Structural parameters from electron diffraction

Structural parameters and interatomic distances derived from electron diffraction (7) (77JST(42)l2i) and X-ray diffraction (8) studies (76AX(B)3178) provide unequivocal evidence that pyrazine is planar with >2a symmetry. There is an increased localization of electron density in the carbon-nitrogen bonds, with carbon-carbon bonds being similar in length to those in benzene. ... 

Table 2 Structural parameters and conformations of 1,2,4-trioxolane rings obtained from x-ray diffraction compared with data for 1,2,4-trioxolane from electron diffraction. ...

Electron diffraction measurements of gaseous 1,2,4-trioxolane point to either a C2 symmetry, as depicted at the bottom of Appendix A, or a Cj symmetry, corresponding to an envelope conformation. However, consideration of steric interactions favors the twisted chair conformation (288c) for the molecule. Table 5 presents the structural parameters derived from electron diffraction, assuming that both C—O distances are equal and that the H atoms are symmetrically placed with respect of the neighboring atoms. Assumptions on the vibrational amplitudes of the ring bonds do not affect the calculated values of Table 5, except for the H—C—H angle . ... 

A conformational analysis of thiacycloalkanes including 2-thiacyclobu-tane, utilizing molecular mechanics for the calculation of the geometries and energies, was made by Allinger and Hickey. The calculated C—C, C—S, C—S—C, C—C—C, and C—C—S bond parameters for the thietane structure, in comparison to those that were derived from electron diffraction, NMR, and microwave measurement, are reported. The experimental and calculated heat of formation of the thiacyclobutane are 14.49 and 14.58 kcal/mol, respectively. 

In crystallography, heavy atom derivatives are required to solve the phase problem before electron density maps can be obtained from the diffraction patterns. In nmr, paramagnetic probes are required to provide structural parameters from the nmr spectrum. In other forms of spectroscopy a metal atom itself is often studied. Now many proteins contain metal atoms, but even these metal atoms may not be suitable for crystallographic or spectroscopic purposes. Thus isomorphous substitution has become of major importance in the study of proteins. Isomorphous substitution refers to the replacement of a given metal atom by another metal that has more convenient properties for physical study, or to the insertion of a series of metal atoms into a protein that in its natural state does not contain a metal. In each case it is hoped that the substitution is such that the structural and/or chemical properties are not significantly perturbed. 

It might be thought that the vibrational analysis for PC1 F5 was redundant, since the electron diffraction data provided complete structural information. This is not quite true the two studies were in fact complementary. In the radial distribution functions obtained from electron diffraction, some of the peaks were ill-resolved their better resolution in order to obtain accurate structural parameters was assisted by the amplitudes of vibration which can be calculated by normal coordinate analysis. The vibrational study was also valuable when, in 1987, the same team tackled the structural characterisation of the analogous arsenic compounds. These presented some experimental difficulties, because they are thermally less stable than their phosphorus analogues they tend to decompose to give As(III) species, e.g. 

Structural parameters for SCBs obtained from different methods of quantum chemistry in comparison with the results from electron diffraction experiment have been reported (Table 1) <2006JST(800)146>. 

All the complexes XXIa-i are air-sensitive colorless solids or liquids. Structural parameters are available mainly from X-ray structure data (76,266) and from electron diffraction and microwave studies (67-69,71,79,80). In nearly all cases the molecules are monomeric in the solid state, in solution, and... 

Force field validation. In addition to ensuring that the force field reproduces results of QC calculations we have compared predictions of MD simulations using this force field with the available experimental data. Gas phase MD simulations using the quantum-chemistry based force field accurately reproduced the gas phase structure of DMNA as determined from electron diffraction studies. Liquid phase MD simulations of DMNA predicted the densities and solubility parameter as well as the activation energy and correlation times associated with molecular reorientation that are in good agreement with experimental data [34], As we will show in Section 4, comparison to structural and thermal data for the three pure crystalline polymorphs of HMX support the overall validity of our formulation and parameterization. 

