Research Profile

by Luisa Bonolis

Richard R. Ernst

Nobel Prize in Chemistry 1991 "for his contributions to the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy".


A Thesis in Nuclear Magnetic Resonance

Richard Ernst was born in 1933 in Winterthur, Switzerland. At the age of 13, his interest in chemistry was sparked by a case of chemicals he found in the attic of his home. The chemicals had belonged to a deceased uncle who had been a metallurgical engineer. At that time, the young Ernst was fond of music; he played violoncello and tried musical composition, but as he recalled in his Nobel lecture, when he discovered the magic of chemistry, an exciting period of his life began: “I became almost immediately fascinated by the possibilities of trying out all conceivable reactions with them, some leading to explosions, others to unbearable poisoning of the air in our house, frightening my parents. However, I survived and started to read all chemistry books that I could get a hand on, first some 19th century books from our home library that did not provide much reliable information, and then I emptied the rather extensive city library. Soon, I knew that I would become a chemist, rather than a composer. I wanted to understand the secrets behind my chemical experiments and behind the processes in nature.”

After high school he enrolled “with high expectations and enthusiasm” at the Eidgenössische Technische Hochschule (ETH) in Zurich and earned his diploma in Chemical engineering in 1956. Ernst then started a Ph.D. thesis and his professor, Hans Günthard, who was a physical chemist, associated him with the young and brilliant scientist Hans Primas, whose work at the time was concerned with high-resolution nuclear magnetic resonance spectroscopy, a technique exploiting the magnetic properties of certain atomic nuclei to analyse the properties of matter.

The NMR phenomenon is based on the fact that nuclei of atoms have magnetic properties deriving from their spins. Nuclear magnetic resonance can be observed when a material in which nuclei have a magnetic dipole moment is placed in a magnetic field. The most notable example is the nucleus of the hydrogen atom, which is a single proton. In the presence of an external magnetic field, Nuclear magnetic moments precess like a top. The nuclear precession angular frequency, or Larmor precession frequency, is directly proportional to the strength of the field and is typically in the range of radio frequency, using magnets that can produce fields of the order of 10 Tesla. Nuclei can occupy only a limited number of energy states in a magnetic field, corresponding to different orientations of their spin angular momentum. A weak oscillating field at a right angle to the static field applies a torque to the nuclear moment, tending to tip them, thus changing their orientation with respect to the static field. Transitions between nuclear energy levels can be induced by an oscillating field of the appropriate frequency, the Larmor frequency. Once the resonance condition is achieved, the alternating field satisfies Bohr's frequency condition for the energy difference between the two states, analogous to the resonance absorption of visible light. However, instead of optical frequencies, one deals here normally with frequencies in the radio range, so that the application of the magnetic resonance method is properly labelled as belonging to the field of radiofrequency spectroscopy. At resonance, the desired tilt of the nuclear moments is produced, and a transition occurs from one Zeeman level to another. Detecting the transitions thus provides knowledge of the Larmor precession frequency, which is proportional to the strength of the field through the gyromagnetic ratio, a constant characteristic of each nucleus or nucleon. The gyromagnetic ratio is also the constant of proportionality between the magnetic moment and the nuclear spin angular momentum. If the spin is known, the simultaneous knowledge of the resonance field and the Larmor frequency directly yields the gyromagnetic ratio, making it possible to determine the nuclear magnetic moment with extraordinary precision.

Isidor Rabi had already described and demonstrated the phenomenon of magnetic resonance in molecular-beam experiments in the late 1930s. Rabi's research had been aimed primarily at measuring nuclear moments - which until then were either unknown or only very approximately estimated from small splittings (hyperfine structure) in atomic spectra. In 1937, simultaneously with Rabi, Felix Bloch had the idea of using the resonance condition to directly measure the magnetic moment of the neutron. Between 1939 and 1940, Bloch and Luis Alvarez performed an ingenious experiment using a neutron beam produced with Ernest Lawrence's 37-inch cyclotron at Berkeley.

In 1945, as a natural follow-on to the wartime effort related to radar research, Felix Bloch and Edward Purcell extended Rabi's magnetic resonance experiments. They worked simultaneously, albeit with different conceptual bases and in different places, at Stanford and Harvard. Whereas beam experiments are performed with vaporised samples of atoms or molecules relatively isolated one from one another, postwar NMR could detect the resonance transitions of nuclei in condensed liquid, solid and gaseous samples. If the sample is inserted into an electrical coil and placed into a strong homogenous magnetic field, magnetism of nuclei can be detected by tuning the radio transmitter and the receiver to the Larmor frequency. The strength of the signal is proportional to the number of nuclei in the sample that produce it. How much energy can be absorbed depends on the population difference between the two states. At equal population, irradiation causes the same number of magnetic moments to absorb and emit energy. No effect will be observed. NMR is a rather sensitive spectroscopy since it is capable of detecting very small population differences.
For their discovery, Bloch and Purcell were awarded the Nobel Prize for Physics in 1952.

