NMR spectrum frequencies

Spin-spin relaxation is the steady decay of transverse magnetisation (phase coherence of nuclear spins) produced by the NMR excitation where there is perfect homogeneity of the magnetic field. It is evident in the shape of the FID (/fee induction decay), as the exponential decay to zero of the transverse magnetisation produced in the pulsed NMR experiment. The Fourier transformation of the FID signal (time domain) gives the FT NMR spectrum (frequency domain, Fig. 1.7). 

FID Free induction decay, decay of the induction (transverse magnetisation) back to equilibrium (transverse magnetisation zero) due to spin-spin relaxation, following excitation of a nuclear spin by a radio frequency pulse, in a way which is free from the influence of the radiofrequency field this signal (time-domain) is Fourier-transformed to the FT NMR spectrum (frequency domain)... 

Despite these simplifications, a typical or F NMR spectrum will nomially show many couplings. Figure BTl 1.9 is the NMR spectrum of propan-1-ol in a dilute solution where the exchange of OH hydrogens between molecules is slow. The underlymg frequency scale is included with the spectrum, in order to emphasize how the couplings are quantified. Conveniently, the shift order matches the chemical order of die atoms. The resonance frequencies of each of the 18 resolved peaks can be quantitatively explained by the four... 

Figure Bl.11.9. Integrated 250 MHz H NMR spectrum of dilute propan-1-ol in dinrethylsulfoxide solvent. Here, the shift order parallels the chemical order. Arr expansion of the H2-I nrultiplet is included, as is the implicit frequency scale, also referenced here to TMS = 0.

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

Figure 1.8. Homonuclear decoupling of the CH protons of 3-aminoacrolein (CDaOD, 25 C, 90 MHz), (a) H NMR spectrum (b) decoupling at Sh = 8.5 (c) decoupling at = 7.3. At the position of the decoupled signal in (b) and (c) interference beats are observed because of the superposition of the two very similar frequencies...

Figure 2.21. HFI NOE difference spectra (b, c) and FIFI NOESY diagram (d) of a-pinene (1) with /-/ NMR spectrum (a) for comparison [(CD3)2CO, 10% v/v, 25 °C, 200 MHz, section from <5 = 0.85 to 2.34 ]. Vertical arrows in (b) and (c) indicate the irradiation frequencies in the HH NOESY plot (d), cross-signals linked by a dotted line show the NOE detected in (c)...

Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sfflnple through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compar ed with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted as a spectrum, which consists of a series of peaks at characteristic frequencies. Its interpretation can furnish structural information. Each type of spectroscopy developed independently of the others, and so the data format is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 

The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance frequencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a paiticulai isotope of a single element. The frequency for H is 200 MHz at 4.7 T, but that of is 50.4 MHz. Thus, when recording the NMR spectrum of an... 

It is curious that the chair- boat problem, which is most associated with small, liquid-state molecules, arises in the context of solid-state research (B3, II). Although the paucity of useful experiments militates against a definitive solution here E3), the frequency independence of the NMR second moment (E2), the absence of an observable free-induc-tion decay (Tj <25 fis) in the pulsed NMR spectrum (El), and the smoothness of the absorption mode itself (SI), all argue against the... 

Two-dimensional NMR spectroscopy may be defined as a spectral method in which the data are collected in two different time domains acquisition of the FID tz), and a successively incremented delay (tj). The resulting FID (data matrix) is accordingly subjected to two successive sets of Fourier transformations to furnish a two-dimensional NMR spectrum in the two frequency axes. The time sequence of a typical 2D NMR experiment is given in Fig. 3.1. The major difference between one- and two-dimensional NMR methods is therefore the insertion of an evolution time, t, that is systematically incremented within a sequence of pulse cycles. Many experiments are generally performed with variable /], which is incremented by a constant Atj. The resulting signals (FIDs) from this experiment depend... 

The frequency-domain spectrum is computed by Fourier transformation of the FIDs. Real and imaginary components v(co) and ifi ct>) of the NMR spectrum are obtained as a result. Magnitude-mode or powermode spectra P o)) can be computed from the real and imaginary parts of the spectrum through application of the following equation ... 

The basic principle underlying the development of images is simple (Lauterber, 1973). Consider a body cavity containing two pools of water in different quantities. In a uniform magnetic field, the NMR spectrum will consist of a single peak, since all the water molecules will process at the same frequency, irrespective of their spatial location. If, however, a linear field gradient is applied in the x -direction, the Larmor frequency of the water will increase linearly across the sample as a function of the x -coordinate, thereby creating a one-dimensional profile, or spectrum, of the sample (Fig. 7.21). 

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