Organic Structure Determination Using 2-D NMR Spectroscopy, Second Edition: A Problem-Based Approach PDF
Preview Organic Structure Determination Using 2-D NMR Spectroscopy, Second Edition: A Problem-Based Approach
Organic Structure Determination Using 2-D NMR Spectroscopy A Problem-Based Approach Second Edition Jeffrey H. Simpson Department of Chemistry Massachusetts Institute of Technology Cambridge, Massachusetts, USA AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON(cid:129)NEWYORK(cid:129)OXFORD PARIS(cid:129)SANDIEGO(cid:129)SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO AcademicPressisanimprintofElsevier AcademicPressisanimprintofElsevier 525BStreet,Suite1900,SanDiego,CA92101-4495,USA 225WymanStreet,Waltham,MA02451,USA Radarweg29,POBox211,1000AEAmsterdam,TheNetherlands TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UK Secondedition2012 Copyright(cid:1)2012ElsevierInc.Allrightsreserved. Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmittedinanyformorbyany meanselectronic,mechanical,photocopying,recordingorotherwisewithoutthepriorwrittenpermissionofthe publisher. PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRightsDepartmentinOxford,UK: phone(+44)(0)1865843830;fax(+44)(0)1865853333;email:[email protected] submityourrequestonlinebyvisitingtheElsevierwebsiteathttp://elsevier.com/locate/permissions,andselecting ObtainingpermissiontouseElseviermaterial Notice Noresponsibilityisassumedbythepublisherforanyinjuryand/ordamagetopersonsorpropertyasamatterof productsliability,negligenceorotherwise,orfromanyuseoroperationofanymethods,products,instructionsor ideascontainedinthematerialherein.Becauseofrapidadvancesinthemedicalsciences,inparticular,independent verificationofdiagnosesanddrugdosagesshouldbemade LibraryofCongressCataloging-in-PublicationData Simpson,JeffreyH. Organicstructuredeterminationusing2-DNMRspectroscopy:a problem-basedapproach/JeffreyH.Simpson.e2nded. p.cm. ISBN978-0-12-384970-0(pbk.) 1.Molecularstructure.2.OrganiccompoundseAnalysis.3.Nuclear magneticresonancespectroscopy.I.Title. QD461.S4682012 547’.122edc23 2011038670 BritishLibraryCataloguinginPublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary ISBN:978-0-12-384970-0 ForinformationonallAcademicPresspublications visitourwebsiteatelsevierdirect.com PrintedandboundinUSA 12131415 10987654321 Dedicated to Edward Worcester teacher, coach, philosopher 1935e2011 Preface Thesecondeditionofthisbookcomeswithanumberofnewfigures,passages,andproblems. Increasing the number of figures from 290 to 448 has necessarily impacted the balance between length, margins, and expense. It is my hope that the book has not lost any of its readabilityandaccessibility.Ifirmlybelievethatmostoftheconceptsneededtolearnorganic structure determination using nuclear magnetic resonance spectroscopy do not require an extensive mathematical background. It is my hope that the manner in which the material contained in this book is presented both reflects and validates this belief. Thesecondeditionowesmuchofitsimprovementtotheeffortsofothers.Mostnotably,Letitia YaooftheUniversityofMinnesotalaboredmightilytoimprovethe2ndeditionmanuscript.A number of researchers at the Massachusetts Institute of Technology assisted in generating samples and collecting some of the data that appear in this edition. In this regard, I wish to thank Jason Cox, Rick Danheiser, John Essigmann, Shaun Fontaine, Tim Jamison, Deyu Li, Ryan Moslin, Julia Robinson, and Tim Swager. As before, a number of Elsevier personnel have also assisted in bringing this edition to fruition. Those at Elsevier who helped with this edition include Gavin Becker, Joy Fisher Williams, Anita Koch, Emily McCloskey, MohanapriyanRajendran,LindaVersteeg-Buschman,andRickWilliamson.Ithankthosewho reviewed the 1st edition and shared their comments. I thank my family for supporting me during manuscript preparation, editing, and proofing. Since the publication of the first edition, I have received many emails from readers. These emailshavebeenoverwhelminglypositive,gratifyinglysuggestingthatthebookfillsanichein the near continuum of NMR books available today. I am interested in finding out how I may haveerredinpresentinganymaterialcontainedhereinsothatImaycorrecterrorsandthereby improve the book. As always, I encourage readers to send me email with comments and suggestions. My email address is [email protected]. Lastly, I cannot resist suggesting how best to digest the material contained in this book (this philosophy can also be applied to other learning endeavors). If we have the luxury of not havingtoreadandworkcontinuously(i.e.,ifwearenotworkingtosatisfyadeadline),wewill be well served by taking breaks in between reading and working problems. We balance our workwithotherinterestsandtrynottoletourfriendshipslanguish.Despitetherigorsofwork, xiii