The analysis of a sample provides a spectrum of frequencies typical of that specific sample. Nuclear magnetic resonance spectroscopy is basically a graph of the detected intensity against the wavelength or frequency. The new NMR technique made accessible the resonance of nuclear magnetic moments that were previously either unknown or only approximately measured, but the real novelty was that discoveries of splittings and structure in NMR lines showed the nucleus to be a sensitive probe within the atomic environments in which the nuclear moments are embedded, providing information reflecting basic structural and dynamic properties of the matter. Similar nuclei (e.g., hydrogen nuclei) in different positions in a molecule experience slightly different magnetic fields because the applied field is slightly affected by the local magnetic fields created by electrons in a molecule. Therefore, in the simplest case, different protons in different parts of a molecule will have slightly different resonance frequencies. The partial shielding of the nuclei from the applied magnetic field by the electrons surrounding the nuclei produces what is known as the chemical shift and can be used to identify certain functional groups in molecules.

Being able to measure the shifts in resonant frequency caused by the structural and electronic environment of a nucleus made NMR a powerful tool for chemical analysis. Coupling constants were another of the most important parameters obtained from NMR experiments, providing structural information, particularly with respect to the relative orientations of neighbouring bonds. In allowing detailed studies of the topology, dynamics and three-dimensional structure of small and large molecules in solution and the solid state, NMR provides unique information about the way molecules move and interact with each other. In the early 1950s NMR spectroscopy became a fundamental tool, used within practically all branches of chemistry, at universities as well as industrial laboratories. More generally, NMR had a revolutionary impact on condensed matter physics, biochemistry and medicine, far exceeding its importance in nuclear physics.

The development of new applications of NMR derived from parallel improvements in instrumentation and methodology. In the late 1950s, when Ernst was a Ph.D. student, the field of nuclear magnetic resonance was in its infancy, but high-resolution techniques were being developed in order to study in detail effects such as chemical shifts and interactions between different nuclei in a molecule leading to splitting of resonance lines, effects which made spectra more complex, but provided useful information.

With Primas, Ernst worked both on the development of the theoretical background for the experiments they wanted to perform as well as on the design and construction of advanced electronic equipment for improved NMR spectrometers. Ernst's own work dealt with the construction of high sensitivity radio frequency preamplifiers for a proton resonance spectrometer. On the theoretical side, he was concerned with stochastic (random) resonance, a way of boosting undetectable signals by adding white noise resonating with frequencies corresponding to the frequencies of the original signal. Ernst completed his Ph.D. in physical chemistry with a thesis on instrumentation and a theoretical study of NMR with a stochastic signal that would excite a wide range of frequencies simultaneously.

From this work, Ernst derived an understanding of the treatment of nuclear spin systems, that became a precursor to his later work that led to the Nobel Prize.


Fourier Transform Spectroscopy: a New Methodology in NMR

When Ernst finished his thesis in 1962, he wanted to find some stronger motivation for his work and thus decided to find an industrial job in the United States. Among numerous offers, he chose Varian Associates in Palo Alto, where many of Bloch's students were working. The close collaboration between Bloch's group and research at Varian led to the introduction of the first commercial NMR spectrometer in 1953, basically an instrument for researchers. The first model intended for routine use by organic chemists was introduced in 1961.


One of the main problems in NMR is the low sensitivity compared with other spectroscopic techniques, which severely limits its application and had worried researchers since its discovery. The magnitude of the energy changes involved in NMR spectroscopy is very small. This means that sensitivity can be a limitation when looking at very low concentrations. One way to increase sensitivity would be to record many spectra and then add them together. However, using a conventional continuous wave instrument the time needed to collect the spectra is very large.
During the early years, the main emphasis had been put on perfecting instruments. In the early 1960s, several groups also started to consider improvements of measurement techniques. In continuous wave standard technique, the resonance condition is matched for one frequency at a time, by sweeping the magnetic field through resonance with a monochromatic frequency, in order to observe the resonant absorption signals. Continuous wave NMR spectrometers are similar in principle to optical spectrometers. The sample is held in a strong magnetic field, and the frequency of the source is slowly scanned (in some instruments, the source frequency is held constant, and the field is scanned). Continuous wave spectroscopy is inefficient since it probes the NMR response at individual frequencies in succession.

