COSMIC(90): An improved molecular mechanics treatment of hydrocarbons and conjugated systems |
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Authors: | S David Morley Raymond J Abraham Ian S Haworth David E Jackson Martin R Saunders Jeremy G Vinter |
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Institution: | (1) Department of Pharmaceutical Sciences, University of Nottingham, University Park, NG7 2RD Nottingham, U.K.;(2) The School of Chemistry, The University of Liverpool, P.O. Box 147, L69 3BX Liverpool, U.K.;(3) Oxford Centre for Molecular Sciences and Physical Chemistry, Oxford University, South Parks Road, OX1 3QZ Oxford, U.K.;(4) SmithKline Beecham Pharmaceuticals, The Frythe, AL6 9AR Welwyn, Herts, U.K.;(5) Baylay's Glen, Fore Street, Weston, SG4 7AS Hitchin, Herts, U.K.;(6) Present address: Department of Chemistry, University College London, WC1H 0AJ London, U.K.;(7) University Chemical Laboratories, University of Cambridge, CB2 1EW Cambridge, U.K. |
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Abstract: | Summary Four modifications to the COSMIC molecular mechanics force field are described, which greatly increase both its versatility and the accuracy of calculated conformational energies. The Hill non-bonded van der Waals potential function has been replaced by a two-parameter Morse curve and a new H-H potential, similar to that in MM3, incorporated. Hydrocarbon energies in particular are much improved.A simple iterative Hückel pi-electron molecular orbital calculation allows modelling of conjugated systems. Calculated bond lengths and rotational barriers for a series of conjugated hydrocarbons and nitrogen heterocycles are shown to be as accurate as those determined by the MM2 SCF method.Explicit hydrogen-bonding potentials for H-bond acceptor-donor atom pairs have been included to give better hydrogen bond energies and lengths. The van der Waals radii of protonic hydrogens are reduced to 0.5 Å and the energy well depth is increased to 1.0 kcal mol-1.Two new general atom types, N+
sp
2
and O-
sp
3
, have been introduced which allow a wide variety of charged conjugated systems to be studied. A minimum of parameterisation is required, as the new types are easily included in the Hückel scheme which automatically adjusts bond and torsional parameters according to the defined bond-order relationships. |
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Keywords: | COSMIC Non-bonded potential Morse curve Hü ckel calculation Force field |
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