Planar trans

Extensive calculations have also been performed on 1,2-dilithioethylene, for which the stable isomers are predicted to be the planar trans form 139 and a doubly bridged form 140, both in the singlet state (204). 

Complexes with type II structures are compiled in Table II along with their S—S distances and S—S vibrational frequencies. There are several examples of complexes of structure type Ila (planar trans end-on bridging coordination). Vibrational spectroscopy 174) indicates that this structure occurs in [(CN)5Co(S2)Co(CN)5] (7). Trans arrangement of the metal atoms has been proved by X-ray structure determination for [(NH3)5Ru(S2)Ru(NH3)5]Cl4 2H20 15, 44). 

Owing to the inequality of the bond angles, a decrease in the trans population increases 0lnl2. Hence, the positive temperature coefficient for this ratio denotes a lower energy for the trans state. This energy is attributed to favorable interaction between CH3 pairs separated by 380 pm in the planar (trans) conformation. The larger value observed for the ratio in a less polar medium is in the direction predicted for enhanced electrostatic interaction within a chain of partially ionic Si—0 bonds. 

The model parameters are varied systematically within reasonable limits to fit the experimental results. The minimum positions of the rotational bond angles probably do not deviate more than 5° from planar trans and from symetrically staggered gauche, respectively. Entropy contributions to the free energies of the rotational isomers are discussed with respect to the influence on the temperature coefficient. 

All ester groups in the PET chain are assigned confidently to be planar trans. The restriction of bonds 1 and 3 to the trans states and bond 2 to a choice between cis and trans leads to the simple statistical weight matrices. The length of the span of the terephthaloyl residue in PET guarantees independence of the conformations of successive repeating units of the chain. 

Now let us consider two specific point groups (1) and (2) point group is planar trans-CtHtClt jj qj... 

In an extensive ab initio study, the preferred conformers for substituted pyrroles are predicted to be, inter alia NCCH trans and C(2)CCH cis for 2- and 3-methylpyrroles [i.e. a methyl C—H bond eclipsing the pyrrole C(2)—C(3) bond) NCCO cis and C(2)CCO cis for 2- and 3-hydroxymethylpyrroles pyramidal at nitrogen for aminopyrroles planar OH trans for hydroxypyrroles planar trans for vinylpyrroles, and planar cis (syn) for formylpyr-roles. In agreement with experiment the energy difference between the two rotamers for 3-formylpyrrole is much smaller than for the 2-isomer (79NJC473). 

Schemes 3 and 4 show the topological arrangements for trien in both octahderal and square planar (trans) geometries. These stereochemical arrangements are available for any of the linear tetramine ligands (50)—(56), but the R,S nomenclature must be assigned for each individual polyamine. The R,S nomenclature shown in Scheme 3, applicable to trien and 3,2,3-tet, must be systematically reversed for 2,3,2-tet.

Table 9.3. In-plane ligand field splitting (Dq ) or transition energies (cm ) for square planar trans-octahedral transition metal complexes with tetraaza macrocyclic ligands 1 .

Figure 13.19 X-ray molecular structure of a metallomesogen based on square planar trans-VACAi linkers.11...

Scheme 2.13 Rough sketch of the molecule 2,4-difluorobenzyl 2,2-dimethylpropionate (2,4-DFP) used as model compound for the repeating unit of the corresponding polymer. Rotations over CO— CH2Car (V/) ) and OCH2—Car Car (V/)2) bonds are drawn in their planar trans conformation which is represented by

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