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Nuclear Induction at Stanford and the Transition to Varian

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Nuclear Induction at Stanford and the Transition to Varian
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  Packard, Martin E.:Nuclear Induction atStanford and the Transitionto Varian Martin E. Packard Varian Associates, Palo Alto, CA, USA; Institute for Genetic DiseaseControl in Animals, Davis, Davis, CA THE BEGINNING—EARLY JANUARY 1946 We glimpsed our first nuclear induction signal one chillynight in early January 1946. The term ‘nuclear induction’was coined because we were looking for a small voltageinduced in the receiver coil by the precessing nuclei. The‘we’ were Professors Felix Bloch, William W. Hansen, and thegraduate student, Martin E. Packard. It was chilly because theexperiment was set up in the cyclotron laboratory which wasin a leaky, unheated, glassed-over light well in the basementof the original sandstone quad of Leland Stanford JuniorUniversity.The glimpse followed a fruitless two hours of adjustmentas we tried to optimize the equipment and parameters tomatch the expected conditions for resonance. The experimentalconditions were set on the assumption that the thermalrelaxation time,  T  1 , would be very long, perhaps days, and thetransverse relaxation,  T  2 , would be very short, correspondingto the dipole–dipole interaction of the neighboring protons.For these reasons, the sample of ordinary water was‘presoaked’, i.e. polarized for 24h in a special outrider gapof the fringe field of the cyclotron, and the radiofrequency (rf)level in the probe adjusted to be a few gauss. The strengthof the magnetic field was adjusted for resonance at a valuecorresponding to the rf frequency of 7.76 megacycles. The 4or 5-in. magnet was connected in series with the cyclotronmagnet, thus using the cyclotron power supply and regulator.The 60 cycle sweep was broad enough amply to cover theputative linewidth of about 5 G. The radiofrequency (rf) levelin the probe was adjusted to be a few gauss and set to theresonant frequency by means of a surplus US Army frequencymeter.The sample was removed by Bloch from the fringe field of the cyclotron magnet and placed in the nuclear induction probewhich was centered in the gap of the small magnet (Figure 1).Nothing was seen. As a last ditch attempt, the current throughthe cyclotron and small magnet was taken much higher thanexpected for resonance and allowed to drift downward at theslow rate determined by the time constant of the cyclotronmagnet.I was watching the oscilloscope screen and saw a weak signal move across the screen, much as one would expect aradar signal to do. The lineshape of the signal was uniquein that it was positive on the right-hand side of the screenand negative on the left-hand side. The pattern was repeated, Figure 1  The magnet and probe assembly of the first nuclear inductionapparatus used by Bloch, Hansen, and Packard at Stanford, circa January4, 1946. The knob turned the ‘paddle’ to balance out the leakage. Theclamps held the laminated pole pieces thus releasing the euphoria which comes to researchers whenthey finally succeed. Thus in a few brief minutes we werecompensated for the work and disappointments of the previousthree months.It was quite clear that our earlier estimate of the thermalrelaxation time was several orders of magnitude too high.During the next day or two Bloch reviewed his equationsand had the explanation for what we had seen; he namedthe condition as adiabatic rapid passage. He understoodthe role of the rapid tumbling of the water molecules inreducing the thermal relaxation time, although he did notanticipate the beautiful description later developed by NicholasBloembergen. 1 Bloch often stressed how treacherous was thetheory of liquids as contrasted to solids and gases. We wereall happy that ordinary water had been the sample of choice.A short letter to the Editor 2 was prepared and appeared in Physical Review , along with the results of Professor EdwinM. Purcell, Henry C. Torrey, and Robert V. Pound. 