Perpendicular conformations

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

Draw all reasonable resonance contributors for both planar and perpendicular conformers of benzyl cation. Identify the site(s) of the positive charge in each. Which cation would you expect to be more stable Which is the more stable Compare energies of planar and perpendicular conformers of benzyl cation. 

Display electrostatic potential maps for both bisected and perpendicular conformers of cyclopropylcarbinyl cation. For which is the charge more delocalized Is the more delocalized cation also the lower-energy cation ... 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

Reactivity of electrophilic addition (bromination) of MeO CCCCOjMe [33] suggests the predominance of the perpendicular conformation. Triple bonds are usually less reactive toward electrophiles than double bonds. PhCCH is brominated 3,000 times more slowly than PhCH=CH2. However, MeO CCCCO Me is brominated... 

The molecules in Table 6 may be divided into three groups based on their dihedral angles. For most of these molecules the dihedral angle is close to 60°. In 2,3-di-terf-butyl-1,3-butadiene (2,3-TB) the dihedral angle is close to 90°, corresponding to an approximately perpendicular conformation, while the dihedral angle in 2-tert-butyl- 1,3-butadiene (2-TB) is determined to be 32.1°. 

At the B3LYP/6-31G and B3LYP/6-311+G levels the perpendicular conformation of a,a-dicyclopropylcyclobutylmethylium cation is not a mini-... 

Figure 12. Estimation of the rotational barrier for the interconversion of bisected and perpendicular conformations of the a, a-dicyclopropylcyclobutylmethyl cation.

In the eclipsed conformation the stabilizing interaction is ocx — p+ and in the perpendicular conformation the principal stabilizing interaction is the 0ch P+ interaction. Arguing as before we conclude the following ... 

Subject to nonbonded interactions, these compounds adopt the boat conformation 29 (Scheme 18). The phenyl substituents in the 2-position proved to prefer in the axial orientation the perpendicular conformation (0 = 90°), and in the equatorial position the parallel conformation 0 = 0°). A 5-substituent was generally found in an equatorial orientation (76MI1 79BSB223). In 2-Et-5,5-di-Me-l,3-dioxan-4,6-dione, the ethyl substituent was present in the equatorial parallel conformation (0 = 0°) (79BSB223). 

The elusive ethylene dication (C2H4) has been observed in the gas phase, and its potential surface has been well characterized by ab initio theory at MP3/6-31G /6-31G level. The perpendicular conformer (D2d) has been shown to be lower in energy than the planar conformation (D2h) by an unusually large value, 28.1 kcal/mol. The other possible isomeric carbodications also have very low-activation barriers for the conversion into the perpendicular conformer. ... 

Figure B3.3. Interaction diagrams for (a) staggered and (b) perpendicular conformations of hydrazine.

Structural formulas of the ZP and ZP-I dyads are shown in Fig. 1. The absorption spectra of ZP-I dyads indicate that the excitation energies of the ZP S2 and S, states are practically the same throughout the ZP-I series. The peak position of the Soret absorption band of the ZP-I dyads are only slightly red shifted from that of free ZP, and are practically the same as that of ZnAr3P. These results indicate a rather weak D-A electronic interaction in the ground state of ZP-I systems despite of their directly linked structure, which can be ascribed to the nearly perpendicular conformation between ZP and I molecular planes [2],... 

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