Planar conformation calculations

Figure 8 Vectorially calculated dipole moments for the three planar conformations of l,l -thiocarbonyl-bis-...

Two Hell UPS spectra of poly(3-hexylthiophene), or P3HT, compared with the DOVS derived from VEH band structure calculations 83], arc shown in Figure 5-14. The general chemical structure of poIy(3-a ky thiophcne) is sketched in Figure 5-4. The two UPS spectra, were recorded at two different temperatures, +190°C and -60 "C, respectively, and the DOVS was derived from VEH calculations on a planar conformation of P3HT. Compared to unsubslitutcd polythio-phene, the main influence in the UPS spectra due to the presence of the hexyl... 

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]. 

The calculations were performed into two basis sets, with full geometiy optimization except for the torsional angles a and 6. Two non planar conformations were considered, which correspond to minima on the potential energy surface into the GVB approximation [21]. In these conformations, the molecule adopts a pyramidal conformation, as in methanal. In addition, the hydroxilic group is rotated up or down the OCO plane. 

In Table 6, the formation energy values for these two preferred conformations are given, together with the corresponding values for the planar conformations syn and anti. It is seen that one of the minima is only slightly more stable than the other when calucated with the larger basis, but much more stable than the planar conformations in accordance with the GVB calculations [21]. 

The idea that the most stable conformation of 42 and 43 may be helical is supported by a molecular mechanics calculation using Discover III with a PCFF force held (MSI, ver. 3.00). Figure 4.31a shows the total energy of a 42 oligomer with 21 Si repeat units as a function of the Si-Si-Si-Si dihedral angle. The respective P- and M-helical conformations of 42 near dihedral angles of 160° and 200° are more stable than a trans planar conformation of... 

Table I lists the proportions of the non-planar conformers of TIN and TINS that are present in various solvents and the contribution to the total absorbance made by the non-planar conformer, %A (X), at the excitation wavelength, X, calculated using Equation 1, together with relevant solvent parameters.

The spectrum of TIN in PS is red-shifted compared to its spectrum in PMMA and so the relative proportion of each form could not be calculated using this method. However, the similarity between these two spectra suggests that a comparable proportion of TIN molecules in PS assume a non-planar conformation. The absorption spectrum of MeTIN in a PMMA film consists of a single absorption band (see Figure 9). This band is similar to that observed for MeTIN in solution suggesting that MeTIN exists almost entirely in a non-planar conformation in this polymer. 

The reactivity of dienes in Diels-Alder reactions is also controlled by the diene conformation. The two planar conformations of 1,3-butadiene are referred to as s-trans and s-cis (equation 16). Calculations have shown the s-trans conformation to be 2-5 kcalmol-1 more stable than the s-cis conformation. Open-chain dienes can only react in their cisoid conformation. Thus, 2-substituted dienes are generally more reactive than 1,3-butadiene due to their stronger preference for the s-cis conformation. 1 -Cis substituted 1,3-butadienes are almost exclusively in the s-trans conformation and are not reactive in Diels-Alder reactions. Highly substituted dienes may, however, be present in the s-cis conformation during a sufficient amount of time to participate in Diels-Alder reactions, even if a 1 -cis substituent is present62. 

In the case of thioanisole cation-radical, ESR spectroscopy (Alberti et al. 1984) and B3LYP calculations (Baciocchi and Gerini 2004) convincingly indicate that the planar conformation is by far the most stable. In the cation-radical, the thiomethyl group remains, in expectation, an electron-donating substituent. 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

The planar conformation of 27 with symmetiy is calculated to be 76.1 kcal/mol higher in energy than the bowl by ab initio calculations. However, this structure is not the TS for bowl-to-bowl inversion since it exhibits two imaginary... 

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

Limited knowledge exists with respect to detailed molecular structures of simple pyrans and heteropyrans. The semiempirical CNDO/2 calculations, using a full geometry optimization procedure, predict 4 and 5 to exhibit small deviations of the heterocyclic rings from the planar conformations.34 Unfortunately, similar calculated data for thiopyrans 6, 7, and 9 were not presented.45 Accounts of several X-ray diffraction investigations on compounds possessing pyran or thiopyran fragments are available.46 51... 

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