantum mechanical particle in a one dimensional box . \[ \lambda _{abs} = \frac{8mL^{2}c}{h\left ( 2p+1 \right )} \] or Equation (11) \[ L=\sqrt{\frac{\lambda _{abs}h\left ( 2p +1 \right )}{8mc}} \] R $('#pageFiles').css('display', 'none'); << All six dyes: 1,1'-Diethyl-4,4'-cyanine iodide, 1,1'-Diethyl-4,4'-carbocyanine iodide, 1,1'-Diethyl4,4'-dicarbocyanine iodide, 1,1'-Diethyl-2,2'-cyanine iodide, 1,1'-Diethyl-2,2'-carbocyanine iodide and 1,1'-Diethyl-2,2'-dicarbocyanine iodide and methanol are considered highly toxic and irritate the skin. Phys. Figure 3 shows how pi electrons for dye A (a 6 electron system) and dye B (a 8 electron system) are contained in the box when it is in the ground state. 0000005032 00000 n The results from a classic experiment in the undergraduate physical chemistry laboratory, the particle-in-a-box model for spectroscopic transitions of conjugated dyes, is compared to computational results obtained using a molecular mechanics structural approach and the extended Hckel molecular orbital picture. For the recalibration include a plot of the known wavelengths as a function of the measured wavelengths and the linear fit with your report. Educ. The energy level of a particle in a 1D box is expressed as: 2 2 =withn=1,2,3.. 8 2 = 9.109x10-31kg = mass of an electron = 6.626x10-34Js = Plank's constant L = Length of the box 2. 0000012334 00000 n calculate the empirical parameter and to find the length of the conjugated chain using the free The model for these molecules will be the "particle-in-a-box" which we will consider in class. Reference: IMPORTANT! If we look at their structures carefully, we see that only dye 4 has a para- substituted ring, with the carbon chain and the nitrogen atom para- to each other, while dyes 1, 2, and 3 have an ortho-substituted ring, with the carbon chain and the nitrogen atom ortho- to each other (7). where: E is the change in energy, h is Plancks constant, m is the mass of the electron, L is the length of the box and ni and nf are the initial and final quantum numbers. In conclusion, we find that PIB is predictive as long as we apply it to a homologous series. This spectrum will be your background spectrum (Io) for this dye. >> Dye A has 6 electrons and dye B has 8 electrons in their respective pi-bond systems. Dye A Dye B. 0000011917 00000 n Hence, gamma should be calculated using only compounds from a homologous series (9). 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Repeat steps 3 5 until spectra have been recorded for all the dye solutions. An in-house MATLAB subroutine was used to determine the optimum gamma value and calculate the PIB or theoretical wavelengths of the series of four dyes. << Kuhn, H. J. Chem. obj Click here to download the Mathcad module and start the lab, Wear old clothing to lab! . Title The Particle-In-a-Box Model for describing the electronic transitions in conjugated dye molecules using UV visible spectroscopy. Lab 5: Modeling Conjugated Dyes with the Multielectron Particle-in-a-Box CH109 - Semester 2017 1 Lab #5: Modeling Conjugated Dyes with the Multielectron Particle-in-a-Box 1. Wear old clothing to lab! The Particle-In-A-Box approximation Electrons in the -electron system of a conjugated aromatic compound are not restricted to specific nuclei but are free to move throughout the system. result of selective photon absorption. The molar absorptivities (extinction coefficients) of these dyes are quite high, so plan your dilutions to minimize the use of the solutes and solvent. Question: Particle In a Box Lab Help please 1.On a single graph, plot the absorbance spectra of the three dyes. << Make plots of the absorbance spectra for dyes A F. You may combine the plots in one or two page graphs. Please turn in your lab writeup as a printed copy. If the particle in a box is used to represent these molecules then the pi electrons need to be distributed into the box. 61 0 obj << /Linearized 1 /O 63 /H [ 1086 387 ] /L 42495 /E 10562 /N 8 /T 41157 >> endobj xref 61 31 0000000016 00000 n 0000001961 00000 n Consider the following series of cyanine dyes (I- is not shown in each case below): Although the theoretical max gives a similar trend we see in our max measurements, it seems that there is a good agreement between the experimental and theoretical max for dyes 1, 2, and 3, and not for dye 4. 