xiv Preface I still find time to bewith my family, to garden, to camp in winter in the White Mountainsof NewHampshire(sometimesbelow(cid:1)20(cid:3)F/(cid:1)29(cid:3)C),todrawastilllifewithoilpastels,toplay the electric guitar, to drink beer and throw a Frisbee(cid:1), to troll for landlocked salmon and togue on Sebec Lake, and to occasionally pull an all-nighter while anchored near the Isles of Shoals six miles off the coast of Maine and New Hampshire. Life is hurtling by; we must make the most of it. Jeff Simpson Epping, NH, USA July, 2011 Preface to the First Edition IwrotethisbookbecausethisbookdidnotexistwhenIbegantolearnabouttheapplicationof nuclear magnetic resonance spectroscopy to the elucidation of organic molecular structure. This book started as 40 two-dimensional (2-D) nuclear magnetic resonance (NMR) spectroscopy problem sets, but, with a little cajoling from my original editor (Jeremy Hayhurst), I agreed to include problem-solving methodology in chapters 9 and 10, and after that concession was made, the commitment to generate the first 8 chapters was a relatively small one. Two distinct features set this book apart from other books available on the practice of NMR spectroscopyasappliedtoorganicstructuredetermination.Thefirstfeatureisthatthematerial is presented with a level of detail great enough to allow the development of useful ‘NMR intuition’skills, and yet is given at a level that can be understood by a junior-level chemistry major, or a more advanced organic chemist with a limited background in mathematics and physical chemistry. The second distinguishing feature of this book is that it reflects my contention that the best vehicle for learning is to give the reader an abundance of real 2-D NMR spectroscopy problem sets. These two features should allow the reader to develop problem-solving skills essential in the practice of modern NMR spectroscopy. Beyondtheloftygoal of makingthereadermoreskilled atNMR spectruminterpretation,the bookhasotherpassagesthatmayprovideutility.Theinclusionofanumberofpracticaltipsfor successfully conducting NMR experiments should also allow this book to serve as a useful resource. I would like to thank D.C. Lea, my first teacher of chemistry, Dana Mayo, who inspired me tostudyNMRspectroscopy,RonaldChristensen,whotookmeunderhiswingforawholeyear, Bernard Shapiro, who taught the best organic structure determination course I ever took, DavidRice,whotaughtmehowtowriteapaper,PaulInglefieldandAlanJones,whohadmore faith in me than I had in myself, Dan Reger who was the best boss a new NMR lab manager could have and who let me go without recriminations, and, of course, Tim Swager, who inspired me to amass the data sets that are the heart of this book. I thank Jeremy Hayhurst, JasonMalley,DerekColeman,andPhilBugeauofElsevier,andJodiSimpson,whograciously agreed to come out of retirement to copyedit the manuscript. I also wish to thank those who xv xvi Preface to the First Edition reviewed the book and provided helpful suggestions. Finally, I have to thank my wife, ElizabethWorcester,andmychildren,Grant,Maxwell,andEva,forputtingupwithmeduring manuscript preparation. Any errors in this book are solely the fault of the author. If you find an error or have any constructive suggestions, please tell me about it so that I can improve any possible future editions. As of this writing, e-mail can be sent to me at [email protected]. Jeff Simpson Epping, NH, USA January, 2008 CHAPTER 1 Introduction Chapter Outline 1.1 What Is Nuclear Magnetic Resonance? 1 1.2 Consequences of Nuclear Spin 2 1.3 Application of a Magnetic Field to a Nuclear Spin 4 1.4 Application of a Magnetic Field to an Ensemble of Nuclear Spins 7 1.5 Tipping the Net Magnetization Vector from Equilibrium 12 1.6 Signal Detection 13 1.7 The Chemical Shift 14 1.8 The 1-D NMR Spectrum 14 1.9 The 2-D NMR Spectrum 16 1.10 Information Content Available Using NMR Spectroscopy 18 Problems for Chapter One 19 1.1 What Is Nuclear Magnetic Resonance? Nuclear magnetic resonance (NMR) spectroscopy is arguably the most important analytical technique available to chemists. From its humble beginnings in 1945, the area of NMR spectroscopy has evolved into many overlapping subdisciplines. Luminaries have been awardedseveralrecentNobelprizes,includingRichardErnstin1991,JohnPoplein1998,and Kurt Wu¨thrich in 2002. Nuclear magnetic resonance spectroscopy is a techniquewherein a sample is placed in ahomogeneous1(constant)magneticfield,irradiated,andamagneticsignalisdetected.Photon bombardment of the sample causes nuclei in the sample to undergo transitions2 (resonance) between their allowed spin states. In an applied magnetic field, spin states that differ energetically are unequally populated. Perturbing the equilibrium distribution of the spin-state population is called excitation.3 The excited nuclei emit a magnetic signal called a free induction decay4 (FID) which we detect with analog electronics and capture digitally. The 1 Homogeneous. Constant throughout. 