Force field method, prediction

Vibrational frequencies of 1,4-benzodioxin using the density functional theory (DFT) method, as well as the conventional HF and MM3 force-field methods, were calculated to evaluate the frequency prediction capability of each computational method and get a better understanding of the vibrational spectra . 

The conformational energies of the lower members of POM, 2,4-dioxapentane and 2,4,6-trioxaheptane are estimated by the empirical force field method. The gauche states of the Internal rotation around the skeletal C—0 bonds are successfully predicted to be of lower energies in both molecules. In order to calculate the unperturbed dimension and dipole moment of POM, RIS approximations are made by using the results obtained from the force field calculations on 2,4,6-trioxaheptane. Although these parameters are significantly different from those estimated earlier, they reproduce the observed values fairly well. 

Roth and coworkers41 42 have chosen carefully recalibrated force fields to predict reliable heats of formation with errors intended by the authors to be as small as 0.5 kcal mol"1. They started with the MM2 force field of Allinger98 and added to this parameters for the C(sp2)—C(sp2) and the C(cyclopropyl)—C(sp2) single bonds of reference compounds such as substituted 1,3-butadienes and vinylcyclopropanes41. Particular care was given to the correct description of the torsion potential of the reference compounds. The modified MM2 force field (MM2ERW) developed by Roth and coworkers describes polyenes and cyclopolyenes in terms of localized bond structures without any reference to quantum chemical methods such as the Pariser-Parr-Pople (PPP) approach (see Section II.C)41. 

There are various methods for the prediction of stereoselectivities, both in racemate separation (thermodynamics, usually computed by force field methods) and enantioselective catalysis (reactivity, usually computed by quantum mechanics)18. Promising recent developments, primarily based on force field and statistical methods, but also involving QM modeling, are based on stereocartography, the computation of the chirality content and the evaluation of chirophores.151 154... 

Semi-empirical models, like force field methods, perform best when their parameters are good, that is, for well-known systems. They cannot predict completely new structures. Even for known compounds, their results are not always dependable. Thus, semi-empirical calculations tend to overstress diradical mechanisms in cycloadditions.17 AMI and PM3 underestimate frontier interactions with respect to steric repulsions. Basicities of anions are overestimated and their nucleophilicities underestimated. Optimizations may give unreasonable structures. Usually, but not systematically, PM3 gives more reliable structures and AMI more realistic energies.18... 

An additional advantage of the force field method 32) is its power to predict the energy levels of conformations which are not populated and even complete rotational potentials of bonds. Again, statisfying agreement with results from dynamic nmr-measurements for a series of crowed hydrocarbons was found 36). Knowledge of the shape of rotational potentials proved to be helpful for the interpretation of entropy effects in these series of compounds and in their thermolysis reactions. 

Ken pioneered the modeling of transition states with force field methods. Before modem tools existed for locating transition structures in all but the simplest reactions, his group used ab initio calculations to find the geometries of transition states and to determine force constants for distortions away from these preferred geometries. These force constants could then be used in standard molecular mechanics calculations, in order to predict how steric effects would affect the geometries and energies of the transition structures when substituent were present. 

Force field methods are primarily geared to predict two properties geometries and... 

The molecular configuration is a function of time. Molecular systems are not stationary molecules vibrate, rotate, and tumble. Force field calculations and the properties predicted by them are based on a. stationary model. What is needed is some way to predict what motions the atoms within a molecule will undergo at various temperatures. Molecular dynam-ics (MD) simulations use classical mechanics—force field methods—to study the atomic and molecular motions to predict macroscopic properties. "... 

Quantum chemistry approaches to zeolites are complemented by an active research community that uses classical force-field methods to study molecular adsorption and diffusion in zeolites and similar materials. This topic was comprehensively reviewed by Keil, Krishna, and Coppens in 2000.262 For more recent examples of activity in this area, see References 263-270. Examples of impressive agreement between adsorption isotherms and molecular dilfusivities predicted with calculations of this type and experimental data are available.271,272 There appear to be many future opportunities for linking the detailed understanding of multi-component adsorption and diffusion that is now emerging from this area with detailed quantum chemistry approaches to reactivity at active sites inside zeolites. 

Huige and Hezemans179 180 have performed extensive molecular mechanics calculations using the consistent force-field method on various oligo- and polyisocyanides. The hexadecamer of ferf-butyl isocyanide was calculated to have a helical middle section and disordered ends. The dihedral angle N=C—C=N in the middle section was found to be 78.6°, and the number of repeat units per helical turn was 3.75. The latter number is in agreement with circular dichroism calculations using Tinoco s exciton theory (3.6—4.6) and De Voe s polarizability theory (3.81). The molecular mechanics calculations further predicted that the less bulky polymers 56 and 57 form helical polymers as well, whereas a disordered structure was calculated for poly(methyl isocyanide) (55). 

In microscopic approaches the solvent molecules are described as true discrete entities but in some simplified form, generally based on force-field methods (Allinger, 1977). These theories may be of the semicontinuum type if the distant bulk solvent is accounted for, or of the fully discrete type if the solvent description includes a large number of molecules. As an example, the spectrum of formaldehyde in water has been examined using a combination of classical molecular dynamics and ab initio quantum chemical methods and sampling the calculated spectrum at different classical conformations (Blair et al., 1989 Levy et al., 1990). These calculations predict most of the solvent shift as well as the line broadening. 

S-endo cyclization because of a strong conformational preference for the s-trans form of 49 [21]. It is interesting to note that both force field methods are unable to correctly predict the outcome in this and several other similar cases. The ROMP2/... 

---