Microwave Spectroscopy and Electron Diffraction

The electron diffraction structures of 3,5-dimethylpyrazole (19) and 3,5-bis(trifluoromethyl)-pyrazole (20) have been determined 93JST(29i)2l l . The last pyrazole is a compound with a pungent odor which is too volatile to collect diffractometric data even in a sealed capillary. 

Finally, in the case of tris(pyrazol-l-yl)-5-triazine (TPT) (21) the structure has been determined both in the solid state (x-ray) and in the gas phase (electron diffraction) 94MI 30I-02 , both being remarkably similar and close to the conformation of minimum energy (AMI calculations). 

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The geometries of oxiranes have been determined mainly by X-ray diffraction on crystalline natural products, the oxirane ring being widespread in nature (Section 5.05.5.3). However, the detailed structure of the parent compound (Figure 1) has been secured by microwave spectroscopy and electron diffraction studies (64HC(l9-l)l). The strain in this... 

The S-S bond between two divalent sulfur atoms plays an important role as the main stabilizer of the tertiary structure of many proteins. The simplest chemically stable compounds of this class are HSSH and CH3SSCH3. The structures of these two disulfanes have been established by microwave spectroscopy and electron diffraction experiments. 

The purpose of this brief survey was to demonstrate that, despite the criticisms which may be made of the use of any semi-empirical quantum technique for structural and conformational studies, the CNDO/2 and Extended CNDO/2 formalisms are definitely reliable tools for theoretical conformational analyses in inorganic and coordination chemistry. Moreover, if these tools are combined with the most suitable experimental techniques (i.e. microwave spectroscopy and electron diffraction) in that field, many problems of geometry and conformation can be solved in a way that neither of these approaches could have accomplished alone. 

TABLE 5. Comparison of the structural parameters of gaseous 1,2,4-trioxolane determined by microwave spectroscopy and electron diffraction ... 

Equilibrium geometries for upwards of four thousand small molecules have been determined experimentally in the gas phase, primarily by microwave spectroscopy and electron diffraction. In the best cases, the experimental techniques are able to provide bond lengths and angles to within a few thousandths of an A and a few tenths of a degree, respectively. For larger systems, lack of data usually prohibits complete stmcture determination, and some geometrical variables may have been assumed in the reported stmcture. 

Several bond lengths and angles for various monocyclic thietanes, thietes, and their derivatives as determined by X-ray crystallography, microwave spectroscopy, and electron diffraction were presented in CHEC(1984) and CHEC-II(1996). 

Several bond lengths and bond angles for various fused thietanes and thietes and their derivatives were determined by X-ray crystallography, microwave spectroscopy, and electron diffraction, and are presented in CHEC(1984) and CHEC-II(1996). Since 1995, the following structures have been published 5-(2,5-dimethylphenyl)-3,3-diphe-nyl-l-thia-5-aza-spiro[3.4]octan-6-one 3 <2003CC2218> (+)(Ay,4/ )-4-methyl-l,6-diphenyl-2-thia-6-azabicy-... 

Electron diffraction (71PMH(3)27) is predominantly a gas-phase method. By this method, mixtures of conformations can be detected, but rotational barriers can only be estimated in special cases. Microwave spectroscopy and electron diffraction have given many valuable structural determinations in the gas phase, especially for the parent heterocycles and simple derivatives. 

Microwave spectroscopy and electron-diffraction studies have produced very accurate experimental data for small molecules in the gas phase, which provide stringent tests of theories of molecular structure. 

V. P. Spiridonov, A. B. Altmann, A. G. Gershikov, and G V. Romanov, Abstracts, Second Conference on the Determination of Molecular Structure by Microwave Spectroscopy and Electron Diffraction, Tubingen, 1980. 

The large differences in the Si-N=C bond angles reported in different papers may be due to the different averaging process over intramolecular motion as these systems were investigated by microwave spectroscopy and electron diffraction. The bond angles gradually decrease as going from silicon to chlorine. 

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