3 TheNobel committee correctly overlooked this small priority of two weeks, as the two experiments were really conceived andimplemented independently. The 1951 Nobel Prize in Physicswas jointly awarded to Bloch and Purcell.We might have been perhaps a week earlier had I not chosento drive to Oregon to visit my parents over Christmas. I hadnot seen them for some time because of World War II. PROLOGUE The work of assembling the apparatus was started in lateSeptember 1945. However that was not the start of the project.Felix (we usually used his first name) told us the idea came tohim during a concert which he attended while working withProfessor Frederick Terman 4 ∗ at the Harvard Radiation Lab,not the MIT lab. A project he was working on involved findinga coating that would absorb rather than reflect radar signals;the precursor of the Stealth Bomber of the 1990s.A question which is often asked is what is the srcin of an idea. In Bloch’s story we will start with Professor IsadorI. Rabi’s 5 classical experiments. For several years Bloch hadbeen focusing on measuring the value of the magnetic moment eMagRes, Online © 2007 John Wiley & Sons, Ltd.This article is © 2007 John Wiley & Sons, Ltd.This article was previously published in the  Encyclopedia of Magnetic Resonance  in 2007 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470034590.emrhp0136  2  MARTIN E. PACKARD of the neutron. He and Professor Luis W. Alvarez 6 of theUniversity of California at Berkeley carried out a measurementof the neutron moment in terms of the proton cyclotronfrequency. The extension of this method to several nucleirequired an accurate and less cumbersome way to measurethe magnetic field.Bloch, of course, was familiar with Professor Rabi’s 7 classical measurement of proton and deuteron moments in amolecular beam apparatus and with Professor C. J. Gorter’s 8 unsuccessful attempt to observe resonance in a very puresolid. Felix Bloch pioneered the method of polarizing thermalneutrons 9 by passing them through a slab of magnetizediron. I was not aware of his early work in magnetism andthe elucidation of the ‘Bloch wall’ until the arrival of thesemiconductor age. Bloch’s insight led him to a simple way of measuring magnetic fields by observing resonance in matter of ‘normal density’ as contrasted to the molecular beams of Rabi,an important step in expanding the measurement of nuclearmoments. Conversations with Bill Hansen, while they wereboth at Harvard and later at Stanford, showed the feasibilityof the idea and provided the practical basis for the apparatus.Shortly after the Japanese capitulation in the summer of 1945, I was introduced to Professor Bloch by my Westinghousesupervisor and Stanford physics alumnus, Dr. Daniel Alpert.Our group had been sent from Pittsburgh to work on improvingthe mass spectrometer separation of uranium-235 for theManhattan project. Our laboratory was next to the 184-in.cyclotron magnet in the hills above the University of Californiaat Berkeley.After listening to Professor Bloch explain his nuclearinduction idea, I made an immediate decision to join hisgroup, i.e. Bloch and Hansen. A week later my wife, Barbara,and I moved to Palo Alto and I was a graduate student andteaching assistant at Stanford. The admission procedure wasmore streamlined at that time.The distribution of tasks was rather simple and required verylittle interaction between us.Bloch prepared the magnet, starting with a small clumsy C-shaped lecture demonstration magnet. Laminated pole pieceswere attached to allow the use of a large sinusoidal sweep fordisplay of the signal. The magnetic field was measured witha conventional galvanometer and flip coil, the area of whichcould only be estimated to a few per cent.Hansen designed and supervised the construction of the rf receiver portion of the system. This included the probe, thediode detector, and audio preamplifier. This was all containedin a brass box with an appendage, which we called theprobe, that allowed the transmitter, receiver coils, and thesample to be placed between the pole pieces of the magnet.He also conceived and implemented the famous cross-coilconfiguration.Bill Hansen was a genius in designing systems thatwould move but were not overconstrained, like linear motionusing ball bearings in one V groove and a flat. He calledthis approach ‘kinematic design’. His experience with theorthogonal nature of fields in microwave cavities and thedecoupling needed in Doppler radar systems led him todecouple the transmitter and the receiver coil by making themorthogonal. Even at that he chose to use a diode detectorwithout any rf amplification, to avoid saturation.His first configuration, which used wires for constraints,allowed for rotation, but was sensitive to vibration and hence Figure 2  This first nuclear induction probe was constructed byHansen. The crossed wires provided kinematic constraints on rotationto achieve a geometric balance. The design proved to be noisy and wassuperseded by the ‘paddle’ was too noisy. A redesign with the ‘paddle’ for flux steeringwas the solution (Figure 2).My task was to design and construct the rf source, theaudio amplifier, and the display, which was a DuMont 5-in.oscilloscope.The jargon which developed came from our individualexperience in the radar business. The techniques which weused were well known. We were familiar with the detectionof weak signals in the presence of a strong source and theuse of orthogonal decoupling. The sweep field reduced