Molecular structure, electron diffraction

Selp HM (1973) Electron diffraction theory and accuracy. In Sutton LE (ed) Molecular structure by diffraction methods, vol 1. Chem Soc Lond, UK, Chap I, pp 7-57... 

Halides R MX and RMX are formed by all the elements P to Bi. In general they have pyramidal molecular structures. The rather high melting points of MeBiCl (242°C) and Me BiCl (116 C) suggest that these compounds may be associated in the solid state. Phosphorus forms a complete set of halides Me PX, (Table 4.4). The fluorides are monomeric, volatile species, which have trigonal bipyramidal structures. Electron diffraction studies show that the fluorine atoms preferentially occupy the axial positions. The axial P—bonds are longer than the equatorial P—n.m.r. studies confirm these findings for Me PF ... 

The 3D MoRSE code is closely related to the molecular transform. The molecular transform is a generalized scattering function. It can be used to predict the intensity of the scattered radiation i for a known molecular structure in X-ray and electron diffraction experiments. The general molecular transform is given by Eq. (22), where i(s) is the intensity of the scattered radiation caused by a collection of N atoms located at points r. ... 

These cover the following topics (a) theoretical methods, with emphasis on the utility of such methods b) molecular dimensions, as determined by X-ray, electron diffraction and microwave spectra (c) molecular spectra, covering NMR, IR, UV, mass and photoelectron spectra [d) thermodynamic aspects, such as stability, ring strain, aromaticity, shape and conformation of saturated and partially saturated rings (c) tautomerism, including prototopic and ring-chain (/) betaine and other unusual structures. 

The allyl radical would be expected to be planar in order to maximize n delocalization. Molecular structure parameters have been obtained from EPR, IR, and electron diffraction measurements and confirm that the radical is planar. ... 

Figure 17.16 Molecular structure and dimensions of gaseous molecules of chlorine oxides as determined by microwave spectroscopy (CI2O and CIO2) or electron diffraction (CI2O7).

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

A comprehensive and critical compilation has been published relatively recently on gas-phase molecular geometries of sulfur compounds including sulfoxides and sulfones5. This book covers the literature up to about 1980 and contains virtually all structures determined experimentally, up to that date, either by electron diffraction or microwave spectroscopy. Here we shall highlight only some of the most important observations from that source5 and shall discuss recent results in more detail. 

The electron diffraction analysis of l,2-bis(methylsulfonyl)ethane, CH3S02CH2CH2S02CH338, yielded a limited amount of structural information. However, this substance has also been studied by X-ray crystallography5, 52, and the two sets of data offer a possibility for comparison. The molecular model is shown in Figure 14. 

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. 

The ordered structure and molecule orientation in the monolayers, as suggested by the Hardy model, have been studied by various means. Electron diffraction techniques, for example, including both reflection and transmission, have been employed to examine the molecular orientation of adsorbed monolayers or surface hlms. The observations from these studies can be summarized as follows [3]. 

Later, successful determination of the molecular structure of the free allyl radical was achieved by high-temperature electron diffraction, augmented by mass spectrometry studies (Vaida et al., 1986). The structural parameters obtained for the allyl radical were rcc 142.8 pm, rcH 106.9 pm, accc 124.6°, ccH 120.9°. This was the first electron diffraction study of an unstable organic molecule. 

K. W. Hedberg, Part II. Determination of Some Molecular Structures by the Method of Electron Diffraction. A. Adamantane, PhD Dissertation, Chemistry, Cal Tech, 1948. 

Refiy, B., Kolonits, M., Schulz, A., Klapotke, T.M. and Hargittai, M. (2000) Intriguing Gold Trifluorides — Molecular Structure of Monomers and Dimers An Electron Diffraction and Quantum Chemical Study. Journal of the American Chemical Society, 122, 3127—3134. 

The discussion in this section relates to structural studies carried out by calculation, x-ray diffraction, or electron diffraction techniques and will concentrate on how the molecular structures are influenced... 

Electron Diffraction. - Three gas-phase molecular structures have been determined by electron diffraction. They are the hypophosphite (80)224 trif1uorophosphine sulphide,225 and dichlorotrifluorophosphorane.224... 

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