NMR基础知识 英文版.pdf

P1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to come1Getting Started1.1The TechniqueThis book is not really intended to give an in-depth education in all aspects of the NMR effect(there arenumerous excellent texts if you want more information)but we will try to deal with some of the morepertinent ones.The first thing to understand about NMR is just how insensitive it is compared with many otheranalytical techniques.This is because of the origin of the NMR signal itself.The NMR signal arises from a quantum mechanical property of nuclei called spin.In the texthere,we will use the example of the hydrogen nucleus(proton)as this is the nucleus that we will bedealing with mostly.Protons have a spin quantum number of1/2.In this case,when they are placedin a magnetic field,there are two possible spin states that the nucleus can adopt and there is an energydifference between them(Figure 1.1).The energy difference between these levels is very small,which means that the population differenceis also small.The NMR signal arises from this population difference and hence the signal is also verysmall.There are several factors which influence the population difference and these include the natureof the nucleus(its gyromagnetic ratio)and the strength of the magnetic field that they are placed in.The equation that relates these factors(and the only one in this book)is shown here:?E=hB2=Gyromagneticratioh=PlancksconstantB=MagneticfieldstrengthBecause the sensitivity of the technique goes up with magnetic field,there has been a drive to increasethe strength of the magnets to improve sensitivity.Unfortunately,this improvement has been linear since the first NMR magnets(with a few kinks hereand there).This means that in percentage terms,the benefits have become smaller as development hascontinued.But sensitivity has not been the only factor driving the search for more powerful magnets.You also benefit from stretching your spectrum and reducing overlap of signals when you go to higherfields.Also,when you examine all the factors involved in signal to noise,the dependence on field is toEssential Practical NMR for Organic Chemistry S.A.Richards and J.C.Hollerton 2011 John Wiley&Sons,Ltd.ISBN:978-0-470-71092-0P1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to come2Essential Practical NMR for Organic ChemistryNo fieldApplied magnetic fieldM=-M=+0energyFigure 1.1Energy levels of spin1/2nucleus.the power of 3/2 so we actually gain more signal than a linear relationship.Even so,moving from 800to 900MHz only gets you a 20%increase in signal to noise whereas the cost difference is about 300%.In order to get a signal from a nucleus,we have to change the populations of each spin state.We dothis by using radio frequency at the correct frequency to excite the nuclei into their higher energy state.We can then either monitor the absorption of the energy that we are putting in or monitor the energycoming out when nuclei return to their low energy state.The strength of the NMR magnet is normally described by the frequency at which protons resonate init the more powerful the magnet,the higher the frequency.The earliest commercial NMR instrumentsoperated at 40megacycles(in those days,now MHz)whereas modern NMR magnets are typically tentimes as powerful and the most potent(and expensive!)machines available can operate at fields of1GHz.1.2InstrumentationSo far,we have shown where the signal comes from,but how do we measure it?There are two maintechnologies:continuous wave(CW)and pulsed Fourier transform(FT).CW is the technology used inolder systems and is becoming hard to find these days.(We only include it for the sake of historicalcontext and because it is perhaps the easier technology to explain).FT systems offer many advantagesover CW and they are used for all high field instruments.1.3CW SystemsThese systems work by placing a sample between the pole pieces of a magnet(electromagnet orpermanent),surrounded by a coil of wire.Radio frequency(r.f.)is fed into the wire at a swept set offrequencies.Alternatively,the magnet may have extra coils built onto the pole pieces which can be usedto sweep the field with a fixed r.f.When the combination of field and frequency match the resonantfrequency of each nucleus r.f.is emitted and captured by a receiver coil perpendicular to the transmitterP1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to comeGetting Started3RF receiverRF generatorSweep generatorSweep generatorFigure 1.2Schematic diagram of a CW NMR spectrometer.coil.This emission is then plotted against frequency(Figure 1.2).The whole process of acquiringa spectrum using a CW instrument takes typically about 5min.Each signal is brought to resonancesequentially and the process cannot be rushed!1.4FT SystemsMost spectrometers used for the work we do today are Fourier transform systems.More correctly,theyare pulsed FT systems.Unlike CW systems,the sample is exposed to a powerful polychromatic pulseof radio frequency.This