the1/   f   noise by shifting from the dc region. The signal-to-noiseratio was derived from a calculation of the Johnson 10 noisein the resonant receiver coil and the nuclear magnetizationdetermined from the Boltzmann factor. THE LULL AFTER THE FIRST SUCCESS We moved the system into a room adjacent to the cyclotronand reassembled the system, without immediate success. Blochwas to present a talk on the experiment at the local weeklyPhysics Department colloquium. It seemed like a nice idea toshow how simple the nuclear induction effect was by providinga demonstration as part of the talk. That was a mistake; I wasunable to make it work, much to my embarrassment. Bloch inhis usual nice way compensated for my confusion and lack of success. THE BLOCH VERSUS PURCELL METHOD At Stanford we viewed the phenomenon as one of expectation values which could be handled by classicalmechanics and electrical theory, while Purcell viewed thephenomenon as a spectroscopic one with transitions takingplace between the Zeeman levels.The balance of the cross-coil system left only a quadratureleakage which selected for the out-of-phase component of the signal; thus we viewed the dispersion component. Thebalanced bridge of Purcell and co-workers favored the eMagRes, Online © 2007 John Wiley & Sons, Ltd.This article is © 2007 John Wiley & Sons, Ltd.This article was previously published in the  Encyclopedia of Magnetic Resonance  in 2007 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470034590.emrhp0136  MARTIN E. PACKARD  3 absorption component. The initial experiments of Bloch andPurcell appeared to many to be quite different, whilst in realitythe signals observed by the two groups were of the samephenomenon but only in quadrature to each other.Strangely, this misconception persisted for a very longtime. 11 Even 30 years after the initial experiments, I waschallenged by a foreign visitor with the myth about the Blochmethod and the Purcell method.Bloch’s equations proved to be the most useful approachfor us in understanding the details of the nuclear resonances,particularly for steady state and pulses in the rotating frame. 12 However, the effects of spin–spin coupling required thespectroscopic approach to explain the observed splitting of the lines. THE SCIENTIFIC DIRECTION OF THE LABORATORY A review of many of the projects provides insight as to themain thrust of the laboratory and serves to identify the manyable graduate students who obtained their Ph.D. degrees underBloch.Although the work of the laboratory was never highlyorganized and very few group meetings were held, Blochhad a simple objective and projects were selected to furtherthat goal. He felt that the values of the nuclear momentsof the isotopes would be important in understanding nuclearforces. Neutron Measurements In the paper with Luis W. Alvarez prepared in 1939on the neutron magnetic moment, the authors wrote: ‘Thenow rather accurately known values  . . .  of the magneticmoments of proton, neutron, and deuteron are of considerableinterest for nuclear theory.’ At that time, the deuteron momentwas known to an accuracy of 1% to be the sum of theproton and the neutron moments. The accuracy was nothigh enough to test the theories rigorously. Higher accuracywould require a more accurate magnetic field determination,which Bloch later achieved with his nuclear inductionmethod.One of the first applications of nuclear induction was toimprove the accuracy of the neutron moment measurementin terms of the proton. This measurement was done with theStanford cyclotron by Professor Hans Staub, F. Bloch and Dr.D. Nicodemus. 13 The magnet field was automatically stabi-lized accurately at the proper value for neutron resonance by anuclear induction magnetic field controller 14 which I devisedand constructed.The neutron experiments at Stanford and Berkeley hadtheir genesis in the magnetic polarizing techniques proposedin 1936 by Bloch. 9 One of the earliest neutron experimentswas by Dr. Edward Fryer whose Stanford dissertation wason the cross-section of moderated neutrons. 15 Ed Fryer waslater to join Varian as manager of its Quantum ElectronicsDivision, where he worked with magnetometers and frequencystandards.Emery Rogers, who later played a key role at Varian inbuilding market acceptance by the chemists for the use of nuclear magnetic resonance (NMR), measured the sign of the moment of the neutron and proton using the Stanfordcyclotron. The Nuclear Magnetic Moment Spectrometer of Hansen Bill Hansen designed a spectrometer which was to automatethe search for magnetic moments. Its most prominent featurewas a table-top Bitter magnet, named after its designer, Profes-sor Francis Bitter of the Massachusetts Institute of Technology.The automated nuclear induction spectrometer was dropped infavor of the individual creativity and labors of graduate stu-dents.Warren Proctor’s initial assignment was to work on thespectrometer. Warren eased himself out of this approach andwent on to discover the nitrogen chemical shift with Dr. FuChun Yu 16 a , b and to report on the magnetic moments of manyisotopes. Bill Hansen devoted his remaining energy and life todemonstrating the electron linear accelerator. He had workedbefore the war on producing high voltages for an X-ray source.The rhumbatron, which was the heart of the klystron, wasfirst demonstrated as part of a device for producing highvoltages. 