Nuclear magnetic resonance spectroscopy

The nuclear magnetic resonance spectroscopy ( NMR spectroscopy of English nuclear magnetic resonance) is a spectroscopic method for studying the electronic environment of individual atoms and interactions with neighboring atoms. This allows the elucidation of the structure and dynamics of molecules and concentration determinations.

The method is based on the nuclear magnetic resonance of a resonant interaction between the magnetic moment of atomic nuclei of the sample is in a strong static magnetic field with a high frequency alternating magnetic field. It is only those isotopes spectroscopy available which have a non-zero nuclear spin and therefore a magnetic moment in the ground state. For example: 1H; 2D; 6Li; 10B; 11B; 13C; 15N; 17O; 43Ca; etc.

• 5.1 Chemical shift and integration of signals
• 5.2 Scalar coupling

• 9.1 Scalar coupling
• 9.2 Example spectra

History

On the prehistory of nuclear magnetic resonance spectroscopy see history and development of nuclear magnetic resonance.

Felix Bloch and Edward Mills Purcell reported for the first time after 1946 signals of nuclear magnetic resonance, for which they received the Nobel Prize in 1952. In her Nobel Prize lecture, they showed first spectra with the detection of the chemical shift ( on the example of ethanol ), thus the actual magnetic resonance spectroscopy began. It developed into an important method in the chemical structure elucidation. First, the continuous-wave (CW) has been mainly used method in which the resonances were excited one after the other by varying the frequency or the field. 1947 Varian ranged Russell and Felix Bloch a patent, a first for the nuclear magnetic resonance spectrometer. The first commercial magnetic resonance spectrometer built 1952, Varian Associates in Palo Alto. Around 1955, the Japanese company also built Jeol NMR spectrometer. The American biophysicist Mildred Cohn began in the early sixties, the nuclear magnetic resonance spectroscopy for the elucidation of metabolic processes on a molecular level. Your groundbreaking pioneer work in this field led to methods and applications that are still used by many researchers.

Since the CW technique was characterized by a poor signal -to-noise ratio, a pulse Fourier transform NMR spectrometer developed from the mid- 1960s, Richard R. Ernst ( Nobel Prize in Chemistry 1991), Varian (FT- NMR ), which allowed a much faster absorption of the spectra. At the same time measuring the mean as compared to the CW spectrometer, a significant increase in sensitivity, and thus the signal to noise ratio. Already in the years 1949 and 1950 were examined by Hahn and Torrey, the first pulse method. The first commercial nuclear magnetic resonance pulse spectrometer were the mid-1960s by the German company Bruker (founded by Günther Laukien, one of the pioneers in Germany NMR ) in Karlsruhe built by a group led by Bertold Bludgeon and Manfred Holz. It was followed by the introduction of broadband decoupling and multi- pulse method. From an idea by Jean Jeener 1970s multi-pulse experiments were developed with a systematic variation waiting time between two pulses from the beginning, which led to the two-dimensional spectra after Fourier transformation over two time ranges.

Kurt Wüthrich and others built this 2-D and multi- dimensional NMR in a significant analysis technique in biochemistry from, especially for the structural analysis of biopolymers such as proteins. Wüthrich received for this work in 2002 the Nobel Prize in Chemistry. In contrast to X-ray structure analysis, the nuclear magnetic resonance spectroscopy provides structures of molecules in solution. Of particular importance is the ability to obtain detailed information on the molecular dynamics with the aid of relaxation parameters.

Physical background

Quantum mechanical foundations

And nuclei particles, which have a nuclear spin have a rotating carrier having a magnetic moment, which is often referred to. The magnetic moment of atomic nuclei can not take any, but only certain, described by quantum mechanics orientations in an external magnetic field. The number of possible orientations is determined by the nuclear spin quantum number determined (see: multiplicity ). For each nuclear spin quantum number, there are guidelines and each orientation is associated with a nuclear magnetic spin quantum number.

Examples:

• Hydrogen nucleus with nuclear spin: two orientations and
• Deuterium nucleus with nuclear spin: three orientations and

Without an external magnetic field, the states marked with the same energy (see degeneration ( quantum mechanics) ). In the presence of an external magnetic field created energy differences ( Zeeman effect). Nuclear magnetic resonance phenomena based on the excitation of the nuclear spin transitions between such states. The energy required for this is proportional to the strength of the external magnetic field and the gyromagnetic ratio of the observed nucleus:

This energy is applied by irradiation with resonant electromagnetic waves. The resonance frequency is referred to as the NMR Larmor frequency and is in the radio wave range. Conventional NMR spectrometer operating at proton resonance frequencies between 300 MHz and 1 GHz.

