Dimer pairs

The structures of the acetic acidS0) and of the propionic acid71 inclusions of 26 (Fig. 20, type III) are isomorphous to each other. The increased guest volume with respect to formic acid yields 1 1 stoichiometry, with no H-bonds between host and guest molecules in either case. The tunnel where the dimers of guests are situated (see Fig. 32 a) is functionally the same as in the case of the self-dimerized pairs of the formic acid guests. 

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

Petrenko, A., K. Redding et al. (2005). The influence of the structure of the radical cation dimer pair of aromatic molecules on the principal values of a g-tensor DFT predictions. Chem. Phys. Lett. 406 327-331. 

Table 5.19. Energies ofsingle monomers (/f mon) and dimer pairs (/s pair) leading to evaluation ofV(pair), VPW, and %npw (Eqs. (5.51) and (5.52)) for the open trimer and cyclic pentamer o/ HF (note [see note 61] that, for these comparisons, V(pair), Vtrue, and Vpw are all expressed with respect to the energies of monomers frozen in the geometry of the complex)...

The structural analysis of seco-zincporphyrazine 160 reveals the formation of C, symmetric dimer pairs in the solid state wherein one of the amido oxygen atoms of one molecule occupies the apical site of the square... 

Figure 39. Back-to-back dimer pairs of 160. [Adapted from (40).]...

The extended structure of [Mn(ebbg)(OH2)] [N03]3 [50,51], is difficult to ascertain, owing to the five co-ordinate Mn(III) geometry and the non co-planarity of the cations and anions. Approximate sheet architectures can be discerned in planes parallel to (-1 3 1) and (1 -2 -2). The latter is shown in Fig. 25. In this view, the cations form dimeric pairs, which are linked through N03 anions and coordinated H20 molecules to form chains aligned in the [0 1 -1] direction. The... 

As discussed above, the Si(001) surface is reconstructed into dimers, a side view of which is shown in Fig. 7. In addition to the reconstruction of the surface atoms there is significant distortion of the subsurface region. This distortion blocks channels which enter into the bulk, and causes an excess of backscattered ions over what would be expected from atomic rows in the ideal crystal. This local distortion is the property monitored by backscattered ion intensities. The dimer pairs also tend to form in rows, which is the source of the (2 X 1) LEED pattern observed for this surface. 

The V02+ phthalocyanine [VO(pc)] has been the subject of patents as it is useful in photoelectrophoretic and xerographic imaging.887 It can exist in at least three solid phases888 and the structure of so-called phase II was determined.889 It is composed of sheets of parallel and overlapping [VO(pc)] molecules and no discrete dimer pair exists. The vanadium is five-coordinate and square pyramidal, and is 0.575 A above the plane of the four nitrogens. The V—N distances do not differ significantly with a mean of 2.026 A the V=0 distance is 1 1.580 A. 

Weak ionic and hydrogen bonds occur between a 3 dimer pairs In the deoxygenated state. 

By preparing mixed dimer pairs, analogous to DNA base pairs, it is possible in principle to control the orientation of neighboring molecules, and of neighboring arrays. A biomimetic nitroaniline analog could conceivably be prepared by cocrystallizing derivatives of 2-aminopyrimidine. 

Linear response polarizability theory of spectral bandshapes was applied to the numerical analysis of the chiroptical spectra obtained for DNA-acridine orange complexes [85]. After analysis of various models of conformation, it was concluded that a dimer-pairs repeating sequence model was best able to account for the observed spectral trends. In another work, the CD induced in the same band system was studied at several ionic strengths [86]. The spectra were able to be interpreted in terms of the long-axis-polarized electronic transitions of the dyes, with the induced CD being attributed to intercalated and non-intercalated dye species superimposed by degenerate vibronic exciton interactions between these. 

The single polypeptide-chain protomers (however, see later) are compactly folded, forming dome-shaped structures approximately 4.2 nm high x 4.0 nm wide x 3.9 nm thick. Each subunit contains one Mn2+, one Ca2+, and one carbohydrate binding-site,172,278-280 believed, until recently, to be situated in a prominent cavity in the molecular model.280 Two such subunits are joined with their flattened bases proximal, to form roughly ellipsoidal domes. The dimers pair across one another, to form tetramers as depicted in Fig. 2. 

Hypervalent Cl Sn—Cl bridges are also typical for pentacoordinate tin atoms. For example, the crystal structure of 20 and 21 comprise dimeric pairs of molecules bridged by weak intermolecular Cl Sn interactions. The Cl —Sn distances in 20 (2.475 A ), 21 (average 3.869 A ) and in MeSn(Ar)Cl2 (Ar = 3-methyl-4-nitropyridine-N-oxide) (3.934 A ) are longer than in most comparable structures The Sn NMR resonance (—285.7 ppm) for 21 suggests that it possesses a five-coordinate structure in solution . 

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