Conformation in the crystal

The widespread adoption of high-temperature dyeing methods has also allowed the use of simple disazo structures, such as Cl Disperse Orange 13 (4.89) and Cl Disperse Orange 29 (4.90), as economic dyes giving chiefly yellow and orange hues. The latter dye is known to exist in the syn conformation in the crystal [95] the unsubstituted parent dye prefers the anti conformation. A few monoazo and disazo disperse dyes have absorption bands in the near infrared [96]. 

The infrared absorption spectra of the same polymer in the crystalline and amorphous states may differ because of the following two reasons (i) Specific intermolecular interactions may exist in the crystalline polymer which lead to sharpening or splitting of certain bands and (ii) Some specific conformations may exist in one but not the other phase, which may lead to bands characteristic exclusively of either crystalline or amorphous material. For example in polyethylene terepthalate), the 0CH2CH20 portion of each repeat unit is restricted to the all trans-conformation in the crystal, but... 

The original postulation of general acid catalysis by tyrosine-248 rested on closeness of that group to the amide function of the substrate in the crystal. However, the conformation in the crystal and solution states may be different in regard to the position of tyrosine-248 (Riordan and Muszynska, 1974). Thus, mechanistic inferences concerning that group may not be warranted. 

The bridged biphenyl (472) (67ZN(B)1296> has a rigid twist-boat-chair conformation in the crystal (72CC985) and is an eight-membered heterocycle with two torsional constraints in a 1,3 relationship. It has been resolved by chromatography on cellulose acetate, and AG for racemization is 120 kJ mol-1 (67M1323). 

Figure 9-6 Bond lengths, angles, and arrangement of carbons and bromines in a crystal of 1,8-bis(bromomethyl)naphthalene, 1, as determined by x-ray diffraction. Notice that the preferred conformation in the crystal has the bromines on opposite sides of the naphthalene ring.

Many diasteromeric peptides of type (R,S)-27 and (S,S)-28 (Scheme 1) could be crystallized and their absolute configuration determined based on the known ( -configuration of L-phenylalanine. Table 1 shows a compilation of the relevant torsional angles (4>1/ P1) derived from crystal structures of the peptides shown.108 It is interesting to note that the (S,S)-diastereomers show a high prevalence for P-tum type I conformations in the crystal state. 

Absolute asymmetric synthesis was observed in the solid-state photoreaction of benzoylbenzamide 59 to phthalide 60 however, the reaction mechanism was completely different from that of thioester 57. [35] Recrystallization of these amides 59a-c from the chloroform-hexane solution afforded colorless prisms in all cases. X-ray crystallographic analysis revealed that all prochiral amides 59a-c adopted orthorhombic chiral space group P2 2 2 and were frozen in chiral and helical conformation in the crystal lattice. 

We now extend the structural basis set for cobalt(III) hexaamines with one additional structure with relatively long Coin-N bonds. The [Co(tmen)3]3+ cation (tmen = 2,3-dimethylpropane-2,3-diamine) is a highly strained species with long Com-N bonds because of the four methyl substituents (see Fig. 17.12.1). The structure of the cation has been determined by an X-ray diffraction study, and the conformation in the crystal has been defined as 065 (see Section 17.3 for the nomenclature of the conformers). Due to the elongation of the Com-N bonds to 1.997 A, there is a remarkable shift in the ligand field spectra (the first d-d transition ( Ai- ) is at 515 nm vs 470 nm for [Co(en)3p ) and the redox potential (-0.18 V vs +0.28 V)[56>231]. 

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