The spectroscopic exhibitions of the structural non-rigidity effects in the series of the complex molecular systems with a few large amplitude motions are investigated with the application of the combination of original experimental and theoretical approaches which were tested by us earlier for the study of the floppy molecules. The characteristic times of intramolecular rearrangements are estimated for the fluorocontaining compounds having the most structural non- rigidity among the R2NPX2 (R equals Me, Et; X equals F, Cl, Br) systems. Using ab initio calculations of the potential energy surfaces, the tendency of the MeOPX2 (X equals F, Cl) molecules is revealed with respect to two kinds of the large amplitude motions. They are internal rotations of the CH3 group around the C-O bond and rotations of the CH3 and PX2 groups relative to the P-O bond. On the basis of the nonempirical dynamic calculations, the features of the vibrational spectrum of the MeSiF3 molecule are studied for the region of the torsion vibrations of the CH3 and SiF3 groups around the C- Si bond.