Structural studies bond lengths

An X-ray diffraction study, when possible, is the ultimate structure proof. Bond lengths are known for certainty to 0.001 A and bond angles to 0. T. Although the literature abounds with X-ray data for heterocycles there are no X-ray spectra for a 4,5-dialkyl substituted, nonionic 1,2,3-thiadiazole. However, an X-ray structure has been reported for a 1,2,3-benzothiadiazole (5), and the bond lengths and bond angles are listed in Table 1 (79AX(B)3114>. 

The molecular structure and bond length given above for PCl are from the electron diffraction studies of Hedberg and Iwasahi (1 ). The adopted values are in good agreement within experimental error with the following structural data reported by Kisliuk and Townes (1 ) from microwave studies 2.043 0.003 A, the angle Cl-P-Cl - 100 6 20. The vibrational frequencies are... 

The molecular structure and bond lengths given above for PF were recently determined by Hansen and Bartell (4) from electron-diffraction studies. These results Indicate that PF has a trigonal bipyramid structure with nonequivalent axial and equatorial bonds. Wyatt et al. (5) determined rotational constants for PF from a study of the infrared vibration-rotation band -1 ... 

In all of the above crystal studies, bond lengths and bond angles were found to be similar to those of the corresponding purines. As with all X-ray diffraction work, it can not be assumed that a structure found in an anhydrous crystal would necessarily recur in a hydrated one (which, unfortunately, cannot always be obtained), nor do crystal studies necessarily reflect the tautomeric equilibrium attained in aqueous solution. 

Atom specific and allows for study of closed shell systems which are inaccessible via valence excited state methods extended X-ray absorption fine structure (EXAFS) involves ionized scattered e and provides structural information (bond lengths and number of scatterers) X-ray absorption near-edge structure (XANES) involves transitions to bound states and is dependent on type of edge ... 

Crystal structure parameters have been obtained for MCl (M = Th, Pa, U, or Np) and MBr (M = Th or Pa) by a comparison of quantitative X-ray powder data with single crystal studies. Bond lengths for the halides were calculated, and the covalent radii of the ions estimated at 1.71 (Th), 1.64 (Pa), and 1.61 A (U and Np), respectively. CfF3 exhibited a high-temperature... 

Fig. 1. A. The molecular structure of the tetrahedral complex, Zn(imidazole)2Cl2, determined from X-ray diffraction studies. Bond lengths and bond angles are shown. Taken from Ref. 6) with permission.

According to X-ray studies, 1,2,4,5-tetrazine is planar. Its structural parameters, bond lengths C-N 133.4 and N-N 132.1 pm, bond angles N-C-N 127.2 and C-N-N 115.6°, indicate a delocalized structure, which is best described by the two equal-energy KekulI structures la and lb ... 

These charge-transfer structures have been studied [4] in terms a very limited number of END trajectories to model vibrational induced electron tiansfer. An electronic 3-21G-1- basis for Li [53] and 3-21G for FI [54] was used. The equilibrium structure has the geometry with a long Li(2)—FI bond (3.45561 a.u.) and a short Li(l)—H bond (3.09017 a.u.). It was first established that only the Li—H bond stietching modes will promote election transfer, and then initial conditions were chosen such that the long bond was stretched and the short bond compressed by the same (%) amount. The small ensemble of six trajectories with 5.6, 10, 13, 15, 18, and 20% initial change in equilibrium bond lengths are sufficient to illustrate the approach. 

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

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