Fourier-transform NMR

In modem instruments called pulsed Fourier transform (FT) spectrometers, the magnetic field is held constant and an rf pulse of short duration excites all the protons simultaneously. The rf pulse covers a range of frequencies, so each nucleus can absorb the frequency it requires to come into resonance (flip its spin) and produce a signal—called a free induction decay (FID)—at a frequency corresponding to A . The intensity of the FID signal decays as the nuclei lose the energy they gained from the rf pulse. 

A computer measures the change in intensity over time and converts it into intensity-versus-frequency data, in a mathematical operation known as a Fourier transform, to produce a spectmm called a Fourier transform NMR (FT-NMR) spectrum. An FT-NMR spectmm can be recorded in about 2 seconds—and large numbers of FIDs can be averaged in a few minutes— using less than 5 mg of compound. The NMR spectra in this book are FT-NMR spectra that were taken on a spectrometer with an operating frequency of 300 MHz. 

In the following three sections, we will get our feet wet with simple concepts of Fourier transform NMR and the time and frequency domains which are related by the transform. Detailed treatments will be given in Chapter II. 

A nuclear spin system behaves much like a damped harmonic oscillator. Suppose a mass is attached to one end of a spring and this mass-spring system is lowered into a liquid which provides damping. Attach the upper end of the spring to a motor which will vibrate the mass-spring system. As the frequency of the motor is varied through the natural resonance frequency of the system the response is a resonance curve as shown. 

The amplitude of the oscillation is a maximum when the driving frequency equals the natural resonance frequency and the width of the resonance line depends on the degree of damping of the system. If the liquid in which the system is immersed is not viscous, the line will be narrow. On the other hand, the line will be quite broad for a viscous liquid. The line-shape produced by such a damped harmonic oscillator is a Lorentzian lineshape. The full width at half height of such a 

If we take the same damped mass and spring system and, instead of driving the system with a motor, simply displace the mass and let it go, we find that it will oscillate with its natural resonance frequency and the amplitude of the oscillation will decay in time. The decay will be exponential with the amplitude proportional to exp(-t/T2p where the viscosity of the liquid will determine T. It is interesting to note that if you pluck the system, its oscillation and decay contains all the information of the system s behavior just as the frequency dependence of the amplitude in the driven simple harmonic oscillator experiment does. In the one case, the full width at half height of the resonance curve is 2/T, while in the other, the decay time constant is T.  

To summarize, all of the information of the system is available from either means of exciting the resonance, driving it and sweeping the frequency, or hitting it with an impulse for its time response. The first experiment is performed in the frequency domain and the second in the time domain. The mathematical transformation of one representation into the other is the Fourier transform. The time domain response and the frequency domain response are called Fourier transform 

The earliest efforts at combating the signal-to-noise problem involved signal accummulation in a small computer or CATing (Computer of Average Transients). The spectrum was measured, the result stored, and the 

After the pulse the magnetization will be tipped away from the equilibrium direction by some angle, a, the flip angle, such that a = where represents the strength of the transmitter field, Xp the 

The FID which results when a single type of (e.g. a proton decoupled singlet) is shown in Fig. 5a for P(OMe) and appears as an exponentially decaying sine wave whose frequency is the difference between the center frequency of the excitation pulse (sometimes called the carrier wave) and the Lamor frequency of our signal. For the case where several absorptions occur the decay will appear somewhat differently such as in Fig. 5b for which the 

The reader is recommended to consult the literature [5] for the details of these operations. Given that all of the necessary spectral infor- 

T2 is related to the observed line widths and contains, in addition to the spin-spin relaxation times, T2, a contribution from the magnetic field homoge- 

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Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

From the Heisenberg uncertainty principle as stated in Equation (1.16) estimate, in cm and Hz, the wavenumber and frequency spread of pulsed radiation with a pulse length of 30 fs, typical of a very short pulse from a visible laser, and of 6 ps, typical of pulsed radiofrequency radiation used in a pulsed Fourier transform NMR experiment. 

Fourier transform spectroscopy in the radio frequency region has been applied most importantly in pulsed Fourier transform NMR spectroscopy, which is not a subject which... 

Fig. 3. A block diagram schematic representation of a Fourier transform nmr spectrometer, ie, a superconducting magnetic resonance system.

Fig. 2. Distribution of sdicon centers in soluble sdicate solutions from Si Fourier-transform nmr spectroscopy (7,37), where (—) represents (--------),...

FT NMR Fourier transform NMR, the newer and more sensitive, less time consuming basic technique of NMR detection, almost exclusively used... 

C fourier-transform nmr commercially available following first ob.servation of C nmr signal by P. C. Lauterbur and by C, H, Holm in 1957,... 

