DREIDING method

DREIDING is an all-purpose organic or bio-organic molecule force field. It has been most widely used for large biomolecular systems. It uses five valence terms, one of which is an electrostatic term. The use of DREIDING has been dwindling with the introduction of improved methods. 

The same situation is expected for stereoisomers of the dendrimer 6 (G = 1), as revealed by a careful inspection of Figure 10 in which the dependence of the fractal dimension of the 12 isomers on probe radii is depicted. Due to the large number of atoms of this open-shell dendrimer, a low level theoretical method, the Dreiding force-field, was used for the optimizations of the geometries of these 12 isomers. ... 

In his constant search for better methods of teaching he made a 60-minute color-sound movie Techniques of Organic Chemistry, and developed a set of precise plastic molecular models, which are larger than, but have the same relative dimensions as, Dreiding models. Unlike the latter, however, the Fieser models have been so inexpensive to manufacture that even undergraduate students have been able to afford a set. 

In table 2, the last two values in the present results column (D-MM), for each molecule, were obtained using the Dreiding II force field to represent the zeolite. The first of the two values, for instance 11.7 in case of ethane, were obtained considering a loading of just one molecule of alkane, in order to compare with the results of Tiltiloye et al. [24]. From table 2 it can be seen that the MD and MC methods furnish comparable results of adsorption energy although the MD values are slightly better when compared with the experiments. 

For the molecules investigated, the MD and MC methods furnish similar adsorption energies although the MD results are slightly better when compared to the experiments. Since the MC and MD simulations have been performed in different ensembles but with the same force field, the difference in the results of the two simulations may be partly due to finite size effects. However, temperature fluctuations during the MD simulations in the NVE ensemble may also contribute to this difference. For the linear alkanes the MM results are qualitatively correct but only when a specific force field is used to describe the zeolite. However, for the branched alkane the MM results are comparable to the MD and MC ones even when a generic force field, such as Dreiding n, is used to represent the zeolite. 

Lin and Marks have considered olefin insertion into the actinide-hydrogen bonds of Cp2An(H)(OR) complexes [Cp = ti -(CH3)5C5 An = Th, U R = achiral or chiral group].Calculations included molecular mechanics analysis of the interaction of Cp U(H) [l(S)-entfo]-bornoxide with norbor-nene. The DREIDING and MMP2 force fields were used to calculate the relative energetics of olefin approach trajectories toward the U center. Three reaction pathways were considered. The authors note that whereas this method is approximate, the energetically favored pathway corresponds to the stereo-chemically favored course of the reaction. 

Nonbonded interactions consist of van der Waals (VDW) and electrostatic potentials. Examples of the valence force field approach include UFF or DREIDING [54], MM2/MMP2 [55], AMBER [56], and CHARMM [57]. The parameters of the potentials can be determined from either experiments or ab initio quantum chemical methods [58]. 

Intramolecular C—insertion is, essentially, a method for specific remote functionalization of hydrocarbons. An important implication of this for synthetic strategy is that the C—H insertion process can dissolve symmetry, thus leading from a simple precursor to a much more complex product. Both the clovene synthesis, by Dreiding, and the pentalenolactone E synthesis, by Cane, take advantage of this idea. It is further illustrated by the pentalenolactone E synthesis reported by Taber (Scheme 1). In the key step, P-keto ester (139), which has a single stereogenic center, is transformed into the tricyclic ketone (140), which has four stereogenic centers. 

Force-field calculations have given the atomic co-ordinates for 3a-hydroxy-5a-androstan-17-one ( androsterone ) with about the same accuracy as X-ray measurements, the errors being no greater than 0.1 A. The value of this method lies partly in its speed (10 minutes of IBM 360 computer time) and partly in the fact that no crystal is required. Variations of energy with conformation can also be computed. The authors stress the likely dangers in using Dreiding models, which may differ markedly in shape from actual molecules because they fail to simulate torsional and van der Waals effects. 

This method is based on the Burchart 1.01-Dreiding 2.21 force field in the Cerius2 package.1451 The Burchart force field is used to treat the frameworks, and the Dreiding II force field is used to treat the intra- and inter-molecular interactions. On the basis of known structure parameters, some parameters which are not addressed in the force field are added. 

Dreiding et al.s have also used essentially the same method for preparation of a-methylene-/3-methylbutyrolactones. Thus /3 bromotiglic ester (3) reacts with cyclohexanone under Reformatsky conditions to give (4) in 78% yield. The ester... 

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