Structural Data and Spectra

X-Ray structural data and recent high level theoretical calculations confirm that this neutral, diamagnetic dithiadiazole is an aromatic six k-electron ring system. The gas-phase infra-red and photoelectron spectra of S2N2CO have also been reported. ... 

Noteworthy also is the extensive compilation of early data on layered MX2 given by Wilson and Yoffe [37], who worked out a group-by-group correlation of transmission spectra of the compounds to available electrical and structural data and produced band models in accord with a molecular orbital approach. 

The complex [Ru(bpy)2(2-Me0C6H4PPh2-P,0)] + exhibits a significant red shift of the absorption and emission spectra that is concentration dependent. From NMR spectroscopic data, it is concluded that this observation arises from equilibration with an aqua complex. Structural data and the solution properties of [Ru(bpy)2(2-Pr0CgH4PPh2-P,0)] have been reported." These results expand on an earlier report of [Ru(bpy)2(2-R0CgH4PPh2-P,0)] (R = Me, Et)." ... 

Dithiolenes have been found to stabilize nickel(III) and a number of structural investigations have been performed on nickel(III) dithiolene complexes. Structural data and physical properties of selected compounds are collected in Table 120. The EPR spectra of the [NiS4] unit have been extensively studied in order to decide whether the unpaired electron resides mainly on the metal or on the ligand3202,3203,3210,3212-3217 giving rise to a true nickel(III) complex (422) or to a nickel(II)-stabilized ligand radical complex (423). 

This paper reports the synthesis, representative single-crystal X-ray structural data, optical spectra, and preliminary measurements of powder SHG efficiencies of the new compounds. 

The fact that, in the case of solution spectra, the spectroscopic data are obtained from the solvated species while the corresponding molecular mechanics structure usually represents the naked species is somewhat unsatisfactory. This is one reason for some observed, albeit small, differences between structural parameters in solution and in the solid state. Nevertheless, the accuracy is often surprisingly high, and this might be explained by the fact that the parameterization of the force field is generally based on crystal structural data, and the calculated structures therefore repre-... 

Al ordering in a dachiardite framework was examined based on the structural data and Si MAS NMR spectra. Obtained ordered distribution model of dachiardite contains four Al atoms per unit cell, and its space group was P2/m instead of the conventional C2/m. Al atoms are located on T3 and T4 sites which construct 4 rings. Because Al content in a mined sample is larger than four, small amount of Si atoms should be substituted by Al atoms and some extent of Al atoms move to another site in order to satisfy the Loewenstein s rule. Simulated NMR spectrum based on this substitution was in excellent agreement with the observed ones. 

Three forms are present in the commercial solid, evidenced by the three components of methylene and mefliyl carbon resonances. Recrystallization from ethyl acetate results in only one form. Polymorphism was also recognized in the case of caffeine [3232]. The C CPMAS spectrum of solid caffeine (Figure 12-12) shows five carbonyl resonances whereas there are three C=0 groups. The assignment of signals in the spectra of phenobarbital or caffeine is not straightforward, it requires X-ray diffraction structural data and theoretical calculations of shielding constants for the forms present in the crystals. 

Fig. 4 Structure (right) and spectra (left) of open and closed conformations of myoglobin. Both x-ray diffraction and infrared absorption data were recorded on single crystals of MbCO at pH 6 (top) and pH 4 (bottom). Displacement of the distal histidine (blue) from the heme pocket in the open structure leads to a shift of the C-O stretch frequency from 1945 cm to 1966 cm and may facilitate entry and exit of small molecules. The solid curve represents the contribution of an individual protein conformer to the IR band, based on the 2.7 cm homogeneous linewidth expected from IR photon echo measurements.The heme is rendered in red. the bound CO in white, the distal histidine in blue, and the remainder of the protein in green. Atomic coordinates for structural models of MbCO at pH 6 and pH 4 were obtained from the Protein Data Bank files Ivxf and Ispe, respectively.(View this art in color at www.dekker.com.)...

