Trans-, structural notation

Chemical identity may appear to present a trivial problem, but most chemicals have several names, and subtle differences between isomers (e.g., cis and trans) may be ignored. The most commonly accepted identifiers are the IUPAC name and the Chemical Abstracts System (CAS) number. More recently, methods have been sought of expressing the structure in line notation form so that computer entry of a series of symbols can be used to define a three-dimensional structure. For environmental purposes the SMILES (Simplified Molecular Identification and Line Entry System, Anderson et al. 1987) is favored, but the Wismesser Line Notation is also quite widely used. 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

The cis-trans notation does not work well here, and structure 11 has the Z configuration and 12 the configuration. In the older chemical literature, these stereoisomers were designated as syn and anti forms, but these names are really no better than cis and trans. 

In the case of ring-fused systems the cis/trans notation is used to designate the geometric nature of the ring junction. Structures (25) and (26) represent partial structures of the ring-fused compounds (13) and (14) respectively, which emphasise that the ring junctions utilise cis- or frans-orientated bonds. 

The terminology and notation that have been used to describe coordination compounds have been derived with one notable exception from the terms and symbols developed to describe the stereochemistry of carbon compounds. The terms ois, trans endo, exo dextro, d, D, (+) and leva, l, L (-) all have been used to describe the stereochemistry of coordination compounds in a close analogy with organic compounds (see Figure 1). As the descriptions of the chemistry and structures of coordination systems have become more varied and complex, the meanings of these terms have become less precise, as in the example of a ois or trans tricarbonyl octahedral compound (see Figure 2). The terms fao and mer were coined to indicate the facial and meridional disposition of substituted octahedral structures. 

Figure 2 shows several all-trans and mono-czs isomers of -carotene which have been identified. The ail-trans isomer is unique in that it contains a stretched conj ugated system with C21, synunetry. The symmetry notations of nAJ, nAj, nB and nB are appropriate for this structure (Tavan and Schulten,... 

The corresponding rate constants In solution have been calculated and compared in Table 5 with the rate constants of the Isomers a. and 3 of the Indolinospiropyrans I and II In a polymethylmethacrylate matrix, the notation a and 6 being given to different conformations of the metastable trans-merocyanlne structure. 

Woody plants synthesize lignin fiom 3-(4-hydroxyphenyl)-2-propenol (trans-4-coumaryl alcohol = 1.1, grasses), 3-(4-hydroxy-3-metho5OThenyl)-2-propenol (trans-cotriferyl alcohol = 1.2, pines), and3-(4-hydroxy-3,5-dimethoxyphenyl)-2-propenol (trans-sinapyl alcohol = 1.3, deciduous) by flee radical crosslinking initiated by errzymatic dehydrogenation (4). Structures of these alcohols and the notation for the carbon atoms of the C, repeat unit... 

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