Functional Groups in Molecular Mechanics

To return to the present case, force fields have been devised that include a charge distribution in alkanes at the atomic level, and other force fields that are based on the assumption that the amount of charge distribution at the atomic level in alkanes is negligible for most purposes, and therefore electrostatics can be neglected in molecular mechanics calculations on alkanes. In our earliest molecular mechanics calculations we concluded that since charges in alkanes were certainly small, we would omit them until there was evidence that they needed to be included. It is true that alkanes do have very small dipole moments, for example, propane (0.083 and isobutane (0.132D). If one wishes to reproduce these in molecular mechanics, then one would have to include electrostatics. But to our knowledge, this is the only improvonent that electrostatics would make in the molecular mechanics of alkanes. The foreseeable improvement would be quite small, so we continue to omit electrostatics for alkanes. 

Machida, Principles of Molecular Mechanics, Kodansha and Wiley, co-publication, Tokyo and New York, 1999. (b) U. Burkert and N. L. Allinger, Molecular Mechanics, American Chemical Society, Washington, D.C., 1982. (c) A. R. Leach, Molecular Modeling Principles and Applications, Pearson Education, Essex, England, 1996. (d) T. Schlick, Optimization Methods in Computational Chemistry, Reviews in Computational Chemistry, Vol. 3, K. B. Lipkowitz and D. B. Boyd, Eds., Wiley, New York, 1992, p. 1. 

Wilson, Jr., J. C. Decius, and P. C. Cross, Molecular Vibrations, The Theory of Infrared and Raman Vibrational Spectra, Dover, New York, 1955, 1980. (b) J. M. HoUas, Modem Spectroscopy, 2nd ed., WUey, Chichester, UK, 1987, 1991. 

Burkert and N. L. AUinger, Molecular Mechanics, American Chemical Society, Washington, D.C., 1982, pp. 42-46. 

Pauling, The Nature of the Chemical Bond and The Structure of Molecules and Crystals, 2nd ed., Cornell University Press, Ithaca, NY, 1948, p. 189. 

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