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Expanding molecular dynamics simulations to the NMR time scale. I. Studies of conformational interconversions of 1, 1‐difluoro‐4, 4‐dimethylcycloheptane using MM3‐MD
Author(s) -
Li Fanbing,
Cui Weili,
Allinger Norman L.
Publication year - 1994
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540150709
Subject(s) - pseudorotation , chemistry , ring (chemistry) , molecular dynamics , twist , cyclohexane conformation , crystallography , force field (fiction) , fluorine , ring flip , computational chemistry , stereochemistry , molecule , hydrogen bond , geometry , physics , mathematics , organic chemistry , quantum mechanics
A molecular dynamics (MD) simulation of 35,000 picoseconds (ps) has been carried out to study the conformational interconversions of 1,1‐difluoro‐4,4‐dimethylcycloheptane at room temperature using the MM3 force field. The exchange between axial and equatorial fluorine atoms was the only conformational interconversion that occurred, and it took place via the process of pseudorotation. Ring inversions (twist–chair < twist–boat < twist–chair) were not observed. The axial‐equatorial exchange of the two fluorine atoms took place five times during the MD trajectory of 35,000 ps. The two CH 3 groups occupied symmetrical positions (exchangeable by a C 2 ‐like rotations, where C 2 ‐like means it would be C 2 if the fluorines were not present) in the MM3 structures, and during most of the time of the MD trajectory. The methyls occasionally moved off the C 2 ‐like axis in the simulated process, mostly because the C 2 ‐like axis was momentarily moved so that it did not pass through the ring atom to which the two CH 3 groups are bonded. A C 2 ‐like symmetry of the twist‐chair conformation was maintained approximately during most of the MD simulation. The conformational geometry with the highest energy obtained during the axial‐equatorial exchange process was found and used to locate the transition state. The energy barrier for this axial‐equatorial exchange was calculated to be 4.7 kcal/mol, and it compares with the value (5.0 kcal/mol) determined by dynamic nuclear magnetic resonance (NMR). © 1994 by John Wiley & Sons, Inc.