Structurally closely related spider silks, namely, major ampullate silk (MaS), minor ampullate silk (MiS), and cocoon silk have been investigated by Cloutier et al. using solid-state NMR. These silks exhibit different mechanical properties that are not well explained with current structural data. NMR spectra confirm that the secondary structures of these silks are similar, but that differing relaxation dynamics exist at the alanine, glycine and glutamine residue level. It has been shown that silk extensibility is related to a more mobile structural organization. 

Experiments produce data on the structure, reartivity, and spectra of matter. Often these data are collected for condensed phases at room temperature. The common quantum chemistry software packages, however, are designed for gas phase molecules at 0° K. Within these limitations, theory can still be a useful help in interpreting some experiments. 

Spectroscopic databases are a very valuable tool for the identification of known and unknown substances. In most spectroscopic laboratories they are available and frequently used. Retrieval of data and spectra similarity searches are established tools for the fast identification of unknown compounds. The spectroscopic information stored in the databases offers the generation of structure-spectra correlations, which can be used for predicting spectral features of new compounds. Effective spectrum prediction tools are available for C NMR and H NMR, and will become available for IR spectroscopy in the near future. The prediction of mass spectra is still a challenge. 

Classifier structures resulting from the training were verified in a blind test. To evaluate the reliability and performance of the NSC it was subjected to a blind test using unknown data containing spectra measured for various sizes and locations of the disbonds (from 50% to over 100% of the probe size). 

A relatively small training set of 744 NMR chemical shifts for protons from 1 20 molecular structures was collected from the literature. This set was designed to cover as many situations of protons in organic structures as possible. Only data from spectra obtained in CDCl, were considered. The collection was restricted to CH protons and to compounds containing the elements C, H, N, 0, S, F, Cl, Br. or I. 

In contrast to IR and NMR spectroscopy, the principle of mass spectrometry (MS) is based on decomposition and reactions of organic molecules on theii way from the ion source to the detector. Consequently, structure-MS correlation is basically a matter of relating reactions to the signals in a mass spectrum. The chemical structure information contained in mass spectra is difficult to extract because of the complicated relationships between MS data and chemical structures. The aim of spectra evaluation can be either the identification of a compound or the interpretation of spectral data in order to elucidate the chemical structure [78-80],... 

The substance chaetochromin A, structure A, has been shown by X-ray dif action to have the absolute configuration indicated in the structure. The CD spectra of A and the related compounds cephalochromin (B) and ustilaginoidin (C) are shown in the figure. Deduce the absolute stereochemistry of cephalochromin and ustilaginoidin trom these data, and draw perspective structures indicating the absolute configuration. 

Comparison of the photoelectron spectra and electronic structures of M-NS and M-NO complexes, e.g., [CpCr(CO)2(NX)] (X = S, O), indicates that NS is a better a-donor and a stronger r-acceptor ligand than NO. This conclusion is supported by " N and Mo NMR data, and by the UV-visible spectra of molybdenum complexes. 

Snyder and his co-workers assigned structures 48 and 49 to these j6-hydroxythiophene derivatives on the basis of chemical evidence and infrared and nuclear magnetic resonance spectral data. Infrared and nuclear magnetic resonance spectra further indicate that compounds of type 49 exist as dimers, probably hydrogen bonded, when R = OC2H5 or CH3, but as monomeric enols when R = H. ... 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

The tautoraerism of certain difunctional derivatives of l-thia-3,4-diazole has received considerable attention. Pala assigned structure 156 to 2,5-dimercapto-l-thia-3,4-diazole on the basis of infrared spectral data, and Thorn" reached the same conclusion by comparing its ultraviolet spectrum (measured in ethanol) with those of the four possible methylated derivatives. However, the infrared spectrum of a chloroform solution of the parent compound showed bands at 2600-2550 cm indicating an SH group and the probable occurrence of form 157 under these conditions, and this conclusion is supported by the occurrence of SH bands in solid state spectra obtained by Swiss investigators. For a summary of earlier work on these compounds, see reference 187. 

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