The molecular structure has been derived from electron-diffraction studies by Sharbaugh et al. (6), Bowen (7), and Anderson (8) and from microwave data by Sheridan and Gordy (9). The adoped parameters are an average of those reported by (8, 9). Infrared spectra of the gas were observed by several investigators (1 , 11., and Raman spectra of the liquid were... 

Early electron diffraction studies (8, 9) and more recent microwave measurements (1, 1 ) have shown that the 8OI2 molecule has 0 symmetry. We adopt structural parameters from the microwave work of Davis and Gerry (11). The bond length and angle 32 35... 

A CNDO/2 calculation on the (Me2NAlH2)3 molecule suggests that the structural parameters obtained from electron diffraction are superior to... 

All the structural information for carbonyl difluoride relates to the gas phase there are no solid state studies. Carbonyl difluoride exhibits C y symmetry its characteristic structural parameters have been established from electron diffraction and microwave spectroscopic data, and are summarized in Table 13.17. Of these data, those of Nakata et al. [1480] are the most precise. 

Structural parameters and interatomic distances derived from electron diffraction <77JST(42)121> and x-ray diffraction studies <76AX(B)3178> were given in CHEC-I. The molecular structure of pyrazine has been determined by combined analyses of data obtained by gas-phase electron diffraction (ED) and liquid-crystal NMR (LCNMR) <88JA2758>. The NMR spectrum gives structural information because the solute is partially oriented in the liquid-crystal solvent. The structural parameters determined from the ED, LCNMR data and in a joint analysis of both are listed in Table 2. There the C—C bonded distance is fixed since LCNMR data give no information on the absolute size of the molecule. Since pyrazine itself has no dipole moment, it should not show a microwave (pure rotation) spectrum. 

Halogen Derivatives. Structural studies of CCl2=CF2, (CF3)3CCF (both by electron diffraction), and CH2DF (by microwave) have been reported. A detailed vibrational study of four isotopic species of BrClFCH has been presented, and used to calculate a generalized valence force field, employing structural parameters from the electron-diffraction study. ... 

But in spite of all the advantages of the SEFS method as compared to both diffraction and spectroscopic methods of structure analysis, this technique has not yet been applied to analyze the local atomic structure of surfaces and thin films. This is explained by the diversity and complexity of the processes forming the secondary electron spectrum and the corresponding fine structures, and the resulting difficulties in their theoretical description and in the mathematical formalization of the problem of determining local atomic structure parameters from the experimental data. 

When the molecule under study contains elements of very different atomic numbers, the parameters relating to lighter atoms may be very uncertain. As a result of 3) and 4), in the worst cases even qualitative conclusions derived from electron diffraction on molecular geometry (such as the structural formula, equilibrium symmetry, and the presence of conformers) can be in error. 

Chapter 1, p. 18) r° if the parameters were derived from electron-diffraction intensities alone rz if they were derived from rotational constants alone r v if they were derived utilizing a combination of both types of data. The method includes a way of converting rg to r ° distances, but it does not provide a procedure for converting r to r , the isotope-substitution distance from microwave spectroscopy. The first structures to be refined by the above workers using these methods were those of butadiene, acrolein, and glyoxal. The principal results for these planar molecules, in which the double bonds are situated trans to each other, are listed in Table 2. This work establishes with certainty for the first time the differences between the C-C bond lengths in these three molecules. 

A paper on the use of cubic force fields obtained ab initio as aids in the determination of equilibrium structures from electron diffraction data uses sulfur dioxide as an example. There are several ways in which solutions can be found to problems for which the primary data do not provide unequivocal answers. All involve the use of additional data, and in the methodology described here scale factors for the force constants are refined, along with the geometrical parameters. The outcome is the equilibrium geometry, in this case the S = O distance of 143.08(1) pm and the OSO angle of 119.32(1)°. 

The absolute values of the structural parameters given in Table 3 were obtained by fitting the C—C-bond length to the corresponding value obtained from electron diffraction [51]. The nmr structural parameters of Table 3 were derived from the pair of coupling constants Dh h Somewhat different structural... 

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