pulse is very short and so contains a spread of frequencies(this is basic Fouriertheory and is covered in many other texts).The result is that all of the signals of interest are excitedsimultaneously(unlike CW where they are excited sequentially)and we can acquire the whole spectrumin one go.This gives us an advantage in that we can acquire a spectrum in a few seconds as opposedto several minutes with a CW instrument.Also,because we are storing all this data in a computer,wecan perform the same experiment on the sample repeatedly and add the results together.The numberof experiments is called the number of scans(or transients,depending on your spectrometer vendor).Because the signal is coherent and the noise is random,we improve our signal to noise with eachtransient that we add.Unfortunately,this is not a linear improvement because the noise also builds upalbeit at a slower rate(due to its lack of coherence).The real signal to noise increase is proportional tothe square root of the number of scans(more on this later).So if the whole spectrum is acquired in one go,why cant we pulse really quickly and get thousandsof transients?The answer is that we have to wait for the nuclei to lose their energy to the surroundings.This takes a finite time and for most protons is just a few seconds(under the conditions that we acquirethe data).So,in reality we can acquire a new transient every three or four seconds.After the pulse,we wait for a short whilst(typically a few microseconds),to let that powerful pulseebb away,and then start to acquire the radio frequency signals emitted from the sample.This exhibitsitself as a number of decaying cosine waves.We term this pattern the free induction decay or FID(Figure 1.3).P1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to come4Essential Practical NMR for Organic Chemistry1.81.71.61.51.41.31.21.11.00.90.80.70.60.50.40.30.20.1secFigure 1.3A free induction decay.Obviously this is a little difficult to interpret,although with experience you can train yourself toextract all the frequencies by eye.(only kidding!)The FID is a time domain display but what wereally need is a frequency domain display(with peaks rather than cosines).To bring about this magic,we make use of the work of Jean Baptiste Fourier(17681830)who was able to relate time-domain tofrequency-domain data.These days,there are superfast algorithms to do this and it all happens in thebackground.It is worth knowing a little about this relationship as we will see later when we discusssome of the tricks that can be used to extract more information from the spectrum.There are many other advantages with pulsed FT systems in that we can create trains of pulses tomake the nucleiperform dances whichallow themto revealmoreinformation about theirenvironment.Ray Freeman coined the rather nice term spin choreography to describe the design of pulse sequences.If you are interested in this area,you could do much worse than listen to Ray explain some of theseconcepts or read his book:Spin Choreography Basic Steps in High Resolution NMR(Oxford UniversityPress,ISBN 0-19-850481-0)!Because we now operate with much stronger magnets than in the old CW days,the way that wegenerate the magnetic field has changed.Permanent magnets are not strong enough for fields above90MHz and conventional electromagnets would consume far too much electricity to make them viable(they would also be huge in order to keep the coil resistance low and need cooling to combat the heatingeffect of the current flowing through the magnet coils).The advent of superconducting wire made higherfields possible.(The discovery of superconduction was made at Leiden University,by Heike Kamerlingh Onnesback in 1911 whilst experimenting with the electrical resistance of mercury,cooled to liquid heliumtemperature.His efforts were recognised with the Nobel Prise for Physics in 1913 and much later,aP1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to comeGetting Started5crater on the dark side of the moon was named after him.The phenomenon was to have a profoundeffect on the development of superconducting magnets for spectrometers years later when technologieswere developed to exploit it.)Superconducting wire has no resistance when it is cooled below a critical temperature.For the wireused in most NMR magnets,this critical temperature is slightly above the boiling point of liquid helium(which boils at just over 4K or about 269C).