When Ernst joined Varian Associates, he worked with Weston Anderson, one of Bloch's brilliant students. Anderson was working on methods to improve the NMR sensitivity experimenting with a multiple channel spectrometer concept. Rather than sweeping through all the resonance lines in a spectrum of a particular nuclear species sequentially, as was the standard practice, they realised that simultaneous irradiation of a material with all the frequencies in the spectrum, using a strong radio-wave pulse, might lead to the long-desired breakthrough. In 1964, they tried to find a realisation of this concept. First, an efficient source for the numerous excitation frequencies was required. A repetitive sequence of very short pulses was a rather obvious choice. Applying such a pulse to a set of nuclear spins would simultaneously excite all the single-quantum NMR transitions. After the pulse, the nuclei return to thermal equilibrium inducing a large electrical signal at the NMR frequency in the coil. The signal decreases exponentially with time as a result of relaxation processes and thus is called the free induction decay or FID.

The second problem was the separation of the responses of the various spectral elements. If the NMR sample has a spectrum consisting of several lines, then all the different NMR frequencies are transmitted simultaneously after the radio-frequency pulse. The result is exactly analogous to what happens if two instruments that are not in tune sound the same note simultaneously. A beat note is obtained whose frequency is equal to the difference between the frequencies played by each instrument.

The decaying signal contains the sum of the frequencies from all the target nuclei. It is picked up in the coil as an oscillating field generated by the magnetic moments rotating back to equilibrium. The signal is mixed with a lower frequency signal to produce an interferogram at low frequency. This interferogram is actually what is called the FID. It contains the basic information on frequencies and intensities characteristic of the spectrum but in a form that is not directly interpretable. Using some basic facts from linear response theory, it was clear that the response to the short pulses is just the Fourier transform of the desired spectrum. Fourier transformation (FT) is a mathematical operation employed to transform signals between time (or spatial) domain and frequency domain. Fourier transform methods permit the extraction from the complicated interference pattern of all the desired information on frequencies and amplitude, making it possible to convert the information into a more familiar form. Fourier transformation of the FID yields a frequency spectrum. In this case, the FID signal picked up by the receiver is transformed from the time domain to the frequency domain, giving the desired NMR spectrum. In conventional NMR spectroscopy, it would require thousands of seconds to obtain a complete spectrum with a resolution of 1 hertz. The same basic spectral information can be obtained in 1 second with the use of FT methods, which can be actually applied to many types of spectroscopy.
This completed the concept of what became known as Fourier transform NMR spectroscopy. Anderson's and Ernst's article on the application of Fourier transform spectroscopy to magnetic resonance was published in January 1966, but as Ernst recalled in his Nobel Lecture, “The paper that described our achievements was rejected twice by the Journal of Chemical Physics to be finally accepted and published in the Review of Scientific Instruments. Varian also resisted to build a spectrometer that incorporated the novel Fourier transform concept. It took many years before in the competitive company Bruker Analytische Messtechnik Tony Keller and his coworkers demonstrated in 1969 for the first time a commercial FT NMR spectrometer to the great amazement of Varian that had the patent rights on the invention.”

In practice the Fourier transformation is normally carried out in a digital computer, where the subsequent response is stored and converted to a familiar spectrum. For typical spectra of protons, this new method could increase the sensitivity by a factor of 30 or more. Initially, Ernst and Anderson acquired the free induction decays in a time averaging computer and punched the data on paper tape. The paper tape had to be carried from Palo Alto to IBM in San Jose to transfer the data to a bunch of cards. With the cards they went to the Palo Alto Computer Service Center where the Fourier transformation and the plotting were done. Fortunately the development of Fourier Transform NMR coincided with the development of digital computers and the digital Fast Fourier Transform, which revolutionized the process. Of major importance for the success of more advanced experiments and measurement techniques in NMR was the availability of small laboratory computers that could be hooked up directly to the spectrometer. Between 1966 and 1968, Ernst and Anderson developed numerous computer applications in spectroscopy for automated experiments and improved data processing. Nevertheless, it took several years before the first Fourier transform NMR spectrometer became commercially available. The computerised, pulsed NMR Fourier transform method of spectroscopy, initiated by Ernst and Anderson thus came into common usage and set the stage for the pioneers of Magnetic Resonance Imaging.

The introduction of the technique of pulse excitation and Fourier transformation had a profound impact on the application of high-resolution NMR spectroscopy. FT-NMR makes it possible to study small amounts of material as well as chemically interesting isotopes of low natural abundance. The spectroscopy of a number of nuclear species, which previously had been severely limited by poor sensitivity, became a practical reality. The most notable example was Carbon-13. The low natural abundance (1.1 %) of this isotope contributes to the low sensitivity, but at the same time, the small number of atoms in a limited number of structural situations leads to relatively simple spectra, from which the molecular structures can be determined.
In 1968, Ernst left Varian and returned to Zurich, where he took a position as Privatdozent in physical chemistry at the ETH, continuing his work in liquid-state and later in solid-state NMR.

