Biphenyl twist angle

In 1992, McLendon and coworkers reported results of a study of another series of porphyrin dyads with zinc in one porphyrin and iron in the second. These were related in structure to dyad 3, but the number of phenyl linkers varied from one to three. These workers found an apparent value for y in Eq. 2 of 0.4 A . They explained this result by a theory that attributes the drop in rate constant not to increased distance, but rather to the decrease in conjugation between the porphyrins that occurs at each phenyl ring junction due to the biphenyl twist angle of - 50°. Each phenyl ring results in a drop in rate of approximately six-fold [32]. 

Direct linkage of aromatic halves by a biphenyl bond is also possible, which leads to a large family of molecular glasses. The twist angle of this linkage is 30°, which couples the halves electronically. However, the extent of this coupling depends on the nature of the chromophore halves. Most compounds of this family are based on the common diaminobiphenyl core (Fig. 3.7). The classical... 

Fig. 2 (a) Schematic diagram of single molecule junction conductance measurement, (b) Structures of a subset of the biphenyl series studied, shown in order of increasing twist angle (6) or decreasing conjugation, (c) Biphenyl Junction conductance as a function of molecular twist angle. (Reprinted with permission from [9])... 

F. Leroux, Atropisomerism, biphenyls, and fluorine A comparison of rotational barriers and twist angles, ChemBioChem 5(5) (2004) 644-649. 

In a recent X-ray study of 4,4 -dichlorobiphenyl221 a twist angle of 42 ° has been reported, in exact agreement with the electron-diffraction result of gaseous biphenyl. The 4,4 -dichiorobiphenyl crystal is isostructural with several other 4,4 -derivatives of biphenyl, however, with slightly smaller values for the twist angle. 

For biphenyl itself the energy dependence of the torsional angle has been studied by quantum mechanical methods. Studies based upon jr-electron calculations, taking explicit account of steric effects226-228 led to an energy minimum at a twist angle of 35 40 ° from planarity compared to the best electron-diffraction... 

Comparison of the four molecules biphenyl, 2-fluorobiphenyl, 2,2 -difluoro-biphenyl and perfluorobiphenyl shows that the angle of twist increases in a reasonable way introducing more fluorine atoms in the 2-position (Table 6). A twist angle recently reported for crystalline 2-H-nonafluorobiphenyl237 of 59.5 0 also fits nicely into the picture. 

Empirical evaluation of the twisting powers of hundreds of chiral molecules of various structural types have revealed regular trends in the magnitude of P with solute structure within specific types by interpolation, it is then possible to derive information on the absolute configuration and structure of new chiral molecules (i.e., that aspect of the structure that is responsible for the solute s chirality). Examples of this include calculations of the twist angle between aryl groups in biphenyls [338] and l,r-binaphthyls [340, 342, 343]. Alternatively, the technique can be used to determine solute enantiomeric purities it requires very small quantities of solute and is particularly useful for compounds with extremely low optical rotations. 

Electron densities of ortho- and para-positions of biphenyl are higher than that of meta-posltions. Sterically non-controlled isopropylatlon of biphenyl at low temperature occurred predominantly at ortho and para-posltlons to give 2-and 4-IPBP because of the electrophilic nature of the alkylation. However, selective formation of 4-IPBP over HM is controlled by the sterlc restrictions depending on the elliptical pore of the zeolite and on the conformation of the transition state for the formation of products. The molecule 2-IPBP has approximately 0.75 nm of diameter In a twisted bulky conformation with an angle of 64°[8]. The formation of 2-IPBP is prevented over HM because the corresponding transition state with bulky conformation requires more space than is available at the acidic sites of HM. On the other hand, the formation of 4-IPBP proceeds unhindered because of its smaller transition state. The formation of 3-IPBP Is also less hindered because of flexible conformations at transition states In HM pore. 

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