J spectrum

The algorithm that uses absolute difference is rapid and simple. For each of the j spectra in the library and n points along the abscissa, the sum St is calculated as the absolute difference between the ordinate of the unknown and that of the corresponding spectrum j in the library (equation (10.10)). Then the summations Sj are ranked. The results are presented along with a correlation index. 

Figures 3,4 and 5 present respectively the Sip, Mo3d and NiM/j spectra of the freshly sulfided and of the fourth sample of the differently treated catalysts. In these figures, the peaks...

For 3-bromothiophene-2-aIdehyde (56), a long-range coupling-constant of 0.051 Hz was measured between H-4 and the aldehyde proton.175 From the peak widths of the / spectra, the value of T2 for the aldehyde proton was found to be 11.2 sec. Partial J spectra were obtained for this proton, and also for the ring protons of methyl furoate (57), by restricting the range of modulation frequencies... 

Good quality spectra from this FID are shown on the next page. Tire j spectra in problems 1.2-1.6 were all generated from the same free induction decay (FID) using data processing errors that are common to both experienced and inexperienced operators. 

J/2) from the CH2 protons and the other at 5-94 Hz from the CH proton whose resonance in a normal spectrum is a 1 2 1 triplet with outer components which precess at twice the rate of the components of the CH2 doublet. (148) In fact each spin-echo normally contains a number of different frequency components which can be separated by electronic filtration or by Fourier transformation to yield a set of J-spectra which can then be conveniently displayed in a two-dimensional plot. In practice the second approach is normally used, and thus the set of spin-echoes is subjected to a double Fourier transformation according to equation (2) in which cafe must be taken to combine correctly the real and imaginary transforms. (26)... 

In equation (2) f 1 corresponds to the period of echo formation while t2 corresponds to the time during which the echo is sampled coi and a>2 correspond to the dispersion within individual J-spectra and to the normal spectral spread respectively. The resulting two-dimensional spectrum S((Ui,a)2) is of course made up of real and imaginary parts according (26) to equation (3) where terms such as 5 (co 1,0)2) are defined by equation (4), the superscripts s and c referring to sine and cosine respectively. 

H Spectra.—The total analysis of the 1H n.m.r. spectrum of 11 p-hydroxypro-gesterone, reported briefly last year,42 has now been described in full.43 This is a further illustration of the value of computer-controlled use of a 400 MHz spectrometer to obtain two-dimensional J spectra, and n.O.e. difference and decoupling difference spectra, which provided all the XH chemical shifts and virtually all geminal and vicinal coupling constants. Comparisons with the previously reported findings42 for 1-dehydrotestosterone acetate represent the first step in a correlation of detailed n.m.r. and structural parameters which promises to have widespread application in future studies on steroidal and terpenoid compounds. 220 MHz spectra permit quantitative estimation of C-24 epimeric mixtures of 24-alkyl sterols.44... 

R. Freeman and H. D. W. Hill, "High-resolution study of NMR spin echoes J spectra, " J. Chem. Phys. 54, 301-313 (1971). 

Generally, transformations of Q J) spectra are analogons to those shown in... 

The idea was to construct the mask similar to an undesired shape observed in the spectra, and use CLEAN to replace distorted peaks with those of a perfect Lorentzian shape. Shaka and co-workers [88] showed that the algorithm is able to convert a twisted shape to a double-absorption in 2D phase-sensitive J spectra of complex organic molecules. This was achieved by (1) locating the twisted-shape peaks, (2) simulating double-dispersion signals of the same line width at the same frequency coordinates, and (3) subtraction of the latter from the original spectmm. Effectively, the most intense peaks were in double-absorption while those ignored by CLEAN remained in the twisted shape. 

A similar approach was presented by Keeler in application to heteronuclear J spectra with highly truncated echo modulation [87]. Truncation of signal, used for sensitivity reasons, results in sine wiggles . These artifacts can be suppressed by apodization, although at the expense of resolution. Keeler showed that CLEAN is an inexpensive alternative to the maximum entropy method, which can also remove truncation artifacts without degrading resolution. 

Homonuclear 2D J spectroscopy was first proposed in a succinct communication by Aue, Karhan and Ernst,followed by two further notesl5,16 on the application of the technique to the proton spectra of biomolecules. Several theoretical analyses of the effects of strong proton-proton coupling on 2D J spectra have been produced,17-19 reference 19 including some experimental data for simple spin systems. Such spectra can be analyzed iteratively if necessary, in order to extract accurate values for coupling constants.l9 20 Homo-... 

FIGURE 37. NMR spectra (a-e) of potassium tetrasilicate glass and simulations (f-j). Spectra were recorded with 64 pulses (< tr/6) and delays determined from the T data at (a) 697 °C,... 

Strickler, S. J., Spectra and Structure of the Nitrate and Nitrite Ions. Thesis, Florida State University, January, 1961. van der Waals, J. H., to be published. 

L.D. Hall and S. Sukumar, A versatile strategy for the generalized acquisition of proton spin-echo data measurement of partially relaxed two-dimensional J spectra, J. Magn. Resonance 40, 405 (1980). 

R. Freeman, G.A. Morris, and D.L. Turner, Proton-coupled carbon-13 J spectra in the presence of strong coupling, I, J. Magn. Resonance 26, 373 (1977). 

Ledbetter MP, Crawford CW, Pines A, Wenuner DE, Knappe S, Kitching J, Budker D (2009) Optical detection of NMR J-spectra at zero magnetic field. J Magn Reson 199 25-29... 

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