3. Now find the a value: a = (2j + 2)L = (2(3) + 2)(1.727 x 10-10) = 1.3816 x 10-9 m Since the s value (700 nm) has been known from the spectrum of , 1, 1 Diethyl -2, 2 dicarboncyanine iodide, use this to find the a values of other two wavelengths. Autschbach, J. J. Chem. Educ. 0000037485 00000 n [CDATA[*/ You MUST fill out the cover sheet fully and correctly to receive credit. /Creator lowest-energy electronic state (called the ground state) to a higher-energy electronic state Polymethine dyes 1,1-diethyl-2,2-cyanine iodide (Eastman, CAS# 977968), 1,1-diethyl-2,2-carbocyanine chloride (Eastman, CAS# 2768903), 1,1-diethyl-2,2-dicarbocyanine iodide (Eastman, CAS# 14187316), and 1,1-diethyl-44-carbocyanine iodide (Eastman, CAS# 4727508), corresponding to dyes 1, 2, 3, and 4, were used as received. impinging photon must match the energy difference between the initial state and some excited Click here to view this article (Truman addresses and J. Chem. Use the spectrophotometer to find the absorbance maximum of the dye 0 This weeks dry lab will consist of a single Mathcad module to work through. Phys. << /Length 5 0 R /Filter /FlateDecode >> 0000007744 00000 n CAUTION! /Parent For any function, f(x), if f(x) = -f(x) then f(x) must be equal to zero. \[ A = \ln \left ( \frac{I_{o}}{I} \right ) \]. Oxford Press: UK. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. obj The values calculated for the three dyes experimented on are, 352nm, 486 nm, and 622 nm, which supports the data given and supports the equation used to calculate the predicted wavelengths. 1. The experiment has been revised to suit our laboratory environment. Moog, R. S. J. Chem. N = P + 3 = # of conjugated electrons in cyanine chain We will use methanol as the solvent. Specifically. 10 Marketing-Management: Mrkte, Marktinformationen und Marktbearbeit (Matthias Sander), Handboek Caribisch Staatsrecht (Arie Bernardus Rijn), Big Data, Data Mining, and Machine Learning (Jared Dean), Junqueira's Basic Histology (Anthony L. Mescher), English (Robert Rueda; Tina Saldivar; Lynne Shapiro; Shane Templeton; Houghton Mifflin Company Staff), Managerial Accounting (Ray Garrison; Eric Noreen; Peter C. Brewer), Applied Statistics and Probability for Engineers (Douglas C. Montgomery; George C. Runger), Auditing and Assurance Services: an Applied Approach (Iris Stuart), Frysk Wurdboek: Hnwurdboek Fan'E Fryske Taal ; Mei Dryn Opnommen List Fan Fryske Plaknammen List Fan Fryske Gemeentenammen. While the particle in the box model1-4 can be used to rationalize the trend in lmax, it does not explain the other effects. /Type 7. Journal of Chemical Education, 63(9), 756. The calculated length of 1, 1 Diethyl -2, 2 dicarboncyanine was 1.727 x 10-10 m. The accepted length of was 1.23 x 10-10. Particle in a Box is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts. 0 0000007372 00000 n 0 0000010580 00000 n /Resources To test the robustness of the PIB model against four conjugated polymethine dyes, 1,1-diethyl-2,2-cyanine iodide (dye 1), 1,1-diethyl-2,2-carbocyanine chloride (dye 2), 1,1-diethyl-2,2-dicarbocyanine iodide (dye 3), and 1,1-diethyl-4,4-carbocyanine iodide (dye 4), I measured their absorption spectra 523 +/- 1 nm, 604 +/- 1 nm, 706 +/- 1 nm, and 706 +/- 1 nm at room temperature using a UV-Vis spectrophotometer, and calculated their PIB approximations and lengthening parameter gamma using an in-house MATLAB subroutine. When we run the routine, we see that for dye 4, the absolute percent error is 0.05% when run with dyes 5 and 6 (Table 2), versus 21.7% obtained when it is grouped with dyes 1, 2, and 3, suggesting that our hypothesis is correct. 0000047672 00000 n 0000010333 00000 n determine the wavelength of maximum absorbance (max) for each dye. 1 Abstract In this experiment, the penetration length can be determined experimentally and refined with experimental data. 0000040702 00000 n $('document').ready(function() { Atkins, J,: Paula, J. 0000037260 00000 n ( G o o g l e) For each dye, a 104 M stock dye solution was prepared in reagent grade methyl alcohol (Fisher, CAS# 67561). 