Trans-, crystal structure

Neoprene AC (1947). This polychloroprene was developed to provide better viscosity stability and resistance to discolouration, but it cures much more slowly at room temperature. It is a fast crystallizing grade and contains about 90% trans-, 4 structure. 

Neoprene WHV-A. It is a non-peptizable and mercaptan-modified polychloro-prene elastomer. It is a slow-crystallizing, high molecular weight type and contains only 85% trans-, 4 structure. It is generally used in blends with low molecular weight crystallizing polychloroprene types to increase solution viscosity. 

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

Both these hydrides insert alkenes and alkynes the crystal structure of [Rh(NH3)5(C2H5)]2+Br2 shows the ethyl group has a trans-influence comparable to that of the hydride [93]. 

Crystal structures have been determined (Figure 2.56) for RhCl3(PEt2Ph)3 and RhBr3(PMe2Ph)3 (in both cases wer-isomer) - in each case, the Rh-X bond length shows the trans-influence of a tertiary phosphine [110]. 

Interestingly, the crystal structures of 3-substituted thietane and thietane dioxides180 showed that in the solid state they exist in the puckered structure, with the S—O bond equatorial in the oxides and the 3-substituent axial for the trans-isomers, contrary to what has been quoted before66,195. Thus, the claim that conformer 5c is predominant in the solutions of the trans-isomer needs to be re-examined. 

The two -OH groups in l,2 5,6-di-0-cyclohexylidene-wii/o-inositol and its di-O-isopropylidene analog are trans. The X-ray crystal structure of the latter compound suggests that the ring is in a skew conformation with the 0-3 and 0-4 -OH groups both in ax positions, but NMR studies and ab initio calculations indicate that a mixture of the skew and chair conformations, with 0-3 and 0-4 both in eq positions, is present. Formation of a dibutylstannylene acetal presumably locks these two compounds in the latter conformation. 

The synthesis of the mesogenic trans-4-n-alkyl-(4 -cyanophenyl)-cyclohexanes (PCHn) was described by Eidenschink et al. [65] in 1977. Most of the compounds exhibit a nematic phase close to room temperature. The chemical structure of the mesogenic PCHn is shown in Fig. 6. During the past few years, the crystal structures of some mesogenic phenylcyclohexanes were published [66-70]. Selected crystallographic and molecular data of the investigated compounds PCHn are presented in Table 4. 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

In 1985, Astheimer et al, [134] described the crystal structure of the mesogenic 4,4 -di(7S0-hexoxybenzalazine, Here, the centrosymmetric molecule is fully elongated with the hexoxy group in an aU-trans conformation and the phenyl rings are exactly coplanar. 

Jones, P.G. (1993) Crystal structure of tetra-n-butylammonium trans-dichlorobis(pentafluorophenyl)aurate (III), (C4H9)4N(Cl2Au(C6F5)2). Zcitschrift jur KristaUographie, 208(2), 362-365. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxane have been determined. In this case (unlike the cyclotetrasiloxane in Fig. 26), the cis isomer does contain an intramolecular hydrogen bond, and intermolecular hydrogen bonds link the molecules into cyclic pairs 57. The trans isomer cannot form intramolecular hydrogen bonds, but forms cyclic tetramers which are further hydrogen-bonded to form infinite sheets 58 (280). 

Lundquist, K. Stomberg, R. On the occurrence of structural elements of the lignan type (P-P structures) in lignins. The crystal structures of (+ )-pinoresinol and ( )-trans-3,4-divanillyltetrahydrofuran. Holzforschung 1988, 42, 375-384. 

Karhunen, P Rummakko, P. Pajunen, A. Brunow, G. Synthesis and crystal structure determination of model compounds for the dibenzodioxocine structure occurring in wood lignins. J. Chem. Soc., Perkin Trans. 1996, 1, 2303-2308. 

Z = 8 Dx = 3.09 R = 0.076 for 1,253 intensities. The crystal structure contains two symmetry-independent molecules. The two l-ascorbate ions differ significantly only in the orientations of the terminal hydroxyl groups of the side chains, with C-6 - 0-6 to C-5 - 0-5 gauche and trans, respectively. The unprotonated oxygen-atoms are 0-1 and 0-3, with C-O distances ofbetween 127 and 133 pm. One Tl+atom has four oxygen atoms at distances of 258 to 298 pm the other has three, at distances of 258,268, and 300 pm. The 0-3 atoms have two Tl+ contacts,... 

In conclusion, whether or not this species crystallizes in an allene-like or a trans-planar structure may depend on the crystal packing.43... 

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