Chemistry
FT NMR
FT NMR, or Fourier Transform Nuclear Magnetic Resonance, is a powerful analytical technique used to study the structure and dynamics of molecules. It involves applying a strong magnetic field to a sample, which causes the nuclei of certain atoms to resonate at characteristic frequencies. By analyzing the resulting signals, valuable information about the chemical environment and interactions within the sample can be obtained.
Written by Perlego with AI-assistance
Related key terms
1 of 5
11 Key excerpts on "FT NMR"
eBook - PDFPhysico-chemical Applications Of Nmr: A Practical Guide
A Practical Guide
- Andrei Borisovitch Koudriavtsev, A B Kudryavtsev, Wolfgang Linert(Authors)
- 1996(Publication Date)
- World Scientific(Publisher)
Chapter 2 Experimental Techniques Employed in NMR 2.1. Introduction In this Chapter the problems of setting up an NMR experiment and the preliminary analysis of the acquired data will be discussed. This Chapter is written mainly for people who are directly involved in using the NMR instrument themselves - for those who are merely the customers of an NMR laboratory it is quite sufficient to know the material of Chapter 1 and to look into Section 4 of this Chapter for information. Problems arising from the regression analysis of the results obtained from an actual physico-chemical NMR experiment will be discussed in Chapter 3. Physico-chemical experiments are usually concerned with the analysis of some dependence of a measured parameter on external factors. Therefore they require prolonged measurements which can create financial problems, especially when a modern expensive NMR FT spectrometer is used. Deuterated solvents for High Resolution FT NMR spectroscopy are also very expensive. Should a CW spectrometer be used, it would be possible to use nondeuterated solvents such as CC1 4 , or actually any other solvent, because field frequency stabilisation may be achieved by employing the TMS signal. However, there is only a small probability of finding a research grade CW spectrometer still in working order. Thus physico-chemical NMR experiment are commonly conducted using a Fourier Transform instrument. This means that such experiments must be of an optimal design and that mainly proton resonance (which does not require prolonged accumulation of data) should be employed. 2.2. Principles of Fourier Transform NMR spectroscopy Fourier Transform NMR spectroscopy was developed as an advanced pulse NMR method in which a nuclear induction signal appearing after a short r.f. field, H|, pulse is the source of the spectral information. This signal is induced in the coils L, of a probehead (Fig.2.1A) by the rotating vector of nuclear magnetisation (or 45
eBook - ePubInstrumental Analytical Chemistry
An Introduction
- James W. Robinson, Eileen M. Skelly Frame, George M. Frame II(Authors)
- 2021(Publication Date)
- CRC Press(Publisher)
One coil applies a constant RF frequency; the second coil detects the RF emission from the excited nuclei as they relaxed. These systems are simple and rugged, but limited in resolution and capability. They are used for teaching and for simple NMR determinations. For complex organic structural determination, a high-resolution NMR is required. The increased sensitivity of FTNMR is so critical to the measurement of 13 C and other less abundant nuclei as well as to increased proton sensitivity that all modern NMR spectrometers are Fourier Transform (FT) instruments. The term NMR spectrometer therefore is used in the remainder of the chapter to mean a pulsed FT system. Figure 5.15 (a) Basic components of an NMR spectrometer (Williams, 1992. Copyright Wiley-Vch Verlag GmbH & Co. KGaA. Used with permission.). (b) Schematic diagram of a CW NMR spectrometer with a permanent magnet. (c) Schematic diagram of an FTNMR spectrometer, Akitt and Mann, Used with permission.). (d) Magnet and assembly for a modern NMR spectrometer. The superconducting coils (the primary solenoid and the shim coils, marked SC) are submerged in a liquid helium dewar (He) which is suspended in an evacuated chamber. A liquid nitrogen dewar (N 2) surrounds that to reduce the loss of the more expensive He. The levels of cryogenic fluids are measured with level sensors, marked LS. The room temperature shim coils (RS) and probe (P) are mounted in the bore of the magnet. A capped sample tube, shown inserted into the probe at the top, is introduced and removed pneumatically from the top of the bore. (e) A schematic probe assembly. A sample tube (uncapped, with a liquid sample as indicated by the meniscus) is shown inserted at the top of the probe. The RF coil for the observed nucleus (OC) is mounted on a glass insert closest to the sample volume. The RF coil for the lock signal (LC) is mounted on a larger glass insert