Two-dimensional FT-NMR

In 1971, Basko Polje and Jean Jeener proposed a novel experiment in which new information could be extracted by applying two separate pulses to a sample and analysing the two-dimensional data set by a double Fourier transformation. At the time, the proposal did not attract much attention, partially because of the limited computing power available and the complexity of double Fourier transformations. Ernst extended Jeener's suggestions, publishing the first experimental results in 1975. With his students he began a new series of developments that opened the application of NMR to increasingly larger molecules.

At a constant field strength, the resonance frequency of a particular nucleus depends on its chemical environment. This relatively small chemical shift effect (usually expressed in parts per million or ppm) is the main reason why an NMR spectrum contains such an abundance of detail about chemical structure. This detail can be used if each resonance line can be assigned to its corresponding nucleus. A great spectral simplification can be obtained by spreading the one-dimensional nuclear magnetic resonance spectrum in two independent frequency dimensions. The power of 2-D spectroscopy lies in its ability to resolve overlapping spectral lines, to enhance sensitivity, and in addition to make it feasible to measure internuclear distances and scalar coupling constants in molecules that are too complex for a 1-D approach. A 2-D NMR experiment adds an additional dimension to the spectra by varying the length of time the system is allowed to evolve following the first pulse. The technique involves the collection of data as a function of two independent time domains, followed by a double Fourier transformation. The resulting two-dimensional spectrum contains one intensity axis and two frequency axes. These two-dimensional experiments correlate signals based on couplings between nuclei in a molecule, thereby allowing the analyst to more clearly determine where the atoms lie in a molecule and in general to get information not available through 1-D methods. It provides the visual representation of the mutual relations between nuclei in molecules in terms of their proximity and their spatial arrangement in the chemical bonding network.
2-D FT NMR extended the range of applications of NMR spectroscopy, particularly with respect to large, complex molecules such as DNA and proteins. Later developments expanded the 2-D experiments to three or more dimensions. A technological innovation of great importance was the introduction of superconducting solenoids, which made possible an increase in the frequency at which hydrogen NMR signals could be observed. As a result, sensitivity was greatly increased, enabling the study of much smaller samples.

The introduction of the multidimensional techniques, coupled with the technological development of superconducting magnets with high fields, allows NMR to challenge X-ray diffraction methods for the determination of the three-dimensional structure of large molecules, in particular for proteins and nucleic acids.

Magnetic Resonance Imaging

A completely new application of NMR was initiated in 1972, when Paul Lauterbur at the State University of New York described a method of producing cross-sectional images of medical and other objects with NMR. A similar idea was independently proposed by Peter Mansfield at Nottingham, with particular application to the study of solid samples. The fundamental idea is to put the object one wishes to image in a magnetic field that varies in a known and controllable manner with position. Since the resonance frequency is proportional to the strength of the magnetic field, different parts of the object would have different resonance frequencies. The size of the resonance signal at each frequency indicates how many nuclei are at that frequency, i.e., at the position at which the corresponding magnetic field occurs.

Fundamental medical imaging methods with a variety of techniques were soon developed by several groups. In 1975, Ernst and his students entered the field, publishing a new method for collecting image information. It was an extension of the method of two-dimensional spectroscopy, which was readily extended to three dimensions and in a modified form later became a major feature of most magnetic resonance imaging (MRI) techniques. The basic idea of image formation is simple, but translating the idea into a practical imaging system involves major engineering and design work. For this reason the first commercial imaging apparatus did not appear until the early 1980s.

Richard Ernst's vital role in the conceptual developments of the methodology of high-resolution NMR spectroscopy, which are the key to the analysis of the structure of large biological molecules, has been recognised by numerous honours, culminating in 1991, when he was awarded the Nobel Prize in Chemistry.


Bibliography

Bax A. and Lerner L. (1986) Two-Dimensional Nuclear Magnetic Resonance Spectroscopy. Science 232 (4753): 960-967
Ernst R. R. (1991) Autobiography. Available at http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1991/ernst-bio.html
Hill H. (1993) Richard R. Ernst. In K.J. Laylin (ed.), Nobel Laureates in Chemistry. 1901-1992. American Chemical Society and the Chemical Heritage Foundation, pp. 759-765
Pake G. () Nuclear magnetic resonance in bulk matter. Physics Today 46(10):46-50
Rigden J. S. (1986) Quantum states and precession: The two discoveries of NMR. Reviews of Modern Physics 58(2): 433-448
Slichter C. P. (1998) The Golden Anniversary of Nuclear Magnetic Resonance NMR: Fifty Years of Surprises. Proceedings of the American Philosophical Society 142(4): 533-556


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