17 The linear electron accelerator became the basis of SLAC.Dr. Henry Kaplan of the Stanford Medical School demon-strated its use for cancer treatment. 18 Dr. Edward L. Ginztonat Varian Associates commercialized the small accelerator asthe modality of choice for radiation cancer therapy.Hansen died on May 23, 1949 at a young age from emphy-sema which many of us felt was caused by vapors producedwhile welding beryllium. The Chemical Shift of Nitrogen Warren Proctor and Fu Chun Yu used a dissolvedsample of ammonium nitrate as a source of nitrogen intheir measurement of the nitrogen moment. 16 b The res-onance was found, but to our surprise and mild dis-may consisted of two peaks, one for the NH 4 + and theother for NO 3 − . This effect was quickly attributed to adiamagnetic shielding which was explained by Lamb, 19 and later as a paramagnetic shift by Professor NormanRamsey. 20 This chemical effect, which was to become the mostimportant aspect of NMR for the chemists, forced ashift in the basic strategy of the laboratory. If the valueof the magnetic moments was to be useful in nuclearforce theory, the effect of the chemical shift must beremoved. Erwin Hahn and Spin Echoes Dr. Erwin Hahn brought his beautiful spin echo technique 21a – c to Stanford from Urbana with the puzzle aboutthe structure in the echo when observing organic liq-uids. Felix worked closely with Erwin on the theoreticalaspects of the chemical structure in the echoes. Althoughthe spin echo technique was elegant, it was not widelyused until the advent of magnetic resonance imaging(MRI) which is now an important imaging modality inmedicine. †‡§ eMagRes, Online © 2007 John Wiley & Sons, Ltd.This article is © 2007 John Wiley & Sons, Ltd.This article was previously published in the  Encyclopedia of Magnetic Resonance  in 2007 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470034590.emrhp0136  4  MARTIN E. PACKARD Erwin’s pulse technology pointed the way for observingNMR by the use of pulses to tip the spins. RussellVarian’s proton free precession idea 22 a , b was an alternativemethod of tipping the spins. Both concepts were preludesto Weston Anderson and Richard Ernst’s pulsed Fouriertransform methods. 23 The Fourier transform brought increasedsensitivity and flexibility which was so important to thechemists and later for MRI. Richard Ernst received theNobel Prize in Chemistry in 1991 for his work with Fouriertransforms, which he started while at Varian Associates in1962. Other Moments and Graduate Students Elliot Leventhal measured the magnetic moment of deu-terium precisely in terms of the proton. 24 This experi-ment was the first to use a double tuned receiver andtransmitter coil which allowed the use of two rf frequen-cies set to resonate simultaneously the nuclei of light andheavy water. Irradiating the sample with two frequenciesbecome important to the chemist for magnet stability, cal-ibration, and to control and identify spin–spin interac-tions.Over the years, many nuclear magnet moments wereobserved by the graduate students. Proctor’s early work waswith nitrogen, but he reported on many other isotopes. 16a , 25a – c Harry E. Weaver reported on 12 other isotopes. 26a – d Carson Jeffries measured the proton cyclotron frequencyin terms of the proton moment, 27 , 28 thus providing thelink to the earlier neutron measurements of Alvarez andBloch.This interest in magnetic moments of other isotopes contin-ued for several years even after several of us joined Varian.Dr. Arnold Bloom, who joined Varian in 1953 from the Uni-versity of California at Berkeley, provided the values for theearly compilation of nuclear moments in the ‘CRC Handbook of Chemistry and Physics’. Following Bloom’s departure in1966 for spectra physics, the listing was extended by WestonAnderson and Dr. Kenneth Lee. The current table contains115 isotopes that have been determined by nuclear induc-tion.One of Varian’s early products was a variable tunedspectrometer for observing wideline NMR signals in manyisotopes. Harry Weaver became the product championfor this technology as well as Varian Associates’ entryinto electron paramagnetic resonance (EPR or ESR asit is known to many chemists). A stringent test forthe sensitivity of the wideline nuclear magnetic momentspectrometer was to measure the deuterium in ordinarywater. The deuterium signal is nearly a million-timesweaker than the hydrogen signal in ordinary water, asboth the magnetic moment and the natural abundance aresmaller.One special case was our work with Mel Klein of the Liv-ermore Laboratory on measuring the ratio of   6 Li to  7 Li inenriched samples. 