Unsymmetrical structure

The first evidence for an unsymmetrical structure for the benzo-furoxan molecule and for the tautomerism of Eq. (3) was provided in 1961 by several groups of workers," using proton resonance... 

It must be taken into account that in all papers published between 1916 and 1922 the structure of methyldiarylpyrylium salts is erroneously indicated as a symmetrical 2,6-diarjd-4-methylpyrylium structure. The unsymmetrical structure (2,4-diaryl-6-methyl) was recognized by Gastaldi, then by Schneider and Ross, and finally by Dilthey and Fischer. ... 

Tetraazafulvalenes bearing two pyrazole subunits could be prepared by an original way. Tlius, treatment of benzylidene acetophenone with iso-pentylnitrite leads to an A, A -dihydroxy-bipyrazolyl-A, A -oxide, which in turn can be oxidized to TAF of type 100 (72CC961, 79JOC3211). Another type of oxidative dimerization was observed by the reaction of the electron-rich l-methyl-2,4-bis(dimethylamino)imidazole with silver salts (83TL3563). A bis-cation was isolated in 30% yield in the presence of sodium tetrafluo-roborate an unsymmetrical structure 101 was predicted from its NMR data (Scheme 40). 

Pyrido- and pyrimido[l, 4]diazepinones have been produced by condensing pyridinediamines and a pyrimidinediamine with esters of /i-oxo acids. Because of the unsymmetrical structures of the diamines the reactions may take two courses, as exemplified by the action of ethyl acetoacetate on pyridine-2,3-diamine.308... 

When m-RuCl2(DMSO)4 is stirred in methanol containing traces of water (to catalyse the formation of intermediate aqua species) Ru2C14(DMSO)5 isformed this has the unsymmetrical structure (DMSO)2C1RuQl(-C1)3Ru(DMSO)3 based on face-sharing octahedra [110]. 

An unsymmetrical structure is indicated by reactions like those in Figure 4.37 (as well as X-ray diffraction [165a]). 

These general effects are summarized in Table 5.1. As can be seen in the table, unsymmetrical structure and flexible links adversely affect the glass transition. However, a combination of flexible and polar substituents can increase solubility without deleterious effect on Ts. 

N O N and N N O , for these have bond angles between 90 and 125°. The structures N N O and N O N ) fr0m N N+ 0 and N 0++ N, respectively, would both be linear, and so compatible with the known arrangement. An a priori decision between them is difficult, although previously advanced arguments favor the unsymmetrical structure. Band spectra should soon decide the question. 

They found that the order for the M—0 stretching modes (M—0 force constants) was (02)Pd(02) > Pd(Oj) and (02)Ni(02) > Ni(02), and since the 0-0 force constant increased as the M-0 force constant decreased, reversibility could not be equated with shorter 0—0 bond length. To test the proposed isosceles model they predicted the various absorptions that would be expected on the basis of the symmetrical structure (a) and unsymmetrical structure (c), Fig. 8. If the isotopic ratio of 0 0 is 1 1, then three bands would be expected for structure (a), with relative intensities 1 2 1, and four bands would be expected for structure (c), with relative intensities 1 1 1 1, the two oxygen atoms now being in different environments. These workers 189) then obtained the IR spectrum for the cocondensation product from the reaction between nickel and O2 (4.2—10°K), in isotopic ratio 0 =1 1, and found three bands with relative inten-... 

These new derivatives were isolated in good yields (60-94%) as high boiling liquids and were fully characterized by NMR spectroscopy (1H, 13C, and 11B) and elemental analysis. The proton NMR of the starting material 1 shows a well-resolved multiplet and quintet for the trimethylene bridge. Upon monosubstitution, however, three complex multiplets are observed, indicative of the unsymmetrical structures of these derivatives. Also, the nonequivalence of the N-C carbon atoms is clearly apparent in the 13C NMR spectra of 2-4. 

Bis[2-(diphenylphosphino)phenyl]phenylphosphine (TP) is a very inflexible ligand with a rigid backbone and gives the trinuclear complex [Au3Cl3(TP)] (452), that has a compact unsymmetrical structure in the solid state which is retained in solution owing to the fixation of the gold atoms through intermetallic contacts.2651... 

Fig. 6. Route of synthesis of nonionic contrast agents, a Synthesis of iohexol with a symmetrical structure. R = R = 2,3-propandiol.b Synthesis of iopromide with an unsymmetrical structure. Ri = methyl. R2 = 2,3-propandiol. R2=acylated 2,3-propandiol...

The unsymmetrical structure of (—)-sparteine creates an additional stereogenic centre at the lithium cation. As a consequence, two diastereotopic transition states for the interaction of isopropyllithium/)—)-sparteine (11) have to be taken into consideration. 

These observations suggest that the environment of heme inside the micellar cavity is more asymmetric than that in simple solution this asymmetry is much larger inside the protein cavity [45]. An unsymmetrical structural disposition of the heme complex inside the micellar cavity has been proposed [67]. Besides, the increase in the spread of the heme methyl signals in the five-coordinate 2-methyl imidazole ferrous heme complex might also arise from the decrease in symmetry... 

Although an intramolecular singlet excimer ( Ar ) fluorescence of l,3-bis(4-cyano-phenyl)propan-l-ol was observed in MTHF at r.t., no CR band of l,3-bis(4-cyanophenyl)-propan-l-ol was observed during PR and y-R. An unsymmetrical structure of 1,3-bis (4-cyanophenyl)propan-l-ol does not inhibit Ar formation [112]. Although two Np-chro-mophores have been suggested to form intramolecular Np 2 in l,3-di- 3-naphthylpro-panes , no CR band of 1,3-di-a-naphthylpropane and 1,3-di-l-pyrenyl-propane was... 

In the light of Angeli s [65] investigations, which led to the conclusion that the azoxy group has an unsymmetrical structure (—N==N—), and not as formerly... 

The production of these compounds is considered to be evidence of the unsymmetrical structure, Se SeCl2, for selenium monochloride. 

A possible justification for frontside attack in electrophilic substitution is that ab initio molecular orbital calculations for the CH5+ cation, the species that would be formed if H+ attacked methane, indicate that the most stable structure would not be a trigonal bipyramid, in which carbon uses a p orbital to bond to two protons, but would be a relatively unsymmetrical structure that has a smallest H—C—H bond angle of about 37° (Figure 4.10).85 For further discussion of SB2 substitution on carbon, see Section 10.3.86... 

Solid-state MAS 29Si NMR data for the pentacoordinate bis(siliconates) 64 and 65 and mono(siliconate) [PhSiF4], K+ 18-crown-6 are also summarized in Table 15 for comparison. The solid-state spectrum of 64 shows an uncharacterizable multiplet, possibly due to the slightly unsymmetrical structure. In contrast to the absence of peaks in solution,... 

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