If all the 1H or 13C atoms would have the exact same Larmor frequency, the NMR method for structural analysis would be of little interest. In fact, the resonant frequencies depend on the individual, active nuclear magnetic fields. These local magnetic fields can vary in strength from the main magnetic field, for example, by the influence of the electronic environment of a nucleus or by magnetic interaction between neighboring nuclei. Due to these properties, the nuclear magnetic resonance spectroscopy is used for the structure elucidation of molecules.

Measuring method of the nuclear magnetic resonance spectroscopy

For the measurement the sample is placed in a homogeneous magnetic field, the so-called main magnetic field. The sample is surrounded by an induction coil which generates a high frequency alternating electromagnetic field perpendicular to the main magnetic field. Then, varying the strength of the main magnetic field until the resonance occurs ( continuous-wave method, obsolete). Alternatively, the magnetic field strength is kept constant and the frequency of the incident alternating field are varied ( engl. continuous field, out of date). When the resonance occurs, so the sample absorbs energy from the alternating field, the current, which is required for the construction of the alternating field changed. This can be measured.

Modern measurement method no longer emit continuous alternating fields into the sample, but radio wave pulses. A short pulse excites the radio wave thereby to a frequency band whose frequency range is inversely proportional to the pulse duration of the Fourier relationship. This will be in the sample, all transitions that fall within this frequency band simultaneously excited. With proper choice of pulse duration and peak power, the magnetization of the excited nuclear spins to be brought into the transverse plane perpendicular to the main magnetic field. After the end of the pulse, this transverse magnetization oscillates for a short time perpendicular to the main magnetic field. Here, each nuclear spin oscillates with its own individual Larmor frequency. This sum of these oscillations is detected as electric current by electromagnetic induction with the same induction coil, which was used for transmission of the excitation pulse. The received signal is digitized and recorded. With the aid of fast Fourier transform, it is possible to extract the individual Larmor frequencies of the sum of the oscillations in order to obtain an NMR spectrum. That is why modern NMR methods bear the name PFT -NMR for pulsed Fourier Transform NMR Spectroscopy. For this method the main magnetic field is static, usually it is generated with the aid of superconducting electromagnets, which are cooled by liquid helium and nitrogen.

A special method is the NMR CIDNP spectroscopy, wherein the sample is irradiated to generate free radicals, or heated with light.

Relaxation

With the PFT -NMR exist three relaxation processes, on the one hand limit the performance of the PFT -NMR, but on the other hand provide unique information on the molecular dynamics and material inhomogeneities. The relaxation is the process of return of the nuclear spins from the excited state to the thermal equilibrium with the release of the absorbed energy in the excitation as heat. Used between two NMR experiments ( scans) does not wait for the complete relaxation, as is available for each other only a certain, lower magnetization and hence signal intensity. If a short Repetitionzeit is desired, the excitation angle must be reduced (on the so-called Ernst angle ) to still get the strongest possible signal. The relaxation is the dephasing of transverse magnetization due to entropic effects related to the magnetic dipole -dipole interaction of adjacent nuclei. No energy is emitted, since the pins remain in the excited state, but the transverse magnetization vector running from a first to a fan and finally a circle apart so that no NMR signal is more induced in the detection coil. If there are additional magnetic field inhomogeneities in the sample, either by imperfections of the main magnetic field or by susceptibility differences within the sample, the accelerated relaxation is observed instead of relaxation.

The relaxation or relaxation limits the lifetime of the NMR signal immediately after excitation. The NMR signal is measured about a damped oscillation, as FID ( Free Induction Decay ). In practice, the relaxation time is measured by the spin echo method.

The relaxation limited how fast NMR experiments can be performed in succession. The relaxation and the relaxation depend strongly on the density and viscosity / rigidity of the sample.

Sensitivity of NMR spectroscopy

An inherent problem of NMR spectroscopy is its relatively low sensitivity ( poor signal -to-noise ratio). This is due to the fact that the energy differences between the states is small and the population differences between the states in thermal equilibrium are very low ( Boltzmann distribution).