eBook - PDFChemical Analysis
Modern Instrumentation Methods and Techniques
- Francis Rouessac, Annick Rouessac, John Towey(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Chemical Analysis: Modern Instrumentation Methods and Techniques , Third Edition. Francis Rouessac and Annick Rouessac, translated by John Towey. © 2022 John Wiley & Sons Ltd. Published 2022 by John Wiley & Sons Ltd. Companion Website: www.wiley.com/go/Rouessac/Analysis3e Nuclear Magnetic Resonance Spectroscopy Chapter 15 N uclear magnetic resonance (NMR) is a spectroscopic method based on the study of the nuclei of certain elements. It has become irreplaceable in a variety of areas, such as chemistry, biology, the food industry or medical imaging (MRI). It is one of the most powerful methods to obtain structural information on organic or inorganic compounds. A succession of technological advances has led to a growing number of applications for it, whether analytical or not, in industry, ranging Chapter 15: Nuclear Magnetic Resonance Spectroscopy 388 from oil exploration to the identification of food products. This chapter is limited to a simplified study of NMR in solution in the field of organic compounds. 15.1 GENERAL INTRODUCTION Nuclear magnetic resonance refers to a property of matter that has given its name to powerful method of materials investigation, in particular to identify the structure of organic or inorganic compounds. NMR spectrometers are therefore often found in research laboratories, although there are routine applications based on this property, using simpler and more robust dedicated instruments (see Figure 15.31). Organic chemists quickly understood the interest of NMR, which has accelerated its development and favoured the appearance of many technical improvements.
eBook - PDF- Satinder Ahuja, Neil Jespersen(Authors)
- 2006(Publication Date)
- Elsevier Science(Publisher)
Chapter 10 Nuclear magnetic resonance spectroscopy Linda Lohr, Brian Marquez and Gary Martin 10.1 INTRODUCTION Nuclear magnetic resonance (NMR) is amenable to a broad range of applications. It has found wide utility in the pharmaceutical, medical and petrochemical industries as well as across the polymer, materials science, cellulose, pigment, and catalysis fields to name just as a few examples. The vast diversity of NMR applications may be in part due to its profound ability to probe both chemical and physical properties in-cluding chemical structure as well as molecular dynamics. This gives NMR the potential to have a great breadth of impact compared with other analytical techniques. Furthermore, it can be applied to liquids, solids or gases. In some ways, it is a ‘‘universal detector’’ in that it detects all irradiated nuclei in a sample regardless of the source. Signals appear from all components in a mixture, proportional to their concentration. NMR is therefore a natural compliment to separation techniques such as chromatography, which provide a high degree of component selectivity in a mixture. NMR is also a logical compliment to mass spectrometry, since it can provide critical structural information. Compared to other solid-state techniques, NMR is exquisitely sensitive to small changes in local electronic environments, such as discerning individual polymorphs in a crystalline mixture. Beyond the qualitative molecular information afforded by NMR, one can also obtain quantitative information. Depending on the sample, NMR can measure relative quantities of components in a mixture as low as 0.1–1% in the solid state. NMR limits of detection are much lower in the liquid state, often as low as 1000:1 down to 10,000:1. In-ternal standards can be used to translate these values into absolute quantities. Of course, the limit of quantitation is not only dependent on Comprehensive Analytical Chemistry 47 S.
