Intensity anomalous

Albrecht M G and Greighton J A 1977 Anomalously intense Raman spectra of pyridine at a silver electrode J. Am. Chem. Soc. 99 5215-17... 

The intensity differences obtained in the diffraction pattern by illuminating such a crystal by x-rays of different wavelengths can be used in a way similar to the method of multiple isomorphous replacement to obtain the phases of the diffracted beams. This method of phase determination which is called Multiwavelength Anomalous Diffraction, MAD, and which was pioneered by Wayne Hendrickson at Columbia University, US, is now increasingly used by protein cystallographers. 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

P212121 Z = 4 D = 2.01 R = 0.04 for 1,611 intensities. The compound is a minor product in the synthesis of methyl tyveloside. The pyranose conformation is a distorted 4, with Q = 66 pm 6= 162° (p=H8a. The (methylthio)carbonyl side-chain is extended. The C-S bond-lengths are 174.8, 179.1 pm. The C-I bond-length is 215.2 pm. The absolute configuration was confirmed by using the anomalous-scattering factors of the iodine atoms. 

The anomalous dual fluorescence emission of p-A V-dimethylamino benzoni-trile (DMABN) in polar solvents was first reported by Ernst Lippert in 1962. Emission spectra of DMABN in solvents of different polarity show a dual emission, where the red-shifted emission is stronger relative to the primary emission when the solvent polarity increases. Furthermore, it can be observed that overall emission intensity is reduced in more polar solvents, but higher solvent viscosity increases the emission intensity. Spectra of DMABN in different solvents are shown in the chapter of Tomin in this book [1]. 

Anomalous X-ray diffraction or resonant scattering refers to the modification of its intensity due to absorption processes involving interactions between the X-ray beam and the atoms in the sample. This interaction combines the chemical and short-range order sensitivity of absorption with the long-range order sensitivity of diffraction. This results in a chemical selectivity, i.e. it is possible to differentiate elements with close atomic numbers or even cations with the same number of electrons like Rb+ and Sr2+... 

The X-ray intensity diffraction data of the given crystal do not allow one to specify which of the two sets describes the actual crystal structure and thus the absolute configuration of the molecule when there is no effect of anomalous X-ray dispersion. Under such conditions Friedel s law holds, which states that the X-ray intensity diffraction pattern of a crystal is centrosymmetric whether the crystal structure is centrosymmetric or not. This does not mean that a false crystal structure containing a center of symmetry is obtained as the solution of the structural problem, but rather that the X-ray analysis cannot differentiate between the two enantiomeric structures. A simple mathematical analogy is provided by the two possible square roots of a number Vj = x. 

Now consider the effect of anomalous scattering on the relative intensities of the diffracted rays in Scheme 2a and b when atom Y scatters anomalously with an intrinsic phase lead A< >(Y), and atom W scatters normally. Under such circumstances, the wave scattered by atom Y in Scheme 2a would lead that of atom W by a phase difference of + A< >(Y), and the wave scattered by atom Y in Scheme 2b would lag behind that of atom W by - + A(Y). These two phase differences are unequal in magnitude, so the corresponding amplitudes of their resultant waves, and the subsequent intensities, will be different, leading to a breakdown of Friedel s law. 

Chew and Wang(39) have pointed out the possibility of double resonance, that is, that the frequencies of both the excitation and inelastically scattered radiation are resonant. They presented the results of calculations which indicate that double resonance can have a significant effect on the angular intensity distribution of inelastically scattered radiation. This case is of some practical interest, particularly in Raman studies, where coincidence may lead to anomalous Raman band intensities, if both the excitation and the shifted frequency are resonant. 

Although mass bias effects relative to total ion intensity may be corrected using a working curve (Fig. 12), anomalous mass bias in natural samples due to the presence of other elements cannot be corrected. Figure 13 shows the effects of anomalous mass bias in a 400 ppb Fe ultra pure standard that has been doped with varying concentrations (up to 75 ppb) of Mg, Al, or La. Carlson et al. (2001) noted a similar effect of Al on the isotope composition of Mg standard solutions. These matrix elements were chosen because they would not produce isobars on the Fe mass spectrum (Fig. 13D) and for their spread in atomic mass Mg and Al both have masses less than Fe and La is greater. Additionally Mg and Al are major elements (7 and 3" most... 

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