The occupation ratio of the two energy states involved can be expressed by the difference in energy relative to the thermal energy at a given temperature T:

Is the Boltzmann constant. The difference in energy corresponds to the energy of an energy quantum () that a particle of cheaper in the unfavorable state promoted ( fundamental equation of spectroscopy). At a resonant frequency of 600 MHz and a temperature of 0 ° C or 273K, a value of approximately e0, 0001, that is very close to unity. Therefore, almost the same number of nuclei in the excited state as in the ground state already in thermal equilibrium. For comparison, visible light has a factor of about 1 million higher frequencies. Thus have transitions that are excited by visible light, population differences of approximately e100, are so completely in the ground state, which makes the spectroscopy in the visible range are much more sensitive.

The sensitivity is thus essentially of four factors:

• Temperature ( dependence 1 / T)
• Difference of the energy states (proportional to magnetic field strength and isotope -specific gyromagnetic ratio)
• Frequency of the isotope
• Spin quantum number of the observed isotope

The factors and isotopic abundance can be expressed by the relative sensitivity. In this case, 1H is used as the reference frequency with the relative one. Thus, for an isotope with spin quantum number and the gyromagnetic ratio at the same temperature, the same magnetic field and with the same isotopic abundance, relative sensitivity:

Multiplying this value with the natural abundance of the isotope, one obtains the absolute sensitivity. More rarely, the relative sensitivity is given to 13C as a reference.

To increase the sensitivity, various measures are taken:

• Measurement sensitive as possible nuclides (especially 1H)
• Magnetic enrichment nuclides whose natural frequency is low (eg 13C or 15N ). This is done for example in proteins often.
• Signal accumulation by multiple measurement of a sample and addition of all spectra
• Increasing the magnetic field strength by using stronger ( superconducting ) magnets.
• Lowering the temperature of the sample
• Reduction of electronic noise by cooling the receiver ( Cryo ).
• Use of hyperpolarization methods to increase the population differences artificial.
• Taking advantage of the nuclear Overhauser effect ( NOE )
• Transfer of magnetization sensitive nuclei ( 1H) to less sensitive (13C ) ( cross-polarization CP)

Lowering the temperature and increasing the magnetic field strength change the thermal population of the states of equilibrium, so that more magnetic transitions may be excited. With the aid of a hyperpolarization occupation imbalance can be generated, which is highly different from the thermal balance, and in which the lowest energy state is almost completely occupied.

In order to optimize signal detection low-noise electronics are used. The use of electrical resonant circuits is limited to the detection of a narrow frequency band in the range of the expected Larmor frequency, ie the detection of spurious signals and noise from the other frequency bands is suppressed.

The signal accumulation is used to improve the signal -to-noise ratio. NMR measurement is carried out twice in an identical manner and the measured signals of the individual measurements can be added. Through this accumulation, the NMR signal strength increases by a factor, while random noise increases only by a factor. As between NMR experiments should be the complete relaxation of the spins and the relaxation of organic matter may take several tens of seconds, the signal accumulation can lead to a considerable extension of the measurement period.

For typical measurements are depending on the experiment and measuring time is about 10 nmol to 1 micromol substance necessary ( typical sample volume: 1 mL of a solution with a concentration of 10 mol / L to 1 mmol / L).

Resolution

The attainable resolution of the spectrometer is inversely proportional to the pulse length of the FID signal. In addition to the transverse relaxation ( ) of the sample, it is determined by the inhomogeneity of the field in the sample. The balancing of magnetic field inhomogeneities via the so-called shimming. These are weak magnetic fields generated in the spectrometer by means of electric currents in addition to the main magnetic field with which local inhomogeneities can be compensated to some extent. Increased homogeneity of the resulting total magnetic field reduces the relaxation, so that the NMR signal is durable.

To obtain the highest resolution in the spectrum, ie narrow (sharp ) NMR lines and differentiation of closely spaced lines, another technique must be applied, namely the rapid rotation ( in engl. spinning ) of the cell about its longitudinal axis by means of a small air turbine. By this movement of the macroscopic sample, the nuclei in the sample experienced a temporal mean value of the outer field over the sample volume. The remaining after shimming magnetic field inhomogeneities are thus averaged out. So you reach a frequency resolution in the NMR of about 0.1 Hz, which is the distinguishability of extremely small energy differences means, which can then provide a wealth of details from high-resolution NMR spectra.

Areas of application

NMR spectra can be recorded easiest for molecules that are in solution and not interact with paramagnetic substances. NMR spectroscopy of paramagnetic substances and solids is also possible, the interpretation of the spectra, and the preparation of the samples for the measurement, however, are more complex in both cases. With respect to the NMR of solids see also magic-angle spinning.