29 The Tritium Experiment at Los Alamos Professor Bloch arranged for us to measure the magneticmoment of tritium. This was during the winter of 1947, atwhich time small samples of triply heavy water were classi-fied and available only at the National Laboratories. My earlierQ clearance came in handy.We loaded all of our apparatus into a truck and movedit to Los Alamos. The principal piece of equipment wasthe large C-shaped magnet, which was borrowed fromUC Berkeley. This was the same magnet, or at least thecoils, used by Ernest O. Lawrence in his early cyclotronexperiments.The apparatus had to be modified, which I was in norush to do, as our six weeks stay in Los Alamos wasvery pleasant. The laboratory had not yet begun to takeon its modern shape. We stayed in the original FullerLodge, and my wife Barbara had time enough to learnabout the culture of the Pueblo Indians of Santa Claraand San Ildefonso. She collected many art objects whichinclude several pieces of Maria Martinez’s now famous black pottery.Felix managed to spend one weekend skiing with us,where I appreciated his mastery of the European telemark turn. 30 On the trip back from the ski slope, I stopped andtook a 16-mm movie of the beautiful scenery. A youngsoldier guard spotted me. I had some trepidation that thismight have been a serious infraction of security, but nothinghappened.The actual experiment was simple and done quickly aswe needed only to adjust the rf to provide for simultaneousdisplay of the signals. The frequencies were measured with asurplus US Army frequency meter. 31a – c After the experimentwas completed, R. Spence, the chemist, reported that thepressure, due to beta decay, in the fractional sample washigher then he had expected. I still wonder what would havehappened if the experiment had been delayed and the sampleruptured. Al Graves was heavily irradiated by the accidentwhich occurred when two  235 U samples were gingerly movedtogether by hand to test criticality. We were not oblivious tothe risks of radioactive materials and were careful, but notparanoid. Sensitivity The need for increased sensitivity was always presentand remained a challenge. David Garber 32 worked on aphotographic technique for averaging multiple sweeps toreduce low frequency noise. The concept was sound but itwas not applied until much later when Varian introducedthe all-digital C-1024. At that time I was Division Man-ager at Varian and forecast that only 10 units would besold. In actuality, hundreds were bought and used by thechemists. Lineshapes The accurate measurement of the resonant frequencyrequired that we set the resonant condition well withinthe linewidth. As the resolution increased, we saw thatthe absorption lineshape was not a simple Lorentzian butasymmetric and followed by ‘wiggles’. My solution wasto follow our experience from the radar field and use asine wave sweep and set on the center of symmetry. I eMagRes, Online © 2007 John Wiley & Sons, Ltd.This article is © 2007 John Wiley & Sons, Ltd.This article was previously published in the  Encyclopedia of Magnetic Resonance  in 2007 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470034590.emrhp0136  MARTIN E. PACKARD  5 suggested this approach to Felix, who thought a moment andagreed.The solution of the Bloch equations for the steady-state con-dition was trivial, but a real challenge for other conditions. Dr.Boris Jacobsohn and Rowald Wangsness 33 developed an ana-lytical solution which included the wiggles. Years later Dr.Arnold Bloom tried to solve the equations using the big ana-log computer at Stanford, with no results. The machine lackedthe necessary stability. THE BEGINNING OF HIGH RESOLUTION The sensitivity and the resolution improved steadily overthe first years. Two of the standard experiments which wereused to impress visitors were to observe the signal from afinger placed in the probe and to demonstrate one’s strengthby pressing on both sides of the C-configured magnet to causethe signal to shift across the oscilloscope screen.Bloch’s hope was that the true value of the magneticmoments could be determined by applying a calculatedcorrection of the chemical shift. This seemed possible for thelighter isotopes but the precision had to be very high. The All-New 12in. Electromagnet Harry Weaver was assigned the job of designing a magnetwhich would have larger pole pieces for homogeneity, a