540 The experiment showed that finding the wavelength of maximum absorbance of a conjugated dye can be used to find the empirical parameter and the length of a conjugated system within the dye. Each spectrum shows a major peak, a shoulder peak at a smaller wavelength, and a baseline with minimum noise (Figure 1). To verify our hypothesis, we plot the literature and theoretical max against p for dye 4 along with 1,1-diethyl-4,4-cyanine iodide (dye 5) and 1,1-diethyl-4,4-dicarbocyanine iodide (dye 6), two dyes that fall within the homologous series containing dye 4 (8). The experimental procedure that was followed can be found in the practical manual. If (q1,q1,q3,) is equal to zero then probability of any two electrons having the same identical set of quantum numbers is zero. R The literature procedures call for the use of methanol as a solvent.3,4 If you wish to explore the effect of solvent it is suggested that you use other polar organic solvents (e. g., tetrahydrofuran, acetonitrile, etc.) Learning more about how organic compounds absorb radiation. j = 3 double bonds endobj &P?ED Hp*^hZ{6@Lb"cx:+R+. In all four dyes we are forcing the conjugation through the chain. 1,1'-Diethyl-2,2'-dicarbocyanine iodide (Dye C) We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Title: Particle in a Box: Absorption Spectrum of Conjugated Dyes A ( pinacynol This principle of expansion will be repeatedly utilized in the quantum mechanical treatment of more complex chemical systems later in the semester. dyes. Discuss the observed colors and the absorption spectra. 2000, 77, 637-639. 7. 0000011264 00000 n subscribers only). $('#commentText').css('display', 'none'); The majority of colors that we see result from transitions between electronic states that occur as a result of selective photon absorption. 1965, 42, 490. /MediaBox How spectroscopic measurements may be used to derive some molecular properties (in this case the length of a conjugated pi-bonded system in dye molecules). /Nums These electrons can be. All of the conjugated dyes used have the largest Unfortunately, this experiment cannot be done with the equipment that we have in the laboratory, but it is possible to test a number of hypotheses using only a UV-Vis spectrometer, the dyes in Table 1 and common laboratory solvents. Using this wavelength, Oj[Fb)WdRjn>3rMyN-b-~MvE_hRnt1BaUy32NV&$ZnHxO#\2FDTT|['7W4`9?|'`#T1 Lhk~6I$e-v:LA" y"?vSr@pBNVft0V'j5IuGz {_(.Xk5D Z* ^lke0rC`>>n'nlM4D} yM]7= 5Nb53%5blWpb)+ps"J(CNR7PFgG^l-J{me@d>RaS>K]NCt*iU9Vf,?L,[Qw>TZd]5,)rlM5h l1+)\N lOxl%z2LNL7Br)" R ?9A{;~7#r1*>O iXPPX:8vvvH6ZXk/. 28, 721. http://www.chemistry.nmsu.edu/studntres/chem435/Lab5/. Some references from the Journal of Chemical Education are included here to help you get started,5-12 and please discuss your ideas with the instructor. 0000007892 00000 n subscribers only). 0000008846 00000 n Any group that produces more waste than that will be penalized accordingly. The general approach to this experiment is adapted from D. P. Shoemaker, C. W. Garland, and J. W. Nibler, Experiments in Physical Chemistry, 6th edition, McGraw Hill Co. Inc, NY, 1996, p378. Our experimental max for dyes 1, 2, and 3 agree with their respective literature values within their uncertainties. /Type _________________________________________________________________________________________________. Figure 2 shows the potential energy of the conjugated pi-bonded systems for dyes A and B. /FlateDecode As an extension to this project, the conjugation length of cyanine dyes has been used to discuss the particle-in-a-box principles, following a lab that was published earlier. We can read off the potential energy of the particle at any point in the box by looking at the level of the floor of the box at that point. However, this trend is not observed with dye 4, whose absorption spectrum overlaps with that of dye 3. 