The high-resolution nuclear magnetic resonance spectroscopy in solution is now used on a large scale for the following tasks:

• For non-destructive detection of constituents of a sample,
• For the determination of molecular structures (from small molecules to proteins and nucleic acid fragments ),
• To study interactions among molecules.

In addition to spectroscopic studies conveys the determination of nuclear spin relaxation information about the structure and dynamics of materials. In liquids, for example, the microstructure and dynamics is difficult to be explored with traditional methods, can molecular distances between neighbors and molecular reorientation, which are typically in the pico to nanosecond range, are determined by relaxation time measurements.

Different nuclear spin relaxation in various biological tissues forms the basis for the used as an imaging diagnostic method in the medical magnetic resonance imaging ( MRI ). An application of nuclear magnetic resonance, which has gained for the neurosciences, such as neurology and neuropsychology, an extraordinary importance, is the functional magnetic resonance imaging. Magnetic resonance imaging methods are generally used, except in medical diagnostics, increasingly applications in engineering and geosciences.

Another important area of ​​application is the study of translational molecular dynamics, ie diffusion and flow motion of molecules or molecular aggregates in liquids and solids by field gradient NMR. With the so-called diffusion -ordered spectroscopy - ( DOSY ) can be measured in mixtures of the translational mobility of the NMR spectroscopy identified, the individual components.

Methods for structure elucidation in organic chemistry

Chemical shift and integration of signals

The resonant frequencies are not as absolute values ​​but as a chemical shift relative to a reference substance, known as the standard, specified. The chemical shift is defined as

Thereby being independent of the field strength of the magnets currently used. Since the chemical shift values ​​are very small, they are reported in ppm. As a standard for 1H and 13C spectra of organic solutions of the respective cores, the resonance frequency is used in tetramethylsilane (TMS). While in the past few milligrams of each sample were added to TMS, reference is nowadays usually on the known chemical shift of residual protons of the deuterated solvent.

The chemical shifts of hydrogen nuclei in organic molecules is influenced by the type of functional groups. Depending on the structure of the molecule, the chemical shifts of the same soft functional groups from each other easily, so that the NMR spectrum is characteristic for a substance. In addition, they are influenced by neighboring molecules in the sample, so that in various solvents or in the pure substance different absolute and relative resonance frequencies of the protons of a sample may occur. In strong interactions between substance and solvent occurred while differences of several ppm. Using Shoolery control the chemical shift can be estimated.

The relative number of a particular signal is based on hydrogen atoms is easier 1H spectroscopy proportional to the surface area ( the integral ) of the respective signal.

By evaluating this integral can thus be determined for example, how many hydrogen atoms of a molecule are located on methyl groups, aromatics, of carboxyl groups on double bonds, etc.. This knowledge is extremely important in organic chemistry for the determination of structures. An overview of the allocation of certain functional groups ( groups of atoms, groups of substances ) to values ​​of the chemical shift provides the following table.

Scalar coupling

Spin states are split by located in their neighborhood more nuclei with zero spin in energetically different levels, the number of which depends on the number of possible different orientations of the individual spins. This scalar coupling is mediated by the spins of the bond between the particles forming the electron couples, its effect is typically up to four bonds observed. The spacing of the lines is independent of the applied field, and is therefore expressed as an absolute difference in frequency (in Hertz). For nuclei that are chemically and spin- symmetric equal ( magnetically equivalent ), the coupling is not visible.

From the observed coupling pattern of the spectroscopist, the neighborhood relations of the individual cores, and thus in many cases open up the complete structure of a compound.

Sometimes it can be helpful to suppress some or all couplings for structure determination. For this purpose, a radio signal with the frequency of a nucleus or an entire group of nuclei ( broadband decoupling ) is irradiated, the rest of the spectrum behaves as if not the corresponding core available. 13C spectra are standard 1H - decoupled, as they would otherwise often uninterpretable by the overlap of the coupling pattern of the individual cores. Moreover, the low sensitivity of the method is improved by the absence of splitting.

In solid or highly viscous samples, the direction-dependent part of the scalar coupling and the dipolar coupling, no longer averaged to zero. Such samples show large, field-independent splittings or line broadening of several kHz for 1H spectra.

Principle of operation

The Kernsuszeptibilitäten are substantially smaller than the diamagnetic and paramagnetic susceptibilities (factor: 104). When Dia and para magnetism, the electrons of the atom for the susceptibility are responsible. In cores, the susceptibility can be determined by the Langevin - Debye formula.

