Bent structures

The molecule of sulphur dioxide has a bent structure. Both S—O distances are equal and short and since sulphur can expand its outer quantum level beyond eight, double bonds between the atoms are likely i.e. 

In the case of 2-silaallene, a bent structure also seems to be energetically favored. 

The long and controversial dispute of Bamberger and Hantzsch (see Sec. 1.1) led the majority of azo chemists to neglect the configuration of azo compounds by formulating stretched azo groups ( —N=N —) instead of bent structures... 

As the o-complexes in these azo coupling reactions are steady-state intermediates (Wheland intermediates, named after Wheland s suggestion in 1942), their stereochemistry cannot be determined directly. Bent structures like that in Figure 12-6 can, however, be isolated in electrophilic substitutions of 1,3,5-triaminobenzene... 

There have a number of computational studies of hypothetical RMMR species [10-13, 40, 411. The simplest compounds are the hydrides HMMH. Some calculated structural parameters and energies of the linear and trans-bent metal-metal bonded forms of the hydrides are given in Table 1. It can be seen that in each case the frans-bent structure is lower in energy than the linear configuration. However, these structures represent stationary points on the potential energy surface, and are not the most stable forms. There also exist mono-bridged, vinylidene or doubly bridged isomers as shown in Fig. 2... 

In 1986, Schaefer et al. reported the first extensive ab initio calculations of S3 [61]. SCF and CISD calculations indicated that the global minimum is the ring D3h form. The more sophisticated CASSCF and MR-CISD calculations, on the other hand, favoured the open bent structure of 2 symmetry. The preference of the open form was subsequently confirmed by several other theoretical investigations [62-65]. The I>3h structure is calculated to be 31-44 kj mol less stable than the open C2V structure, depending on the method of calculation and the level of theory applied [56, 59-65]. The best estimate of the S-S bond length of the Czv form is 193.2 pm (CCSD(T)) [65] and the bond angle is 117°. Compared with the multireference calculations, DFT methods adequately describe the structure and PES of S3. 

Tilt angle of the long molecular axes with respect to the layer normal in the crystal structures. These compounds show a bent structure. 

Table 18) must show a bent structure. This may be one reason for the interlocking and the occurrence of the 21-axis and the antiferroelectricity. But bent structures are possible as well as by gauche conformations like in the described solid state of compound 4-[(S)-2-methylheptyloxy]phenyl 4 -oct-ylbiphenyl-4-carboxylate [153]. 

Maier and co-workers condensed formaldehyde and elemental silicon at 12 K in an argon matrix and photolyzed the mixture to form silaketene H2SiCO, which is similar in structure to the silylene-CO adduct mentioned above. The reactants first form siloxiranylidene 49 (which equilibrates with an unknown species postulated as the planar/linear silaketene 50 when exposed to 313-nm-wavelength light) and then forms complex 51 when photolyzed at 366 nm (Scheme 15). This species could also be formed by photolyzing diazidosilane 52 in the presence of CO, and complex 51 equilibrates with SiCO (53) and H2. The CO infrared shift for this bent structure was calculated at 2129 cm , which is shifted —80cm from the calculated value of free CO, at 2210 cm. The experimentally observed value was reported at 2038-2047 cm at 12 K. 

The stannaketenimine (entry 3, Table V) shown below was found to have a bent structure with an angle at the dicoordinate carbon atom of I540 S7 The coordination geometry of the tin atom is distorted owing to the short contact with one of the fluorine atoms of the trifluoromethyl substituents. The bond between the tin atom and the doubly bonded carbon... 

The complex [Au2(PPh3)2] has been prepared by reduction of [AuI(PPh3)] with sodium napht-halide. Its core unit is isoelectronic with mercurous halides Hg2X2 but, curiously enough, it has a dinuclear trans bent structure with d(Au—An) = 2.76 A and a Au—Au—P angle of 129°.3325 There is no confirmation as yet of the original preliminary report. 

After cocondensation of SiO (1226 cm 1) with alkali metal atoms like Na or K, new bands are detected at 1014 cm 1 (Na) or 1025 cm 1 (K). They can only be attributed to an SiO" anion because of the red shift of the SiO stretching vibration (with respect to that of uncoordinated SiO) and because of different isotopic splittings (28/29/30SiO, Si16/180) [21]. The formation of an ionic species M+(SiO) (M = Na, K) is in line with the results of quantum chemical calculations for the SiO anion (SiO d = 1.49 A, SiO" d = 1.55 A, "electron affinity" SiO + e + 1.06 eV —> SiO") [20]. Taking simple Coulomb interactions into consideration this species is very likely to have a strongly bent structure. The same situation occurs in gaseous NaCN (<(NaNC) = 81.2°) [22],... 

A different strategy for measuring protease activity is based on the property of xanthene dyes to form H-type dimers (see Sect. 6.2.3) when they are in close proximity. These dimers are accompanied with a characteristic quenching of their fluorescence and, particularly for rhodamines, with a blue shift in the absorption spectrum [121, 122]. The probe D-NorFES-D designed to measure activity of elastase in HL-60 cells consists of an undecapeptide derivatized with one tetramethylrhodamine dye on each side. The sequence contains proline residues to create a bent structure and bring the two fluoro-phores in close proximity. Intact D-NorFES-D shows 90% of its fluorescence quenched plus a blue shift of the absorption spectrum. After addition of the serine protease elastase, an increase in the fluorescence and a bathochromic shift of the absorption spectrum is observed, resulting in an increase in the emission ratio [80],... 

The X-ray structural analyses reveal the planar structures around the heavier group 14 elements of all the heavy ketones as well as the case of the carbon analog, ketones. This is in sharp contrast to the trans-bent structure in... 

---