Conformation staggered position

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

The conformational energies and helix parameters are calculated for the various states of polymethylphenylsilylene chain. The calculations are performed for the three different relative dispositions of the phenyls attached to three successive silicon atoms. The minima are invariably shifted from perfectly staggered positions. 

Nearly all side-chain prediction methods depend on the concept of side-chain rotamers. From conformational analysis of organic molecules, it was predicted long ago [157, 158] that protein side chains should attain a limited number of conformations because of steric and dihedral strain within each side chain and between the side chain and the backbone (dihedral strain occurs because of Pauli exclusion between bonding molecular orbitals in eclipsed positions) [159]. For sp3-sp3 hybridized bonds, the energy minima for the dihedral are at the staggered positions that minimize dihedral strain at approximately 60°, 180°, and —60°. For sp3-sp2 bonds, the minima are usually narrowly distributed around +90° or —90° for aromatics and widely distributed around 0° or 180° for carboxylates and amides (e.g., Asn/Asp y2 and Glu/Gln /3). 

In simpler substituted ethanes in symmetrical conformations, the librational minima will be nearer to each other and they coincide at the perfectly staggered position in ethane, which has no librational barrier. If the simpler ethane has an unsymmetrical conformation, e.g. gauche butane, one librational minimum may disappear or be merely a point of inflection, so that the sixfold potential becomes an unsymmetrical threefold one with more substituted carbon-carbon bonds with other than simple methyl substituents, the six possible minima should be considered as three librational pairs. Whether any librational pair is so unsymmetrical that it can be considered as a single minimum, and thus simplified, can be decided by calculation, or perhaps by intuition. The libration may not be suitable for experimental study, but awareness of the... 

The results of studies of the related fluoroparaffins t-butyl fluoride and tris(trifluoromethyl)methane i (see Table 15) show that the C-F lengths in these molecules compare favourably with their counterparts in Table 14, particularly when allowance is made for the high nominal e.s.d. plus parameter correlation, in t-butyl fluoride. The C-C distances are quite normal for paraffins, suggesting that the electronegativity of fluorine has little effect except on the bonds directly involving fluorine. However, F F non-bonded interactions in (CF3)3CH influence the conformation of the molecule the CF groups are found to be rotated 18° from their staggered positions, and they have 10° torsional amplitudes of vibration about the C-C bonds. 

Figure 2.2 shows the conformational energy plotted as a function of the torsion angle and the energy difference between the stable staggered position. The energy barrier (eclipsed) is equal to 11.8 kj mol" which may be compared with the thermal energy at room temperature, RT 8.31 X 300 J mol 2.5 k] mol". ... 

Figure 2.4 Conformational energy of -butane as a function of torsion angle of the central carbon-carbon bond. The outer carbon-carbon bonds are assumed to be in their minimum energy states (staggered positions).

According to the ED data the prevailing conformer was syn-syn. The presence of the syn-anti form (up to 30%) could not be excluded. Local Csv symmetiy was assumed for the CF3 groups. The CF3 groups were fixed at the staggered position with respect to the P-S bonds. The nozzle was at room temperature. 

Conformer I is more stable than conformer II by 0.1(3) kcal mol (enthalpy), whereas con-former II is favored by 0.2(2) cal mol K in entropy. The dihedral angle H-N-C-C is assnmed to be either 60° or 180°. Amino gronps were assnmed to be in exactly staggered position. H(a) is hydrogen-bonded to F. 

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