Earlier NMR resonances were determined with a bridge circuit in resonant circuits. Bloch and colleagues used two identical resonant circuits, that is, two coils and two capacitors to make an adjustment with a bridge circuit; a coil as a transmitter as a receiver. It is possible to make a bridge circuit with only one coil. This method was used by Purcell.

Prior to sample measurement, the bridge with the frequency to be measured is adjusted. Using equations from physics there is a resonant circuit and a bridge circuit to calculate the phase shift between current and voltage, the impedance and the Stromlosigkeitsbedingungen a bridge.

In the coil now comes into a substance tubes. A magnetic field ( with a permanent magnet or electromagnet ) is generated horizontal to the coil axis. At a certain frequency and a certain magnetic field strength and only in the presence of a substance sample ( with corresponding atomic nuclei ) of the resonant circuit is detuned. In the oscilloscope or a recorder this upset is visible.

Very significant was the determination of the spatial magnetization by the applied magnetic field. Bloch led for all spatial directions by calculations and could the vibration- dependent susceptibilities for the spatial directions can be derived ( Bloch equations ). However, remained unanswered the question of the relaxation time, that is, the time duration to the excited magnetic resonance falls back to the ground level. By means of paramagnetic salts could then be calculated on for about three seconds, the relaxation time of pure hydrogen protons.

Hydrogen nuclei could be detected by resonant circuit upset at very low frequencies (a few kHz) and a very weak magnetic field. Interestingly, the method for the structural analysis of complex molecules but only at high frequencies (above 60 MHz) and stronger magnetic fields (1.4 Tesla ), since then distinguish the different chemical shifts of hydrogen atoms complicated connections clearer. However, to not only see a single signal on the oscilloscope, but several different hydrogen nuclei (or other nuclei ) as a whole frequency band must be irradiated.

Formerly - until the 1970s - took advantage of the NMR spectrometer, the continuous-wave method (CW) to scan the spectrum of a complex compound.

Today, the pulse - Fourier transform ( PFT ) is common. Here, a high-frequency pulse is irradiated. This pulse comprises a full band of vibration.

The already mentioned dependence of the energy levels of the nuclear spins of the molecular structure occurs because in the first place forth by the interaction of the electron structure of the molecules with the external magnetic field: This results in the electron shell, an induction current, which in turn generates a magnetic field, which is the outer opposite direction. Wherein the magnetic field is weakened in the nucleus, the frequency of the radiation required for the transition is therefore smaller than in the case of a naked nucleus. The difference is called chemical shift and is usually expressed in relation to the necessary for the bare nucleus frequency. Chemical shifts are usually in the range of 0-5000 ppm.

The magnetic field at the nucleus is influenced by the orientation of further magnetic moments in the vicinity. For example, a core is with two alignment options in the vicinity, it can strengthen or weaken the field. This leads to a splitting of the signal, it is called a coupling. Because the chemical shift depends essentially on the electron density at the nucleus, one can expect similar shifts for atomic nuclei in chemically similar environments. Of the coupling is obtained in addition information about neighbor relationships between different nuclei in a molecule. Both taken together provides important information about the structure of the whole molecule.

Atomic nuclei with an odd protons and / or neutrons number possess a nuclear spin I could completely and half-integer values ​​(e.g., 1 /2, 1, 3/2, ..., 9 /2) Accept: in the so-called uu cores is I = n ( thus only integer values ​​: 1,2,3, ...) while in gu - ug - and nuclei I = (2n 1 ) / 2 (ie half-integer: 1/ 2, 3 / 2, 5/ 2, ...), is in isotopes with an even number of protons and neutrons ( called gg cores ) I = 0 Non-zero nuclear spins are associated with a magnetic dipole moment. The magnitude of this dipole moment is described by the gyromagnetic ratio of the isotope in question. In an external, static magnetic field, nuclear magnetic moments align according to the rules of quantum mechanics. Has a nucleus with I = ½ the form of a ball, nuclei with I > ½ have an ellipsoidal shape and have therefore additionally an electric quadrupole moment " eQ ", which can interact with the electric field gradient (see also nuclear quadrupole resonance spectroscopy). This additional strong electrical interaction possibility leads to wide NMR resonance lines, which are complicated to be interpreted as the small, structured by well resolvable coupling the resonance lines of the spin ½ nuclei.