4 Be sure to select solvents that do not absorb light in the same wavelength region as your dyes. Theobald, R. S. (1964). The theoretical max also shows a similar trend. Particle in a Box Experiment Therefore, we can replace the electron system by free electrons moving in a one dimensional box of length . trailer << /Size 92 /Info 59 0 R /Root 62 0 R /Prev 41147 /ID[] >> startxref 0 %%EOF 62 0 obj << /Type /Catalog /Pages 60 0 R /OpenAction [ 63 0 R /XYZ null null null ] /PageMode /UseNone >> endobj 90 0 obj << /S 210 /Filter /FlateDecode /Length 91 0 R >> stream You MUST bring a completed pre-lab before you will be allowed to start the lab. 9. %PDF-1.4 Hb```f``c`e``ce@ ^yLPeP%-By:gTL9=\\.yJ[wO6>;2X2A8-,%%7RhhChX@9HK%X(^7\{S.T9]^VktH_gbf-[ty&lbt )\ endstream endobj 91 0 obj 281 endobj 63 0 obj << /Type /Page /Parent 60 0 R /Resources 64 0 R /Contents [ 69 0 R 73 0 R 78 0 R 80 0 R 82 0 R 84 0 R 86 0 R 88 0 R ] /MediaBox [ 0 0 612 792 ] /CropBox [ 0 0 612 792 ] /Rotate 0 >> endobj 64 0 obj << /ProcSet [ /PDF /Text ] /Font << /F2 74 0 R /TT2 66 0 R /TT4 70 0 R >> /ExtGState << /GS1 89 0 R >> /ColorSpace << /Cs5 67 0 R >> >> endobj 65 0 obj << /Type /FontDescriptor /Ascent 905 /CapHeight 0 /Descent -211 /Flags 32 /FontBBox [ -628 -376 2000 1010 ] /FontName /Arial-BoldMT /ItalicAngle 0 /StemV 133 >> endobj 66 0 obj << /Type /Font /Subtype /TrueType /FirstChar 32 /LastChar 150 /Widths [ 278 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 556 0 0 0 0 0 0 0 333 0 0 0 0 0 0 722 722 722 722 667 0 0 0 0 0 0 0 0 0 778 667 778 722 667 611 0 0 0 0 0 0 0 0 0 0 0 0 556 611 556 611 556 333 611 611 278 278 0 278 889 611 611 611 0 389 556 333 611 0 778 556 556 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 556 ] /Encoding /WinAnsiEncoding /BaseFont /Arial-BoldMT /FontDescriptor 65 0 R >> endobj 67 0 obj [ /CalRGB << /WhitePoint [ 0.9505 1 1.089 ] /Gamma [ 2.22221 2.22221 2.22221 ] /Matrix [ 0.4124 0.2126 0.0193 0.3576 0.71519 0.1192 0.1805 0.0722 0.9505 ] >> ] endobj 68 0 obj 764 endobj 69 0 obj << /Filter /FlateDecode /Length 68 0 R >> stream << the best fit is determined to be equal to 1 and the length of the box of , 1, 1 Diethyl -2, 2 R Band breadth of electronic transitions and the particle-in-a-box model. Donot put all 11 spectra on a single graph as this will be a big mess! http://www.chemistry.nmsu.edu/studntres/chem435/Lab5/ You only need to turn in your work (with enough context to understand what you are answering), and not the entire Mathcad file. 0000037506 00000 n endobj Shoemaker, D. P.; Garland, C. W. and Nibler, J. W. Experiments in Physical Chemistry, 7th Ed. 1,1'-Diethyl-2,2'-carbocyanine iodide (Dye B) If the particle in a box is used to represent these molecules then the pi electrons need to be distributed into the box. McGraw-Hill: New York, 2009; 393-, Copyright 2023 StudeerSnel B.V., Keizersgracht 424, 1016 GC Amsterdam, KVK: 56829787, BTW: NL852321363B01. Abstract Particle in a one dimensional box laboratory experiments have traditionally used chemicals like polyenes or cyanine dyes as model systems. 0 Molecules of a colored object absorb visible light photons when they are excited from their lowest-energy electronic state (called the ground state) to a higher-energy electronic state (called an excited state). White light is a mixture of all wavelengths of the visible spectrum. Do not get the dye solutions or methanol on your skin or in your eyes. 1,1'-Diethyl-4,4'carbocyanine iodide (Dye E) 0000044317 00000 n In reality the pi electrons may be considered to reside across the system of conjugated pi-bond system that extends from one nitrogen atom to the other, Figure 1. Ephoton = h v = Emolecule = Eupper state Elower State (1) Educ. conjugated chain backbone (including the nitrogen atoms). trailer <<3068649AEBAF11DF80810026B0D9EA12>]>> startxref 0 %%EOF 106 0 obj<>stream 0000004510 00000 n The wavelength of maximum absorbance (lmax) for the cyanine family of conjugated dyes, representative members of which are shown in Table 1, has a marked dependence on the number of conjugated carbons,1,2 while some also exhibit changes in lmax with solvent polarity, dye concentration and other factors. 0000002119 00000 n The purpose of this experiment is to determine the value of the empirical parameter and Note: In the following work completely clean the cuvette between samples. In the case of electrons in the particle in a box there are two quantum numbers, n and spin. However, our new gamma calculated using only dyes 1, 2, and 3 is 1.2400, which is the same as the one calculated using dyes 1, 2, 3, and 4. This paper describes an experiment in which beta-carotene and lutein, compounds that are present in carrots and spinach respectively, are used to model the particle in a one dimensional box system. 