A geometrical algorithm to search the conformational space (GASCOS) of flexible molecules

We report a geometrical algorithm to search the conformational space (GASCOS), with the goal to evaluate, in a computationally effective way, all possible conformations of a flexible molecule. The present report itself consists of two parts. The first part is primarily devoted to the mathematical principles on which our approach is based. The basic principles are illustrated in terms of the conformational aspect of small normal hydrocarbons: CnH2n+2. In the second half of the paper we apply the geometrical approach to a set of specific chemical problems namely, partially fluorinated propanes are used in order to determine the steric requirement and, therefore, the spatial proximity of the terminal atoms during multiple internal rotations. In one type of application the torsion about the bond was regarded as a continuous variable by choosing the increment in torsional angle (Delta theta) to be small, such as 5 degrees. In this approach we were able to assess the spatial requirement and therefore any possible collision of rotating moieties. In an other type of application Delta theta was assumed to be as large as 120 degrees, thus the internal rotational angle was regarded as a variable that can assume only discrete values (g(+), a, g(-)). In this fashion, the program could generate an input file for empirical force field, semi-empirical MO or ab initio molecular computations within the framework of multidimensional conformational analysis (MDCA). In other words GASCOS could become the driver program for any energy calculating software. Finally, when an unusually short distance was obtained by the GASCOS method, it was then possible to consider such an occurrence as an indicator of a probable conformational catastrophe. (C) 1999 Elsevier Science B.V. All rights reserved.