eBook - PDFBioactive Natural Products
Detection, Isolation, and Structural Determination, Second Edition
- Steven M. Colegate, Russell J. Molyneux, Steven M. Colegate, Russell J. Molyneux(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
NMR spectroscopy has been the single most important physi-cal method for the determination of molecular structures for more than 40 years. The power of the technique lies not only in defining the numbers and types of nuclei present in an organic molecule, but also in describing their individual chemical environments and, more importantly, the way they are interconnected. Driven by its potential to determine the structures of organic compounds, NMR spectroscopy has seen substantial development in the six decades since the first experiments. In particular, the implementation of the pulsed Fourier transform (FT) method 1 and, subsequently, the concept of multidimensional experiments 2 provided the seeds for vibrant growth. There are currently hundreds of multipulse experiments available to the NMR spectroscopist. However, only a small proportion of these procedures are regularly employed for the solution of molecular struc-tures. The most useful experiments have been the subject of numerous reviews. 3–15 This chapter aims at introducing NMR spectroscopy to the reader who is unfamiliar with the technique. Within the context of the multidisciplinary nature of bioactive natural product research, the chapter will briefly review the most utilized NMR experiments with the aim of highlighting the types of information that each can provide. Theoretical and experimental details of each procedure will be kept to a minimum with references to the literature for readers who require further informa-tion. Section 3.2, introduces the essential concepts. Subsequently, the most useful NMR techniques will be discussed approximately in the order that they would be applied to solve the structure of an unknown compound. Sample spectra have been chosen with the aim of clearly demonstrating each technique without the need for detailed argument.
eBook - PDFFourier Transforms in NMR, Optical, and Mass Spectrometry
A User's Handbook
- A.G. Marshall, F.R. Verdun(Authors)
- 2016(Publication Date)
- Elsevier Science(Publisher)
315 90.0 80.0 70.0 60.0 50.0 40.0 3θ!θ 20.0 PPM Figure 8.27 2D-FT/NMR INADEQUATE spectrum of a natural product provided by R. Doskotch. Virtually all of the carbon-bonded skeleton of the molecule can be inferred from the two-carbon fragments identified from this single spectrum (see text). (Data provided by C. E. Cottrell.) 8.6 NMR Imaging The basic principle of magnetic resonance imaging (MRI) is that the proton NMR Larmor frequency (usually of H2O) is directly proportional to applied magnetic field strength (Eq. 8.5). Thus, if the applied magnetic field strength varies monotonically with position across the object, then the corresponding NMR spectral magnitude vs. frequency will provide a direct measure of the number of nuclei vs. spatial position along the gradient. 316 Figure 8.28 shows the (one-dimensional) projection image of a simple object, based on continuous application of a magnetic field linear z -gradient during FT/NMR excitation and detection. One can think of such an image as a shadow of the object. Figure 8.28 is the simplest example of frequency-encoding, in which the field gradient is left on during detection—see below. By varying the direction of the field gradient, one can produce shadows or projections from several viewing directions, and then reconstruct the object by back-projection from those shadows, as in X-ray CAT scanning. Some of the earliest MR images were in fact produced in this way. z A Ώ :o Signal Figure 8.28 Schematic one-dimensional projection image of an object consisting of two water-filled test tubes of circular cross-section. The image is simply the l H NMR spectrum, whose magnitude reflects the relative number of protons and whose frequency is determined by the magnetic field strength at a given z-position. However, Fourier transform methods provide a much more direct means for obtaining a direct MR image without the use of back-projection.
eBook - PDFCharacterization of Food
Emerging Methods
- Anilkumar G. Gaonkar(Author)
- 1995(Publication Date)
- Elsevier Science(Publisher)
Slightly more of nuclear magnets occupy the lower energy state in thermal equilibrium. Thus, net magnetic moment of the ensemble of nuclear magnets aligns parallel to the magnetic field. We call the net magnetic moment a macroscopic magnetization, M. In NMR, the behavior of the macroscopic magnetization is detected. NMR resonance frequencies are in the radio frequency (r.f.) range (~MHz) which depends on the strength of the magnetic field used and the gyromagnetic ratio of the nucleus investigated. When an r.f. pulse (a pulse of oscillating magnetic field with a radio frequency) is introduced, magnetic resonance (net absorption) is induced on the basis of the resonant condition. 2.2. Fourier transform NMR To visualize the spin behavior in FT-NMR, it is easy to use the classical description, where the nuclei are regarded as an ensemble of small magnets spinning at their Larmor frequency along the direction (z-axis) of the applied magnetic field, Bo. The macroscopic magnetization points to the z-axis. Instead of the laboratory frame (x,y,z-axis), let us consider a rotating frame of reference which rotates with an angular frequency tar relative to the laboratory. In this frame of reference, the nuclear magnets appear to precess with the angular frequency mo- mr. If o3 r is Figure 2. The revolution of macroscopic magnetization in a rotating frame of reference: (a), application of additional magnetic field (BI) along x'-axis as a 90 ~ pulse which tips the macroscopic magnetization into the x'y'-plane; (b), as they precess in the x'y'-plane, the macroscopic magnetization diminishes because nuclear magnets actually precess at slightly different frequencies which causes dephasing. 120 Figure 3. The acquired NMR signal: (a), Free Induction Decay (FID) in the time domain response.Two transient signals each 90 ~ out of phase comprise the real and imaginary part of the FID; (b), its frequency domain response using Fourier transformation.
