Chain-like isomer

The relative stabilities of RSSR and R2S=S for R=OH and SH have been determined by Steudel and co-workers [23, 24]. At the MP2/6-311G //HF/ 6-311G +ZPE level of theory, (HO)2S=S (thiosulfurous acid) is just 13 kj mol less stable than the chain-like isomer HOSSOH (dihydroxydisul-fane) [23]. For R=SH, (HS)2S=S is less stable than the corresponding trisul-fane HSSSH by 132 kJ mor (MP2/6-31G -//HF/4-31G) [24]. 

Vibrational data have been reported for silylidene, H2C=Si and D2C=Si, Table 11. ° An ab initio calculation has been reported for the vibrational wavenumbers of the 2-silaketenylidene radical, CSiO, ° and for the ground and first excited states of the 2-silaketenyl radical. Similar calculations for isomers of the interstellar molecule SiC2N suggest that only the chain-like isomers are likely to be stable. ... 

However, a few moments of reflection (or access to a molecular model kit) should convince you that these are in fact equivalent to structures written previously. In particular, the first one, like isomer I, has a five-carbon chain in which no carbon atom is attached to more than two other carbons. The second structure, like II, has a four-carbon chain with one carbon atom bonded to three other carbons. Structures I, II, and III represent the three possible isomers of CsH12 there are no others. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

The symmetrical thiosulfoxide isomer of tetrasulfane (HS)2S=S is also of Cs symmetry and by 123 kJ mol" less stable than the chain-like ground state of C2 symmetry (at the MP2/6-31GV/HF/4-31G level of theory) [51]. However, at the much higher G3(MP2) level the energy difference is only 93 kJ mol [52]. Similarly, the two hexasulfane isomers (HSS)2S=S (Cs symmetry) and (HS)(HSSS)S=S (Q symmetry) have recently been calculated at the G3X(MP2) level to be by 53 and 54 kJ mol respectively, less stable than the helical chain of C2 symmetry [52]. The decreasing energy difference be-... 

According to ab initio MO calculations [50] protonation of S2O produces the planar chain-like molecule ds-SSOH+ as the isomer of lowest energy. The tmns-isomei is by only 2 kj mol less stable and S-protonated as well as 0-protonated cyclic isomers are next highest in energy. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

The aromatic bromides which contain bromine in the benzene nucleus are either colourless liquids or crystals, which in contrast with the side-chain substituted isomers in part possess an aromatic odour, and their vapours do not attack the eyes and nostrils. The bromine is held very firmly in them, more firmly than in the aliphatic bromides, and cannot he detected by stiver nitrate. While the aliphatic bromides, as mentioned under bromethyl, decompose with ammonia, alcohol, alkalies, etc., to form amines, ethers, alcohols, etc., respectively, these reagents do not act on the aromatic bromides. The bromides containing the bromine in the side-chain, behave like their aliphatic analogues. 

However, in addition to the chain-like and cyclic species discussed so far the presence of branched rings and chains in sulfur vapor and in liquid sulfur has also been discussed [108] but no conclusive experimental evidence for such isomers is presently available. For example, an isomer of cyclo-S ... 

In 1988, it became obvious that the composition of sulfur vapor is even more complex than had been thought before Lenain et al. published a Raman spectroscopic study of the sulfur vapor composition at temperatures of up to 700 °C [20]. 8ignals for the cyclic species 8s, 87, and 8e as well as for the acyclic molecules 84, 83, and 82 were observed and spectroscopic evidence for the presence of two conformational isomers of 84 and for either chain-like (8 ") or branched-bonded species (8 =8 with n>4) was presented. These authors came to the conclusion that the thermodynamic data of Rau et al. [24] are reliable except for the enthalpy of 87 formation from 8s which was said to be too high. 

Vibrational frequencies of diatomic S2 and 82 and triatomic S3, 83 , and 83 are listed in Secs. 2.1 and 2.2, respectively. The IR spectra of the 84 molecule produced in Ar matrices suggest the formation of two open-chain isomers [ 1745]. 84 vapor consists of green-absorbing (k ax, 530 nm) and red-absorbing (560-660 nm) molecules. Raman spectra show that the former is a chain-like rra z -planar molecule of C2h symmetry whereas the latter is a branched-ring molecule [1746]. 