The most commonly used for chemical structure elucidation isotopes are therefore nuclei with spin ½. These include the nuclides 1H, 13C, 15N, 19F, 29Si and 31P. Spin ½ nuclei can assume only two discrete states, namely, either parallel or anti- parallel to the external magnetic field. Intermediate positions are quantum mechanically forbidden. The two possible arrangements correspond to two different energy states.

The energy difference between these two states is proportional to the strength of the magnetic field at the nucleus. The proportionality factor is the gyromagnetic ratio of the isotope in question. Transitions between the two orientations of the nuclear moments can be caused by the irradiation of resonant magnetic fields. The resonant frequency of the energy splitting between the two spins is referred to as the Larmor frequency and is proportional.

Illustrate this can be through the adjacent diagram. Here we think of a coordinate system with the external magnetic field along the z- axis. A nucleus with a spin of ½ targeted with a spin vector is either parallel or anti- parallel to the external field. If one takes the vectors of several atoms in this coordinate system, creates two cones, one for each of parallel and anti- parallel. Due to the energy difference between the parallel and antiparallel orientation of the nuclear magnetic moments are in thermal equilibrium a population difference between the two orientations. This follows high temperature approximating the Boltzmann distribution and causes an excessive magnetization in the positive direction along the z- axis.

The NMR signal is caused by the fact that the sample to be tested in a magnetic field is exposed to a radio frequency pulse. The spins of the individual atoms are influenced by the magnetic field of the pulse, so that the total vector, resulting from the spinning cones shown, is tilted. It is no longer parallel to the z - axis but is deflected by an angle which is proportional to the duration and intensity of the radio frequency pulse. Are typical pulse lengths of about 1-10 microseconds. Maximum transverse magnetization perpendicular to the z - axis is achieved with a deflection angle of 90 °.

This transverse magnetization behaves like a magnetic gyro and precesses in the plane perpendicular to the static magnetic field. This precession movement makes itself felt as a very weak alternating magnetic field, which is measured by a suitable amplifier. After end of the resonant radiation, two processes occur, so-called relaxation, a, through which the transverse magnetization decreases again. The NMR signal is thus after the end of the radio frequency pulse as free induction decay (FID, of English: free induction decay ) were measured. The time constant of the free induction decay depends on the transverse relaxation time, as well as of the homogeneity of the magnetic field. For highly mobile liquids in homogeneous magnetic fields, they can be in the range of several seconds. The FID will be modulated by the frequency difference due to chemical shift and coupling. By a Fourier transform, the distribution of the different frequencies can be calculated from the FID. This is the NMR spectrum. NMR spectrum gives in many cases a unique "fingerprint" of the particular molecule. Along with information from other measurements, such as mass spectrometry can be determined from the spectra of the chemical structure of an unknown substance.

Commercial NMR spectrometer for chemical structure elucidation typically operate at magnetic flux densities of 7-21 Tesla. For 1H resonance frequencies then match ( Larmor frequencies ) between 300 and 900 MHz. Since 1H is the most important NMR core, the field strength of spectrometers is usually expressed in its Larmor frequency. For 1H, the splitting of the spectra due to different chemical shifts is approximately 10 ppm. This therefore corresponds to a maximum bandwidth of about 3 kHz at an NMR frequency of 300 MHz. The frequency bandwidth of the NMR spectra of the chemical shift due to increases in proportion to the magnetic field. The chemical shift itself is defined as the ratio of the difference of the resonant frequency of the core in a given chemical environment, the resonant frequency in a reference connection to the resonance frequency itself. This allows an easy comparison of the NMR spectra were measured at different fields. For hydrogen and carbon tetramethylsilane (TMS ) as a reference substance., The range of chemical shifts of carbon and many other cores is considerably wider than for hydrogen and may be several 100 ppm. In some very heavy nuclei such as 207Pb chemical shifts in the range of percent are observed.

Pulse - Fourier transform NMR

Nowadays all modern NMR spectrometers with the pulse technique. In this, a single radio -frequency pulse (RF - pulse) or a sequence of RF pulses is sent to a sample which is located in a strong magnetic field. After the decay of the pulse in the receiver electronics (dead time) is the decay of the magnetization (English Free Induction Decay, FID), that is, detects their return to the equilibrium state of the thus induced in the receiver coil voltage as a function of time. By Fourier transformation of this signal is time transformed in the computer into the frequency spectrum (signal intensity as a function of frequency).

This measurement technique has the CW method previously used ( engl. continuous wave) (see above) almost completely displaced.

