Figure 11.S Possible conformations of N2H4 with pyramidal N. Hydrazine adopts the gauche C l form with N N 145 pm, H-N- II 108°. and a twist angle of 95 as shown in the lower diagram. |

As a consequence of these various possible conformations, the polymer chains exist as coils with spherical symmetry. Our eventual goal is to describe these three-dimensional structures, although some preliminary considerations must be taken up first. Accordingly, we begin by discussing a statistical exercise called a one-dimensional random walk. [Pg.43]

For any given protein, the number of possible conformations that it could adopt is astronomical. Yet each protein folds into a unique stmcture totally deterrnined by its sequence. The basic assumption is that the protein is at a free energy minimum however, calometric studies have shown that a native protein is more stable than its unfolded state by only 20—80 kj/mol (5—20 kcal/mol) (5). This small difference can be accounted for by the favorable [Pg.209]

As the ring size increases, the number of possible conformations increases further so that many alternative diamond-lattice conformations are available. [Pg.149]

The level of theory necessary for computing PES s depends on how those results are to be used. Molecular mechanics calculations are often used for examining possible conformers of a molecule. Semiempiricial calculations can give a qualitative picture of a reaction surface. Ah initio methods must often be used for quantitatively correct reaction surfaces. Note that size consistent methods must be used for the most accurate results. The specific recommendations given in Chapter 18 are equally applicable to PES calculations. [Pg.175]

Notice molecular symmetry at work. The Huckel rr-electron model is in many ways a blunt instrument, because we would get exactly the same answers for either of the following possible conformers of hexatriene (Figure 7.3). [Pg.125]

STO-3G calculations find the corresponding transition state to be more stable than other possible conformations by several kilocalories per raole. The origin of the preference for this transition-state conformation is believed to be a stabilization of the C=0 LUMO by the a orbital of the perpendicularly oriented substituent. [Pg.175]

Fig. 8-1. Schematic representations of the interaction of the (R)NapEtNH enantiomer guest with a chiral pyridine-18-crown-6 host (S,S)-1 and possible conformations of the (R)NapEt com- |

The model (23) is simple to study, but unfortunately not very widely applicable. In general, one is more interested in situations where the interfaces are free to fold around and to assume every possible conformation. A second possible approach is to switch over to a lattice formulation. This has been done by a number of groups [231-233]. The resulting models are very [Pg.668]

Polymers can be crystalline, but may not be easy to crystallize. Computational studies can be used to predict whether a polymer is likely to crystallize readily. One reason polymers fail to crystallize is that there may be many conformers with similar energies and thus little thermodynamic driving force toward an ordered conformation. Calculations of possible conformations of a short oligomer can be used to determine the difference in energy between the most stable conformer and other low-energy conformers. [Pg.311]

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