Conformation planar

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

M7 A double bond can be forced to adopt a planar conformation through the use of appropriate chiral raints. 

The barriers in Fig, 4-10 are high because it is difficult to twist ethylene out of its normal planar conformation. The energy is the same at the midpoint and the end points in Fig, 4-10 because, on twisting an ethylene molecule 180" out of its normal conformation, one obtains a molecule that is indistinguishable from the original. The molecule has 2-foid torsional syinnteiry. 

Neighboring C—H bonds are eclipsed in any planar cy cloalkane Thus all planar conformations are destabi lized by torsional strain... 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

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

From studies reported in the references in Table 5 (Section 4.04.1.3.1) the dihedral angle between a phenyl and a pyrazole ring in the crystalline state, falls between 4° and 22° when the phenyl group is in the 3- or 4-position. The planar conformation of C-formylpyrazoles (57) and the resonance interaction between them (87) has already been discussed in connection with H NMR (Section 4.04.1.3.3(i)) and IR studies (Section 4.04.1.3.7(iii)). 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

FIGURE 3.11 Nonpl anar ("puckered") conformation of cyclobutane. The nonplanar conformation avoids the eclipsing of bonds on adjacent carbons that characterizes the planar conformation. 

With these results, we see an example of a phenomenon that occurs from time to time a less accurate model chemistry will produce a better answer than more accurate ones. In our case, fortuitous cancellation of errors at the HF/STO-3G levels leads to the correct identification of the planar conformation as a transition state. However, a more accurate model chemistry is needed to properly study this system. ... 

The 6-6-5 membered ring of the IQ system assumes a planar conformation All carbon or nitrogen atoms deviate from the plane, but no more than 0.028 A, from the... 

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... 

The photoelectron spectra of 11 //-dibenz[Z>,e]azepine52 and 5//-dibenz[6,/]azepine53 have been measured. The results for the latter suggest that in the gas phase, the molecule has an almost planar conformation. A comparison of the photoelectron spectra of 5-methyl- and 5-acetyl-5 H-dibenz(7 ,/]azepine supports the important contribution of the amide mesomer to the latter.32... 

Unsymmetrically substituted pentadienyl anions populate six planar conformations, which are in equilibration13 a 18. The energy barrier for a torsion in the potassium compound (R = primary alkyl) was estimated to be approximately 35 keal/mol for the 1,2-bond and 15 keal/mol for the 2,3- and 3,4-bonds. The barriers are much lower in the lithium compound. Not only the rate, but also the position of the equilibrium is greatly influenced by the cation from trapping experiments18 it was concluded that the exo-VJ anion is most stable for lithium and the exo-U form for potassium. 

Fig. 1 a, b. Projection along the chain axis and side view of models of syndiotactic polystyrene in the a) trans-planar conformation b) s(2/l)2 helical conformation... 

Fig. 2 a, b. Side view and projection along the chain axis of models of isotactic polystyrene in the a) s(3/l) helical conformation b) nearly /raw-planar conformation, proposed for the crystalline gels [12]... 

In other cases, the transition induced by thermal treatments are from a metastable to a stable form and are therefore irreversible. This is observed, for instance, for the transition from the y form of s-PS (with helical conformation of the chain) toward the a form (having a trans-planar conformation of the chain) which has been shown to occur, in reasonable times, only above T 180 °C [8,9],... 

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

Well-defined differences exist, for instance, between the spectra of samples in the a and (3 crystalline forms (although both contain chains of trans-planar conformation) as shown by the expanded FTIR spectra, in two different regions, of Fig. 19 [110]. 

At the same time, Ciajolo et al, [136] presented the crystal structure of azinobis(ethylidyne-p-phenylene)dipropionate. The crystal structure contains two crystallographically independent molecules with an almost planar conformation, the phenyl rings being slightly rotated with respect to the average molecular plane. The molecules are arranged in layers. 

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]. 

Despite the planar conformation of nitrogens, several possibilities are available for the introduction of chirality. It is possible to prepare benzim-idazolylidenes, triazolylidenes, imidazolylidenes or unsubstituted-backbone imidazolinylidenes with a stereogenic center on one or two N-substituents (carbenes I and II). The other possibility is to relay the imidazolinylidenes backbone stereogenecity via the N-substituents or to combine stereogenic N-substituents with a chiral backbone (carbenes III or IV). It is at least possible to prepare bis-carbenes of type V with one (or two) stereogenic link between the two carbenes (Fig. 7). 

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]. 

Table 6 Energy values (in a.u.) of M(n,7r ) excited state of formic acid for the two minima and two planar conformations.

The geometrical parameters found for these four conformations are gathered in Table 7. It is seen that the carbon atom of the preferred conformations exhibits an sp hybridization because of the destabilization of the x electronic system by the x antibonding orbital, whereas the carbon atom of the planar conformations shows mostly an sp one. 

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