Experimental sizes

• The chemical shift of a resonant magnetic field at the nucleus is from the local -dependent, in turn depends on the chemical environment of the observed nucleus.
• The intensity of a response is usually proportional to the number of eliciting cores as long as the population differences are not changed by strong coupling or susceptibility differences.
• With the relaxation of excited states, a distinction between the longitudinal relaxation time ( spin-lattice relaxation) and transverse relaxation time ( spin-spin relaxation). Longitudinal relaxation times determine the adjustment of the equilibrium magnetization. The transverse relaxation times determine the line width of the resonant lines. Relaxation effects provide information about existing interactions and molecular motions.
• Spatially neighboring nuclei interact with each other via magnetic dipole -dipole interaction ( dipolar coupling). This interaction vanishes in isotropic solutions in the time average.
• Indirectly, can interact with each other cores via chemical bonds. This scalar coupling is responsible for the splitting of the signals into multiplets and provides a material basis for molecular structure determination by NMR dar. The distance between two adjacent lines of a multiplet is called the coupling constant, which is measured in Hertz.

One-dimensional NMR spectroscopy

The one-dimensional NMR spectroscopy is the structural elucidation of the chemical method most commonly used. For her, the chemical shift of an atom of a reference substance is measured. 1H and 13C are the nuclei that are most often measured in the organic chemistry, as well as 15N, 31P, 19F, and many other NMR-active isotopes can be spectroscopically.

The appearance of the spectra depends critically on the recording mode. 1H spectra are not included broadband decoupled in general. To ensure that all hydrogen atoms have the opportunity to couple their spin with other nuclei, the so-called spin -spin coupling. This produces at the characteristic chemical shift of an atom, a characteristic of the surrounding area of the split signal can be derived from the information about the molecular structure.

13C, 15N, 31P, 19F, and other cores are frequently incorporated 1H broadband decoupled, so that the splitting of the signals is absent due to the coupling to 1H nuclei.

Scalar coupling

The nucleus of an atom can interact with an adjacent nucleus. This can be done (via the bonding electrons between the nuclei ) either directly ( through space ) or indirectly. In a liquid sample, the direct ( dipolar ) interactions averaged by the rapid movement of the nuclei of the room. Receive remains the scalar coupling, which is mediated by the fact that the ( always paired (↓ ↑) ) interact differently occurring spins of the binding electrons with the nuclear spins on both sides of the binding.

Has a core of the condition α (↑ ), then the (↑ ) electron is ejected from the binding it, so to keep the other core more. More from there outgoing bonds are also spin polarized (to a lesser extent ). If so, another core reached, an energy difference between the α and β by state results, in turn, its interaction with the electrons of the bond. Such couplings are usually about max. three to four bonds detectable in conjugated π - systems as well as on.

The NMR signal of the first core is thereby a doublet ( for, or lines) is split and that the second core also, rising by the same amount, because the difference in energy (so-called coupling constant ) between αα ( = ββ ) and αβ ( = βα ) must be the same. By the same strong splitting the neighborhood of the two atoms is then detected in the molecule. Splits a single core still further another, then each of its lines split accordingly again.

Splits a single core to two (or in general ) like neighboring nuclei ( from the point of view of the origin core, the same chemical environment and spin symmetry have ), we obtain a triplet, since the middle lines of the " doublet of doublet " (or quartet, quintet coincide, etc., ie lines, or lines for nuclei with ). The relative intensities of the lines are presented ( for - cores) from the -th row of the Pascal triangle Schens, ie 1:2:1 or 1:3:3:1. Are the coupled nuclei not alike, that is, their coupling constants are different, so the middle lines do not coincide, then obtained eg etc. a doublet of triplet

Example spectra

As a simple example is propane ( H3C - CH2-CH3 ): The CH2 group in the propane has two adjacent methyl groups (- CH3). This corresponds to six adjacent, equivalent H atoms. That the signal is split into a septet. The methyl protons split by the two methylene protons of the triplet, the 4J coupling to the other three methyl protons is invisible, because they are magnetically equivalent, as no 2J coupling can be observed in the methyl groups.

In a molecule are several different eg methyl groups present, then the multiplets superimposed frequently, making them quickly interpretable. To dissolve better in such cases is, this often resorted to multi-dimensional NMR techniques such as COSY. Since the splitting is not field dependent, the distance between the signals of chemically different protons but already can be superpositions resolved by applying a higher field.