2 Transition. Thechange inthespinstate ofone ormore NMR-activenuclei. 3 Excitation. Theperturbation ofspins fromtheir equilibrium distributionof spin-statepopulations. 4 Freeinductiondecay,FID.TheanalogsignalinducedinthereceivercoilofanNMRinstrumentcausedbythe xycomponentofthenetmagnetization.SometimestheFIDisalsoassumedtobethedigitalarrayofnumbers corresponding totheFID’s amplitude asa functionof time. OrganicStructureDeterminationUsing2-DNMRSpectroscopy.DOI:10.1016/B978-0-12-384970-0.00001-6 Copyright(cid:1)2012ElsevierInc.Allrightsreserved. 1 2 Chapter 1 digitizedFID(s)is(are)processedcomputationallyto(wehope)revealmeaningfulthingsabout our sample. Althoughexcitationanddetectionmaysoundverycomplicatedandesoteric,wearereallyjust tweaking the nuclei of atoms in our sample and getting information back. How the nuclei behaveoncetweakedconveysinformationaboutthechemistryoftheatomsinthemoleculesof our sample. TheacronymNMRsimplymeansthatthenuclearportionsofatomsareaffectedbymagnetic fields and undergo resonance as a result. 1.2 Consequences of Nuclear Spin Observation of the NMR signal5 requires a sample containing atoms of a specific atomic numberandisotope,i.e.,aspecificnuclidesuchasprotium,thelightestisotopeoftheelement hydrogen, also commonly referred to as simply a proton. A magnetically active nuclide will havetwoormoreallowednuclearspinstates.6Magneticallyactivenuclidesarealsosaidtobe NMR-active. Table 1.1 lists several NMR-active nuclides in approximate order of their importance to chemists. An isotope’s NMR activity is caused by the presence of a magnetic moment7 in its nucleus. Thenuclearmagneticmomentarisesbecausethepositivechargeprefersnottobewelllocated, as described by the Heisenberg uncertainty principle (see Figure 1.1). Instead, the nuclear chargecirculates.Becausethechargeandmassarebothinherenttotheparticle,themovement ofthechargeimpartsmovementtothemassofthenucleus.Themotionofallrotatingmassesis Table 1.1: NMR-active nuclides. Nuclide Element-Isotope Spin NaturalAbundance(%) FrequencyRelativeto1H 1H Hydrogen-1 ½ 99.985 1.00000 13C Carbon-13 ½ 1.108 0.25145 15N Nitrogen-15 ½ 0.37 0.10137 19F Fluorine-19 ½ 100. 0.94094 31P Phosphorus-31 ½ 100. 0.40481 2H(or2D) Deuterium-2 1 0.015 0.15351 5 Signal.An electrical current containinginformation. 6 Spinstate.Syn. spinangular momentum quantumnumber. Theprojection of themagnetic momentofa spin ontothez-axis.Theorientationofacomponentofthemagneticmomentofaspinrelativetotheappliedfield axis(fora spin-½ nucleus,this canbe +½ore½). 7 Magneticmoment.Avectorquantityexpressedinunitsofangularmomentumthatrelatesthetorquefeltbythe particletothemagnitudeand direction ofan externally appliedmagnetic field. The magneticfield associated withacirculating charge. Introduction 3 Figure 1.1: The structure of an atom with the positive charge unequally distributed in the nucleus inside the electron cloud. expressed in units of angular momentum. In a nucleus, this motion is called nuclear spin.8 Imaginethemotionofthenucleusasbeinglikethatofawildanimalpacingincirclesinacage. Nuclear spin (see column three of Table 1.1) is an example of the motion associated with zero-point energy in quantum mechanics, whose most well-known example is perhaps the harmonic oscillator. Thesmallsizeofthenucleusdictatesthatthespinningofthenucleusisquantized;thatis,the quantummechanicalnatureofsmallparticlesforcesthespinoftheNMR-activenucleustobe quantized into only a few discrete states. Nuclear spin states are differentiated from one anotherbasedonhowmuchtheaxisofnuclearspinalignswithareferenceaxis(theaxisofthe applied magnetic field, see Figure 1.2). We can determine how many allowed spin states there are for a given nuclide by multiplying the nuclear spin number (I) by 2 and adding 1. For a spin-½ nuclide, there are therefore 2 (1/2)þ1¼2 allowed spin states. In the absence of an externally applied magnetic field, the energies of the two spin states of a spin-½ nuclide are degenerate9 (the same). The circulation of the nuclear charge, as is expected of any circulating charge, gives rise to a tiny magnetic field called the nuclear magnetic moment (m) e also commonly referred to as a spin (recall that the mass puts everything into a world of angular momentum). Magnetically active nuclei are rotating masses, each with a tiny magnet, and these nuclear magnets interact with other magnetic fields according to Maxwell’s equations. 8 Nuclear spin.The circular motionof thepositive chargeof anucleus. 9 Degenerate. Two spinstates are saidtobe degeneratewhen theirenergies are thesame.
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