(It should be noted that new superconducting materialsare being investigated all the time.At the time of writing,some ceramic superconductors can becomesuperconducting at close to liquid nitrogen temperatures although these can be tricky to make intocoils.)When a superconducting magnet is energised,current is passed into the coil below its criticaltemperature.The current continues to flow undiminished,as long as the coil is kept below the criticaltemperature.To this end,the magnet coils are immersed in a Dewar of liquid helium.Because heliumis expensive(believe it or not,it comes from holes in the ground)we try to minimise the amount that islost through boil off,so the liquid helium Dewar is surrounded by a vacuum and then a liquid nitrogenDewar(temperature 196C).A schematic diagram of a superconducting magnet is shown in Figure1.4.Obviously,our sample cant be at 269C(it wouldnt be very liquid at that temperature)so therehas to be very good insulation between the magnet coils and the sample measurement area.In the centre(room temperature)part of the magnet we also need to get the radiofrequency coils andsome of the tuning circuits close to the sample.These are normally housed in an aluminium cylinderwith some electrical connectors and this is referred to as the probe.The NMR tube containing thesample is lowered into the centre of the magnet using an air lift.The tube itself is long and thin(often5mm outside diameter)and designed to optimise the filling of the receive coil in the probe.We wouldcall such a probe a 5mm probe(for obvious reasons!).It is also possible to get probes with differentdiameters and the choice of probe is made based on the typical sample requirements.At the time ofwriting,common probes go from 1mm outside diameter(pretty thin!)to 10mm although there are someother special sizes made.superconducting solenoidliquid N2(77 Kelvin)liquid He(4 Kelvin)vacuumsampleprobe(Tx,Rx coils,electronics)Figure 1.4Schematic diagram of a superconducting NMR magnet.P1:JYSc01JWST025-RichardsSeptember 27,201017:16Printer:Yet to come6Essential Practical NMR for Organic ChemistryProbes are designed to look at a specific nucleus or groups of nuclei.A simple probe would be aproton,carbon dual probe.This would have two sets of coils and tuning circuits,one for carbon theother for proton.Additionally there would be a third circuit to monitor deuterium.The reason for usinga deuterium signal is that we can use this signal to lock the spectrometer frequency so that any drift bythe magnet will be compensated by monitoring the deuterium resonance(more on this later).Thereisavastarrayofprobesavailabletodomanyspecialistjobsbutfortheworkthatwewilldiscussin this book,a protoncarbon dual probe would perform most of the experiments(although having afour nucleus probe is better as this would allow other common nuclei such as fluorine or phosphorus tobe observed).The last thing to mention about probes is that they can have one of two geometries.They can benormalgeometry,inwhichcasethenonprotonnucleuscoilswouldbeclosesttothesampleorinversegeometry(the inverse of normal!).We mention this because it will have an impact on the sensitivityof the probe for acquiring proton data(inverse is more sensitive than normal).Most of the time thisshouldnt matter unless you are really stuck for sample in which case it is a bigger deal.1.4.1Origin of the Chemical ShiftEarly NMR experiments were expected to show that a single nucleus would absorb radio frequencyenergy at a discrete frequency and give a single line.Experimenters were a little disconcerted to findinstead,some fine structure on the lines and when examined closely,in some cases,lots of linesspread over a frequency range.In the case of proton observation,this was due to the influence ofsurrounding nuclei shielding and deshielding the close nuclei from the magnetic field.The observationof this phenomenon gave rise to the term chemical shift,first observed by Fuchun Yu and WarrenProctor in 1950.There were some who thought this to be a nuisance but it turned out to be the effectthat makes NMR such a powerful tool in solving structural problems.There are many factors that influence the chemical shift of an NMR signal.Some are through bondeffects such as the electronegativity of the surrounding atoms.These are the most predictable effectsand there are many software packages around which do a good job of making through bond chemicalshift predictions.Other factors are through space and these include electric and magnetic field effects.These are much harder things to predict as they are dependant on the average solution conformation ofthe molecule of interest.In order to have a reliable measure of chemical shift,we need to have a reference for the value.In proton NMR this is normally referenced to tetramethyl silane(TMS)which is notionally given achemical shift of zero.Spectrum 1.1 shows what a spectrum of TMS would look like.You will notice that the spectrum runs backwards compared with most techniques(i.e.,0 is at theright of the graph).This is because the silicon in TMS shields the protons from the magnetic field.Mostother signals will come to the left of TMS.For some years,there was a debate about this and there weretwo different scales in operation.The scale shown here is the now accepted one and is called.Theolder scale(which you may still encounter in old literature)is called and it references TMS at 10,soyou need a little mental agility to make the translation between the two scales.The scale itself is quotedin parts per m