Schleyer force field

The Schleyer-Mislow program BIGSTRN (26b) likewise is extensively used. This program incorporates the so-called Engler-Schleyer force field (26a) as well as AUinger s 1971 force field as standard options and is unique in that it uses pattern-search methods of energy minimization. This method is effective at the outset but slows down as the geometry approaches a minimum (Table 2). 

Several modifications of the Engler-Schleyer force field have appeared. White (27) added olefin parameters and used an efficient two-stage Newton-Raphson minimization modification. The accuracy of the White force field regarding heat of formation calculations is... 

The observed low barrier of conformational interconversion in manxane (18, 12 2 kcal/mol) was satisfactorily reproduced by the Schleyer force field (125). This molecule also involves a number of close intramolecular nonbonded H H contacts (126). The flipping of one methylene bridge is transmitted readily through space to the... 

Later, DeTar (241) tentatively implemented parameters for alcohols and carboxyl groups in the Schleyer force field to treat esterification, this time using the orthoacid model (77) ... 

Carbonium ions are well-defined transition states in many reactions. Indeed, one of the earliest applications of MM method to the reactivity problem was concerned with carbonium ions. At present, only the Schleyer force field (26b) and its predecessor (253) are capable of handhng carbonium ions, although the parameterization principle used earlier can be readily improved upon to the present standards. Schleyer s measure of steric strain in carbonium ions, Ajtrain (the difference in steric energies of free carbonium ion and its... 

One of the most talked-about dream targets of synthesis, dodeca-hedrane (123), has been studied repeatedly by the MM method (128,235). Recent MM2 calculations (234) predict a heat of formation of 22.15 kcal/mol, incidentally equal to the average from the two previous values 45 .28 by the Allinger 1971 force field, and -0.22 kcal/mol by the Schleyer force field (26a). In the If, structure a very high strain energy is predicted by these two force fields because of the perfect eclipsed orientation of all the C—C as well as C—H bonds. Ermer calculated twisted conformations for this molecule with the CFF and confirmed that the I, structure was the GMEC (324,324a). 

Figure 2. Strain energies of conformers of substituted bicyclo[3.3.1]nonanes 2 and 22-25 calculated using the Schleyer force field (24,33).

W L. Jorgensen, OPLS force fields, in 77ie Encydopedia of Computational Chemistry, Vol. 3, P. v. R. Schleyer,... 

AD MacKerell Jr. Protein force fields. In PvR Schleyer, NL Allmger, T Clark, I Gasteiger, PA Kollman, HP Schaefer III, PR Schreiner, eds. The Encyclopedia of Computational Chemistry, Vol 3. Chichester, UK, Wiley 1998, pp 2191-2200. 

The problem of the preferred conformation of cyclodecane has been extensively studied by Dunitz et al. (46). In the crystals of seven simple cyclodecane derivatives (mono- or 1,6-disubstituted cyclodecanes) the same conformation was found for the ten-membered ring (BCB-conformation, Fig. 9). It follows from this that the BCB-conformation is an energetically favourable conformation, possibly the most favourable one. Numerous force field calculations support this interpretation Of all calculated conformations BOB corresponded to the lowest potential energy minimum. Lately this picture has become more complicated, however. A recent force field calculation of Schleyer etal. (21) yielded for a conformation termed TCCC a potential energy lower by 0.6 kcal mole-1 than for BCB. (Fig. 9 T stands for twisted TCCC is a C2h-symmetric crown-conformation which can be derived from rrans-decalin by breaking the central CC-bond and keeping the symmetry.) A force field of... 

Chemical reactivity differences may be calculated if for the transition state of a rate-determining step of a reaction a structural model can be given which is describable by a force field with known constants. We give only two examples. Schleyer and coworkers were able to interpret quantitatively a multitude of carbonium-ion reactivities (63, 111) in this way. Adams and Kovacic studied the pyrolysis of 3-homoadamantylacetate (I) at 550 °C and considered as transition state models the two bridgehead olefins II and III (112). From kinetic data they estimated II to be about 2 kcal mole-1 more favourable than III. 

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

EAS (Engler, Andose, Schleyer) [184] is quite an old force field designed to model alkanes exclusively. The harmonic potential is used for the bond stretching and cubic anharmonic for the valence angle bending. No out of plane, electrostatic or cross terms are included. The nonbonded interactions are represented by the Buchingham potential. 

Perhydrotriquinacene 358) has a gas-phase enthalpy of formation of —102.39 kJ mol-1,379 suggesting that the latent strain in the pentagonal dodeca-hedrane molecule should be rather lower than that predicted by Allinger s force field calculations380 and somewhat more comparable to the molecular mechanics prediction advanced by Schleyer.381 The existing discrepancy arises because both... 

The situation is more complicated in homoadamantane (7) where a casual study of the molecular model would suggest that the molecule ought to assume two strain-free enantiomeric C2 conformations (6 and 6 ). Schleyer s force-field calculation (22) has predicted, however, that the achiral C2v conformer (7) will be more stable than the twisted conformers, and this has been borne out by a dynamic NMR study (23), as well as by CD spectral analyses of its mono- and C2 diketone derivatives (24). 

We are indebted to Professor Schleyer for supplying us with unpublished results obtained using his force-field. 

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