0000008638 00000 n [ Thus the percentage difference was approximately 40%. 2007, 84, 1840-1845. 0 electron model. %PDF-1.3 These dyes are typically $100-$300 per gram and so it is important not to waste the materials. Educ. I found a large absolute percent error in the PIB approximation for dye 4 with respect to the experimental, and a breakdown in the positive linear association between the number of carbons framed by nitrogen atoms in the dyes and the experimental wavelengths when dye 4 is added to dyes 1, 2, and 3. >> And the general solution for an equation of this form is: (x) = A \sin (kx) + B \cos (kx) (x) = Asin(kx)+ Bcos(kx) However, looking at the boundary conditions can help narrow this down. \[-\frac{\hbar^{2} }{2m}\frac{\partial ^{2}\psi}{\partial x ^{2}}+ V\left ( x \right ) \psi = E \psi\] 7 You should just be able to see a little color. Some trajectories of a particle in a box according to Newton's laws of classical mechanics (A), and according to the Schrdinger equation of quantum mechanics (B-F). 0000044295 00000 n The potential energy of a system needs to be defined to calculate the energy levels. 0000001720 00000 n \[\psi \left ( q_{1}, q_{2}, q_{3}, q_{4}, \right ) = - \psi \left ( q_{2}, q_{1}, q_{3}, q_{4}, \right ) \] >> Furthermore, when we calculate the theoretical max using a lengthening parameter, gamma, the chemical connectivity of dye 4, being different from that of dyes 1, 2, and 3, may contribute to a less-than-accurate gamma. Click here to view this article (Truman addresses and J. Chem. %PDF-1.3 % Click here to view this article (Truman addresses and J. Chem. McRae, E.G. Im in the Class of 2020, and a week ago I turned in my final lab report for college. Objective: The purpose of this experiment is to obtain the visible spectra of several cyanine dyes and then interpret them to a simple model of the electronic structure of the system: the Particle in a Box. Click here to view this article (Truman addresses and J. Chem. 0000047452 00000 n a = (2j + 2)L, C: 1,1-Diethyl-2,2-dicarbocyanine Iodide, N = 2j + 2 = 2(4) + 2 = 10 pi electrons The different in energy between these two levels is exactly equal to the energy a photon needs to excite the dye. Educ. Assume that each bond has a length of 0.139 nm and estimate the length of the box from the number of bonds. Elsevier. modeled as one-dimensional particles. Table 1. dicarboncyanine is 1 x 10-10 m. Identify the wavelength of the maximum absorbance, max, for each dye. 0000034252 00000 n 0000007549 00000 n The literature and experimental major wavelengths (max) absorbed for dyes 1, 2, and 3 agree within the experimental absolute uncertainty; however, the literature max absorbed for dye 4 falls outside the experimental absolute uncertainty (Table 1). The region that pi electrons occupied is said to be the. Dye A has 6 electrons and dye B has 8 electrons in their respective pi-bond systems. Particle in a box Pre-Lecture Assignment You must show all working to receive full credit. You will measure the absorption spectra of a series of conjugated dyes and then usethe Particle-In-A-Box (theoretical model)to explain the observed spectra. 0000000967 00000 n This variation may have been due to the methanol buffer in which the dye is stored. if({{!user.admin}}){ /Annots The base 4. 1997, 74, 1317-1319. Be sure to read the questions raised in the texts3,4 as they may give you insight into the problem and suggest issues that you should address as part of your discussion. %PDF-1.2 % 0000047792 00000 n R 4: Absorption Spectrum of Conjugated Dyes is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. conjugated bonds between the nitrogen atoms of the dye molecules. Therefore, if = 1, this 8 determine the length of a conjugated carbon chain for a set series of dyes by a UV/VIS Therefore to understand how organic dyes produce color it is necessary to understand how these compounds absorb light. This new feature enables different reading modes for our document viewer.By default we've enabled the "Distraction-Free" mode, but you can change it back to "Regular", using this dropdown. 0000037568 00000 n 0000005322 00000 n ; McGraw-Hill: New York, 2003, p. 380-385. /Page We do this by determining the length of conjugated region.

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