X-rays geometries

The ylide 44, prepared from the corresponding diazine and tetracyanoethylene oxide, rearranges in methanol the give the 1,3-diazepine 45 <96TL1587>. The x-ray geometry for 45 is reported. 

The results, presented in Table 8, show that in most cases the conformer with the lowest steric energy indeed corresponds to the experimentally most favored one. In addition, several molecules containing the N—C—N moiety were retrieved from the Cambridge Structural Database and calculated with the new parameter set. A comparison between MM2 and X-ray geometries (selected structural parameters only) for two conformers of 1,4,5,8-tetraazadecalin (25, 26) is provided in Table 9 and shows good fit between the experimental and calculated data. 

Galasso and coworkers (03CP(288)33) have carried out MP2 and B3LYP calculations on compounds DILLEP 1, SUXWUD and SUXXAK (Scheme 21) and, at the same time, determined the 13C NMR and the photoelectron spectra. The theoretical results satisfactorily reproduced the experimental data including the X-ray geometries (VI.A). An important paper reports a theoretical study of alkyl-n and aryl-7i interactions (02JOC7057). The authors extended their calculations to Wilcox s... 

Nuclear Overhauser effect (NOE) experiments clarified the preference of the cis-trans-geometry in solution for cyclic lactams 19. For 19a-c, X-ray geometries (Section 14.10.3.1) retain in solution, and NOEs were observed between the methylene protons next to the ring carbonyl and the NCH2 protons, whereas no such NOE was observed in 19d <2002CC2656>. 

An x-ray geometry is considered to be reasonably well reproduced if the RMS deviation of the atomic positions RMSxfz is less than 0.3 A. A chain geometry is taken to be well reproduced if the RMS deviation of the torsion angles at rotatable bonds RMSTa is less than 15°. A 3D molecular model is considered to be free of nonbonded interactions if the close contact ratio CCR (the ratio of the smallest nonbonded distance to the smallest acceptable value for this distance) is greater than 0.8. 

Each program was used to generate conformational ensembles for the compounds in the data set. To find out whether the conformational search method under consideration is able to reproduce biologically active conformations within the ensembles of conformations, the computed conformations were compared to their x-ray geometries (receptor-bound conformations) by applying a distance measure in Cartesian space (i.e., the RMS rz deviation of the corresponding atoms). 

The programs described above were used to generate ensembles of conformations for each ligand in the test set. Finally, all conformations were compared to their corresponding bioactive geometry. A hit was defined when at least one conformation was found within an ensemble with an RMS fz deviation to the x-ray geometry less than or equal to 0.5 A. 

What is the efficiency of a 50-mm-long proportional counter filled with a mixture of xenon (20 pereent) and methane at a pressure of 1 atm for a parallel beam of 10-keV X-rays (geometry of Fig. 12.11) ... 

Table 1 Calculated and x-ray geometries of the eight-membered rings of disaccharide 3,2 -0-isopropyIidene acetals.

Table 18.1 Optimized stmctuial parameters of Cr2(02CCH3)4(OH2)2 (1) by X-B3LYP (X = R. BS, HS, and AP) and calculated J values between Cr(II) ions at the optimized and X-ray geometries...

The optimized geometry by the AP method shortens the Fe-Fe bond in comparison with the X-ray geometry, and the BS result looks accurate than the AP result. This is just due to the SCE. The slight difference between AP and X-ray geometry seems to originate in the DPT functional set. In other words, the results indicate that we have to be careful about the SCE to discuss the reliability of the functional sets, especially for the biradical systems [36]. 

The X-ray geometry of trans-2-cycloocten-l-yl 3,5-dinitrobenzoate illustrates the combination of structural deformations in this highly reactive (but isolable ) molecule. The trans-double bond suffers from -20° twisting and 22° out-of-plane bending. The C-C = C-C torsion angle is -138° (Figure 2.24). "... 

In this type of hyperconjugation, donor and acceptor interactions are balanced, and often there is no dominating effect. As a result, the importance of sacrificial hyperconjugation in neutral hydrocarbons has been initially controversial. However, now the effects of hyperconjugation on X-ray geometries of nentral molecules are well-documented. °... 

Hyperconjugative interactions in ground state from X-ray geometries Laube, T., Ha, T. K. (1988). Detection of hyperconjugative effects in experimentally determined structures of neutral molecules. Journal of the American Chemical Society, 110, 5511-5517. 

Solid State. - The X-ray geometry of 1,3-dicyanobenzene has been compared with a HF/6-31+G(d) structure in the gas-phase. The distortion of the benzene ring in 1,3-dicyanobenzene has been compared and discussed in relation to the o- and /i-isomers of dicyanobenzene. The differences in the C-C bond lengths of the phenyl ring have been analysed in terms of p. The electron density and its Laplacian were analysed in terms of the topological properties at the BCPs. 

Table 6. Tensor values for the two methylene positions in syndiotactic poly(propylene) calculated by IGLO. In this case, the X-ray geometry was used as basis for the IGLO calculations...

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