The analysis of the data in Table. 1 shows that the kinetic models, most likely, do not confirm the experimental results although they formally describe some of the experimental observations in Fig. 1 and Table 1. For some of the representatives among n-, iso- and cjc/o-paraffins no relationship of on [RFI] has been established Nos 4, 5, 7, 11, 16, 19, 20, and 24 or this dependence is within the limits of the graphic presentation Nos 1, 2, 3, 6, 8, 15, 21, 22, and 23. A good correlation has been found for Nos 9, 10, 12, 13, 14, 17, and 18, since all these are No-parrafins with 1,2 or 3 CHj- groups in the side chains. These isomers contain always hardly... 

Besides, in the hydrophobic part, a surfactant may have homologous chains and positional isomers for example, cocoamidopropyl betaine may contain alkyl chains with a carbon pair number from 8 to 18, the majority being those with a carbon number of 12 and 14. Aromatic groups are usually bonded in alkyl chain, like in alkylphenols (nonionic) or alkyl-benzenesulfonates (anionic). Also, the hydrophilic part may contain oligomers of ethylene oxide or propylene oxide. In this article, cationic polymers have been included as a subclass of cationic surfactant based on the classification made by Richmond (1990), although many authors consider this polymer to be another ingredient of cosmetic products. Another subclass, quaternary ammonium compounds (four alkyl chains around a nitrogen atom) will be frequently mentioned components. 

Simple oxaziridines and diaziridines do not absorb in the near UV. Lack of absorption was one argument to distinguish between true three-membered ring structures and unsaturated open chain isomers like nitrones or hydrazones. 

Diaziridines, discovered in 1958, six years after the oxaziridines, were almost immediately realized to be structural analogs of oxaziridines. Like these they showed oxidizing properties unexpected for other classes of organic nitrogen compound. Properties in common with oxaziridines include the rearrangement to open chain isomers on heating above 100 °C (for several diaziridines), and their hydrolytic behavior in acidic media, which leads to carbonyl compounds with conservation of the hetero-hetero bond. 

The mechanism of the polymerization contains ionic intermediate steps. The free H+ goes to a carbenium ion and, as shown in route B, rearranges to form tetrapropylene. It is highly likely that this actual tetrapropylene exists only in very small concentrations. The product variety is explained by the rearrangement of the carbenium ion to dodecene isomers according to route C. In addition, short-chain olefins formed by fragmentation (route D). Polymerization proceeds at almost 100% to mono olefins. Aromatics, paraffins, and diolefins exist only in trace amounts. The propylene tetramer is best characterized by its distillation range. 

In addition to the branched rings and chains, cyclic Ss conformations of lower symmetry than Did are also likely components of liquid sulfur. For example, the following exo-endo isomer of Ss (Cs symmetry) is by just 28 kJ mor (AG°29s) less stable than the ground state conformation and therefore its relative concentration in liquid sulfur and sulfur vapor at the boiling point will also be 1% of all Ss species [35]. 

A peptoid pentamer of five poro-substituted (S)-N-(l-phenylethyl)glycine monomers, which exhibits the characteristic a-helix-like CD spectrum described above, was further analyzed by 2D-NMR [42]. Although this pentamer has a dynamic structure and adopts a family of conformations in methanol solution, 50-60% of the population exists as a right-handed helical conformer, containing all cis-amide bonds (in agreement with modeling studies [3]), with about three residues per turn and a pitch of 6 A. Minor families of conformational isomers arise from cis/trans-amide bond isomerization. Since many peptoid sequences with chiral aromatic side chains share similar CD characteristics with this helical pentamer, the type of CD spectrum described above can be considered to be indicative of the formation of this class of peptoid helix in general. 

Studies of the effect of permeant s size on the translational diffusion in membranes suggest that a free-volume model is appropriate for the description of diffusion processes in the bilayers [93]. The dynamic motion of the chains of the membrane lipids and proteins may result in the formation of transient pockets of free volume or cavities into which a permeant molecule can enter. Diffusion occurs when a permeant jumps from a donor to an acceptor cavity. Results from recent molecular dynamics simulations suggest that the free volume transport mechanism is more likely to be operative in the core of the bilayer [84]. In the more ordered region of the bilayer, a kink shift diffusion mechanism is more likely to occur [84,94]. Kinks may be pictured as dynamic structural defects representing small, mobile free volumes in the hydrocarbon phase of the membrane, i.e., conformational kink g tg ) isomers of the hydrocarbon chains resulting from thermal motion [52] (Fig. 8). Small molecules can enter the small free volumes of the kinks and migrate across the membrane together with the kinks. 

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