- Denis Rousseau(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The echo amplitude will then reflect the true spin-spin relaxation time T 2 . In actual experiments one uses a series of multiple 180° pulses developed by Carr and Purcell (1954) to avoid difficulties due to diffusion of the spins 42 TRUMAN R. BROWN AND KÂMIL UÖURBIL Ml V ' M 2 ' (a) Mz V Δωτ (b) Mi Δωτ (c) Fig. 20. Events in the formation of an echo following a 90°-τ-180° pulse se-quence. See text for details. from one field value to another. For more details and possible pulse se-quences, we refer the interested reader to the book on Fourier transform N M R by Farrar and Becker (1971). V. Applications In this section we will present examples of the use of N M R in the areas of structural determination, dynamics, kinetics, cellular studies of metabolism, and a variety of other applications. A. Structure Nuclear magnetic resonance (NMR) spectroscopy has been extensively used to obtain structural information of varying complexity and sophistication. The routine use of N M R for identification of organic compounds stems from the sensitivity of N M R parameters to structure. The resonance fre-quencies of a methyl carbon or methyl protons give rise to peaks which are well separated from those of a methylene carbon or protons, respectively. 1. NUCLEAR MAGNETIC RESONANCE 43 Therefore, just the knowledge of the chemical shifts provides considerable constraint on the possible structures available to the molecule under exami-nation. On identification of the observed resonances with particular nuclei, traditionally through chemical modifications, more detailed structural in-formation can be obtained using the measured scalar coupling constants T x , T 2 and the N O E to construct a model. Of course, it must be kept in mind that what is being observed is the average-solution structure. When molecules with an aromatic moiety are being studied, structural information is also contained in the chemical shifts of the observed reso-nances.
eBook - PDF- J.R.J. Paré, J.M.R. Bélanger(Authors)
- 1997(Publication Date)
- Elsevier Science(Publisher)
experiment contains a new 180 ~ pulse. In this way, each FID collects a further and further echo. Plotting the amplitude of the signal generated by each echo as a function of time allows the determination of the T2 relaxation time, not affected by experimental imperfect conditions (Figure 24k). The much faster relaxation time for individual echoes is the T2* relaxation time (Figure 24k). 220 Chapter 6. NMR Spectroscopy 6.7. INSTRUMENTAL AND EXPERIMENTAL CONSIDERATIONS A schematic diagram of a NMR spectrometer is presented in Figure 25. Depending on the type of spectrometer some elements may be missing or additional ones may be present. 6.7.1. The spectrometer One way to characterize the spectrometers is in relation with their type of magnet. Thus, there are permanent, resistive (electromagnets) and superconducting magnets. Another way is to specify the way of excitation of the sample. In this respect there are Continuous Wave (CW) and Pulse systems (PFT). Two different constructive types of CW spectrometers are also possible: with Magnetic Field Sweep (at fixed frequency) and with Frequency Sweep (at fixed magnetic field). Another way to characterize the NMR spectrometers is based on the mathematical treatment of data. Thus, there are spectrometers either with or without mathematical treatments. Nowadays Fourier Transform (FT) is a standard feature of spectrometers and some advanced systems have additional non-FT processing methods (Linear Prediction, Maximum Entropy Method, Hilbert Method, Bayesian Analysis). Still another way to characterize an NMR spectrometer is according to the field strength. Thus, there are Low Resolution and High Resolution spectrometers. The High Resolution are usually referred as low-, medium-, high- and ultra high-field spectrometers. NMR imaging and microscopy are already considered separated topics, and usually (because of the vastness of the field) treated separately in most discussions.
