Trans form, dimers

The ratio of cis (27) to trans (26) dimers produced in this reaction has been found to be solvent and concentration dependent, as can be seen in Table 10.3. The data in Table 10.3 show that the cis dimer predominates in all three solvents. It is also seen that as the acenaphthylene concentration is increased, proportionally more cis dimer is formed. This effect is slightly more pronounced in cyclohexane than in benzene (see Figure 10.2) Increases in the yield of dimer with increasing concentration would of course be expected... 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Cycloadditions, although relatively rare, are not unknown in heterocyclic systems. The first documented example was the photodimerization of pyrid-2-one a reinvestigation of this reaction has established that three other isomeric [ 4 + 4] dimers are formed in low yield in addition to the originally reported trans-anti dimer (264).215 Attempts to effect analogous cycloadditions in a series of l,T-polymethylenedipyrid-2-ones were unsuccessful, [ 2 + 2] and [ 4 + 2] additions being preferred.216 Thus,... 

Another example of the very subtle effect of a ligand is provided by the dimerization of norbornadiene (31, 52). With (PPh3)2Ni(CO)2, under irradiation, the exo,trans,exo dimer is formed stereoselectively (example... 

Dimeric (methylthioalkylmethyleneamino)diorganoboranes produced in this reaction (mixed cis-trans forms) are in equilibrium with their monomers in hot chloroform solution. 

The sensitivity to temperature is reminiscent of that seen in Ron s study (188) of o-methoxy-m-cinnamic acid, 129. When crystals of this substance are exposed to light at low temperatures (< — 80°C), there occurs a gradual isomerization of the molecules to the trans form, 130, with formation of a little of the (2 + 2) photodimer, 131, of the latter. If this irradiated material is warmed in the dark to + 60°C, recooled, and reirradiated for a short while, there is a jump in the amount of dimer. This behavior is interpreted as follows ... 

Figure 4.11. The tilt of the hexagons determined the contour of the menisci. The shape of the meniscus is indicated by the + symbol the larger this symbol, the higher the meniscus at that point on the face. There are at least two ways to form dimers of the [1,2], [11.2.i>21.2.1], [1,2,3], and [1,2,3,4] hexagons. In (a), (c), and (d), the favored cm configuration matches the contours of the menisci well the disfavored trans configuration matches the contours of the menisci less well. In (b), the [11.2.1,21.2.1] hexagons float almost parallel to the interface, and there is little or no preference between the cis and...

K4[Mn2(CN)4(NO)4] has been isolated in cis and trans forms.14 Evidence for a dimeric structure with Mn—Mn bonds derives from the diamagnetism of the compounds, the IR spectra and cis-trans conversions. Yellow crystals of the cis are obtained from the reaction in liquid... 

Hieber and co-workers 93 94) studied the reaction [Ni(NO)X]4 + 4L = 2 [NiX(NO)L]2 and [Ni(NO)X]4 + 8L = 4 NiX(NO)L2 (where X is Br and I, and L = PR3, P(OR)3, ASR3, SbR3, and NHR2). The dimeric derivatives generally have the trans structure, but the one with tricyclo-hexylphosphine was obtained in both cis and trans form. The two isomers can be recognized 10) from the number of NO stretching bands in the IR spectrum the trans isomer with symmetry has only one active band, while the cis isomer with Cjja symmetry has two bands. 

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