higherfield for sensitivity, and a full yoke for stability. The windingswere high impedance as the current was regulated usingsurplus modulators which had eight 304TL vacuum tubes.This basic magnet design was used by Varian in all the high-resolution research NMR spectrometers until the advent of thesuperconducting magnet 34 which was also Harry’s contributionto the chemist. Helium-3 It was natural to try to measure the magnet moment of  3 He as part of the effort to reduce or remove the effectof the chemical shift. Again it was hoped that calculations,particularly of the lighter isotopes, would allow a correctionto give the true magnetic moment.Bloch obtained a very small sample of   3 He. I blew the glassfor the sample-handling system and prepared the sample vialfilled with FeO 2 35 as a means of reducing the relaxation time.The sample vial was evacuated and the helium transferred. Itremained for H. L. Anderson 36 to provide the value, as wesaw no signal. Whether this was due to the very small sampleor to a flaw in the relaxation process we never knew.Harry Weaver’s big 12in. electromagnet was placed inservice sometime during 1950. James T. Arnold, a graduatestudent, and I set out to utilize its size and field strengthto improve sensitivity and resolution to measure the magnetmoments of the light isotopes better and to correct for theeffect of the surrounding electrons.With these narrow lines, hence low rf fields and betterdecoupling between the transmitter and receiver coils, wechose to use an rf amplifier. The leakage was adjusted topresent the absorption mode. The operating frequency waschosen to be 30 Mc s − 1 (MHz) because that was the frequencyof the surplus radar intermediate frequency (if) strips. Stanfordscience benefitted from the mountain of surplus gear whichwas scattered about the old and condemned dormitory, SequoiaHall. Thirty megahertz and 7000 G became the de factostandard for several years.The pole pieces of the magnet were not finely machinedor polished, which was somewhat of a blessing. Jim Arnoldbecame a master at moving the probe within the gap to find alocal region of high homogeneity. We tried some rudimentaryelectric shimming to improve the homogeneity. It remainedfor Dr. Edwin Jaynes 37 to provide the theoretical basis forthe orthogonal electric shims and for others to make thempractical by moving them from a spherical geometry to theflat pole pieces. 38a – d Another major improvement in homogeneity came withthe spinning sample idea of Bloch’s which was successfullydemonstrated by Arnold and Anderson. 39a – c Bloch reportedthat the idea came to him as he was stirring a cup of teamidday at home! HIGH RESOLUTION IN ORGANIC COMPOUNDSThe Alcohol Experiment Although we were able to measure the shifts, for example,between water and mineral oil, the standard of groups inthe East, 40 this was set aside following the suggestion of Dr. Shrinvas Dharmatti, a postdoc from the Tata Institute inBombay. He asked what would happen if we looked at anorganic liquid.We first tried ethyl alcohol, which was one of our favoritesubstances, and were rewarded with the view of three separatepeaks. This led us to publish a comparison of the area under thepeaks in various alcohols. This paper, although not importantin itself, attracted considerable attention. 41a – d It was thispicture which later brought Dr. James Shoolery 42 to Varian,an event which contributed so much to the development of NMR spectroscopy for and by the chemists.Felix Bloch did not choose to place his name on the seminalhigh-resolution paper of organic compounds even though itwould have been appropriate. Although he never objected toour foray into chemistry, that was not his major interest.The spectrum shown in the first alcohol paper wasdeliberately broadened by selecting an inhomogeneous spotin the magnet, but with great care we could observe threelines in the methyl group and four in the methylene group.The hydroxyl group remained only one peak (Figure 3).This splitting was the result of the spin–spin coupling whichturned out to be so important to the chemists as it providedinformation about the number and closeness of near neighbors. Hydrogen Bonding of the Hydroxyl Group in Alcohol An important demonstration of the intermolecular exchangewas noted by Jim Arnold who observed that the separationbetween the hydroxyl and methylene peak was temperaturedependent. This we did not understand and decided to publisha short letter describing the effect. We were very quickly eMagRes, Online © 2007 John Wiley & Sons, Ltd.This article is © 2007 John Wiley & Sons, Ltd.This article was previously published in the  Encyclopedia of Magnetic Resonance  in 2007 by John Wiley & Sons, Ltd.DOI: 10.1002/9780470034590.emrhp0136
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