eBook - PDF- Oleg Jardetzky, G. C. K. Roberts, Bernard Horecker, Nathan O. Kaplan, Julius Marmur(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
Since for a 90° pulse [Eq. (11-10)] yH 1 t = n/2 (11 -37) the condition for satisfying Eq. (H-35) is t « 1 /4A (11 -38) (A) (8) F. FOURIER TRANSFORM NMR SPECTROSCOPY 47 which means that short intense RF pulses must be used in FT NMR, as mentioned in Sections A and B. From Eq. (11-34), the magnetization of nucleus i precesses about H e ff with a frequency Q t = [(co,-— co) 2 4-(yH^Y 12 and thus rotates relative to the rotating frame. The instantaneous value of the magnetization along a fixed axis in the rotating frame (i.e., the detected signal) therefore oscillates with a frequency |co £ — co|, as well as decaying with a time constant T 2 , as shown in Fig. 11-16. In a real sample, the signal detected in response to an H l pulse will consist of the superposition of a number of such decays of varying modulation frequency and T 2 . This complex signal is converted to a normal N M R spectrum (i.e., absorption vs. frequency) by the process of Fourier transformation. 2. Fourier Transformation The Fourier theorem, which can be proved quite generally and is found in standard mathematical reference works, states that any function of time f(t) can be transformed into a function of frequency F(oo) and vice versa, by multiplication with a complex exponential, and integration over the independent variable: where i = (—1) 1 / 2 . Since the two functions can be readily calculated from each other, they are often spoken of as being the same function, f(t) being the function in the time domain and F(co) in the frequency domain. They are also known as Fourier transforms (FT) of each other and the process of calcu-lating one from the other as Fourier transformation. It can readily be shown that if f(t) is a sine wave of frequency co, its Fourier transform is a delta function at co; this is the basis of the widespread use of the Fourier transform in waveform analysis. Thus each nucleus having a different chemical shift, which contributes a component oscillating at a>i — co to the FID, will contribute a line at |co £ — co in the frequency domain spectrum after FT. The Fourier transform of an exponential is a Lorentzian, with a width at half-height
eBook - PDF- Don Shillady(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
For a real sample these would not be clean waves but would contain the information from many different chemical shift environments. The time scale could also be as much as 10 times longer in MRI. The two components are available for Fourier transformation with M x as the real part and M y as the imaginary part of the complex transform integral. Essentials of Nuclear Magnetic Resonance 437 COMPLEX FOURIER TRANSFORM In the previous use of a Fourier transform for infrared spectra, we only needed the real part but the complex form of the Fourier transform is ideally suited for two dimensions that are orthogonal by using M x as the real part and plotting M y as the imaginary part just to indicate the orthogonality of M x and M y vector components. f ( v ) ¼ ð þ1 1 f ( t ) e i v t dt ¼ ð þ1 1 f ( t ) cos ( v t ) i sin ( v t ) ½ ĸ dt With very fast computer control, the complex FID can be sampled and the transform calculated in real time for several reasons using the trick of sampling the FID at time delays that correspond to a phase shift to obtain the sin( v t ) sample as sin( v t ) ¼ cos v t þ p 2 = Þ ð . Another feature of the pulse acquisition is that it is repeated many times to average the results. Thus we have a situation where a computer controls the timing of RF pulses perpendicular to the z -axis of the main magnet and many programs are available to carry out a number of experiments with the same spectrometer. As such modern NMR spectrometers are ‘‘ programmable experiments. ’’ 2D-NMR COSY Now we come to the part we have been working toward ever since we noted the spin-Hamiltonian approach needed a way to fi nd the coupling constants.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