T. C. Farrar, E. D. Becker, Pulse and Fourier Transform NMR. Introduction to Theory and Methods, Academic Press, New York, 1971. 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

FT-NMR (Section 13.4) Fourier-transform NMR a rapid technique for recording XMR spectra in wrhich all magnetic nuclei absorb at the same time. 

Formate ion, bond lengths in, 757 electrostatic potential map of, 757 Formic acid, bond lengths in. 757 pKa of, 756 Formyl group, 697 p-Formyl benzoic acid, p/C, of, 760 Fourier-transform NMR spectroscopy (FT-NMR), 447-448 Fractional crystallization, resolution and, 307... 

Conventional use has been made of the radioisotope C, and details need hardly be given here. Illustrative examples include the elucidation of pathways for the anaerobic degradation of amino acids (Chapter 7, Part 1) and purines (Chapter 10, Part 1). Some applications have used C with high-resolution Fourier transform NMR in whole-cell suspensions, and this is equally applicable to molecules containing the natural or the synthetic P nuclei. As noted later, major advances in NMR have made it possible to use natural levels of C. 

FAB-SSIMS Fast atom bombardment static FTNMR Fourier-transform NMR... 

Spectroscopic Analysis. Infrared (IR) spectroscopic analysis was performed on a Beckman Microlab 620 MX computing spectrometer. Samples were cast on a sodium chloride pellet or made into a pellet with potassium bromide. and 13C NMR spectra were obtained using a JEOL HNM-FX 270 MHz Fourier transform NMR spectrometer. Samples were dissolved in deuterium chloroform and chemical shifts were referenced to an internal standard of tetramethylsilane. 

One of the most promising applications of nuclear magnetic resonance to vitamin Bj.2-chemistry is the use of carbon-13 NMR (145). The use of 13C fourier transform NMR has greatly increased the applicability of 13C NMR in that it permits one to obtain high quality spectra with natural abundance 13C. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

Instrumentation. The NMR Process. Chemical Shift. Spin-spin Coupling. Carbon-13 NMR. Pulsed Fourier transform NMR (FT-NMR). Qualitative Analysis - The Identification of Structural Features. Quantitative Analysis. Applications of NMR Spectrometry. 

The NMR spectrum of calcitriol, recorded on a Varian XL-100/Nicolet TT-100 pulsed Fourier Transform NMR spectrometer, with internal deuterium lock, is shown in Figure 2 (2). The spectrum was recorded using a solution of 0.84 mg of sample dissolved in 50 microliters of CD OD (100%D) containing 1% v/v tetramethylsilane in a 1.7 mm capillary tube. The spectral assignments are given in Table I. 

This lower sensitivity can be pulsed using Fourier-transform NMR, wherein a high-power microsecond pulse of radio frequency energy sets all the carbon nuclei into resonance at once, thus... 

H NMR has long been routinely used to detect the presence of M-H bonds and also to obtain structural information about clusters containing organic ligands. The availability of Fourier transform NMR spectrometers over the last few years has enabled studies involving 13C, 31P and other nuclei also to be carried out. This has made NMR an extremely useful technique for obtaining structural information and, be-... 

Gemini Superconducting Fourier transform NMR systems, VXR series 5, Varian Instruments, Sugar Lane, Texas, USA... 

The introduction of additional techniques such as Pulsed Fourier Transform NMR spectroscopy (PFT-NMR) has considerably increased the sensitivity of the method, allowing many magnetic nuclei which may be in low abundance, including 13C, to be studied. The additional data available from these methods allow information on polymer structure, conformation and relaxation behaviour to be obtained (1.18.20). 

D. Shaw, Fourier Transform NMR Spectroscopy, Studies in Physical and Theoretical Chemistry, Vol. 30, 2nd ed., Elsevier, Amsterdam, 1984. 

This approach has recently been extended to keto tautomers (Bassetti et al., 1988). Fourier-transform nmr spectroscopy is needed to record the signals of the small amounts of the keto isomers that are present. The carbonyl carbons of the keto form are to lower field of the corresponding atoms in the enol by ca. 10 ppm. The data were analysed into substituent factors relative to pentane-2,4-dione. Replacing the methyl groups of this compound with 2-thienyl, phenyl and t-butyl groups caused upfield shifts to the a-carbon of —3.14, —4.4 and —6.5 ppm, respectively, when the substituents were introduced at this site. 

Fig. 3. Proton-decoupled natural abundance Fourier-transform NMR spectra of DPPC (chemical structure at the bottom) in excess H2O as a function of pressure at 52.7 °C (after Ref. 14).

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