Explaining the spectrum of ethanol:

The OH group is only one singlet, when the ethanol is present in aqueous solution. The alcoholic hydrogen atom is slightly acid, and is therefore constantly replaced by hydrogen atoms from the solvent. The result is that no permanent spin-spin - coupling effects. At very low temperatures, this exchange could be slowed enough to leave it there because of the expected CH2 group ( ) triplet arise.

Examples of two-dimensional NMR spectroscopy

All of these variations are based on a receive time series of spectra, and while continuously changing a pulse parameters in duration, thereby changing phase and intensity of the spectra systematically. By re- transformation of the Fourier spectra of the individual points along the time axis is obtained from the one-dimensional two-dimensional spectra. By varying other parameters higher-dimensional spectra are obtained. The required number of spectra and thus measuring time rises exponentially.

Low-field NMR

There are relatively inexpensive low-field NMR instruments ( ≈ 10-40 MHz), equipped with a permanent magnet, although not resolved spectra provide, but are included in the operating costs incomparably more favorable ( no He- cooling). Also, such systems can be designed portable. By analyzing the 1H relaxation times mixing proportions of multi-component systems ( suspensions, semi-crystalline substances ) and, after calibration, and absolute quantities of materials can be quantified, which is of particular interest in the industry. Measurements are carried out rather than in a (expensive) deuterated solvents commonly used in substance. Other than hydrogen nuclei are due to the low sensitivity rarely studied.

Deuterium nuclear magnetic resonance spectroscopy

Deuterium (D, 2H) represents the extent constitutes a specific feature, because the spin is. As a result, the line width of the NMR signal with respect to 1H nuclei is higher. The resonant frequencies are well below those of 1H nuclei ( 61.4 compared with 400 MHz at 9.39 Tesla). The deuterium NMR - spectroscopy is approximately 100 times less sensitive than 1H - NMR spectroscopy. In addition, the proportion of deuterium to hydrogen in organic compounds is very low (approximately 0.0159 %). With modern NMR spectrometers, however, the investigation today not a problem The recordings and evaluations are done with the Fourier Transform method. The interpretation of the spectra is difficult because the chemical shifts are virtually identical to those of 1H.

With the 2H NMR spectroscopy can the deuterium distribution in the individual positions in an organic compounds and the D / H ratio determined. The deuterium distribution can be read directly from the spectra, the D / H ratio can be determined only if one uses a standard with a known D / H ratio. This method is important for the analysis, because such a statement about the origin of an organic compound can be made. This is partly due to the fact that the deuterium content in the world is different and so have the raw materials for natural products mainly including water a slight difference in deuterium content. On the other hand the kinetic isotope effect of importance in the synthetic routes. So, for example, has Ethanol to a different deuterium distribution in the molecule or a different D / H ratio in wines from different regions. The same is true for all natural substances and therefore can become for many of these substances an origin or the type of synthesis, whether natural or synthetic, assign. When produced from fermentation processes ethanol ( so-called R- value ) is found in the D / H ratio to determine the plant origin, ie whether from sugar cane, beet, cereals, maize, potatoes, grapes or apples. In addition, unauthorized sugar additives can be determined. In addition to ethanol, such as glycerol, methanol, organic acids and preservatives may also from 13C spectra of other ingredients in wine, are determined qualitatively and quantitatively

A metal-core nuclear magnetic resonance spectroscopy

In addition to the investigations of organic compounds containing the 1H, 13C, and 19F NMR spectroscopy is the Metal NMR Spectroscopy of importance. This metal -to-metal or metal -ligand bonds in complexes and organometallic compounds can be studied directly. Also, proteins can be investigated with embedded metal ions. Short-lived intermediates that are difficult or impossible to be detected after the reaction, can be detected in the reaction solutions. It requires no special solvents, but can perform the measurements in the reaction solution. Examples are the 6/7Li-, 25Mg, 27Al NMR spectroscopy and measurements of heavy metal nuclei such as 195Pt, 207Pb and 205Tl.

The investigation of electrically conductive metallic solids requires other experimental conditions than that of metal nuclei in solutions. The nuclear spin quantum number of certain metal cores is greater than 1/2 (examples: 6 Li: 1, 7Li: 3/2, 23Na: 3/2, 25Mg: 5/ 2, 59Co: 7/ 2). Such nuclei have an electric quadrupole moment, they relax on a special relaxation mechanism and therefore often have very broad resonance lines, which has implications for the sensitivity of the NMR measurements.