T-shaped structure

Note that the structural form of each compound is implied by the presence (T-shaped structure) or absence (distorted structure) of the perturbed asymmetric stretching mode at about 550 cm . 

Early experimental spectroscopic investigations on Rg- XY complexes resulted in contradictory information regarding the interactions within them and their preferred geometries. Rovibronic absorption and LIF spectra revealed T-shaped excited- and ground-state configurations, wherein the Rg atom is confined to a plane perpendicular to the X—Y bond [10, 19, 28-30]. While these results were supported by the prediction of T-shaped structures based on pairwise additive Lennard-Jones or Morse atom-atom potentials, they seemed to be at odds with results from microwave spectroscopy experiments that were consistent with linear ground-state geometries [31, 32]. Some attempts were made to justify the contradictory results of the microwave and optical spectroscopic studies, and... 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

For bromide and iodide, the nature of the countercation influences the structure of the anionic complex. In fact, when the [(18-C-6)-K]Br and [(18-G-6)-K]I salts are used, the anionic complexes ([ 163—X]—, X = Br, I) remain mononuclear and adopt a T-shaped structure (Figure 11). In both cases, the Hg-X bonds are shorter than those observed in the corresponding dinuclear complexes in agreement with the terminal location of the anion. The reaction of bis(pentafluoro)phenylmercury 164 with [(18-C-6)-K]Br and [(18-C-6)-K]I also afford T-shaped complexes [164-Br] and [164-1]. The Hg-Br (2.93 A) and Hg-I (3.12 A) bonds found in these complexes are longer than those observed in [163-Br] and [ 163—1] indicating that 164 is a weaker Lewis acid than 163.206... 

Thus, according to this alternate approach, CIF3 belongs to the VSEPR class AX3E2. Molecules of this type adopt a T-shaped structure. 

Electrostatic interactions play a dominant role in weak molecular clusters between components, like arenes, exhibiting quadrupole momenta q. The global minimum of the benzene dimer corresponds to structure with monomers located in perpendicular planes (T-shape structure). " This structure is favored by attractive... 

T-shaped structure. The corresponding values obtained here are 16.02 and 14.90 cm for the linear and T-shaped isomers, and are very closed to the ones predicted experimentally. Further, recent LIF experimental studies have determined the binding energy (Do) for both isomers (linear and T-shaped) to be 17.6(2) and 16.9(2) cm , respectively. These findings are in excellent agreement with the present calculations, confirming our theoretical predictions. 

I o find new and more effective medicines, chemists use various models that J. describe how drugs work. By far, one of the most useful models of drug action is the lock-and-key model. The basis of this model is the connection between a drugs chemical structure and its biological effect. For example, morphine and all related pain-relieving opioids, such as codeine and heroin, have the T-shaped structure shown in Figure 14.1. 

Well-expressed correlations between headgroup structure and transfection activity have been found for cholesterol-based cationic derivatives. Polyamine derivatives with T-shaped structure exhibit considerably higher levels of transfection as compared to lipids with linear polyamine headgroups (Fig. 6) [36]. Varying... 

The XY3 molecule has a T-shaped structure axial (ax), equatorial (eq) (103)2 is a dimer bridging (b), terminal (t) XY5 forms a square-based pyramid apical (ap), basal (ba) XY7 has the shape of a pentagonal bipyramid equatorial (eq), axial (ax). 

On the other hand, C02-HBr was found to be different. The bromine faces the carbon with a Br C-O angle of approximately 90°, in what we refer to as an inertially T-shaped structure (Sharpe et al. 1990 Zeng et al. 1992). Unfortunately, the accuracy of our measurements was insufficient to locate the hydrogen. It is reasonable for it to be localized near the oxygen, even though hydrogen bonding is not the dominant interaction. 

Similar effects may occur in molecule C1F3, which has an equilibrium T-shaped structure (G = C2V)- Both gas and liquid-phase 19F NMR measurements have been reported (yielding an AB2-type spectrum),104 and some theoretical analyses exist.10 106... 

In the low-symmetry situation of the T-shaped structure, orbitals d , and dzi must always mix together. (In the cis-divacant structure this is avoided by a suitable choice of axes.) Solution of the relevant determinant gives us e - (1.5 1 /) P 8,z for the two interacting orbitals [see McClure (19) for a more complete discussion]. In Burdett (24) such mixing was ignored. 

The data show that the quadmpole hyperfine patterns of the rotational transitions are different between the two states, due to changes of the relative positions of some of the hyperfine components within the multiplet. The rotational spectrum of a pyrrole dimer is consistent with essentially a T-shaped structure, in which the planes of the two pyrrole monomers form an angle of 55.4(4)° and the nitrogen side of one ring is directed to the 7t-electron system of the other ring establishing a weak H bond <1997JCP504>. 

The C1F3 molecule has a planar distorted T-shaped structure with one short (0.1598 nm) and two long (0.1698 nm) Cl—F bonds. The angle between the two different kinds of Cl—F bonds is 87.48° [86,87]. The infrared and Raman spectra [88] and NMR [89] were measured. Gas chromatography was applied and data were obtained using a polytetrafluoroethylene-packed stainless steel column [90-92]. 

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