H-1-NMR LANTHANIDE-INDUCED SHIFT INVESTIGATION OF HIGHLY FLEXIBLE MOLECULES - A NEW APPROACH

A new method for lanthanide-induced shift (LIS) calculations of highly flexible structures was developed using molecular geometries obtained from the random variation of dihedral angles (up to ten) which are rotating freely and independently of each other. The result of the new method (program JLLISMUL) is a data set containing the specified random dihedral angles and the corresponding agreement factor R (as a criterion for the fitting of the calculated and experimental LIS values). For comparison, the same random procedure was also used for parallel force field calculations using the molecular mechanics program PIMM. The results obtained by the new method are in very good agreement with known or reported preferred conformations of five highly flexible molecules which were used as examples for examining the new strategies of obtaining the preferred conformers. The minima of both the 'LIS R factor hyper surface' and the 'energy hyper surface' are rapidly accessible.